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Submission: On May 06 via api from US — Scanned from CH
Effective URL: https://262.ecma-international.org/
Submission: On May 06 via api from US — Scanned from CH
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Table of Contents
1. Introduction
2. 1 Scope
3. ◢2 Conformance
1. 2.1 Example Normative Optional Clause Heading
2. 2.2 Example Legacy Clause Heading
3. 2.3 Example Legacy Normative Optional Clause Heading
4. 3 Normative References
5. ◢4 Overview
1. 4.1 Web Scripting
2. 4.2 Hosts and Implementations
3. ◢4.3 ECMAScript Overview
1. 4.3.1 Objects
2. 4.3.2 The Strict Variant of ECMAScript
4. ◢4.4 Terms and Definitions
1. 4.4.1 implementation-approximated
2. 4.4.2 implementation-defined
3. 4.4.3 host-defined
4. 4.4.4 type
5. 4.4.5 primitive value
6. 4.4.6 object
7. 4.4.7 constructor
8. 4.4.8 prototype
9. 4.4.9 ordinary object
10. 4.4.10 exotic object
11. 4.4.11 standard object
12. 4.4.12 built-in object
13. 4.4.13 undefined value
14. 4.4.14 Undefined type
15. 4.4.15 null value
16. 4.4.16 Null type
17. 4.4.17 Boolean value
18. 4.4.18 Boolean type
19. 4.4.19 Boolean object
20. 4.4.20 String value
21. 4.4.21 String type
22. 4.4.22 String object
23. 4.4.23 Number value
24. 4.4.24 Number type
25. 4.4.25 Number object
26. 4.4.26 Infinity
27. 4.4.27 NaN
28. 4.4.28 BigInt value
29. 4.4.29 BigInt type
30. 4.4.30 BigInt object
31. 4.4.31 Symbol value
32. 4.4.32 Symbol type
33. 4.4.33 Symbol object
34. 4.4.34 function
35. 4.4.35 built-in function
36. 4.4.36 property
37. 4.4.37 method
38. 4.4.38 built-in method
39. 4.4.39 attribute
40. 4.4.40 own property
41. 4.4.41 inherited property
5. 4.5 Organization of This Specification
6. ◢5 Notational Conventions
1. ◢5.1 Syntactic and Lexical Grammars
1. 5.1.1 Context-Free Grammars
2. 5.1.2 The Lexical and RegExp Grammars
3. 5.1.3 The Numeric String Grammar
4. 5.1.4 The Syntactic Grammar
5. ◢5.1.5 Grammar Notation
1. 5.1.5.1 Terminal Symbols
2. 5.1.5.2 Nonterminal Symbols and Productions
3. 5.1.5.3 Optional Symbols
4. 5.1.5.4 Grammatical Parameters
5. 5.1.5.5 one of
6. 5.1.5.6 [empty]
7. 5.1.5.7 Lookahead Restrictions
8. 5.1.5.8 [no LineTerminator here]
9. 5.1.5.9 but not
10. 5.1.5.10 Descriptive Phrases
2. ◢5.2 Algorithm Conventions
1. 5.2.1 Abstract Operations
2. 5.2.2 Syntax-Directed Operations
3. ◢5.2.3 Runtime Semantics
1. 5.2.3.1 Completion ( completionRecord )
2. 5.2.3.2 Throw an Exception
3. 5.2.3.3 ReturnIfAbrupt
4. 5.2.3.4 ReturnIfAbrupt Shorthands
5. 5.2.3.5 Implicit Normal Completion
4. 5.2.4 Static Semantics
5. 5.2.5 Mathematical Operations
6. 5.2.6 Value Notation
7. 5.2.7 Identity
7. ◢6 ECMAScript Data Types and Values
1. ◢6.1 ECMAScript Language Types
1. 6.1.1 The Undefined Type
2. 6.1.2 The Null Type
3. 6.1.3 The Boolean Type
4. ◢6.1.4 The String Type
1. 6.1.4.1 StringIndexOf ( string, searchValue, fromIndex )
5. ◢6.1.5 The Symbol Type
1. 6.1.5.1 Well-Known Symbols
6. ◢6.1.6 Numeric Types
1. ◢6.1.6.1 The Number Type
1. 6.1.6.1.1 Number::unaryMinus ( x )
2. 6.1.6.1.2 Number::bitwiseNOT ( x )
3. 6.1.6.1.3 Number::exponentiate ( base, exponent )
4. 6.1.6.1.4 Number::multiply ( x, y )
5. 6.1.6.1.5 Number::divide ( x, y )
6. 6.1.6.1.6 Number::remainder ( n, d )
7. 6.1.6.1.7 Number::add ( x, y )
8. 6.1.6.1.8 Number::subtract ( x, y )
9. 6.1.6.1.9 Number::leftShift ( x, y )
10. 6.1.6.1.10 Number::signedRightShift ( x, y )
11. 6.1.6.1.11 Number::unsignedRightShift ( x, y )
12. 6.1.6.1.12 Number::lessThan ( x, y )
13. 6.1.6.1.13 Number::equal ( x, y )
14. 6.1.6.1.14 Number::sameValue ( x, y )
15. 6.1.6.1.15 Number::sameValueZero ( x, y )
16. 6.1.6.1.16 NumberBitwiseOp ( op, x, y )
17. 6.1.6.1.17 Number::bitwiseAND ( x, y )
18. 6.1.6.1.18 Number::bitwiseXOR ( x, y )
19. 6.1.6.1.19 Number::bitwiseOR ( x, y )
20. 6.1.6.1.20 Number::toString ( x, radix )
2. ◢6.1.6.2 The BigInt Type
1. 6.1.6.2.1 BigInt::unaryMinus ( x )
2. 6.1.6.2.2 BigInt::bitwiseNOT ( x )
3. 6.1.6.2.3 BigInt::exponentiate ( base, exponent )
4. 6.1.6.2.4 BigInt::multiply ( x, y )
5. 6.1.6.2.5 BigInt::divide ( x, y )
6. 6.1.6.2.6 BigInt::remainder ( n, d )
7. 6.1.6.2.7 BigInt::add ( x, y )
8. 6.1.6.2.8 BigInt::subtract ( x, y )
9. 6.1.6.2.9 BigInt::leftShift ( x, y )
10. 6.1.6.2.10 BigInt::signedRightShift ( x, y )
11. 6.1.6.2.11 BigInt::unsignedRightShift ( x, y )
12. 6.1.6.2.12 BigInt::lessThan ( x, y )
13. 6.1.6.2.13 BigInt::equal ( x, y )
14. 6.1.6.2.14 BinaryAnd ( x, y )
15. 6.1.6.2.15 BinaryOr ( x, y )
16. 6.1.6.2.16 BinaryXor ( x, y )
17. 6.1.6.2.17 BigIntBitwiseOp ( op, x, y )
18. 6.1.6.2.18 BigInt::bitwiseAND ( x, y )
19. 6.1.6.2.19 BigInt::bitwiseXOR ( x, y )
20. 6.1.6.2.20 BigInt::bitwiseOR ( x, y )
21. 6.1.6.2.21 BigInt::toString ( x, radix )
7. ◢6.1.7 The Object Type
1. 6.1.7.1 Property Attributes
2. 6.1.7.2 Object Internal Methods and Internal Slots
3. 6.1.7.3 Invariants of the Essential Internal Methods
4. 6.1.7.4 Well-Known Intrinsic Objects
2. ◢6.2 ECMAScript Specification Types
1. 6.2.1 The Enum Specification Type
2. 6.2.2 The List and Record Specification Types
3. 6.2.3 The Set and Relation Specification Types
4. ◢6.2.4 The Completion Record Specification Type
1. 6.2.4.1 NormalCompletion ( value )
2. 6.2.4.2 ThrowCompletion ( value )
3. 6.2.4.3 UpdateEmpty ( completionRecord, value )
5. ◢6.2.5 The Reference Record Specification Type
1. 6.2.5.1 IsPropertyReference ( V )
2. 6.2.5.2 IsUnresolvableReference ( V )
3. 6.2.5.3 IsSuperReference ( V )
4. 6.2.5.4 IsPrivateReference ( V )
5. 6.2.5.5 GetValue ( V )
6. 6.2.5.6 PutValue ( V, W )
7. 6.2.5.7 GetThisValue ( V )
8. 6.2.5.8 InitializeReferencedBinding ( V, W )
9. 6.2.5.9 MakePrivateReference ( baseValue, privateIdentifier )
6. ◢6.2.6 The Property Descriptor Specification Type
1. 6.2.6.1 IsAccessorDescriptor ( Desc )
2. 6.2.6.2 IsDataDescriptor ( Desc )
3. 6.2.6.3 IsGenericDescriptor ( Desc )
4. 6.2.6.4 FromPropertyDescriptor ( Desc )
5. 6.2.6.5 ToPropertyDescriptor ( Obj )
6. 6.2.6.6 CompletePropertyDescriptor ( Desc )
7. 6.2.7 The Environment Record Specification Type
8. 6.2.8 The Abstract Closure Specification Type
9. ◢6.2.9 Data Blocks
1. 6.2.9.1 CreateByteDataBlock ( size )
2. 6.2.9.2 CreateSharedByteDataBlock ( size )
3. 6.2.9.3 CopyDataBlockBytes ( toBlock, toIndex, fromBlock,
fromIndex, count )
10. 6.2.10 The PrivateElement Specification Type
11. 6.2.11 The ClassFieldDefinition Record Specification Type
12. 6.2.12 Private Names
13. 6.2.13 The ClassStaticBlockDefinition Record Specification Type
8. ◢7 Abstract Operations
1. ◢7.1 Type Conversion
1. ◢7.1.1 ToPrimitive ( input [ , preferredType ] )
1. 7.1.1.1 OrdinaryToPrimitive ( O, hint )
2. 7.1.2 ToBoolean ( argument )
3. 7.1.3 ToNumeric ( value )
4. ◢7.1.4 ToNumber ( argument )
1. ◢7.1.4.1 ToNumber Applied to the String Type
1. 7.1.4.1.1 StringToNumber ( str )
2. 7.1.4.1.2 RS: StringNumericValue
3. 7.1.4.1.3 RoundMVResult ( n )
5. 7.1.5 ToIntegerOrInfinity ( argument )
6. 7.1.6 ToInt32 ( argument )
7. 7.1.7 ToUint32 ( argument )
8. 7.1.8 ToInt16 ( argument )
9. 7.1.9 ToUint16 ( argument )
10. 7.1.10 ToInt8 ( argument )
11. 7.1.11 ToUint8 ( argument )
12. 7.1.12 ToUint8Clamp ( argument )
13. 7.1.13 ToBigInt ( argument )
14. ◢7.1.14 StringToBigInt ( str )
1. 7.1.14.1 StringIntegerLiteral Grammar
2. 7.1.14.2 RS: MV
15. 7.1.15 ToBigInt64 ( argument )
16. 7.1.16 ToBigUint64 ( argument )
17. 7.1.17 ToString ( argument )
18. 7.1.18 ToObject ( argument )
19. 7.1.19 ToPropertyKey ( argument )
20. 7.1.20 ToLength ( argument )
21. 7.1.21 CanonicalNumericIndexString ( argument )
22. 7.1.22 ToIndex ( value )
2. ◢7.2 Testing and Comparison Operations
1. 7.2.1 RequireObjectCoercible ( argument )
2. 7.2.2 IsArray ( argument )
3. 7.2.3 IsCallable ( argument )
4. 7.2.4 IsConstructor ( argument )
5. 7.2.5 IsExtensible ( O )
6. 7.2.6 IsIntegralNumber ( argument )
7. 7.2.7 IsPropertyKey ( argument )
8. 7.2.8 IsRegExp ( argument )
9. 7.2.9 SS: IsStringWellFormedUnicode ( string )
10. 7.2.10 SameValue ( x, y )
11. 7.2.11 SameValueZero ( x, y )
12. 7.2.12 SameValueNonNumber ( x, y )
13. 7.2.13 IsLessThan ( x, y, LeftFirst )
14. 7.2.14 IsLooselyEqual ( x, y )
15. 7.2.15 IsStrictlyEqual ( x, y )
3. ◢7.3 Operations on Objects
1. 7.3.1 MakeBasicObject ( internalSlotsList )
2. 7.3.2 Get ( O, P )
3. 7.3.3 GetV ( V, P )
4. 7.3.4 Set ( O, P, V, Throw )
5. 7.3.5 CreateDataProperty ( O, P, V )
6. 7.3.6 CreateMethodProperty ( O, P, V )
7. 7.3.7 CreateDataPropertyOrThrow ( O, P, V )
8. 7.3.8 CreateNonEnumerableDataPropertyOrThrow ( O, P, V )
9. 7.3.9 DefinePropertyOrThrow ( O, P, desc )
10. 7.3.10 DeletePropertyOrThrow ( O, P )
11. 7.3.11 GetMethod ( V, P )
12. 7.3.12 HasProperty ( O, P )
13. 7.3.13 HasOwnProperty ( O, P )
14. 7.3.14 Call ( F, V [ , argumentsList ] )
15. 7.3.15 Construct ( F [ , argumentsList [ , newTarget ] ] )
16. 7.3.16 SetIntegrityLevel ( O, level )
17. 7.3.17 TestIntegrityLevel ( O, level )
18. 7.3.18 CreateArrayFromList ( elements )
19. 7.3.19 LengthOfArrayLike ( obj )
20. 7.3.20 CreateListFromArrayLike ( obj [ , elementTypes ] )
21. 7.3.21 Invoke ( V, P [ , argumentsList ] )
22. 7.3.22 OrdinaryHasInstance ( C, O )
23. 7.3.23 SpeciesConstructor ( O, defaultConstructor )
24. 7.3.24 EnumerableOwnProperties ( O, kind )
25. 7.3.25 GetFunctionRealm ( obj )
26. 7.3.26 CopyDataProperties ( target, source, excludedItems )
27. 7.3.27 PrivateElementFind ( O, P )
28. 7.3.28 PrivateFieldAdd ( O, P, value )
29. 7.3.29 PrivateMethodOrAccessorAdd ( O, method )
30. 7.3.30 HostEnsureCanAddPrivateElement ( O )
31. 7.3.31 PrivateGet ( O, P )
32. 7.3.32 PrivateSet ( O, P, value )
33. 7.3.33 DefineField ( receiver, fieldRecord )
34. 7.3.34 InitializeInstanceElements ( O, constructor )
4. ◢7.4 Operations on Iterator Objects
1. 7.4.1 Iterator Records
2. 7.4.2 GetIteratorFromMethod ( obj, method )
3. 7.4.3 GetIterator ( obj, kind )
4. 7.4.4 IteratorNext ( iteratorRecord [ , value ] )
5. 7.4.5 IteratorComplete ( iterResult )
6. 7.4.6 IteratorValue ( iterResult )
7. 7.4.7 IteratorStep ( iteratorRecord )
8. 7.4.8 IteratorClose ( iteratorRecord, completion )
9. 7.4.9 IfAbruptCloseIterator ( value, iteratorRecord )
10. 7.4.10 AsyncIteratorClose ( iteratorRecord, completion )
11. 7.4.11 CreateIterResultObject ( value, done )
12. 7.4.12 CreateListIteratorRecord ( list )
13. 7.4.13 IteratorToList ( iteratorRecord )
9. ◢8 Syntax-Directed Operations
1. 8.1 RS: Evaluation
2. ◢8.2 Scope Analysis
1. 8.2.1 SS: BoundNames
2. 8.2.2 SS: DeclarationPart
3. 8.2.3 SS: IsConstantDeclaration
4. 8.2.4 SS: LexicallyDeclaredNames
5. 8.2.5 SS: LexicallyScopedDeclarations
6. 8.2.6 SS: VarDeclaredNames
7. 8.2.7 SS: VarScopedDeclarations
8. 8.2.8 SS: TopLevelLexicallyDeclaredNames
9. 8.2.9 SS: TopLevelLexicallyScopedDeclarations
10. 8.2.10 SS: TopLevelVarDeclaredNames
11. 8.2.11 SS: TopLevelVarScopedDeclarations
3. ◢8.3 Labels
1. 8.3.1 SS: ContainsDuplicateLabels
2. 8.3.2 SS: ContainsUndefinedBreakTarget
3. 8.3.3 SS: ContainsUndefinedContinueTarget
4. ◢8.4 Function Name Inference
1. 8.4.1 SS: HasName
2. 8.4.2 SS: IsFunctionDefinition
3. 8.4.3 SS: IsAnonymousFunctionDefinition ( expr )
4. 8.4.4 SS: IsIdentifierRef
5. 8.4.5 RS: NamedEvaluation
5. ◢8.5 Contains
1. 8.5.1 SS: Contains
2. 8.5.2 SS: ComputedPropertyContains
6. ◢8.6 Miscellaneous
1. 8.6.1 RS: InstantiateFunctionObject
2. ◢8.6.2 RS: BindingInitialization
1. 8.6.2.1 InitializeBoundName ( name, value, environment )
3. 8.6.3 RS: IteratorBindingInitialization
4. 8.6.4 SS: AssignmentTargetType
5. 8.6.5 SS: PropName
10. ◢9 Executable Code and Execution Contexts
1. ◢9.1 Environment Records
1. ◢9.1.1 The Environment Record Type Hierarchy
1. ◢9.1.1.1 Declarative Environment Records
1. 9.1.1.1.1 HasBinding ( N )
2. 9.1.1.1.2 CreateMutableBinding ( N, D )
3. 9.1.1.1.3 CreateImmutableBinding ( N, S )
4. 9.1.1.1.4 InitializeBinding ( N, V )
5. 9.1.1.1.5 SetMutableBinding ( N, V, S )
6. 9.1.1.1.6 GetBindingValue ( N, S )
7. 9.1.1.1.7 DeleteBinding ( N )
8. 9.1.1.1.8 HasThisBinding ( )
9. 9.1.1.1.9 HasSuperBinding ( )
10. 9.1.1.1.10 WithBaseObject ( )
2. ◢9.1.1.2 Object Environment Records
1. 9.1.1.2.1 HasBinding ( N )
2. 9.1.1.2.2 CreateMutableBinding ( N, D )
3. 9.1.1.2.3 CreateImmutableBinding ( N, S )
4. 9.1.1.2.4 InitializeBinding ( N, V )
5. 9.1.1.2.5 SetMutableBinding ( N, V, S )
6. 9.1.1.2.6 GetBindingValue ( N, S )
7. 9.1.1.2.7 DeleteBinding ( N )
8. 9.1.1.2.8 HasThisBinding ( )
9. 9.1.1.2.9 HasSuperBinding ( )
10. 9.1.1.2.10 WithBaseObject ( )
3. ◢9.1.1.3 Function Environment Records
1. 9.1.1.3.1 BindThisValue ( V )
2. 9.1.1.3.2 HasThisBinding ( )
3. 9.1.1.3.3 HasSuperBinding ( )
4. 9.1.1.3.4 GetThisBinding ( )
5. 9.1.1.3.5 GetSuperBase ( )
4. ◢9.1.1.4 Global Environment Records
1. 9.1.1.4.1 HasBinding ( N )
2. 9.1.1.4.2 CreateMutableBinding ( N, D )
3. 9.1.1.4.3 CreateImmutableBinding ( N, S )
4. 9.1.1.4.4 InitializeBinding ( N, V )
5. 9.1.1.4.5 SetMutableBinding ( N, V, S )
6. 9.1.1.4.6 GetBindingValue ( N, S )
7. 9.1.1.4.7 DeleteBinding ( N )
8. 9.1.1.4.8 HasThisBinding ( )
9. 9.1.1.4.9 HasSuperBinding ( )
10. 9.1.1.4.10 WithBaseObject ( )
11. 9.1.1.4.11 GetThisBinding ( )
12. 9.1.1.4.12 HasVarDeclaration ( N )
13. 9.1.1.4.13 HasLexicalDeclaration ( N )
14. 9.1.1.4.14 HasRestrictedGlobalProperty ( N )
15. 9.1.1.4.15 CanDeclareGlobalVar ( N )
16. 9.1.1.4.16 CanDeclareGlobalFunction ( N )
17. 9.1.1.4.17 CreateGlobalVarBinding ( N, D )
18. 9.1.1.4.18 CreateGlobalFunctionBinding ( N, V, D )
5. ◢9.1.1.5 Module Environment Records
1. 9.1.1.5.1 GetBindingValue ( N, S )
2. 9.1.1.5.2 DeleteBinding ( N )
3. 9.1.1.5.3 HasThisBinding ( )
4. 9.1.1.5.4 GetThisBinding ( )
5. 9.1.1.5.5 CreateImportBinding ( N, M, N2 )
2. ◢9.1.2 Environment Record Operations
1. 9.1.2.1 GetIdentifierReference ( env, name, strict )
2. 9.1.2.2 NewDeclarativeEnvironment ( E )
3. 9.1.2.3 NewObjectEnvironment ( O, W, E )
4. 9.1.2.4 NewFunctionEnvironment ( F, newTarget )
5. 9.1.2.5 NewGlobalEnvironment ( G, thisValue )
6. 9.1.2.6 NewModuleEnvironment ( E )
2. ◢9.2 PrivateEnvironment Records
1. ◢9.2.1 PrivateEnvironment Record Operations
1. 9.2.1.1 NewPrivateEnvironment ( outerPrivEnv )
2. 9.2.1.2 ResolvePrivateIdentifier ( privEnv, identifier )
3. ◢9.3 Realms
1. 9.3.1 CreateRealm ( )
2. 9.3.2 CreateIntrinsics ( realmRec )
3. 9.3.3 SetRealmGlobalObject ( realmRec, globalObj, thisValue )
4. 9.3.4 SetDefaultGlobalBindings ( realmRec )
4. ◢9.4 Execution Contexts
1. 9.4.1 GetActiveScriptOrModule ( )
2. 9.4.2 ResolveBinding ( name [ , env ] )
3. 9.4.3 GetThisEnvironment ( )
4. 9.4.4 ResolveThisBinding ( )
5. 9.4.5 GetNewTarget ( )
6. 9.4.6 GetGlobalObject ( )
5. ◢9.5 Jobs and Host Operations to Enqueue Jobs
1. 9.5.1 JobCallback Records
2. 9.5.2 HostMakeJobCallback ( callback )
3. 9.5.3 HostCallJobCallback ( jobCallback, V, argumentsList )
4. 9.5.4 HostEnqueuePromiseJob ( job, realm )
6. 9.6 InitializeHostDefinedRealm ( )
7. ◢9.7 Agents
1. 9.7.1 AgentSignifier ( )
2. 9.7.2 AgentCanSuspend ( )
8. 9.8 Agent Clusters
9. 9.9 Forward Progress
10. ◢9.10 Processing Model of WeakRef and FinalizationRegistry Targets
1. 9.10.1 Objectives
2. 9.10.2 Liveness
3. 9.10.3 Execution
4. ◢9.10.4 Host Hooks
1. 9.10.4.1 HostEnqueueFinalizationRegistryCleanupJob (
finalizationRegistry )
11. 9.11 ClearKeptObjects ( )
12. 9.12 AddToKeptObjects ( value )
13. 9.13 CleanupFinalizationRegistry ( finalizationRegistry )
14. 9.14 CanBeHeldWeakly ( v )
11. ◢10 Ordinary and Exotic Objects Behaviours
1. ◢10.1 Ordinary Object Internal Methods and Internal Slots
1. ◢10.1.1 [[GetPrototypeOf]] ( )
1. 10.1.1.1 OrdinaryGetPrototypeOf ( O )
2. ◢10.1.2 [[SetPrototypeOf]] ( V )
1. 10.1.2.1 OrdinarySetPrototypeOf ( O, V )
3. ◢10.1.3 [[IsExtensible]] ( )
1. 10.1.3.1 OrdinaryIsExtensible ( O )
4. ◢10.1.4 [[PreventExtensions]] ( )
1. 10.1.4.1 OrdinaryPreventExtensions ( O )
5. ◢10.1.5 [[GetOwnProperty]] ( P )
1. 10.1.5.1 OrdinaryGetOwnProperty ( O, P )
6. ◢10.1.6 [[DefineOwnProperty]] ( P, Desc )
1. 10.1.6.1 OrdinaryDefineOwnProperty ( O, P, Desc )
2. 10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible, Desc,
Current )
3. 10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P, extensible,
Desc, current )
7. ◢10.1.7 [[HasProperty]] ( P )
1. 10.1.7.1 OrdinaryHasProperty ( O, P )
8. ◢10.1.8 [[Get]] ( P, Receiver )
1. 10.1.8.1 OrdinaryGet ( O, P, Receiver )
9. ◢10.1.9 [[Set]] ( P, V, Receiver )
1. 10.1.9.1 OrdinarySet ( O, P, V, Receiver )
2. 10.1.9.2 OrdinarySetWithOwnDescriptor ( O, P, V, Receiver,
ownDesc )
10. ◢10.1.10 [[Delete]] ( P )
1. 10.1.10.1 OrdinaryDelete ( O, P )
11. ◢10.1.11 [[OwnPropertyKeys]] ( )
1. 10.1.11.1 OrdinaryOwnPropertyKeys ( O )
12. 10.1.12 OrdinaryObjectCreate ( proto [ , additionalInternalSlotsList
] )
13. 10.1.13 OrdinaryCreateFromConstructor ( constructor,
intrinsicDefaultProto [ , internalSlotsList ] )
14. 10.1.14 GetPrototypeFromConstructor ( constructor,
intrinsicDefaultProto )
15. 10.1.15 RequireInternalSlot ( O, internalSlot )
2. ◢10.2 ECMAScript Function Objects
1. ◢10.2.1 [[Call]] ( thisArgument, argumentsList )
1. 10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )
2. 10.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument )
3. 10.2.1.3 RS: EvaluateBody
4. 10.2.1.4 OrdinaryCallEvaluateBody ( F, argumentsList )
2. 10.2.2 [[Construct]] ( argumentsList, newTarget )
3. 10.2.3 OrdinaryFunctionCreate ( functionPrototype, sourceText,
ParameterList, Body, thisMode, env, privateEnv )
4. ◢10.2.4 AddRestrictedFunctionProperties ( F, realm )
1. 10.2.4.1 %ThrowTypeError% ( )
5. 10.2.5 MakeConstructor ( F [ , writablePrototype [ , prototype ] ] )
6. 10.2.6 MakeClassConstructor ( F )
7. 10.2.7 MakeMethod ( F, homeObject )
8. 10.2.8 DefineMethodProperty ( homeObject, key, closure, enumerable )
9. 10.2.9 SetFunctionName ( F, name [ , prefix ] )
10. 10.2.10 SetFunctionLength ( F, length )
11. 10.2.11 FunctionDeclarationInstantiation ( func, argumentsList )
3. ◢10.3 Built-in Function Objects
1. 10.3.1 [[Call]] ( thisArgument, argumentsList )
2. 10.3.2 [[Construct]] ( argumentsList, newTarget )
3. 10.3.3 CreateBuiltinFunction ( behaviour, length, name,
additionalInternalSlotsList [ , realm [ , prototype [ , prefix ] ] ]
)
4. ◢10.4 Built-in Exotic Object Internal Methods and Slots
1. ◢10.4.1 Bound Function Exotic Objects
1. 10.4.1.1 [[Call]] ( thisArgument, argumentsList )
2. 10.4.1.2 [[Construct]] ( argumentsList, newTarget )
3. 10.4.1.3 BoundFunctionCreate ( targetFunction, boundThis,
boundArgs )
2. ◢10.4.2 Array Exotic Objects
1. 10.4.2.1 [[DefineOwnProperty]] ( P, Desc )
2. 10.4.2.2 ArrayCreate ( length [ , proto ] )
3. 10.4.2.3 ArraySpeciesCreate ( originalArray, length )
4. 10.4.2.4 ArraySetLength ( A, Desc )
3. ◢10.4.3 String Exotic Objects
1. 10.4.3.1 [[GetOwnProperty]] ( P )
2. 10.4.3.2 [[DefineOwnProperty]] ( P, Desc )
3. 10.4.3.3 [[OwnPropertyKeys]] ( )
4. 10.4.3.4 StringCreate ( value, prototype )
5. 10.4.3.5 StringGetOwnProperty ( S, P )
4. ◢10.4.4 Arguments Exotic Objects
1. 10.4.4.1 [[GetOwnProperty]] ( P )
2. 10.4.4.2 [[DefineOwnProperty]] ( P, Desc )
3. 10.4.4.3 [[Get]] ( P, Receiver )
4. 10.4.4.4 [[Set]] ( P, V, Receiver )
5. 10.4.4.5 [[Delete]] ( P )
6. 10.4.4.6 CreateUnmappedArgumentsObject ( argumentsList )
7. ◢10.4.4.7 CreateMappedArgumentsObject ( func, formals,
argumentsList, env )
1. 10.4.4.7.1 MakeArgGetter ( name, env )
2. 10.4.4.7.2 MakeArgSetter ( name, env )
5. ◢10.4.5 Integer-Indexed Exotic Objects
1. 10.4.5.1 [[GetOwnProperty]] ( P )
2. 10.4.5.2 [[HasProperty]] ( P )
3. 10.4.5.3 [[DefineOwnProperty]] ( P, Desc )
4. 10.4.5.4 [[Get]] ( P, Receiver )
5. 10.4.5.5 [[Set]] ( P, V, Receiver )
6. 10.4.5.6 [[Delete]] ( P )
7. 10.4.5.7 [[OwnPropertyKeys]] ( )
8. 10.4.5.8 IntegerIndexedObjectCreate ( prototype )
9. 10.4.5.9 IsValidIntegerIndex ( O, index )
10. 10.4.5.10 IntegerIndexedElementGet ( O, index )
11. 10.4.5.11 IntegerIndexedElementSet ( O, index, value )
6. ◢10.4.6 Module Namespace Exotic Objects
1. 10.4.6.1 [[GetPrototypeOf]] ( )
2. 10.4.6.2 [[SetPrototypeOf]] ( V )
3. 10.4.6.3 [[IsExtensible]] ( )
4. 10.4.6.4 [[PreventExtensions]] ( )
5. 10.4.6.5 [[GetOwnProperty]] ( P )
6. 10.4.6.6 [[DefineOwnProperty]] ( P, Desc )
7. 10.4.6.7 [[HasProperty]] ( P )
8. 10.4.6.8 [[Get]] ( P, Receiver )
9. 10.4.6.9 [[Set]] ( P, V, Receiver )
10. 10.4.6.10 [[Delete]] ( P )
11. 10.4.6.11 [[OwnPropertyKeys]] ( )
12. 10.4.6.12 ModuleNamespaceCreate ( module, exports )
7. ◢10.4.7 Immutable Prototype Exotic Objects
1. 10.4.7.1 [[SetPrototypeOf]] ( V )
2. 10.4.7.2 SetImmutablePrototype ( O, V )
5. ◢10.5 Proxy Object Internal Methods and Internal Slots
1. 10.5.1 [[GetPrototypeOf]] ( )
2. 10.5.2 [[SetPrototypeOf]] ( V )
3. 10.5.3 [[IsExtensible]] ( )
4. 10.5.4 [[PreventExtensions]] ( )
5. 10.5.5 [[GetOwnProperty]] ( P )
6. 10.5.6 [[DefineOwnProperty]] ( P, Desc )
7. 10.5.7 [[HasProperty]] ( P )
8. 10.5.8 [[Get]] ( P, Receiver )
9. 10.5.9 [[Set]] ( P, V, Receiver )
10. 10.5.10 [[Delete]] ( P )
11. 10.5.11 [[OwnPropertyKeys]] ( )
12. 10.5.12 [[Call]] ( thisArgument, argumentsList )
13. 10.5.13 [[Construct]] ( argumentsList, newTarget )
14. 10.5.14 ValidateNonRevokedProxy ( proxy )
15. 10.5.15 ProxyCreate ( target, handler )
12. ◢11 ECMAScript Language: Source Text
1. ◢11.1 Source Text
1. 11.1.1 SS: UTF16EncodeCodePoint ( cp )
2. 11.1.2 SS: CodePointsToString ( text )
3. 11.1.3 SS: UTF16SurrogatePairToCodePoint ( lead, trail )
4. 11.1.4 SS: CodePointAt ( string, position )
5. 11.1.5 SS: StringToCodePoints ( string )
6. 11.1.6 SS: ParseText ( sourceText, goalSymbol )
2. ◢11.2 Types of Source Code
1. 11.2.1 Directive Prologues and the Use Strict Directive
2. 11.2.2 Strict Mode Code
3. 11.2.3 Non-ECMAScript Functions
13. ◢12 ECMAScript Language: Lexical Grammar
1. 12.1 Unicode Format-Control Characters
2. 12.2 White Space
3. 12.3 Line Terminators
4. 12.4 Comments
5. 12.5 Hashbang Comments
6. 12.6 Tokens
7. ◢12.7 Names and Keywords
1. ◢12.7.1 Identifier Names
1. 12.7.1.1 SS: Early Errors
2. 12.7.1.2 SS: IdentifierCodePoints
3. 12.7.1.3 SS: IdentifierCodePoint
2. 12.7.2 Keywords and Reserved Words
8. 12.8 Punctuators
9. ◢12.9 Literals
1. 12.9.1 Null Literals
2. 12.9.2 Boolean Literals
3. ◢12.9.3 Numeric Literals
1. 12.9.3.1 SS: Early Errors
2. 12.9.3.2 SS: MV
3. 12.9.3.3 SS: NumericValue
4. ◢12.9.4 String Literals
1. 12.9.4.1 SS: Early Errors
2. 12.9.4.2 SS: SV
3. 12.9.4.3 SS: MV
5. ◢12.9.5 Regular Expression Literals
1. 12.9.5.1 SS: BodyText
2. 12.9.5.2 SS: FlagText
6. ◢12.9.6 Template Literal Lexical Components
1. 12.9.6.1 SS: TV
2. 12.9.6.2 SS: TRV
10. ◢12.10 Automatic Semicolon Insertion
1. 12.10.1 Rules of Automatic Semicolon Insertion
2. 12.10.2 Examples of Automatic Semicolon Insertion
3. ◢12.10.3 Interesting Cases of Automatic Semicolon Insertion
1. 12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in
Statement Lists
2. ◢12.10.3.2 Cases of Automatic Semicolon Insertion and “[no
LineTerminator here]”
1. 12.10.3.2.1 List of Grammar Productions with Optional Operands
and “[no LineTerminator here]”
14. ◢13 ECMAScript Language: Expressions
1. ◢13.1 Identifiers
1. 13.1.1 SS: Early Errors
2. 13.1.2 SS: StringValue
3. 13.1.3 RS: Evaluation
2. ◢13.2 Primary Expression
1. ◢13.2.1 The this Keyword
1. 13.2.1.1 RS: Evaluation
2. 13.2.2 Identifier Reference
3. ◢13.2.3 Literals
1. 13.2.3.1 RS: Evaluation
4. ◢13.2.4 Array Initializer
1. 13.2.4.1 RS: ArrayAccumulation
2. 13.2.4.2 RS: Evaluation
5. ◢13.2.5 Object Initializer
1. 13.2.5.1 SS: Early Errors
2. 13.2.5.2 SS: IsComputedPropertyKey
3. 13.2.5.3 SS: PropertyNameList
4. 13.2.5.4 RS: Evaluation
5. 13.2.5.5 RS: PropertyDefinitionEvaluation
6. 13.2.6 Function Defining Expressions
7. ◢13.2.7 Regular Expression Literals
1. 13.2.7.1 SS: Early Errors
2. 13.2.7.2 SS: IsValidRegularExpressionLiteral ( literal )
3. 13.2.7.3 RS: Evaluation
8. ◢13.2.8 Template Literals
1. 13.2.8.1 SS: Early Errors
2. 13.2.8.2 SS: TemplateStrings
3. 13.2.8.3 SS: TemplateString ( templateToken, raw )
4. 13.2.8.4 GetTemplateObject ( templateLiteral )
5. 13.2.8.5 RS: SubstitutionEvaluation
6. 13.2.8.6 RS: Evaluation
9. ◢13.2.9 The Grouping Operator
1. 13.2.9.1 SS: Early Errors
2. 13.2.9.2 RS: Evaluation
3. ◢13.3 Left-Hand-Side Expressions
1. ◢13.3.1 Static Semantics
1. 13.3.1.1 SS: Early Errors
2. ◢13.3.2 Property Accessors
1. 13.3.2.1 RS: Evaluation
3. 13.3.3 EvaluatePropertyAccessWithExpressionKey ( baseValue,
expression, strict )
4. 13.3.4 EvaluatePropertyAccessWithIdentifierKey ( baseValue,
identifierName, strict )
5. ◢13.3.5 The new Operator
1. ◢13.3.5.1 RS: Evaluation
1. 13.3.5.1.1 EvaluateNew ( constructExpr, arguments )
6. ◢13.3.6 Function Calls
1. 13.3.6.1 RS: Evaluation
2. 13.3.6.2 EvaluateCall ( func, ref, arguments, tailPosition )
7. ◢13.3.7 The super Keyword
1. 13.3.7.1 RS: Evaluation
2. 13.3.7.2 GetSuperConstructor ( )
3. 13.3.7.3 MakeSuperPropertyReference ( actualThis, propertyKey,
strict )
8. ◢13.3.8 Argument Lists
1. 13.3.8.1 RS: ArgumentListEvaluation
9. ◢13.3.9 Optional Chains
1. 13.3.9.1 RS: Evaluation
2. 13.3.9.2 RS: ChainEvaluation
10. ◢13.3.10 Import Calls
1. ◢13.3.10.1 RS: Evaluation
1. 13.3.10.1.1 ContinueDynamicImport ( promiseCapability,
moduleCompletion )
11. ◢13.3.11 Tagged Templates
1. 13.3.11.1 RS: Evaluation
12. ◢13.3.12 Meta Properties
1. ◢13.3.12.1 RS: Evaluation
1. 13.3.12.1.1 HostGetImportMetaProperties ( moduleRecord )
2. 13.3.12.1.2 HostFinalizeImportMeta ( importMeta, moduleRecord
)
4. ◢13.4 Update Expressions
1. 13.4.1 SS: Early Errors
2. ◢13.4.2 Postfix Increment Operator
1. 13.4.2.1 RS: Evaluation
3. ◢13.4.3 Postfix Decrement Operator
1. 13.4.3.1 RS: Evaluation
4. ◢13.4.4 Prefix Increment Operator
1. 13.4.4.1 RS: Evaluation
5. ◢13.4.5 Prefix Decrement Operator
1. 13.4.5.1 RS: Evaluation
5. ◢13.5 Unary Operators
1. ◢13.5.1 The delete Operator
1. 13.5.1.1 SS: Early Errors
2. 13.5.1.2 RS: Evaluation
2. ◢13.5.2 The void Operator
1. 13.5.2.1 RS: Evaluation
3. ◢13.5.3 The typeof Operator
1. 13.5.3.1 RS: Evaluation
4. ◢13.5.4 Unary + Operator
1. 13.5.4.1 RS: Evaluation
5. ◢13.5.5 Unary - Operator
1. 13.5.5.1 RS: Evaluation
6. ◢13.5.6 Bitwise NOT Operator ( ~ )
1. 13.5.6.1 RS: Evaluation
7. ◢13.5.7 Logical NOT Operator ( ! )
1. 13.5.7.1 RS: Evaluation
6. ◢13.6 Exponentiation Operator
1. 13.6.1 RS: Evaluation
7. ◢13.7 Multiplicative Operators
1. 13.7.1 RS: Evaluation
8. ◢13.8 Additive Operators
1. ◢13.8.1 The Addition Operator ( + )
1. 13.8.1.1 RS: Evaluation
2. ◢13.8.2 The Subtraction Operator ( - )
1. 13.8.2.1 RS: Evaluation
9. ◢13.9 Bitwise Shift Operators
1. ◢13.9.1 The Left Shift Operator ( << )
1. 13.9.1.1 RS: Evaluation
2. ◢13.9.2 The Signed Right Shift Operator ( >> )
1. 13.9.2.1 RS: Evaluation
3. ◢13.9.3 The Unsigned Right Shift Operator ( >>> )
1. 13.9.3.1 RS: Evaluation
10. ◢13.10 Relational Operators
1. 13.10.1 RS: Evaluation
2. 13.10.2 InstanceofOperator ( V, target )
11. ◢13.11 Equality Operators
1. 13.11.1 RS: Evaluation
12. ◢13.12 Binary Bitwise Operators
1. 13.12.1 RS: Evaluation
13. ◢13.13 Binary Logical Operators
1. 13.13.1 RS: Evaluation
14. ◢13.14 Conditional Operator ( ? : )
1. 13.14.1 RS: Evaluation
15. ◢13.15 Assignment Operators
1. 13.15.1 SS: Early Errors
2. 13.15.2 RS: Evaluation
3. 13.15.3 ApplyStringOrNumericBinaryOperator ( lval, opText, rval )
4. 13.15.4 EvaluateStringOrNumericBinaryExpression ( leftOperand,
opText, rightOperand )
5. ◢13.15.5 Destructuring Assignment
1. 13.15.5.1 SS: Early Errors
2. 13.15.5.2 RS: DestructuringAssignmentEvaluation
3. 13.15.5.3 RS: PropertyDestructuringAssignmentEvaluation
4. 13.15.5.4 RS: RestDestructuringAssignmentEvaluation
5. 13.15.5.5 RS: IteratorDestructuringAssignmentEvaluation
6. 13.15.5.6 RS: KeyedDestructuringAssignmentEvaluation
16. ◢13.16 Comma Operator ( , )
1. 13.16.1 RS: Evaluation
15. ◢14 ECMAScript Language: Statements and Declarations
1. ◢14.1 Statement Semantics
1. 14.1.1 RS: Evaluation
2. ◢14.2 Block
1. 14.2.1 SS: Early Errors
2. 14.2.2 RS: Evaluation
3. 14.2.3 BlockDeclarationInstantiation ( code, env )
3. ◢14.3 Declarations and the Variable Statement
1. ◢14.3.1 Let and Const Declarations
1. 14.3.1.1 SS: Early Errors
2. 14.3.1.2 RS: Evaluation
2. ◢14.3.2 Variable Statement
1. 14.3.2.1 RS: Evaluation
3. ◢14.3.3 Destructuring Binding Patterns
1. 14.3.3.1 RS: PropertyBindingInitialization
2. 14.3.3.2 RS: RestBindingInitialization
3. 14.3.3.3 RS: KeyedBindingInitialization
4. ◢14.4 Empty Statement
1. 14.4.1 RS: Evaluation
5. ◢14.5 Expression Statement
1. 14.5.1 RS: Evaluation
6. ◢14.6 The if Statement
1. 14.6.1 SS: Early Errors
2. 14.6.2 RS: Evaluation
7. ◢14.7 Iteration Statements
1. ◢14.7.1 Semantics
1. 14.7.1.1 LoopContinues ( completion, labelSet )
2. 14.7.1.2 RS: LoopEvaluation
2. ◢14.7.2 The do-while Statement
1. 14.7.2.1 SS: Early Errors
2. 14.7.2.2 RS: DoWhileLoopEvaluation
3. ◢14.7.3 The while Statement
1. 14.7.3.1 SS: Early Errors
2. 14.7.3.2 RS: WhileLoopEvaluation
4. ◢14.7.4 The for Statement
1. 14.7.4.1 SS: Early Errors
2. 14.7.4.2 RS: ForLoopEvaluation
3. 14.7.4.3 ForBodyEvaluation ( test, increment, stmt,
perIterationBindings, labelSet )
4. 14.7.4.4 CreatePerIterationEnvironment ( perIterationBindings )
5. ◢14.7.5 The for-in, for-of, and for-await-of Statements
1. 14.7.5.1 SS: Early Errors
2. 14.7.5.2 SS: IsDestructuring
3. 14.7.5.3 RS: ForDeclarationBindingInitialization
4. 14.7.5.4 RS: ForDeclarationBindingInstantiation
5. 14.7.5.5 RS: ForInOfLoopEvaluation
6. 14.7.5.6 ForIn/OfHeadEvaluation ( uninitializedBoundNames, expr,
iterationKind )
7. 14.7.5.7 ForIn/OfBodyEvaluation ( lhs, stmt, iteratorRecord,
iterationKind, lhsKind, labelSet [ , iteratorKind ] )
8. 14.7.5.8 RS: Evaluation
9. 14.7.5.9 EnumerateObjectProperties ( O )
10. ◢14.7.5.10 For-In Iterator Objects
1. 14.7.5.10.1 CreateForInIterator ( object )
2. ◢14.7.5.10.2 The %ForInIteratorPrototype% Object
1. 14.7.5.10.2.1 %ForInIteratorPrototype%.next ( )
3. 14.7.5.10.3 Properties of For-In Iterator Instances
8. ◢14.8 The continue Statement
1. 14.8.1 SS: Early Errors
2. 14.8.2 RS: Evaluation
9. ◢14.9 The break Statement
1. 14.9.1 SS: Early Errors
2. 14.9.2 RS: Evaluation
10. ◢14.10 The return Statement
1. 14.10.1 RS: Evaluation
11. ◢14.11 The with Statement
1. 14.11.1 SS: Early Errors
2. 14.11.2 RS: Evaluation
12. ◢14.12 The switch Statement
1. 14.12.1 SS: Early Errors
2. 14.12.2 RS: CaseBlockEvaluation
3. 14.12.3 CaseClauseIsSelected ( C, input )
4. 14.12.4 RS: Evaluation
13. ◢14.13 Labelled Statements
1. 14.13.1 SS: Early Errors
2. 14.13.2 SS: IsLabelledFunction ( stmt )
3. 14.13.3 RS: Evaluation
4. 14.13.4 RS: LabelledEvaluation
14. ◢14.14 The throw Statement
1. 14.14.1 RS: Evaluation
15. ◢14.15 The try Statement
1. 14.15.1 SS: Early Errors
2. 14.15.2 RS: CatchClauseEvaluation
3. 14.15.3 RS: Evaluation
16. ◢14.16 The debugger Statement
1. 14.16.1 RS: Evaluation
16. ◢15 ECMAScript Language: Functions and Classes
1. ◢15.1 Parameter Lists
1. 15.1.1 SS: Early Errors
2. 15.1.2 SS: ContainsExpression
3. 15.1.3 SS: IsSimpleParameterList
4. 15.1.4 SS: HasInitializer
5. 15.1.5 SS: ExpectedArgumentCount
2. ◢15.2 Function Definitions
1. 15.2.1 SS: Early Errors
2. 15.2.2 SS: FunctionBodyContainsUseStrict
3. 15.2.3 RS: EvaluateFunctionBody
4. 15.2.4 RS: InstantiateOrdinaryFunctionObject
5. 15.2.5 RS: InstantiateOrdinaryFunctionExpression
6. 15.2.6 RS: Evaluation
3. ◢15.3 Arrow Function Definitions
1. 15.3.1 SS: Early Errors
2. 15.3.2 SS: ConciseBodyContainsUseStrict
3. 15.3.3 RS: EvaluateConciseBody
4. 15.3.4 RS: InstantiateArrowFunctionExpression
5. 15.3.5 RS: Evaluation
4. ◢15.4 Method Definitions
1. 15.4.1 SS: Early Errors
2. 15.4.2 SS: HasDirectSuper
3. 15.4.3 SS: SpecialMethod
4. 15.4.4 RS: DefineMethod
5. 15.4.5 RS: MethodDefinitionEvaluation
5. ◢15.5 Generator Function Definitions
1. 15.5.1 SS: Early Errors
2. 15.5.2 RS: EvaluateGeneratorBody
3. 15.5.3 RS: InstantiateGeneratorFunctionObject
4. 15.5.4 RS: InstantiateGeneratorFunctionExpression
5. 15.5.5 RS: Evaluation
6. ◢15.6 Async Generator Function Definitions
1. 15.6.1 SS: Early Errors
2. 15.6.2 RS: EvaluateAsyncGeneratorBody
3. 15.6.3 RS: InstantiateAsyncGeneratorFunctionObject
4. 15.6.4 RS: InstantiateAsyncGeneratorFunctionExpression
5. 15.6.5 RS: Evaluation
7. ◢15.7 Class Definitions
1. 15.7.1 SS: Early Errors
2. 15.7.2 SS: ClassElementKind
3. 15.7.3 SS: ConstructorMethod
4. 15.7.4 SS: IsStatic
5. 15.7.5 SS: NonConstructorElements
6. 15.7.6 SS: PrototypePropertyNameList
7. 15.7.7 SS: AllPrivateIdentifiersValid
8. 15.7.8 SS: PrivateBoundIdentifiers
9. 15.7.9 SS: ContainsArguments
10. 15.7.10 RS: ClassFieldDefinitionEvaluation
11. 15.7.11 RS: ClassStaticBlockDefinitionEvaluation
12. 15.7.12 RS: EvaluateClassStaticBlockBody
13. 15.7.13 RS: ClassElementEvaluation
14. 15.7.14 RS: ClassDefinitionEvaluation
15. 15.7.15 RS: BindingClassDeclarationEvaluation
16. 15.7.16 RS: Evaluation
8. ◢15.8 Async Function Definitions
1. 15.8.1 SS: Early Errors
2. 15.8.2 RS: InstantiateAsyncFunctionObject
3. 15.8.3 RS: InstantiateAsyncFunctionExpression
4. 15.8.4 RS: EvaluateAsyncFunctionBody
5. 15.8.5 RS: Evaluation
9. ◢15.9 Async Arrow Function Definitions
1. 15.9.1 SS: Early Errors
2. 15.9.2 SS: AsyncConciseBodyContainsUseStrict
3. 15.9.3 RS: EvaluateAsyncConciseBody
4. 15.9.4 RS: InstantiateAsyncArrowFunctionExpression
5. 15.9.5 RS: Evaluation
10. ◢15.10 Tail Position Calls
1. 15.10.1 SS: IsInTailPosition ( call )
2. 15.10.2 SS: HasCallInTailPosition
3. 15.10.3 PrepareForTailCall ( )
17. ◢16 ECMAScript Language: Scripts and Modules
1. ◢16.1 Scripts
1. 16.1.1 SS: Early Errors
2. 16.1.2 SS: IsStrict
3. 16.1.3 RS: Evaluation
4. 16.1.4 Script Records
5. 16.1.5 ParseScript ( sourceText, realm, hostDefined )
6. 16.1.6 ScriptEvaluation ( scriptRecord )
7. 16.1.7 GlobalDeclarationInstantiation ( script, env )
2. ◢16.2 Modules
1. ◢16.2.1 Module Semantics
1. 16.2.1.1 SS: Early Errors
2. 16.2.1.2 SS: ImportedLocalNames ( importEntries )
3. 16.2.1.3 SS: ModuleRequests
4. 16.2.1.4 Abstract Module Records
5. ◢16.2.1.5 Cyclic Module Records
1. ◢16.2.1.5.1 LoadRequestedModules ( [ hostDefined ] )
1. 16.2.1.5.1.1 InnerModuleLoading ( state, module )
2. 16.2.1.5.1.2 ContinueModuleLoading ( state,
moduleCompletion )
2. ◢16.2.1.5.2 Link ( )
1. 16.2.1.5.2.1 InnerModuleLinking ( module, stack, index )
3. ◢16.2.1.5.3 Evaluate ( )
1. 16.2.1.5.3.1 InnerModuleEvaluation ( module, stack, index )
2. 16.2.1.5.3.2 ExecuteAsyncModule ( module )
3. 16.2.1.5.3.3 GatherAvailableAncestors ( module, execList )
4. 16.2.1.5.3.4 AsyncModuleExecutionFulfilled ( module )
5. 16.2.1.5.3.5 AsyncModuleExecutionRejected ( module, error )
4. 16.2.1.5.4 Example Cyclic Module Record Graphs
6. ◢16.2.1.6 Source Text Module Records
1. 16.2.1.6.1 ParseModule ( sourceText, realm, hostDefined )
2. 16.2.1.6.2 GetExportedNames ( [ exportStarSet ] )
3. 16.2.1.6.3 ResolveExport ( exportName [ , resolveSet ] )
4. 16.2.1.6.4 InitializeEnvironment ( )
5. 16.2.1.6.5 ExecuteModule ( [ capability ] )
7. 16.2.1.7 GetImportedModule ( referrer, specifier )
8. 16.2.1.8 HostLoadImportedModule ( referrer, specifier,
hostDefined, payload )
9. 16.2.1.9 FinishLoadingImportedModule ( referrer, specifier,
payload, result )
10. 16.2.1.10 GetModuleNamespace ( module )
11. 16.2.1.11 RS: Evaluation
2. ◢16.2.2 Imports
1. 16.2.2.1 SS: Early Errors
2. 16.2.2.2 SS: ImportEntries
3. 16.2.2.3 SS: ImportEntriesForModule
3. ◢16.2.3 Exports
1. 16.2.3.1 SS: Early Errors
2. 16.2.3.2 SS: ExportedBindings
3. 16.2.3.3 SS: ExportedNames
4. 16.2.3.4 SS: ExportEntries
5. 16.2.3.5 SS: ExportEntriesForModule
6. 16.2.3.6 SS: ReferencedBindings
7. 16.2.3.7 RS: Evaluation
18. ◢17 Error Handling and Language Extensions
1. 17.1 Forbidden Extensions
19. 18 ECMAScript Standard Built-in Objects
20. ◢19 The Global Object
1. ◢19.1 Value Properties of the Global Object
1. 19.1.1 globalThis
2. 19.1.2 Infinity
3. 19.1.3 NaN
4. 19.1.4 undefined
2. ◢19.2 Function Properties of the Global Object
1. ◢19.2.1 eval ( x )
1. 19.2.1.1 PerformEval ( x, strictCaller, direct )
2. 19.2.1.2 HostEnsureCanCompileStrings ( calleeRealm )
3. 19.2.1.3 EvalDeclarationInstantiation ( body, varEnv, lexEnv,
privateEnv, strict )
2. 19.2.2 isFinite ( number )
3. 19.2.3 isNaN ( number )
4. 19.2.4 parseFloat ( string )
5. 19.2.5 parseInt ( string, radix )
6. ◢19.2.6 URI Handling Functions
1. 19.2.6.1 decodeURI ( encodedURI )
2. 19.2.6.2 decodeURIComponent ( encodedURIComponent )
3. 19.2.6.3 encodeURI ( uri )
4. 19.2.6.4 encodeURIComponent ( uriComponent )
5. 19.2.6.5 Encode ( string, extraUnescaped )
6. 19.2.6.6 Decode ( string, preserveEscapeSet )
7. 19.2.6.7 ParseHexOctet ( string, position )
3. ◢19.3 Constructor Properties of the Global Object
1. 19.3.1 AggregateError ( . . . )
2. 19.3.2 Array ( . . . )
3. 19.3.3 ArrayBuffer ( . . . )
4. 19.3.4 BigInt ( . . . )
5. 19.3.5 BigInt64Array ( . . . )
6. 19.3.6 BigUint64Array ( . . . )
7. 19.3.7 Boolean ( . . . )
8. 19.3.8 DataView ( . . . )
9. 19.3.9 Date ( . . . )
10. 19.3.10 Error ( . . . )
11. 19.3.11 EvalError ( . . . )
12. 19.3.12 FinalizationRegistry ( . . . )
13. 19.3.13 Float32Array ( . . . )
14. 19.3.14 Float64Array ( . . . )
15. 19.3.15 Function ( . . . )
16. 19.3.16 Int8Array ( . . . )
17. 19.3.17 Int16Array ( . . . )
18. 19.3.18 Int32Array ( . . . )
19. 19.3.19 Map ( . . . )
20. 19.3.20 Number ( . . . )
21. 19.3.21 Object ( . . . )
22. 19.3.22 Promise ( . . . )
23. 19.3.23 Proxy ( . . . )
24. 19.3.24 RangeError ( . . . )
25. 19.3.25 ReferenceError ( . . . )
26. 19.3.26 RegExp ( . . . )
27. 19.3.27 Set ( . . . )
28. 19.3.28 SharedArrayBuffer ( . . . )
29. 19.3.29 String ( . . . )
30. 19.3.30 Symbol ( . . . )
31. 19.3.31 SyntaxError ( . . . )
32. 19.3.32 TypeError ( . . . )
33. 19.3.33 Uint8Array ( . . . )
34. 19.3.34 Uint8ClampedArray ( . . . )
35. 19.3.35 Uint16Array ( . . . )
36. 19.3.36 Uint32Array ( . . . )
37. 19.3.37 URIError ( . . . )
38. 19.3.38 WeakMap ( . . . )
39. 19.3.39 WeakRef ( . . . )
40. 19.3.40 WeakSet ( . . . )
4. ◢19.4 Other Properties of the Global Object
1. 19.4.1 Atomics
2. 19.4.2 JSON
3. 19.4.3 Math
4. 19.4.4 Reflect
21. ◢20 Fundamental Objects
1. ◢20.1 Object Objects
1. ◢20.1.1 The Object Constructor
1. 20.1.1.1 Object ( [ value ] )
2. ◢20.1.2 Properties of the Object Constructor
1. 20.1.2.1 Object.assign ( target, ...sources )
2. 20.1.2.2 Object.create ( O, Properties )
3. ◢20.1.2.3 Object.defineProperties ( O, Properties )
1. 20.1.2.3.1 ObjectDefineProperties ( O, Properties )
4. 20.1.2.4 Object.defineProperty ( O, P, Attributes )
5. 20.1.2.5 Object.entries ( O )
6. 20.1.2.6 Object.freeze ( O )
7. 20.1.2.7 Object.fromEntries ( iterable )
8. 20.1.2.8 Object.getOwnPropertyDescriptor ( O, P )
9. 20.1.2.9 Object.getOwnPropertyDescriptors ( O )
10. 20.1.2.10 Object.getOwnPropertyNames ( O )
11. ◢20.1.2.11 Object.getOwnPropertySymbols ( O )
1. 20.1.2.11.1 GetOwnPropertyKeys ( O, type )
12. 20.1.2.12 Object.getPrototypeOf ( O )
13. 20.1.2.13 Object.hasOwn ( O, P )
14. 20.1.2.14 Object.is ( value1, value2 )
15. 20.1.2.15 Object.isExtensible ( O )
16. 20.1.2.16 Object.isFrozen ( O )
17. 20.1.2.17 Object.isSealed ( O )
18. 20.1.2.18 Object.keys ( O )
19. 20.1.2.19 Object.preventExtensions ( O )
20. 20.1.2.20 Object.prototype
21. 20.1.2.21 Object.seal ( O )
22. 20.1.2.22 Object.setPrototypeOf ( O, proto )
23. 20.1.2.23 Object.values ( O )
3. ◢20.1.3 Properties of the Object Prototype Object
1. 20.1.3.1 Object.prototype.constructor
2. 20.1.3.2 Object.prototype.hasOwnProperty ( V )
3. 20.1.3.3 Object.prototype.isPrototypeOf ( V )
4. 20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
5. 20.1.3.5 Object.prototype.toLocaleString ( [ reserved1 [ ,
reserved2 ] ] )
6. 20.1.3.6 Object.prototype.toString ( )
7. 20.1.3.7 Object.prototype.valueOf ( )
8. ◢20.1.3.8 Object.prototype.__proto__
1. 20.1.3.8.1 get Object.prototype.__proto__
2. 20.1.3.8.2 set Object.prototype.__proto__
9. ◢20.1.3.9 Legacy Object.prototype Accessor Methods
1. 20.1.3.9.1 Object.prototype.__defineGetter__ ( P, getter )
2. 20.1.3.9.2 Object.prototype.__defineSetter__ ( P, setter )
3. 20.1.3.9.3 Object.prototype.__lookupGetter__ ( P )
4. 20.1.3.9.4 Object.prototype.__lookupSetter__ ( P )
4. 20.1.4 Properties of Object Instances
2. ◢20.2 Function Objects
1. ◢20.2.1 The Function Constructor
1. ◢20.2.1.1 Function ( ...parameterArgs, bodyArg )
1. 20.2.1.1.1 CreateDynamicFunction ( constructor, newTarget,
kind, parameterArgs, bodyArg )
2. ◢20.2.2 Properties of the Function Constructor
1. 20.2.2.1 Function.length
2. 20.2.2.2 Function.prototype
3. ◢20.2.3 Properties of the Function Prototype Object
1. 20.2.3.1 Function.prototype.apply ( thisArg, argArray )
2. 20.2.3.2 Function.prototype.bind ( thisArg, ...args )
3. 20.2.3.3 Function.prototype.call ( thisArg, ...args )
4. 20.2.3.4 Function.prototype.constructor
5. 20.2.3.5 Function.prototype.toString ( )
6. 20.2.3.6 Function.prototype [ @@hasInstance ] ( V )
4. ◢20.2.4 Function Instances
1. 20.2.4.1 length
2. 20.2.4.2 name
3. 20.2.4.3 prototype
5. 20.2.5 HostHasSourceTextAvailable ( func )
3. ◢20.3 Boolean Objects
1. ◢20.3.1 The Boolean Constructor
1. 20.3.1.1 Boolean ( value )
2. ◢20.3.2 Properties of the Boolean Constructor
1. 20.3.2.1 Boolean.prototype
3. ◢20.3.3 Properties of the Boolean Prototype Object
1. 20.3.3.1 Boolean.prototype.constructor
2. 20.3.3.2 Boolean.prototype.toString ( )
3. 20.3.3.3 Boolean.prototype.valueOf ( )
4. 20.3.4 Properties of Boolean Instances
4. ◢20.4 Symbol Objects
1. ◢20.4.1 The Symbol Constructor
1. 20.4.1.1 Symbol ( [ description ] )
2. ◢20.4.2 Properties of the Symbol Constructor
1. 20.4.2.1 Symbol.asyncIterator
2. 20.4.2.2 Symbol.for ( key )
3. 20.4.2.3 Symbol.hasInstance
4. 20.4.2.4 Symbol.isConcatSpreadable
5. 20.4.2.5 Symbol.iterator
6. 20.4.2.6 Symbol.keyFor ( sym )
7. 20.4.2.7 Symbol.match
8. 20.4.2.8 Symbol.matchAll
9. 20.4.2.9 Symbol.prototype
10. 20.4.2.10 Symbol.replace
11. 20.4.2.11 Symbol.search
12. 20.4.2.12 Symbol.species
13. 20.4.2.13 Symbol.split
14. 20.4.2.14 Symbol.toPrimitive
15. 20.4.2.15 Symbol.toStringTag
16. 20.4.2.16 Symbol.unscopables
3. ◢20.4.3 Properties of the Symbol Prototype Object
1. 20.4.3.1 Symbol.prototype.constructor
2. 20.4.3.2 get Symbol.prototype.description
3. ◢20.4.3.3 Symbol.prototype.toString ( )
1. 20.4.3.3.1 SymbolDescriptiveString ( sym )
4. 20.4.3.4 Symbol.prototype.valueOf ( )
5. 20.4.3.5 Symbol.prototype [ @@toPrimitive ] ( hint )
6. 20.4.3.6 Symbol.prototype [ @@toStringTag ]
4. 20.4.4 Properties of Symbol Instances
5. ◢20.4.5 Abstract Operations for Symbols
1. 20.4.5.1 KeyForSymbol ( sym )
5. ◢20.5 Error Objects
1. ◢20.5.1 The Error Constructor
1. 20.5.1.1 Error ( message [ , options ] )
2. ◢20.5.2 Properties of the Error Constructor
1. 20.5.2.1 Error.prototype
3. ◢20.5.3 Properties of the Error Prototype Object
1. 20.5.3.1 Error.prototype.constructor
2. 20.5.3.2 Error.prototype.message
3. 20.5.3.3 Error.prototype.name
4. 20.5.3.4 Error.prototype.toString ( )
4. 20.5.4 Properties of Error Instances
5. ◢20.5.5 Native Error Types Used in This Standard
1. 20.5.5.1 EvalError
2. 20.5.5.2 RangeError
3. 20.5.5.3 ReferenceError
4. 20.5.5.4 SyntaxError
5. 20.5.5.5 TypeError
6. 20.5.5.6 URIError
6. ◢20.5.6 NativeError Object Structure
1. ◢20.5.6.1 The NativeError Constructors
1. 20.5.6.1.1 NativeError ( message [ , options ] )
2. ◢20.5.6.2 Properties of the NativeError Constructors
1. 20.5.6.2.1 NativeError.prototype
3. ◢20.5.6.3 Properties of the NativeError Prototype Objects
1. 20.5.6.3.1 NativeError.prototype.constructor
2. 20.5.6.3.2 NativeError.prototype.message
3. 20.5.6.3.3 NativeError.prototype.name
4. 20.5.6.4 Properties of NativeError Instances
7. ◢20.5.7 AggregateError Objects
1. ◢20.5.7.1 The AggregateError Constructor
1. 20.5.7.1.1 AggregateError ( errors, message [ , options ] )
2. ◢20.5.7.2 Properties of the AggregateError Constructor
1. 20.5.7.2.1 AggregateError.prototype
3. ◢20.5.7.3 Properties of the AggregateError Prototype Object
1. 20.5.7.3.1 AggregateError.prototype.constructor
2. 20.5.7.3.2 AggregateError.prototype.message
3. 20.5.7.3.3 AggregateError.prototype.name
4. 20.5.7.4 Properties of AggregateError Instances
8. ◢20.5.8 Abstract Operations for Error Objects
1. 20.5.8.1 InstallErrorCause ( O, options )
22. ◢21 Numbers and Dates
1. ◢21.1 Number Objects
1. ◢21.1.1 The Number Constructor
1. 21.1.1.1 Number ( value )
2. ◢21.1.2 Properties of the Number Constructor
1. 21.1.2.1 Number.EPSILON
2. 21.1.2.2 Number.isFinite ( number )
3. 21.1.2.3 Number.isInteger ( number )
4. 21.1.2.4 Number.isNaN ( number )
5. 21.1.2.5 Number.isSafeInteger ( number )
6. 21.1.2.6 Number.MAX_SAFE_INTEGER
7. 21.1.2.7 Number.MAX_VALUE
8. 21.1.2.8 Number.MIN_SAFE_INTEGER
9. 21.1.2.9 Number.MIN_VALUE
10. 21.1.2.10 Number.NaN
11. 21.1.2.11 Number.NEGATIVE_INFINITY
12. 21.1.2.12 Number.parseFloat ( string )
13. 21.1.2.13 Number.parseInt ( string, radix )
14. 21.1.2.14 Number.POSITIVE_INFINITY
15. 21.1.2.15 Number.prototype
3. ◢21.1.3 Properties of the Number Prototype Object
1. 21.1.3.1 Number.prototype.constructor
2. 21.1.3.2 Number.prototype.toExponential ( fractionDigits )
3. 21.1.3.3 Number.prototype.toFixed ( fractionDigits )
4. 21.1.3.4 Number.prototype.toLocaleString ( [ reserved1 [ ,
reserved2 ] ] )
5. 21.1.3.5 Number.prototype.toPrecision ( precision )
6. 21.1.3.6 Number.prototype.toString ( [ radix ] )
7. 21.1.3.7 Number.prototype.valueOf ( )
4. 21.1.4 Properties of Number Instances
2. ◢21.2 BigInt Objects
1. ◢21.2.1 The BigInt Constructor
1. ◢21.2.1.1 BigInt ( value )
1. 21.2.1.1.1 NumberToBigInt ( number )
2. ◢21.2.2 Properties of the BigInt Constructor
1. 21.2.2.1 BigInt.asIntN ( bits, bigint )
2. 21.2.2.2 BigInt.asUintN ( bits, bigint )
3. 21.2.2.3 BigInt.prototype
3. ◢21.2.3 Properties of the BigInt Prototype Object
1. 21.2.3.1 BigInt.prototype.constructor
2. 21.2.3.2 BigInt.prototype.toLocaleString ( [ reserved1 [ ,
reserved2 ] ] )
3. 21.2.3.3 BigInt.prototype.toString ( [ radix ] )
4. 21.2.3.4 BigInt.prototype.valueOf ( )
5. 21.2.3.5 BigInt.prototype [ @@toStringTag ]
3. ◢21.3 The Math Object
1. ◢21.3.1 Value Properties of the Math Object
1. 21.3.1.1 Math.E
2. 21.3.1.2 Math.LN10
3. 21.3.1.3 Math.LN2
4. 21.3.1.4 Math.LOG10E
5. 21.3.1.5 Math.LOG2E
6. 21.3.1.6 Math.PI
7. 21.3.1.7 Math.SQRT1_2
8. 21.3.1.8 Math.SQRT2
9. 21.3.1.9 Math [ @@toStringTag ]
2. ◢21.3.2 Function Properties of the Math Object
1. 21.3.2.1 Math.abs ( x )
2. 21.3.2.2 Math.acos ( x )
3. 21.3.2.3 Math.acosh ( x )
4. 21.3.2.4 Math.asin ( x )
5. 21.3.2.5 Math.asinh ( x )
6. 21.3.2.6 Math.atan ( x )
7. 21.3.2.7 Math.atanh ( x )
8. 21.3.2.8 Math.atan2 ( y, x )
9. 21.3.2.9 Math.cbrt ( x )
10. 21.3.2.10 Math.ceil ( x )
11. 21.3.2.11 Math.clz32 ( x )
12. 21.3.2.12 Math.cos ( x )
13. 21.3.2.13 Math.cosh ( x )
14. 21.3.2.14 Math.exp ( x )
15. 21.3.2.15 Math.expm1 ( x )
16. 21.3.2.16 Math.floor ( x )
17. 21.3.2.17 Math.fround ( x )
18. 21.3.2.18 Math.hypot ( ...args )
19. 21.3.2.19 Math.imul ( x, y )
20. 21.3.2.20 Math.log ( x )
21. 21.3.2.21 Math.log1p ( x )
22. 21.3.2.22 Math.log10 ( x )
23. 21.3.2.23 Math.log2 ( x )
24. 21.3.2.24 Math.max ( ...args )
25. 21.3.2.25 Math.min ( ...args )
26. 21.3.2.26 Math.pow ( base, exponent )
27. 21.3.2.27 Math.random ( )
28. 21.3.2.28 Math.round ( x )
29. 21.3.2.29 Math.sign ( x )
30. 21.3.2.30 Math.sin ( x )
31. 21.3.2.31 Math.sinh ( x )
32. 21.3.2.32 Math.sqrt ( x )
33. 21.3.2.33 Math.tan ( x )
34. 21.3.2.34 Math.tanh ( x )
35. 21.3.2.35 Math.trunc ( x )
4. ◢21.4 Date Objects
1. ◢21.4.1 Overview of Date Objects and Definitions of Abstract
Operations
1. 21.4.1.1 Time Values and Time Range
2. 21.4.1.2 Day Number and Time within Day
3. 21.4.1.3 Year Number
4. 21.4.1.4 Month Number
5. 21.4.1.5 Date Number
6. 21.4.1.6 Week Day
7. 21.4.1.7 GetUTCEpochNanoseconds ( year, month, day, hour, minute,
second, millisecond, microsecond, nanosecond )
8. 21.4.1.8 GetNamedTimeZoneEpochNanoseconds ( timeZoneIdentifier,
year, month, day, hour, minute, second, millisecond, microsecond,
nanosecond )
9. 21.4.1.9 GetNamedTimeZoneOffsetNanoseconds ( timeZoneIdentifier,
epochNanoseconds )
10. 21.4.1.10 DefaultTimeZone ( )
11. 21.4.1.11 LocalTime ( t )
12. 21.4.1.12 UTC ( t )
13. 21.4.1.13 Hours, Minutes, Second, and Milliseconds
14. 21.4.1.14 MakeTime ( hour, min, sec, ms )
15. 21.4.1.15 MakeDay ( year, month, date )
16. 21.4.1.16 MakeDate ( day, time )
17. 21.4.1.17 TimeClip ( time )
18. ◢21.4.1.18 Date Time String Format
1. 21.4.1.18.1 Expanded Years
19. ◢21.4.1.19 Time Zone Offset String Format
1. 21.4.1.19.1 IsTimeZoneOffsetString ( offsetString )
2. 21.4.1.19.2 ParseTimeZoneOffsetString ( offsetString )
2. ◢21.4.2 The Date Constructor
1. 21.4.2.1 Date ( ...values )
3. ◢21.4.3 Properties of the Date Constructor
1. 21.4.3.1 Date.now ( )
2. 21.4.3.2 Date.parse ( string )
3. 21.4.3.3 Date.prototype
4. 21.4.3.4 Date.UTC ( year [ , month [ , date [ , hours [ , minutes
[ , seconds [ , ms ] ] ] ] ] ] )
4. ◢21.4.4 Properties of the Date Prototype Object
1. 21.4.4.1 Date.prototype.constructor
2. 21.4.4.2 Date.prototype.getDate ( )
3. 21.4.4.3 Date.prototype.getDay ( )
4. 21.4.4.4 Date.prototype.getFullYear ( )
5. 21.4.4.5 Date.prototype.getHours ( )
6. 21.4.4.6 Date.prototype.getMilliseconds ( )
7. 21.4.4.7 Date.prototype.getMinutes ( )
8. 21.4.4.8 Date.prototype.getMonth ( )
9. 21.4.4.9 Date.prototype.getSeconds ( )
10. 21.4.4.10 Date.prototype.getTime ( )
11. 21.4.4.11 Date.prototype.getTimezoneOffset ( )
12. 21.4.4.12 Date.prototype.getUTCDate ( )
13. 21.4.4.13 Date.prototype.getUTCDay ( )
14. 21.4.4.14 Date.prototype.getUTCFullYear ( )
15. 21.4.4.15 Date.prototype.getUTCHours ( )
16. 21.4.4.16 Date.prototype.getUTCMilliseconds ( )
17. 21.4.4.17 Date.prototype.getUTCMinutes ( )
18. 21.4.4.18 Date.prototype.getUTCMonth ( )
19. 21.4.4.19 Date.prototype.getUTCSeconds ( )
20. 21.4.4.20 Date.prototype.setDate ( date )
21. 21.4.4.21 Date.prototype.setFullYear ( year [ , month [ , date ]
] )
22. 21.4.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ]
] ] )
23. 21.4.4.23 Date.prototype.setMilliseconds ( ms )
24. 21.4.4.24 Date.prototype.setMinutes ( min [ , sec [ , ms ] ] )
25. 21.4.4.25 Date.prototype.setMonth ( month [ , date ] )
26. 21.4.4.26 Date.prototype.setSeconds ( sec [ , ms ] )
27. 21.4.4.27 Date.prototype.setTime ( time )
28. 21.4.4.28 Date.prototype.setUTCDate ( date )
29. 21.4.4.29 Date.prototype.setUTCFullYear ( year [ , month [ , date
] ] )
30. 21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ ,
ms ] ] ] )
31. 21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
32. 21.4.4.32 Date.prototype.setUTCMinutes ( min [ , sec [ , ms ] ] )
33. 21.4.4.33 Date.prototype.setUTCMonth ( month [ , date ] )
34. 21.4.4.34 Date.prototype.setUTCSeconds ( sec [ , ms ] )
35. 21.4.4.35 Date.prototype.toDateString ( )
36. 21.4.4.36 Date.prototype.toISOString ( )
37. 21.4.4.37 Date.prototype.toJSON ( key )
38. 21.4.4.38 Date.prototype.toLocaleDateString ( [ reserved1 [ ,
reserved2 ] ] )
39. 21.4.4.39 Date.prototype.toLocaleString ( [ reserved1 [ ,
reserved2 ] ] )
40. 21.4.4.40 Date.prototype.toLocaleTimeString ( [ reserved1 [ ,
reserved2 ] ] )
41. ◢21.4.4.41 Date.prototype.toString ( )
1. 21.4.4.41.1 TimeString ( tv )
2. 21.4.4.41.2 DateString ( tv )
3. 21.4.4.41.3 TimeZoneString ( tv )
4. 21.4.4.41.4 ToDateString ( tv )
42. 21.4.4.42 Date.prototype.toTimeString ( )
43. 21.4.4.43 Date.prototype.toUTCString ( )
44. 21.4.4.44 Date.prototype.valueOf ( )
45. 21.4.4.45 Date.prototype [ @@toPrimitive ] ( hint )
5. 21.4.5 Properties of Date Instances
23. ◢22 Text Processing
1. ◢22.1 String Objects
1. ◢22.1.1 The String Constructor
1. 22.1.1.1 String ( value )
2. ◢22.1.2 Properties of the String Constructor
1. 22.1.2.1 String.fromCharCode ( ...codeUnits )
2. 22.1.2.2 String.fromCodePoint ( ...codePoints )
3. 22.1.2.3 String.prototype
4. 22.1.2.4 String.raw ( template, ...substitutions )
3. ◢22.1.3 Properties of the String Prototype Object
1. 22.1.3.1 String.prototype.at ( index )
2. 22.1.3.2 String.prototype.charAt ( pos )
3. 22.1.3.3 String.prototype.charCodeAt ( pos )
4. 22.1.3.4 String.prototype.codePointAt ( pos )
5. 22.1.3.5 String.prototype.concat ( ...args )
6. 22.1.3.6 String.prototype.constructor
7. 22.1.3.7 String.prototype.endsWith ( searchString [ , endPosition
] )
8. 22.1.3.8 String.prototype.includes ( searchString [ , position ]
)
9. 22.1.3.9 String.prototype.indexOf ( searchString [ , position ] )
10. 22.1.3.10 String.prototype.lastIndexOf ( searchString [ ,
position ] )
11. 22.1.3.11 String.prototype.localeCompare ( that [ , reserved1 [ ,
reserved2 ] ] )
12. 22.1.3.12 String.prototype.match ( regexp )
13. 22.1.3.13 String.prototype.matchAll ( regexp )
14. 22.1.3.14 String.prototype.normalize ( [ form ] )
15. 22.1.3.15 String.prototype.padEnd ( maxLength [ , fillString ] )
16. ◢22.1.3.16 String.prototype.padStart ( maxLength [ , fillString ]
)
1. 22.1.3.16.1 StringPad ( O, maxLength, fillString, placement )
2. 22.1.3.16.2 ToZeroPaddedDecimalString ( n, minLength )
17. 22.1.3.17 String.prototype.repeat ( count )
18. ◢22.1.3.18 String.prototype.replace ( searchValue, replaceValue )
1. 22.1.3.18.1 GetSubstitution ( matched, str, position,
captures, namedCaptures, replacementTemplate )
19. 22.1.3.19 String.prototype.replaceAll ( searchValue, replaceValue
)
20. 22.1.3.20 String.prototype.search ( regexp )
21. 22.1.3.21 String.prototype.slice ( start, end )
22. 22.1.3.22 String.prototype.split ( separator, limit )
23. 22.1.3.23 String.prototype.startsWith ( searchString [ , position
] )
24. 22.1.3.24 String.prototype.substring ( start, end )
25. 22.1.3.25 String.prototype.toLocaleLowerCase ( [ reserved1 [ ,
reserved2 ] ] )
26. 22.1.3.26 String.prototype.toLocaleUpperCase ( [ reserved1 [ ,
reserved2 ] ] )
27. 22.1.3.27 String.prototype.toLowerCase ( )
28. 22.1.3.28 String.prototype.toString ( )
29. 22.1.3.29 String.prototype.toUpperCase ( )
30. ◢22.1.3.30 String.prototype.trim ( )
1. 22.1.3.30.1 TrimString ( string, where )
31. 22.1.3.31 String.prototype.trimEnd ( )
32. 22.1.3.32 String.prototype.trimStart ( )
33. 22.1.3.33 String.prototype.valueOf ( )
34. 22.1.3.34 String.prototype [ @@iterator ] ( )
4. ◢22.1.4 Properties of String Instances
1. 22.1.4.1 length
5. ◢22.1.5 String Iterator Objects
1. ◢22.1.5.1 The %StringIteratorPrototype% Object
1. 22.1.5.1.1 %StringIteratorPrototype%.next ( )
2. 22.1.5.1.2 %StringIteratorPrototype% [ @@toStringTag ]
2. ◢22.2 RegExp (Regular Expression) Objects
1. ◢22.2.1 Patterns
1. 22.2.1.1 SS: Early Errors
2. 22.2.1.2 SS: CountLeftCapturingParensWithin ( node )
3. 22.2.1.3 SS: CountLeftCapturingParensBefore ( node )
4. 22.2.1.4 SS: CapturingGroupNumber
5. 22.2.1.5 SS: IsCharacterClass
6. 22.2.1.6 SS: CharacterValue
7. 22.2.1.7 SS: GroupSpecifiersThatMatch ( thisGroupName )
8. 22.2.1.8 SS: CapturingGroupName
9. 22.2.1.9 SS: RegExpIdentifierCodePoints
10. 22.2.1.10 SS: RegExpIdentifierCodePoint
2. ◢22.2.2 Pattern Semantics
1. ◢22.2.2.1 Notation
1. 22.2.2.1.1 RegExp Records
2. 22.2.2.2 RS: CompilePattern
3. ◢22.2.2.3 RS: CompileSubpattern
1. 22.2.2.3.1 RepeatMatcher ( m, min, max, greedy, x, c,
parenIndex, parenCount )
4. ◢22.2.2.4 RS: CompileAssertion
1. 22.2.2.4.1 IsWordChar ( rer, Input, e )
5. 22.2.2.5 RS: CompileQuantifier
6. 22.2.2.6 RS: CompileQuantifierPrefix
7. ◢22.2.2.7 RS: CompileAtom
1. 22.2.2.7.1 CharacterSetMatcher ( rer, A, invert, direction )
2. 22.2.2.7.2 BackreferenceMatcher ( rer, n, direction )
3. 22.2.2.7.3 Canonicalize ( rer, ch )
8. 22.2.2.8 RS: CompileCharacterClass
9. ◢22.2.2.9 RS: CompileToCharSet
1. 22.2.2.9.1 CharacterRange ( A, B )
2. 22.2.2.9.2 WordCharacters ( rer )
3. 22.2.2.9.3 UnicodeMatchProperty ( p )
4. 22.2.2.9.4 UnicodeMatchPropertyValue ( p, v )
3. ◢22.2.3 Abstract Operations for RegExp Creation
1. 22.2.3.1 RegExpCreate ( P, F )
2. 22.2.3.2 RegExpAlloc ( newTarget )
3. 22.2.3.3 RegExpInitialize ( obj, pattern, flags )
4. 22.2.3.4 SS: ParsePattern ( patternText, u )
4. ◢22.2.4 The RegExp Constructor
1. 22.2.4.1 RegExp ( pattern, flags )
5. ◢22.2.5 Properties of the RegExp Constructor
1. 22.2.5.1 RegExp.prototype
2. 22.2.5.2 get RegExp [ @@species ]
6. ◢22.2.6 Properties of the RegExp Prototype Object
1. 22.2.6.1 RegExp.prototype.constructor
2. 22.2.6.2 RegExp.prototype.exec ( string )
3. 22.2.6.3 get RegExp.prototype.dotAll
4. ◢22.2.6.4 get RegExp.prototype.flags
1. 22.2.6.4.1 RegExpHasFlag ( R, codeUnit )
5. 22.2.6.5 get RegExp.prototype.global
6. 22.2.6.6 get RegExp.prototype.hasIndices
7. 22.2.6.7 get RegExp.prototype.ignoreCase
8. 22.2.6.8 RegExp.prototype [ @@match ] ( string )
9. 22.2.6.9 RegExp.prototype [ @@matchAll ] ( string )
10. 22.2.6.10 get RegExp.prototype.multiline
11. 22.2.6.11 RegExp.prototype [ @@replace ] ( string, replaceValue )
12. 22.2.6.12 RegExp.prototype [ @@search ] ( string )
13. ◢22.2.6.13 get RegExp.prototype.source
1. 22.2.6.13.1 EscapeRegExpPattern ( P, F )
14. 22.2.6.14 RegExp.prototype [ @@split ] ( string, limit )
15. 22.2.6.15 get RegExp.prototype.sticky
16. 22.2.6.16 RegExp.prototype.test ( S )
17. 22.2.6.17 RegExp.prototype.toString ( )
18. 22.2.6.18 get RegExp.prototype.unicode
7. ◢22.2.7 Abstract Operations for RegExp Matching
1. 22.2.7.1 RegExpExec ( R, S )
2. 22.2.7.2 RegExpBuiltinExec ( R, S )
3. 22.2.7.3 AdvanceStringIndex ( S, index, unicode )
4. 22.2.7.4 GetStringIndex ( S, codePointIndex )
5. 22.2.7.5 Match Records
6. 22.2.7.6 GetMatchString ( S, match )
7. 22.2.7.7 GetMatchIndexPair ( S, match )
8. 22.2.7.8 MakeMatchIndicesIndexPairArray ( S, indices, groupNames,
hasGroups )
8. ◢22.2.8 Properties of RegExp Instances
1. 22.2.8.1 lastIndex
9. ◢22.2.9 RegExp String Iterator Objects
1. 22.2.9.1 CreateRegExpStringIterator ( R, S, global, fullUnicode )
2. ◢22.2.9.2 The %RegExpStringIteratorPrototype% Object
1. 22.2.9.2.1 %RegExpStringIteratorPrototype%.next ( )
2. 22.2.9.2.2 %RegExpStringIteratorPrototype% [ @@toStringTag ]
24. ◢23 Indexed Collections
1. ◢23.1 Array Objects
1. ◢23.1.1 The Array Constructor
1. 23.1.1.1 Array ( ...values )
2. ◢23.1.2 Properties of the Array Constructor
1. 23.1.2.1 Array.from ( items [ , mapfn [ , thisArg ] ] )
2. 23.1.2.2 Array.isArray ( arg )
3. 23.1.2.3 Array.of ( ...items )
4. 23.1.2.4 Array.prototype
5. 23.1.2.5 get Array [ @@species ]
3. ◢23.1.3 Properties of the Array Prototype Object
1. 23.1.3.1 Array.prototype.at ( index )
2. ◢23.1.3.2 Array.prototype.concat ( ...items )
1. 23.1.3.2.1 IsConcatSpreadable ( O )
3. 23.1.3.3 Array.prototype.constructor
4. 23.1.3.4 Array.prototype.copyWithin ( target, start [ , end ] )
5. 23.1.3.5 Array.prototype.entries ( )
6. 23.1.3.6 Array.prototype.every ( callbackfn [ , thisArg ] )
7. 23.1.3.7 Array.prototype.fill ( value [ , start [ , end ] ] )
8. 23.1.3.8 Array.prototype.filter ( callbackfn [ , thisArg ] )
9. 23.1.3.9 Array.prototype.find ( predicate [ , thisArg ] )
10. 23.1.3.10 Array.prototype.findIndex ( predicate [ , thisArg ] )
11. 23.1.3.11 Array.prototype.findLast ( predicate [ , thisArg ] )
12. ◢23.1.3.12 Array.prototype.findLastIndex ( predicate [ , thisArg
] )
1. 23.1.3.12.1 FindViaPredicate ( O, len, direction, predicate,
thisArg )
13. ◢23.1.3.13 Array.prototype.flat ( [ depth ] )
1. 23.1.3.13.1 FlattenIntoArray ( target, source, sourceLen,
start, depth [ , mapperFunction [ , thisArg ] ] )
14. 23.1.3.14 Array.prototype.flatMap ( mapperFunction [ , thisArg ]
)
15. 23.1.3.15 Array.prototype.forEach ( callbackfn [ , thisArg ] )
16. 23.1.3.16 Array.prototype.includes ( searchElement [ , fromIndex
] )
17. 23.1.3.17 Array.prototype.indexOf ( searchElement [ , fromIndex ]
)
18. 23.1.3.18 Array.prototype.join ( separator )
19. 23.1.3.19 Array.prototype.keys ( )
20. 23.1.3.20 Array.prototype.lastIndexOf ( searchElement [ ,
fromIndex ] )
21. 23.1.3.21 Array.prototype.map ( callbackfn [ , thisArg ] )
22. 23.1.3.22 Array.prototype.pop ( )
23. 23.1.3.23 Array.prototype.push ( ...items )
24. 23.1.3.24 Array.prototype.reduce ( callbackfn [ , initialValue ]
)
25. 23.1.3.25 Array.prototype.reduceRight ( callbackfn [ ,
initialValue ] )
26. 23.1.3.26 Array.prototype.reverse ( )
27. 23.1.3.27 Array.prototype.shift ( )
28. 23.1.3.28 Array.prototype.slice ( start, end )
29. 23.1.3.29 Array.prototype.some ( callbackfn [ , thisArg ] )
30. ◢23.1.3.30 Array.prototype.sort ( comparefn )
1. 23.1.3.30.1 SortIndexedProperties ( obj, len, SortCompare,
holes )
2. 23.1.3.30.2 CompareArrayElements ( x, y, comparefn )
31. 23.1.3.31 Array.prototype.splice ( start, deleteCount, ...items )
32. 23.1.3.32 Array.prototype.toLocaleString ( [ reserved1 [ ,
reserved2 ] ] )
33. 23.1.3.33 Array.prototype.toReversed ( )
34. 23.1.3.34 Array.prototype.toSorted ( comparefn )
35. 23.1.3.35 Array.prototype.toSpliced ( start, skipCount, ...items
)
36. 23.1.3.36 Array.prototype.toString ( )
37. 23.1.3.37 Array.prototype.unshift ( ...items )
38. 23.1.3.38 Array.prototype.values ( )
39. 23.1.3.39 Array.prototype.with ( index, value )
40. 23.1.3.40 Array.prototype [ @@iterator ] ( )
41. 23.1.3.41 Array.prototype [ @@unscopables ]
4. ◢23.1.4 Properties of Array Instances
1. 23.1.4.1 length
5. ◢23.1.5 Array Iterator Objects
1. 23.1.5.1 CreateArrayIterator ( array, kind )
2. ◢23.1.5.2 The %ArrayIteratorPrototype% Object
1. 23.1.5.2.1 %ArrayIteratorPrototype%.next ( )
2. 23.1.5.2.2 %ArrayIteratorPrototype% [ @@toStringTag ]
2. ◢23.2 TypedArray Objects
1. ◢23.2.1 The %TypedArray% Intrinsic Object
1. 23.2.1.1 %TypedArray% ( )
2. ◢23.2.2 Properties of the %TypedArray% Intrinsic Object
1. 23.2.2.1 %TypedArray%.from ( source [ , mapfn [ , thisArg ] ] )
2. 23.2.2.2 %TypedArray%.of ( ...items )
3. 23.2.2.3 %TypedArray%.prototype
4. 23.2.2.4 get %TypedArray% [ @@species ]
3. ◢23.2.3 Properties of the %TypedArray% Prototype Object
1. 23.2.3.1 %TypedArray%.prototype.at ( index )
2. 23.2.3.2 get %TypedArray%.prototype.buffer
3. 23.2.3.3 get %TypedArray%.prototype.byteLength
4. 23.2.3.4 get %TypedArray%.prototype.byteOffset
5. 23.2.3.5 %TypedArray%.prototype.constructor
6. 23.2.3.6 %TypedArray%.prototype.copyWithin ( target, start [ ,
end ] )
7. 23.2.3.7 %TypedArray%.prototype.entries ( )
8. 23.2.3.8 %TypedArray%.prototype.every ( callbackfn [ , thisArg ]
)
9. 23.2.3.9 %TypedArray%.prototype.fill ( value [ , start [ , end ]
] )
10. 23.2.3.10 %TypedArray%.prototype.filter ( callbackfn [ , thisArg
] )
11. 23.2.3.11 %TypedArray%.prototype.find ( predicate [ , thisArg ] )
12. 23.2.3.12 %TypedArray%.prototype.findIndex ( predicate [ ,
thisArg ] )
13. 23.2.3.13 %TypedArray%.prototype.findLast ( predicate [ , thisArg
] )
14. 23.2.3.14 %TypedArray%.prototype.findLastIndex ( predicate [ ,
thisArg ] )
15. 23.2.3.15 %TypedArray%.prototype.forEach ( callbackfn [ , thisArg
] )
16. 23.2.3.16 %TypedArray%.prototype.includes ( searchElement [ ,
fromIndex ] )
17. 23.2.3.17 %TypedArray%.prototype.indexOf ( searchElement [ ,
fromIndex ] )
18. 23.2.3.18 %TypedArray%.prototype.join ( separator )
19. 23.2.3.19 %TypedArray%.prototype.keys ( )
20. 23.2.3.20 %TypedArray%.prototype.lastIndexOf ( searchElement [ ,
fromIndex ] )
21. 23.2.3.21 get %TypedArray%.prototype.length
22. 23.2.3.22 %TypedArray%.prototype.map ( callbackfn [ , thisArg ] )
23. 23.2.3.23 %TypedArray%.prototype.reduce ( callbackfn [ ,
initialValue ] )
24. 23.2.3.24 %TypedArray%.prototype.reduceRight ( callbackfn [ ,
initialValue ] )
25. 23.2.3.25 %TypedArray%.prototype.reverse ( )
26. ◢23.2.3.26 %TypedArray%.prototype.set ( source [ , offset ] )
1. 23.2.3.26.1 SetTypedArrayFromTypedArray ( target,
targetOffset, source )
2. 23.2.3.26.2 SetTypedArrayFromArrayLike ( target, targetOffset,
source )
27. 23.2.3.27 %TypedArray%.prototype.slice ( start, end )
28. 23.2.3.28 %TypedArray%.prototype.some ( callbackfn [ , thisArg ]
)
29. 23.2.3.29 %TypedArray%.prototype.sort ( comparefn )
30. 23.2.3.30 %TypedArray%.prototype.subarray ( begin, end )
31. 23.2.3.31 %TypedArray%.prototype.toLocaleString ( [ reserved1 [ ,
reserved2 ] ] )
32. 23.2.3.32 %TypedArray%.prototype.toReversed ( )
33. 23.2.3.33 %TypedArray%.prototype.toSorted ( comparefn )
34. 23.2.3.34 %TypedArray%.prototype.toString ( )
35. 23.2.3.35 %TypedArray%.prototype.values ( )
36. 23.2.3.36 %TypedArray%.prototype.with ( index, value )
37. 23.2.3.37 %TypedArray%.prototype [ @@iterator ] ( )
38. 23.2.3.38 get %TypedArray%.prototype [ @@toStringTag ]
4. ◢23.2.4 Abstract Operations for TypedArray Objects
1. 23.2.4.1 TypedArraySpeciesCreate ( exemplar, argumentList )
2. 23.2.4.2 TypedArrayCreate ( constructor, argumentList )
3. 23.2.4.3 TypedArrayCreateSameType ( exemplar, argumentList )
4. 23.2.4.4 ValidateTypedArray ( O )
5. 23.2.4.5 TypedArrayElementSize ( O )
6. 23.2.4.6 TypedArrayElementType ( O )
7. 23.2.4.7 CompareTypedArrayElements ( x, y, comparefn )
5. ◢23.2.5 The TypedArray Constructors
1. ◢23.2.5.1 TypedArray ( ...args )
1. 23.2.5.1.1 AllocateTypedArray ( constructorName, newTarget,
defaultProto [ , length ] )
2. 23.2.5.1.2 InitializeTypedArrayFromTypedArray ( O, srcArray )
3. 23.2.5.1.3 InitializeTypedArrayFromArrayBuffer ( O, buffer,
byteOffset, length )
4. 23.2.5.1.4 InitializeTypedArrayFromList ( O, values )
5. 23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O, arrayLike )
6. 23.2.5.1.6 AllocateTypedArrayBuffer ( O, length )
6. ◢23.2.6 Properties of the TypedArray Constructors
1. 23.2.6.1 TypedArray.BYTES_PER_ELEMENT
2. 23.2.6.2 TypedArray.prototype
7. ◢23.2.7 Properties of the TypedArray Prototype Objects
1. 23.2.7.1 TypedArray.prototype.BYTES_PER_ELEMENT
2. 23.2.7.2 TypedArray.prototype.constructor
8. 23.2.8 Properties of TypedArray Instances
25. ◢24 Keyed Collections
1. ◢24.1 Map Objects
1. ◢24.1.1 The Map Constructor
1. 24.1.1.1 Map ( [ iterable ] )
2. 24.1.1.2 AddEntriesFromIterable ( target, iterable, adder )
2. ◢24.1.2 Properties of the Map Constructor
1. 24.1.2.1 Map.prototype
2. 24.1.2.2 get Map [ @@species ]
3. ◢24.1.3 Properties of the Map Prototype Object
1. 24.1.3.1 Map.prototype.clear ( )
2. 24.1.3.2 Map.prototype.constructor
3. 24.1.3.3 Map.prototype.delete ( key )
4. 24.1.3.4 Map.prototype.entries ( )
5. 24.1.3.5 Map.prototype.forEach ( callbackfn [ , thisArg ] )
6. 24.1.3.6 Map.prototype.get ( key )
7. 24.1.3.7 Map.prototype.has ( key )
8. 24.1.3.8 Map.prototype.keys ( )
9. 24.1.3.9 Map.prototype.set ( key, value )
10. 24.1.3.10 get Map.prototype.size
11. 24.1.3.11 Map.prototype.values ( )
12. 24.1.3.12 Map.prototype [ @@iterator ] ( )
13. 24.1.3.13 Map.prototype [ @@toStringTag ]
4. 24.1.4 Properties of Map Instances
5. ◢24.1.5 Map Iterator Objects
1. 24.1.5.1 CreateMapIterator ( map, kind )
2. ◢24.1.5.2 The %MapIteratorPrototype% Object
1. 24.1.5.2.1 %MapIteratorPrototype%.next ( )
2. 24.1.5.2.2 %MapIteratorPrototype% [ @@toStringTag ]
2. ◢24.2 Set Objects
1. ◢24.2.1 The Set Constructor
1. 24.2.1.1 Set ( [ iterable ] )
2. ◢24.2.2 Properties of the Set Constructor
1. 24.2.2.1 Set.prototype
2. 24.2.2.2 get Set [ @@species ]
3. ◢24.2.3 Properties of the Set Prototype Object
1. 24.2.3.1 Set.prototype.add ( value )
2. 24.2.3.2 Set.prototype.clear ( )
3. 24.2.3.3 Set.prototype.constructor
4. 24.2.3.4 Set.prototype.delete ( value )
5. 24.2.3.5 Set.prototype.entries ( )
6. 24.2.3.6 Set.prototype.forEach ( callbackfn [ , thisArg ] )
7. 24.2.3.7 Set.prototype.has ( value )
8. 24.2.3.8 Set.prototype.keys ( )
9. 24.2.3.9 get Set.prototype.size
10. 24.2.3.10 Set.prototype.values ( )
11. 24.2.3.11 Set.prototype [ @@iterator ] ( )
12. 24.2.3.12 Set.prototype [ @@toStringTag ]
4. 24.2.4 Properties of Set Instances
5. ◢24.2.5 Set Iterator Objects
1. 24.2.5.1 CreateSetIterator ( set, kind )
2. ◢24.2.5.2 The %SetIteratorPrototype% Object
1. 24.2.5.2.1 %SetIteratorPrototype%.next ( )
2. 24.2.5.2.2 %SetIteratorPrototype% [ @@toStringTag ]
3. ◢24.3 WeakMap Objects
1. ◢24.3.1 The WeakMap Constructor
1. 24.3.1.1 WeakMap ( [ iterable ] )
2. ◢24.3.2 Properties of the WeakMap Constructor
1. 24.3.2.1 WeakMap.prototype
3. ◢24.3.3 Properties of the WeakMap Prototype Object
1. 24.3.3.1 WeakMap.prototype.constructor
2. 24.3.3.2 WeakMap.prototype.delete ( key )
3. 24.3.3.3 WeakMap.prototype.get ( key )
4. 24.3.3.4 WeakMap.prototype.has ( key )
5. 24.3.3.5 WeakMap.prototype.set ( key, value )
6. 24.3.3.6 WeakMap.prototype [ @@toStringTag ]
4. 24.3.4 Properties of WeakMap Instances
4. ◢24.4 WeakSet Objects
1. ◢24.4.1 The WeakSet Constructor
1. 24.4.1.1 WeakSet ( [ iterable ] )
2. ◢24.4.2 Properties of the WeakSet Constructor
1. 24.4.2.1 WeakSet.prototype
3. ◢24.4.3 Properties of the WeakSet Prototype Object
1. 24.4.3.1 WeakSet.prototype.add ( value )
2. 24.4.3.2 WeakSet.prototype.constructor
3. 24.4.3.3 WeakSet.prototype.delete ( value )
4. 24.4.3.4 WeakSet.prototype.has ( value )
5. 24.4.3.5 WeakSet.prototype [ @@toStringTag ]
4. 24.4.4 Properties of WeakSet Instances
26. ◢25 Structured Data
1. ◢25.1 ArrayBuffer Objects
1. 25.1.1 Notation
2. ◢25.1.2 Abstract Operations For ArrayBuffer Objects
1. 25.1.2.1 AllocateArrayBuffer ( constructor, byteLength )
2. 25.1.2.2 IsDetachedBuffer ( arrayBuffer )
3. 25.1.2.3 DetachArrayBuffer ( arrayBuffer [ , key ] )
4. 25.1.2.4 CloneArrayBuffer ( srcBuffer, srcByteOffset, srcLength )
5. 25.1.2.5 IsUnsignedElementType ( type )
6. 25.1.2.6 IsUnclampedIntegerElementType ( type )
7. 25.1.2.7 IsBigIntElementType ( type )
8. 25.1.2.8 IsNoTearConfiguration ( type, order )
9. 25.1.2.9 RawBytesToNumeric ( type, rawBytes, isLittleEndian )
10. 25.1.2.10 GetValueFromBuffer ( arrayBuffer, byteIndex, type,
isTypedArray, order [ , isLittleEndian ] )
11. 25.1.2.11 NumericToRawBytes ( type, value, isLittleEndian )
12. 25.1.2.12 SetValueInBuffer ( arrayBuffer, byteIndex, type, value,
isTypedArray, order [ , isLittleEndian ] )
13. 25.1.2.13 GetModifySetValueInBuffer ( arrayBuffer, byteIndex,
type, value, op [ , isLittleEndian ] )
3. ◢25.1.3 The ArrayBuffer Constructor
1. 25.1.3.1 ArrayBuffer ( length )
4. ◢25.1.4 Properties of the ArrayBuffer Constructor
1. 25.1.4.1 ArrayBuffer.isView ( arg )
2. 25.1.4.2 ArrayBuffer.prototype
3. 25.1.4.3 get ArrayBuffer [ @@species ]
5. ◢25.1.5 Properties of the ArrayBuffer Prototype Object
1. 25.1.5.1 get ArrayBuffer.prototype.byteLength
2. 25.1.5.2 ArrayBuffer.prototype.constructor
3. 25.1.5.3 ArrayBuffer.prototype.slice ( start, end )
4. 25.1.5.4 ArrayBuffer.prototype [ @@toStringTag ]
6. 25.1.6 Properties of ArrayBuffer Instances
2. ◢25.2 SharedArrayBuffer Objects
1. ◢25.2.1 Abstract Operations for SharedArrayBuffer Objects
1. 25.2.1.1 AllocateSharedArrayBuffer ( constructor, byteLength )
2. 25.2.1.2 IsSharedArrayBuffer ( obj )
2. ◢25.2.2 The SharedArrayBuffer Constructor
1. 25.2.2.1 SharedArrayBuffer ( length )
3. ◢25.2.3 Properties of the SharedArrayBuffer Constructor
1. 25.2.3.1 SharedArrayBuffer.prototype
2. 25.2.3.2 get SharedArrayBuffer [ @@species ]
4. ◢25.2.4 Properties of the SharedArrayBuffer Prototype Object
1. 25.2.4.1 get SharedArrayBuffer.prototype.byteLength
2. 25.2.4.2 SharedArrayBuffer.prototype.constructor
3. 25.2.4.3 SharedArrayBuffer.prototype.slice ( start, end )
4. 25.2.4.4 SharedArrayBuffer.prototype [ @@toStringTag ]
5. 25.2.5 Properties of SharedArrayBuffer Instances
3. ◢25.3 DataView Objects
1. ◢25.3.1 Abstract Operations For DataView Objects
1. 25.3.1.1 GetViewValue ( view, requestIndex, isLittleEndian, type )
2. 25.3.1.2 SetViewValue ( view, requestIndex, isLittleEndian, type,
value )
2. ◢25.3.2 The DataView Constructor
1. 25.3.2.1 DataView ( buffer [ , byteOffset [ , byteLength ] ] )
3. ◢25.3.3 Properties of the DataView Constructor
1. 25.3.3.1 DataView.prototype
4. ◢25.3.4 Properties of the DataView Prototype Object
1. 25.3.4.1 get DataView.prototype.buffer
2. 25.3.4.2 get DataView.prototype.byteLength
3. 25.3.4.3 get DataView.prototype.byteOffset
4. 25.3.4.4 DataView.prototype.constructor
5. 25.3.4.5 DataView.prototype.getBigInt64 ( byteOffset [ ,
littleEndian ] )
6. 25.3.4.6 DataView.prototype.getBigUint64 ( byteOffset [ ,
littleEndian ] )
7. 25.3.4.7 DataView.prototype.getFloat32 ( byteOffset [ ,
littleEndian ] )
8. 25.3.4.8 DataView.prototype.getFloat64 ( byteOffset [ ,
littleEndian ] )
9. 25.3.4.9 DataView.prototype.getInt8 ( byteOffset )
10. 25.3.4.10 DataView.prototype.getInt16 ( byteOffset [ ,
littleEndian ] )
11. 25.3.4.11 DataView.prototype.getInt32 ( byteOffset [ ,
littleEndian ] )
12. 25.3.4.12 DataView.prototype.getUint8 ( byteOffset )
13. 25.3.4.13 DataView.prototype.getUint16 ( byteOffset [ ,
littleEndian ] )
14. 25.3.4.14 DataView.prototype.getUint32 ( byteOffset [ ,
littleEndian ] )
15. 25.3.4.15 DataView.prototype.setBigInt64 ( byteOffset, value [ ,
littleEndian ] )
16. 25.3.4.16 DataView.prototype.setBigUint64 ( byteOffset, value [ ,
littleEndian ] )
17. 25.3.4.17 DataView.prototype.setFloat32 ( byteOffset, value [ ,
littleEndian ] )
18. 25.3.4.18 DataView.prototype.setFloat64 ( byteOffset, value [ ,
littleEndian ] )
19. 25.3.4.19 DataView.prototype.setInt8 ( byteOffset, value )
20. 25.3.4.20 DataView.prototype.setInt16 ( byteOffset, value [ ,
littleEndian ] )
21. 25.3.4.21 DataView.prototype.setInt32 ( byteOffset, value [ ,
littleEndian ] )
22. 25.3.4.22 DataView.prototype.setUint8 ( byteOffset, value )
23. 25.3.4.23 DataView.prototype.setUint16 ( byteOffset, value [ ,
littleEndian ] )
24. 25.3.4.24 DataView.prototype.setUint32 ( byteOffset, value [ ,
littleEndian ] )
25. 25.3.4.25 DataView.prototype [ @@toStringTag ]
5. 25.3.5 Properties of DataView Instances
4. ◢25.4 The Atomics Object
1. 25.4.1 WaiterList Objects
2. ◢25.4.2 Abstract Operations for Atomics
1. 25.4.2.1 ValidateIntegerTypedArray ( typedArray [ , waitable ] )
2. 25.4.2.2 ValidateAtomicAccess ( typedArray, requestIndex )
3. 25.4.2.3 GetWaiterList ( block, i )
4. 25.4.2.4 EnterCriticalSection ( WL )
5. 25.4.2.5 LeaveCriticalSection ( WL )
6. 25.4.2.6 AddWaiter ( WL, W )
7. 25.4.2.7 RemoveWaiter ( WL, W )
8. 25.4.2.8 RemoveWaiters ( WL, c )
9. 25.4.2.9 SuspendAgent ( WL, W, minimumTimeout )
10. 25.4.2.10 NotifyWaiter ( WL, W )
11. 25.4.2.11 AtomicReadModifyWrite ( typedArray, index, value, op )
12. 25.4.2.12 ByteListBitwiseOp ( op, xBytes, yBytes )
13. 25.4.2.13 ByteListEqual ( xBytes, yBytes )
3. 25.4.3 Atomics.add ( typedArray, index, value )
4. 25.4.4 Atomics.and ( typedArray, index, value )
5. 25.4.5 Atomics.compareExchange ( typedArray, index, expectedValue,
replacementValue )
6. 25.4.6 Atomics.exchange ( typedArray, index, value )
7. 25.4.7 Atomics.isLockFree ( size )
8. 25.4.8 Atomics.load ( typedArray, index )
9. 25.4.9 Atomics.or ( typedArray, index, value )
10. 25.4.10 Atomics.store ( typedArray, index, value )
11. 25.4.11 Atomics.sub ( typedArray, index, value )
12. 25.4.12 Atomics.wait ( typedArray, index, value, timeout )
13. 25.4.13 Atomics.notify ( typedArray, index, count )
14. 25.4.14 Atomics.xor ( typedArray, index, value )
15. 25.4.15 Atomics [ @@toStringTag ]
5. ◢25.5 The JSON Object
1. ◢25.5.1 JSON.parse ( text [ , reviver ] )
1. 25.5.1.1 InternalizeJSONProperty ( holder, name, reviver )
2. ◢25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] )
1. 25.5.2.1 JSON Serialization Record
2. 25.5.2.2 SerializeJSONProperty ( state, key, holder )
3. 25.5.2.3 QuoteJSONString ( value )
4. 25.5.2.4 UnicodeEscape ( C )
5. 25.5.2.5 SerializeJSONObject ( state, value )
6. 25.5.2.6 SerializeJSONArray ( state, value )
3. 25.5.3 JSON [ @@toStringTag ]
27. ◢26 Managing Memory
1. ◢26.1 WeakRef Objects
1. ◢26.1.1 The WeakRef Constructor
1. 26.1.1.1 WeakRef ( target )
2. ◢26.1.2 Properties of the WeakRef Constructor
1. 26.1.2.1 WeakRef.prototype
3. ◢26.1.3 Properties of the WeakRef Prototype Object
1. 26.1.3.1 WeakRef.prototype.constructor
2. 26.1.3.2 WeakRef.prototype.deref ( )
3. 26.1.3.3 WeakRef.prototype [ @@toStringTag ]
4. ◢26.1.4 WeakRef Abstract Operations
1. 26.1.4.1 WeakRefDeref ( weakRef )
5. 26.1.5 Properties of WeakRef Instances
2. ◢26.2 FinalizationRegistry Objects
1. ◢26.2.1 The FinalizationRegistry Constructor
1. 26.2.1.1 FinalizationRegistry ( cleanupCallback )
2. ◢26.2.2 Properties of the FinalizationRegistry Constructor
1. 26.2.2.1 FinalizationRegistry.prototype
3. ◢26.2.3 Properties of the FinalizationRegistry Prototype Object
1. 26.2.3.1 FinalizationRegistry.prototype.constructor
2. 26.2.3.2 FinalizationRegistry.prototype.register ( target,
heldValue [ , unregisterToken ] )
3. 26.2.3.3 FinalizationRegistry.prototype.unregister (
unregisterToken )
4. 26.2.3.4 FinalizationRegistry.prototype [ @@toStringTag ]
4. 26.2.4 Properties of FinalizationRegistry Instances
28. ◢27 Control Abstraction Objects
1. ◢27.1 Iteration
1. ◢27.1.1 Common Iteration Interfaces
1. 27.1.1.1 The Iterable Interface
2. 27.1.1.2 The Iterator Interface
3. 27.1.1.3 The AsyncIterable Interface
4. 27.1.1.4 The AsyncIterator Interface
5. 27.1.1.5 The IteratorResult Interface
2. ◢27.1.2 The %IteratorPrototype% Object
1. 27.1.2.1 %IteratorPrototype% [ @@iterator ] ( )
3. ◢27.1.3 The %AsyncIteratorPrototype% Object
1. 27.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )
4. ◢27.1.4 Async-from-Sync Iterator Objects
1. 27.1.4.1 CreateAsyncFromSyncIterator ( syncIteratorRecord )
2. ◢27.1.4.2 The %AsyncFromSyncIteratorPrototype% Object
1. 27.1.4.2.1 %AsyncFromSyncIteratorPrototype%.next ( [ value ] )
2. 27.1.4.2.2 %AsyncFromSyncIteratorPrototype%.return ( [ value ]
)
3. 27.1.4.2.3 %AsyncFromSyncIteratorPrototype%.throw ( [ value ] )
3. 27.1.4.3 Properties of Async-from-Sync Iterator Instances
4. 27.1.4.4 AsyncFromSyncIteratorContinuation ( result,
promiseCapability )
2. ◢27.2 Promise Objects
1. ◢27.2.1 Promise Abstract Operations
1. ◢27.2.1.1 PromiseCapability Records
1. 27.2.1.1.1 IfAbruptRejectPromise ( value, capability )
2. 27.2.1.2 PromiseReaction Records
3. ◢27.2.1.3 CreateResolvingFunctions ( promise )
1. 27.2.1.3.1 Promise Reject Functions
2. 27.2.1.3.2 Promise Resolve Functions
4. 27.2.1.4 FulfillPromise ( promise, value )
5. 27.2.1.5 NewPromiseCapability ( C )
6. 27.2.1.6 IsPromise ( x )
7. 27.2.1.7 RejectPromise ( promise, reason )
8. 27.2.1.8 TriggerPromiseReactions ( reactions, argument )
9. 27.2.1.9 HostPromiseRejectionTracker ( promise, operation )
2. ◢27.2.2 Promise Jobs
1. 27.2.2.1 NewPromiseReactionJob ( reaction, argument )
2. 27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve,
thenable, then )
3. ◢27.2.3 The Promise Constructor
1. 27.2.3.1 Promise ( executor )
4. ◢27.2.4 Properties of the Promise Constructor
1. ◢27.2.4.1 Promise.all ( iterable )
1. 27.2.4.1.1 GetPromiseResolve ( promiseConstructor )
2. 27.2.4.1.2 PerformPromiseAll ( iteratorRecord, constructor,
resultCapability, promiseResolve )
3. 27.2.4.1.3 Promise.all Resolve Element Functions
2. ◢27.2.4.2 Promise.allSettled ( iterable )
1. 27.2.4.2.1 PerformPromiseAllSettled ( iteratorRecord,
constructor, resultCapability, promiseResolve )
2. 27.2.4.2.2 Promise.allSettled Resolve Element Functions
3. 27.2.4.2.3 Promise.allSettled Reject Element Functions
3. ◢27.2.4.3 Promise.any ( iterable )
1. 27.2.4.3.1 PerformPromiseAny ( iteratorRecord, constructor,
resultCapability, promiseResolve )
2. 27.2.4.3.2 Promise.any Reject Element Functions
4. 27.2.4.4 Promise.prototype
5. ◢27.2.4.5 Promise.race ( iterable )
1. 27.2.4.5.1 PerformPromiseRace ( iteratorRecord, constructor,
resultCapability, promiseResolve )
6. 27.2.4.6 Promise.reject ( r )
7. ◢27.2.4.7 Promise.resolve ( x )
1. 27.2.4.7.1 PromiseResolve ( C, x )
8. 27.2.4.8 get Promise [ @@species ]
5. ◢27.2.5 Properties of the Promise Prototype Object
1. 27.2.5.1 Promise.prototype.catch ( onRejected )
2. 27.2.5.2 Promise.prototype.constructor
3. 27.2.5.3 Promise.prototype.finally ( onFinally )
4. ◢27.2.5.4 Promise.prototype.then ( onFulfilled, onRejected )
1. 27.2.5.4.1 PerformPromiseThen ( promise, onFulfilled,
onRejected [ , resultCapability ] )
5. 27.2.5.5 Promise.prototype [ @@toStringTag ]
6. 27.2.6 Properties of Promise Instances
3. ◢27.3 GeneratorFunction Objects
1. ◢27.3.1 The GeneratorFunction Constructor
1. 27.3.1.1 GeneratorFunction ( ...parameterArgs, bodyArg )
2. ◢27.3.2 Properties of the GeneratorFunction Constructor
1. 27.3.2.1 GeneratorFunction.length
2. 27.3.2.2 GeneratorFunction.prototype
3. ◢27.3.3 Properties of the GeneratorFunction Prototype Object
1. 27.3.3.1 GeneratorFunction.prototype.constructor
2. 27.3.3.2 GeneratorFunction.prototype.prototype
3. 27.3.3.3 GeneratorFunction.prototype [ @@toStringTag ]
4. ◢27.3.4 GeneratorFunction Instances
1. 27.3.4.1 length
2. 27.3.4.2 name
3. 27.3.4.3 prototype
4. ◢27.4 AsyncGeneratorFunction Objects
1. ◢27.4.1 The AsyncGeneratorFunction Constructor
1. 27.4.1.1 AsyncGeneratorFunction ( ...parameterArgs, bodyArg )
2. ◢27.4.2 Properties of the AsyncGeneratorFunction Constructor
1. 27.4.2.1 AsyncGeneratorFunction.length
2. 27.4.2.2 AsyncGeneratorFunction.prototype
3. ◢27.4.3 Properties of the AsyncGeneratorFunction Prototype Object
1. 27.4.3.1 AsyncGeneratorFunction.prototype.constructor
2. 27.4.3.2 AsyncGeneratorFunction.prototype.prototype
3. 27.4.3.3 AsyncGeneratorFunction.prototype [ @@toStringTag ]
4. ◢27.4.4 AsyncGeneratorFunction Instances
1. 27.4.4.1 length
2. 27.4.4.2 name
3. 27.4.4.3 prototype
5. ◢27.5 Generator Objects
1. ◢27.5.1 Properties of the Generator Prototype Object
1. 27.5.1.1 Generator.prototype.constructor
2. 27.5.1.2 Generator.prototype.next ( value )
3. 27.5.1.3 Generator.prototype.return ( value )
4. 27.5.1.4 Generator.prototype.throw ( exception )
5. 27.5.1.5 Generator.prototype [ @@toStringTag ]
2. 27.5.2 Properties of Generator Instances
3. ◢27.5.3 Generator Abstract Operations
1. 27.5.3.1 GeneratorStart ( generator, generatorBody )
2. 27.5.3.2 GeneratorValidate ( generator, generatorBrand )
3. 27.5.3.3 GeneratorResume ( generator, value, generatorBrand )
4. 27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion,
generatorBrand )
5. 27.5.3.5 GetGeneratorKind ( )
6. 27.5.3.6 GeneratorYield ( iterNextObj )
7. 27.5.3.7 Yield ( value )
8. 27.5.3.8 CreateIteratorFromClosure ( closure, generatorBrand,
generatorPrototype )
6. ◢27.6 AsyncGenerator Objects
1. ◢27.6.1 Properties of the AsyncGenerator Prototype Object
1. 27.6.1.1 AsyncGenerator.prototype.constructor
2. 27.6.1.2 AsyncGenerator.prototype.next ( value )
3. 27.6.1.3 AsyncGenerator.prototype.return ( value )
4. 27.6.1.4 AsyncGenerator.prototype.throw ( exception )
5. 27.6.1.5 AsyncGenerator.prototype [ @@toStringTag ]
2. 27.6.2 Properties of AsyncGenerator Instances
3. ◢27.6.3 AsyncGenerator Abstract Operations
1. 27.6.3.1 AsyncGeneratorRequest Records
2. 27.6.3.2 AsyncGeneratorStart ( generator, generatorBody )
3. 27.6.3.3 AsyncGeneratorValidate ( generator, generatorBrand )
4. 27.6.3.4 AsyncGeneratorEnqueue ( generator, completion,
promiseCapability )
5. 27.6.3.5 AsyncGeneratorCompleteStep ( generator, completion, done
[ , realm ] )
6. 27.6.3.6 AsyncGeneratorResume ( generator, completion )
7. 27.6.3.7 AsyncGeneratorUnwrapYieldResumption ( resumptionValue )
8. 27.6.3.8 AsyncGeneratorYield ( value )
9. 27.6.3.9 AsyncGeneratorAwaitReturn ( generator )
10. 27.6.3.10 AsyncGeneratorDrainQueue ( generator )
11. 27.6.3.11 CreateAsyncIteratorFromClosure ( closure,
generatorBrand, generatorPrototype )
7. ◢27.7 AsyncFunction Objects
1. ◢27.7.1 The AsyncFunction Constructor
1. 27.7.1.1 AsyncFunction ( ...parameterArgs, bodyArg )
2. ◢27.7.2 Properties of the AsyncFunction Constructor
1. 27.7.2.1 AsyncFunction.length
2. 27.7.2.2 AsyncFunction.prototype
3. ◢27.7.3 Properties of the AsyncFunction Prototype Object
1. 27.7.3.1 AsyncFunction.prototype.constructor
2. 27.7.3.2 AsyncFunction.prototype [ @@toStringTag ]
4. ◢27.7.4 AsyncFunction Instances
1. 27.7.4.1 length
2. 27.7.4.2 name
5. ◢27.7.5 Async Functions Abstract Operations
1. 27.7.5.1 AsyncFunctionStart ( promiseCapability, asyncFunctionBody
)
2. 27.7.5.2 AsyncBlockStart ( promiseCapability, asyncBody,
asyncContext )
3. 27.7.5.3 Await ( value )
29. ◢28 Reflection
1. ◢28.1 The Reflect Object
1. 28.1.1 Reflect.apply ( target, thisArgument, argumentsList )
2. 28.1.2 Reflect.construct ( target, argumentsList [ , newTarget ] )
3. 28.1.3 Reflect.defineProperty ( target, propertyKey, attributes )
4. 28.1.4 Reflect.deleteProperty ( target, propertyKey )
5. 28.1.5 Reflect.get ( target, propertyKey [ , receiver ] )
6. 28.1.6 Reflect.getOwnPropertyDescriptor ( target, propertyKey )
7. 28.1.7 Reflect.getPrototypeOf ( target )
8. 28.1.8 Reflect.has ( target, propertyKey )
9. 28.1.9 Reflect.isExtensible ( target )
10. 28.1.10 Reflect.ownKeys ( target )
11. 28.1.11 Reflect.preventExtensions ( target )
12. 28.1.12 Reflect.set ( target, propertyKey, V [ , receiver ] )
13. 28.1.13 Reflect.setPrototypeOf ( target, proto )
14. 28.1.14 Reflect [ @@toStringTag ]
2. ◢28.2 Proxy Objects
1. ◢28.2.1 The Proxy Constructor
1. 28.2.1.1 Proxy ( target, handler )
2. ◢28.2.2 Properties of the Proxy Constructor
1. 28.2.2.1 Proxy.revocable ( target, handler )
3. ◢28.3 Module Namespace Objects
1. 28.3.1 @@toStringTag
30. ◢29 Memory Model
1. 29.1 Memory Model Fundamentals
2. 29.2 Agent Events Records
3. 29.3 Chosen Value Records
4. 29.4 Candidate Executions
5. ◢29.5 Abstract Operations for the Memory Model
1. 29.5.1 EventSet ( execution )
2. 29.5.2 SharedDataBlockEventSet ( execution )
3. 29.5.3 HostEventSet ( execution )
4. 29.5.4 ComposeWriteEventBytes ( execution, byteIndex, Ws )
5. 29.5.5 ValueOfReadEvent ( execution, R )
6. ◢29.6 Relations of Candidate Executions
1. 29.6.1 agent-order
2. 29.6.2 reads-bytes-from
3. 29.6.3 reads-from
4. 29.6.4 host-synchronizes-with
5. 29.6.5 synchronizes-with
6. 29.6.6 happens-before
7. ◢29.7 Properties of Valid Executions
1. 29.7.1 Valid Chosen Reads
2. 29.7.2 Coherent Reads
3. 29.7.3 Tear Free Reads
4. 29.7.4 Sequentially Consistent Atomics
5. 29.7.5 Valid Executions
8. 29.8 Races
9. 29.9 Data Races
10. 29.10 Data Race Freedom
11. 29.11 Shared Memory Guidelines
31. ◢A Grammar Summary
1. A.1 Lexical Grammar
2. A.2 Expressions
3. A.3 Statements
4. A.4 Functions and Classes
5. A.5 Scripts and Modules
6. A.6 Number Conversions
7. A.7 Time Zone Offset String Format
8. A.8 Regular Expressions
32. ◢B Additional ECMAScript Features for Web Browsers
1. ◢B.1 Additional Syntax
1. B.1.1 HTML-like Comments
2. ◢B.1.2 Regular Expressions Patterns
1. B.1.2.1 SS: Early Errors
2. B.1.2.2 SS: CountLeftCapturingParensWithin and
CountLeftCapturingParensBefore
3. B.1.2.3 SS: IsCharacterClass
4. B.1.2.4 SS: CharacterValue
5. B.1.2.5 RS: CompileSubpattern
6. B.1.2.6 RS: CompileAssertion
7. B.1.2.7 RS: CompileAtom
8. ◢B.1.2.8 RS: CompileToCharSet
1. B.1.2.8.1 CharacterRangeOrUnion ( rer, A, B )
9. B.1.2.9 SS: ParsePattern ( patternText, u )
2. ◢B.2 Additional Built-in Properties
1. ◢B.2.1 Additional Properties of the Global Object
1. B.2.1.1 escape ( string )
2. B.2.1.2 unescape ( string )
2. ◢B.2.2 Additional Properties of the String.prototype Object
1. B.2.2.1 String.prototype.substr ( start, length )
2. ◢B.2.2.2 String.prototype.anchor ( name )
1. B.2.2.2.1 CreateHTML ( string, tag, attribute, value )
3. B.2.2.3 String.prototype.big ( )
4. B.2.2.4 String.prototype.blink ( )
5. B.2.2.5 String.prototype.bold ( )
6. B.2.2.6 String.prototype.fixed ( )
7. B.2.2.7 String.prototype.fontcolor ( color )
8. B.2.2.8 String.prototype.fontsize ( size )
9. B.2.2.9 String.prototype.italics ( )
10. B.2.2.10 String.prototype.link ( url )
11. B.2.2.11 String.prototype.small ( )
12. B.2.2.12 String.prototype.strike ( )
13. B.2.2.13 String.prototype.sub ( )
14. B.2.2.14 String.prototype.sup ( )
15. B.2.2.15 String.prototype.trimLeft ( )
16. B.2.2.16 String.prototype.trimRight ( )
3. ◢B.2.3 Additional Properties of the Date.prototype Object
1. B.2.3.1 Date.prototype.getYear ( )
2. B.2.3.2 Date.prototype.setYear ( year )
3. B.2.3.3 Date.prototype.toGMTString ( )
4. ◢B.2.4 Additional Properties of the RegExp.prototype Object
1. B.2.4.1 RegExp.prototype.compile ( pattern, flags )
3. ◢B.3 Other Additional Features
1. B.3.1 Labelled Function Declarations
2. ◢B.3.2 Block-Level Function Declarations Web Legacy Compatibility
Semantics
1. B.3.2.1 Changes to FunctionDeclarationInstantiation
2. B.3.2.2 Changes to GlobalDeclarationInstantiation
3. B.3.2.3 Changes to EvalDeclarationInstantiation
4. B.3.2.4 Changes to Block SS: Early Errors
5. B.3.2.5 Changes to switch Statement SS: Early Errors
6. B.3.2.6 Changes to BlockDeclarationInstantiation
3. B.3.3 FunctionDeclarations in IfStatement Statement Clauses
4. B.3.4 VariableStatements in Catch Blocks
5. B.3.5 Initializers in ForIn Statement Heads
6. ◢B.3.6 The [[IsHTMLDDA]] Internal Slot
1. B.3.6.1 Changes to ToBoolean
2. B.3.6.2 Changes to IsLooselyEqual
3. B.3.6.3 Changes to the typeof Operator
7. B.3.7 Non-default behaviour in HostMakeJobCallback
8. B.3.8 Non-default behaviour in HostEnsureCanAddPrivateElement
33. C The Strict Mode of ECMAScript
34. ◢D Host Layering Points
1. D.1 Host Hooks
2. D.2 Host-defined Fields
3. D.3 Host-defined Objects
4. D.4 Running Jobs
5. D.5 Internal Methods of Exotic Objects
6. D.6 Built-in Objects and Methods
35. E Corrections and Clarifications in ECMAScript 2015 with Possible
Compatibility Impact
36. F Additions and Changes That Introduce Incompatibilities with Prior
Editions
37. G Colophon
38. H Bibliography
39. I Copyright & Software License
ECMA-262, 14TH EDITION, JUNE 2023
ECMASCRIPT® 2023 LANGUAGE SPECIFICATION
ABOUT THIS SPECIFICATION
The document at https://tc39.es/ecma262/ is the most accurate and up-to-date
ECMAScript specification. It contains the content of the most recent yearly
snapshot plus any finished proposals (those that have reached Stage 4 in the
proposal process and thus are implemented in several implementations and will be
in the next practical revision) since that snapshot was taken.
This document is available as a single page and as multiple pages.
CONTRIBUTING TO THIS SPECIFICATION
This specification is developed on GitHub with the help of the ECMAScript
community. There are a number of ways to contribute to the development of this
specification:
* GitHub Repository: https://github.com/tc39/ecma262
* Issues: All Issues, File a New Issue
* Pull Requests: All Pull Requests, Create a New Pull Request
* Test Suite: Test262
* Editors:
* Shu-yu Guo (@_shu)
* Michael Ficarra (@smooshMap)
* Kevin Gibbons (@bakkoting)
* Community:
* Discourse: https://es.discourse.group
* Chat: Matrix
* Mailing List Archives: https://esdiscuss.org/
Refer to the colophon for more information on how this document is created.
INTRODUCTION
This Ecma Standard defines the ECMAScript 2023 Language. It is the fourteenth
edition of the ECMAScript Language Specification. Since publication of the first
edition in 1997, ECMAScript has grown to be one of the world's most widely used
general-purpose programming languages. It is best known as the language embedded
in web browsers but has also been widely adopted for server and embedded
applications.
ECMAScript is based on several originating technologies, the most well-known
being JavaScript (Netscape) and JScript (Microsoft). The language was invented
by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0
browser. It has appeared in all subsequent browsers from Netscape and in all
browsers from Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November
1996. The first edition of this Ecma Standard was adopted by the Ecma General
Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the
fast-track procedure, and approved as international standard ISO/IEC 16262, in
April 1998. The Ecma General Assembly of June 1998 approved the second edition
of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the
first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions,
better string handling, new control statements, try/catch exception handling,
tighter definition of errors, formatting for numeric output and minor changes in
anticipation of future language growth. The third edition of the ECMAScript
standard was adopted by the Ecma General Assembly of December 1999 and published
as ISO/IEC 16262:2002 in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in
conjunction with the World Wide Web where it has become the programming language
that is supported by essentially all web browsers. Significant work was done to
develop a fourth edition of ECMAScript. However, that work was not completed and
not published as the fourth edition of ECMAScript but some of it was
incorporated into the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de
facto interpretations of the language specification that have become common
among browser implementations and added support for new features that had
emerged since the publication of the third edition. Such features include
accessor properties, reflective creation and inspection of objects, program
control of property attributes, additional array manipulation functions, support
for the JSON object encoding format, and a strict mode that provides enhanced
error checking and program security. The fifth edition was adopted by the Ecma
General Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the
fast-track procedure, and approved as international standard ISO/IEC 16262:2011.
Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the
same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General
Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth edition
was being prepared for publication. However, this was preceded by significant
experimentation and language enhancement design efforts dating to the
publication of the third edition in 1999. In a very real sense, the completion
of the sixth edition is the culmination of a fifteen year effort. The goals for
this edition included providing better support for large applications, library
creation, and for use of ECMAScript as a compilation target for other languages.
Some of its major enhancements included modules, class declarations, lexical
block scoping, iterators and generators, promises for asynchronous programming,
destructuring patterns, and proper tail calls. The ECMAScript library of
built-ins was expanded to support additional data abstractions including maps,
sets, and arrays of binary numeric values as well as additional support for
Unicode supplementary characters in strings and regular expressions. The
built-ins were also made extensible via subclassing. The sixth edition provides
the foundation for regular, incremental language and library enhancements. The
sixth edition was adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new
yearly release cadence and open development process. A plain-text source
document was built from the ECMAScript 2015 source document to serve as the base
for further development entirely on GitHub. Over the year of this standard's
development, hundreds of pull requests and issues were filed representing
thousands of bug fixes, editorial fixes and other improvements. Additionally,
numerous software tools were developed to aid in this effort including Ecmarkup,
Ecmarkdown, and Grammarkdown. ES2016 also included support for a new
exponentiation operator and adds a new method to Array.prototype called
includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along
with smaller language and library enhancements, bug fixes, and editorial
updates. Async functions improve the asynchronous programming experience by
providing syntax for promise-returning functions. Shared Memory and Atomics
introduce a new memory model that allows multi-agent programs to communicate
using atomic operations that ensure a well-defined execution order even on
parallel CPUs. It also included new static methods on Object: Object.values,
Object.entries, and Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the
AsyncIterator protocol and async generators. It also included four new regular
expression features: the dotAll flag, named capture groups, Unicode property
escapes, and look-behind assertions. Lastly it included object rest and spread
properties.
ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on
Array.prototype for flattening arrays, Object.fromEntries for directly turning
the return value of Object.entries into a new Object, and trimStart and trimEnd
on String.prototype as better-named alternatives to the widely implemented but
non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it
included a few minor updates to syntax and semantics. Updated syntax included
optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and
U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other
updates included requiring that Array.prototype.sort be a stable sort, requiring
that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying
Function.prototype.toString by requiring that it either return the corresponding
original source text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduced the matchAll method for Strings,
to produce an iterator for all match objects generated by a global regular
expression; import(), a syntax to asynchronously import Modules with a dynamic
specifier; BigInt, a new number primitive for working with arbitrary precision
integers; Promise.allSettled, a new Promise combinator that does not
short-circuit; globalThis, a universal way to access the global this value;
dedicated export * as ns from 'module' syntax for use within modules; increased
standardization of for-in enumeration order; import.meta, a host-populated
object available in Modules that may contain contextual information about the
Module; as well as adding two new syntax features to improve working with
“nullish” values (null or undefined): nullish coalescing, a value selection
operator; and optional chaining, a property access and function invocation
operator that short-circuits if the value to access/invoke is nullish.
ECMAScript 2021, the 12th edition, introduced the replaceAll method for Strings;
Promise.any, a Promise combinator that short-circuits when an input value is
fulfilled; AggregateError, a new Error type to represent multiple errors at
once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a
target object without preserving it from garbage collection, and
FinalizationRegistry, to manage registration and unregistration of cleanup
operations performed when target objects are garbage collected; separators for
numeric literals (1_000); and Array.prototype.sort was made more precise,
reducing the amount of cases that result in an implementation-defined sort
order.
ECMAScript 2022, the 13th edition, introduced top-level await, allowing the
keyword to be used at the top level of modules; new class elements: public and
private instance fields, public and private static fields, private instance
methods and accessors, and private static methods and accessors; static blocks
inside classes, to perform per-class evaluation initialization; the #x in obj
syntax, to test for presence of private fields on objects; regular expression
match indices via the /d flag, which provides start and end indices for matched
substrings; the cause property on Error objects, which can be used to record a
causation chain in errors; the at method for Strings, Arrays, and TypedArrays,
which allows relative indexing; and Object.hasOwn, a convenient alternative to
Object.prototype.hasOwnProperty.
ECMAScript 2023, the 14th edition, introduced the toSorted, toReversed, with,
findLast, and findLastIndex methods on Array.prototype and TypedArray.prototype,
as well as the toSpliced method on Array.prototype; added support for #!
comments at the beginning of files to better facilitate executable ECMAScript
files; and allowed the use of most Symbols as keys in weak collections.
Dozens of individuals representing many organizations have made very significant
contributions within Ecma TC39 to the development of this edition and to the
prior editions. In addition, a vibrant community has emerged supporting TC39's
ECMAScript efforts. This community has reviewed numerous drafts, filed thousands
of bug reports, performed implementation experiments, contributed test suites,
and educated the world-wide developer community about ECMAScript. Unfortunately,
it is impossible to identify and acknowledge every person and organization who
has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition
Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions
Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions
Shu-yu Guo
ECMA-262, Project Editor, 12th through 14th Editions
Michael Ficarra
ECMA-262, Project Editor, 12th through 14th Editions
Kevin Gibbons
ECMA-262, Project Editor, 12th through 14th Editions
1 SCOPE
This Standard defines the ECMAScript 2023 general-purpose programming language.
2 CONFORMANCE
A conforming implementation of ECMAScript must provide and support all the
types, values, objects, properties, functions, and program syntax and semantics
described in this specification.
A conforming implementation of ECMAScript must interpret source text input in
conformance with the latest version of the Unicode Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application
programming interface (API) that supports programs that need to adapt to the
linguistic and cultural conventions used by different human languages and
countries must implement the interface defined by the most recent edition of
ECMA-402 that is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values,
objects, properties, and functions beyond those described in this specification.
In particular, a conforming implementation of ECMAScript may provide properties
not described in this specification, and values for those properties, for
objects that are described in this specification.
A conforming implementation of ECMAScript may support program and regular
expression syntax not described in this specification. In particular, a
conforming implementation of ECMAScript may support program syntax that makes
use of any “future reserved words” noted in subclause 12.7.2 of this
specification.
A conforming implementation of ECMAScript must not implement any extension that
is listed as a Forbidden Extension in subclause 17.1.
A conforming implementation of ECMAScript must not redefine any facilities that
are not implementation-defined, implementation-approximated, or host-defined.
A conforming implementation of ECMAScript may choose to implement or not
implement Normative Optional subclauses. If any Normative Optional behaviour is
implemented, all of the behaviour in the containing Normative Optional clause
must be implemented. A Normative Optional clause is denoted in this
specification with the words "Normative Optional" in a coloured box, as shown
below.
Normative Optional
2.1 EXAMPLE NORMATIVE OPTIONAL CLAUSE HEADING
Example clause contents.
A conforming implementation of ECMAScript must implement Legacy subclauses,
unless they are also marked as Normative Optional. All of the language features
and behaviours specified within Legacy subclauses have one or more undesirable
characteristics. However, their continued usage in existing applications
prevents their removal from this specification. These features are not
considered part of the core ECMAScript language. Programmers should not use or
assume the existence of these features and behaviours when writing new
ECMAScript code.
Legacy
2.2 EXAMPLE LEGACY CLAUSE HEADING
Example clause contents.
Normative Optional, Legacy
2.3 EXAMPLE LEGACY NORMATIVE OPTIONAL CLAUSE HEADING
Example clause contents.
3 NORMATIVE REFERENCES
The following referenced documents are indispensable for the application of this
document. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any
amendments) applies.
ISO/IEC 10646 Information Technology — Universal Multiple-Octet Coded Character
Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and
Amendment 4:2008, plus additional amendments and corrigenda, or successor
ECMA-402, ECMAScript 2015 Internationalization API Specification.
https://www.ecma-international.org/publications-and-standards/standards/ecma-402/
ECMA-404, The JSON Data Interchange Format.
https://www.ecma-international.org/publications-and-standards/standards/ecma-404/
4 OVERVIEW
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing
computations and manipulating computational objects within a host environment.
ECMAScript as defined here is not intended to be computationally
self-sufficient; indeed, there are no provisions in this specification for input
of external data or output of computed results. Instead, it is expected that the
computational environment of an ECMAScript program will provide not only the
objects and other facilities described in this specification but also certain
environment-specific objects, whose description and behaviour are beyond the
scope of this specification except to indicate that they may provide certain
properties that can be accessed and certain functions that can be called from an
ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has
become widely used as a general-purpose programming language. A scripting
language is a programming language that is used to manipulate, customize, and
automate the facilities of an existing system. In such systems, useful
functionality is already available through a user interface, and the scripting
language is a mechanism for exposing that functionality to program control. In
this way, the existing system is said to provide a host environment of objects
and facilities, which completes the capabilities of the scripting language. A
scripting language is intended for use by both professional and non-professional
programmers.
ECMAScript was originally designed to be a Web scripting language, providing a
mechanism to enliven Web pages in browsers and to perform server computation as
part of a Web-based client-server architecture. ECMAScript is now used to
provide core scripting capabilities for a variety of host environments.
Therefore the core language is specified in this document apart from any
particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the
full spectrum of programming tasks in many different environments and scales. As
the usage of ECMAScript has expanded, so have the features and facilities it
provides. ECMAScript is now a fully featured general-purpose programming
language.
4.1 WEB SCRIPTING
A web browser provides an ECMAScript host environment for client-side
computation including, for instance, objects that represent windows, menus,
pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and
input/output. Further, the host environment provides a means to attach scripting
code to events such as change of focus, page and image loading, unloading, error
and abort, selection, form submission, and mouse actions. Scripting code appears
within the HTML and the displayed page is a combination of user interface
elements and fixed and computed text and images. The scripting code is reactive
to user interaction, and there is no need for a main program.
A web server provides a different host environment for server-side computation
including objects representing requests, clients, and files; and mechanisms to
lock and share data. By using browser-side and server-side scripting together,
it is possible to distribute computation between the client and server while
providing a customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own host
environment, completing the ECMAScript execution environment.
4.2 HOSTS AND IMPLEMENTATIONS
To aid integrating ECMAScript into host environments, this specification defers
the definition of certain facilities (e.g., abstract operations), either in
whole or in part, to a source outside of this specification. Editorially, this
specification distinguishes the following kinds of deferrals.
An implementation is an external source that further defines facilities
enumerated in Annex D or those that are marked as implementation-defined or
implementation-approximated. In informal use, an implementation refers to a
concrete artefact, such as a particular web browser.
An implementation-defined facility is one that defers its definition to an
external source without further qualification. This specification does not make
any recommendations for particular behaviours, and conforming implementations
are free to choose any behaviour within the constraints put forth by this
specification.
An implementation-approximated facility is one that defers its definition to an
external source while recommending an ideal behaviour. While conforming
implementations are free to choose any behaviour within the constraints put
forth by this specification, they are encouraged to strive to approximate the
ideal. Some mathematical operations, such as Math.exp, are
implementation-approximated.
A host is an external source that further defines facilities listed in Annex D
but does not further define other implementation-defined or
implementation-approximated facilities. In informal use, a host refers to the
set of all implementations, such as the set of all web browsers, that interface
with this specification in the same way via Annex D. A host is often an external
specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other
words, facilities that are host-defined are often further defined in external
specifications.
A host hook is an abstract operation that is defined in whole or in part by an
external source. All host hooks must be listed in Annex D. A host hook must
conform to at least the following requirements:
* It must return either a normal completion or a throw completion.
A host-defined facility is one that defers its definition to an external source
without further qualification and is listed in Annex D. Implementations that are
not hosts may also provide definitions for host-defined facilities.
A host environment is a particular choice of definition for all host-defined
facilities. A host environment typically includes objects or functions which
allow obtaining input and providing output as host-defined properties of the
global object.
This specification follows the editorial convention of always using the most
specific term. For example, if a facility is host-defined, it should not be
referred to as implementation-defined.
Both hosts and implementations may interface with this specification via the
language types, specification types, abstract operations, grammar productions,
intrinsic objects, and intrinsic symbols defined herein.
4.3 ECMASCRIPT OVERVIEW
The following is an informal overview of ECMAScript—not all parts of the
language are described. This overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by
objects, and an ECMAScript program is a cluster of communicating objects. In
ECMAScript, an object is a collection of zero or more properties each with
attributes that determine how each property can be used—for example, when the
Writable attribute for a property is set to false, any attempt by executed
ECMAScript code to assign a different value to the property fails. Properties
are containers that hold other objects, primitive values, or functions. A
primitive value is a member of one of the following built-in types: Undefined,
Null, Boolean, Number, BigInt, String, and Symbol; an object is a member of the
built-in type Object; and a function is a callable object. A function that is
associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the
definition of ECMAScript entities. These built-in objects include the global
object; objects that are fundamental to the runtime semantics of the language
including Object, Function, Boolean, Symbol, and various Error objects; objects
that represent and manipulate numeric values including Math, Number, and Date;
the text processing objects String and RegExp; objects that are indexed
collections of values including Array and nine different kinds of Typed Arrays
whose elements all have a specific numeric data representation; keyed
collections including Map and Set objects; objects supporting structured data
including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects
supporting control abstractions including generator functions and Promise
objects; and reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators
include various unary operations, multiplicative operators, additive operators,
bitwise shift operators, relational operators, equality operators, binary
bitwise operators, binary logical operators, assignment operators, and the comma
operator.
Large ECMAScript programs are supported by modules which allow a program to be
divided into multiple sequences of statements and declarations. Each module
explicitly identifies declarations it uses that need to be provided by other
modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is
relaxed to enable it to serve as an easy-to-use scripting language. For example,
a variable is not required to have its type declared nor are types associated
with properties, and defined functions are not required to have their
declarations appear textually before calls to them.
4.3.1 OBJECTS
Even though ECMAScript includes syntax for class definitions, ECMAScript objects
are not fundamentally class-based such as those in C++, Smalltalk, or Java.
Instead objects may be created in various ways including via a literal notation
or via constructors which create objects and then execute code that initializes
all or part of them by assigning initial values to their properties. Each
constructor is a function that has a property named "prototype" that is used to
implement prototype-based inheritance and shared properties. Objects are created
by using constructors in new expressions; for example, new Date(2009, 11)
creates a new Date object. Invoking a constructor without using new has
consequences that depend on the constructor. For example, Date() produces a
string representation of the current date and time rather than an object.
Every object created by a constructor has an implicit reference (called the
object's prototype) to the value of its constructor's "prototype" property.
Furthermore, a prototype may have a non-null implicit reference to its
prototype, and so on; this is called the prototype chain. When a reference is
made to a property in an object, that reference is to the property of that name
in the first object in the prototype chain that contains a property of that
name. In other words, first the object mentioned directly is examined for such a
property; if that object contains the named property, that is the property to
which the reference refers; if that object does not contain the named property,
the prototype for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by
instances, methods are carried by classes, and inheritance is only of structure
and behaviour. In ECMAScript, the state and methods are carried by objects,
while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their
prototype contains share that property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five objects have been created by
using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects
contains properties named "q1" and "q2". The dashed lines represent the implicit
prototype relationship; so, for example, cf3's prototype is CFp. The
constructor, CF, has two properties itself, named "P1" and "P2", which are not
visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named "CFP1" in CFp is
shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties
found in CFp's implicit prototype chain that are not named "q1", "q2", or
"CFP1". Notice that there is no implicit prototype link between CF and CFp.
Unlike most class-based object languages, properties can be added to objects
dynamically by assigning values to them. That is, constructors are not required
to name or assign values to all or any of the constructed object's properties.
In the above diagram, one could add a new shared property for cf1, cf2, cf3,
cf4, and cf5 by assigning a new value to the property in CFp.
Although ECMAScript objects are not inherently class-based, it is often
convenient to define class-like abstractions based upon a common pattern of
constructor functions, prototype objects, and methods. The ECMAScript built-in
objects themselves follow such a class-like pattern. Beginning with ECMAScript
2015, the ECMAScript language includes syntactic class definitions that permit
programmers to concisely define objects that conform to the same class-like
abstraction pattern used by the built-in objects.
4.3.2 THE STRICT VARIANT OF ECMASCRIPT
The ECMAScript Language recognizes the possibility that some users of the
language may wish to restrict their usage of some features available in the
language. They might do so in the interests of security, to avoid what they
consider to be error-prone features, to get enhanced error checking, or for
other reasons of their choosing. In support of this possibility, ECMAScript
defines a strict variant of the language. The strict variant of the language
excludes some specific syntactic and semantic features of the regular ECMAScript
language and modifies the detailed semantics of some features. The strict
variant also specifies additional error conditions that must be reported by
throwing error exceptions in situations that are not specified as errors by the
non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of
the language. Strict mode selection and use of the strict mode syntax and
semantics of ECMAScript is explicitly made at the level of individual ECMAScript
source text units as described in 11.2.2. Because strict mode is selected at the
level of a syntactic source text unit, strict mode only imposes restrictions
that have local effect within such a source text unit. Strict mode does not
restrict or modify any aspect of the ECMAScript semantics that must operate
consistently across multiple source text units. A complete ECMAScript program
may be composed of both strict mode and non-strict mode ECMAScript source text
units. In this case, strict mode only applies when actually executing code that
is defined within a strict mode source text unit.
In order to conform to this specification, an ECMAScript implementation must
implement both the full unrestricted ECMAScript language and the strict variant
of the ECMAScript language as defined by this specification. In addition, an
implementation must support the combination of unrestricted and strict mode
source text units into a single composite program.
4.4 TERMS AND DEFINITIONS
For the purposes of this document, the following terms and definitions apply.
4.4.1 IMPLEMENTATION-APPROXIMATED
an implementation-approximated facility is defined in whole or in part by an
external source but has a recommended, ideal behaviour in this specification
4.4.2 IMPLEMENTATION-DEFINED
an implementation-defined facility is defined in whole or in part by an external
source to this specification
4.4.3 HOST-DEFINED
same as implementation-defined
Note
Editorially, see clause 4.2.
4.4.4 TYPE
set of data values as defined in clause 6
4.4.5 PRIMITIVE VALUE
member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or
String as defined in clause 6
Note
A primitive value is a datum that is represented directly at the lowest level of
the language implementation.
4.4.6 OBJECT
member of the type Object
Note
An object is a collection of properties and has a single prototype object. The
prototype may be null.
4.4.7 CONSTRUCTOR
function object that creates and initializes objects
Note
The value of a constructor's "prototype" property is a prototype object that is
used to implement inheritance and shared properties.
4.4.8 PROTOTYPE
object that provides shared properties for other objects
Note
When a constructor creates an object, that object implicitly references the
constructor's "prototype" property for the purpose of resolving property
references. The constructor's "prototype" property can be referenced by the
program expression constructor.prototype, and properties added to an object's
prototype are shared, through inheritance, by all objects sharing the prototype.
Alternatively, a new object may be created with an explicitly specified
prototype by using the Object.create built-in function.
4.4.9 ORDINARY OBJECT
object that has the default behaviour for the essential internal methods that
must be supported by all objects
4.4.10 EXOTIC OBJECT
object that does not have the default behaviour for one or more of the essential
internal methods
Note
Any object that is not an ordinary object is an exotic object.
4.4.11 STANDARD OBJECT
object whose semantics are defined by this specification
4.4.12 BUILT-IN OBJECT
object specified and supplied by an ECMAScript implementation
Note
Standard built-in objects are defined in this specification. An ECMAScript
implementation may specify and supply additional kinds of built-in objects. A
built-in constructor is a built-in object that is also a constructor.
4.4.13 UNDEFINED VALUE
primitive value used when a variable has not been assigned a value
4.4.14 UNDEFINED TYPE
type whose sole value is the undefined value
4.4.15 NULL VALUE
primitive value that represents the intentional absence of any object value
4.4.16 NULL TYPE
type whose sole value is the null value
4.4.17 BOOLEAN VALUE
member of the Boolean type
Note
There are only two Boolean values, true and false.
4.4.18 BOOLEAN TYPE
type consisting of the primitive values true and false
4.4.19 BOOLEAN OBJECT
member of the Object type that is an instance of the standard built-in Boolean
constructor
Note
A Boolean object is created by using the Boolean constructor in a new
expression, supplying a Boolean value as an argument. The resulting object has
an internal slot whose value is the Boolean value. A Boolean object can be
coerced to a Boolean value.
4.4.20 STRING VALUE
primitive value that is a finite ordered sequence of zero or more 16-bit
unsigned integer values
Note
A String value is a member of the String type. Each integer value in the
sequence usually represents a single 16-bit unit of UTF-16 text. However,
ECMAScript does not place any restrictions or requirements on the values except
that they must be 16-bit unsigned integers.
4.4.21 STRING TYPE
set of all possible String values
4.4.22 STRING OBJECT
member of the Object type that is an instance of the standard built-in String
constructor
Note
A String object is created by using the String constructor in a new expression,
supplying a String value as an argument. The resulting object has an internal
slot whose value is the String value. A String object can be coerced to a String
value by calling the String constructor as a function (22.1.1.1).
4.4.23 NUMBER VALUE
primitive value corresponding to a double-precision 64-bit binary format IEEE
754-2019 value
Note
A Number value is a member of the Number type and is a direct representation of
a number.
4.4.24 NUMBER TYPE
set of all possible Number values including the special “Not-a-Number” (NaN)
value, positive infinity, and negative infinity
4.4.25 NUMBER OBJECT
member of the Object type that is an instance of the standard built-in Number
constructor
Note
A Number object is created by using the Number constructor in a new expression,
supplying a Number value as an argument. The resulting object has an internal
slot whose value is the Number value. A Number object can be coerced to a Number
value by calling the Number constructor as a function (21.1.1.1).
4.4.26 INFINITY
Number value that is the positive infinite Number value
4.4.27 NAN
Number value that is an IEEE 754-2019 “Not-a-Number” value
4.4.28 BIGINT VALUE
primitive value corresponding to an arbitrary-precision integer value
4.4.29 BIGINT TYPE
set of all possible BigInt values
4.4.30 BIGINT OBJECT
member of the Object type that is an instance of the standard built-in BigInt
constructor
4.4.31 SYMBOL VALUE
primitive value that represents a unique, non-String Object property key
4.4.32 SYMBOL TYPE
set of all possible Symbol values
4.4.33 SYMBOL OBJECT
member of the Object type that is an instance of the standard built-in Symbol
constructor
4.4.34 FUNCTION
member of the Object type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable code and state
that determine how it behaves when invoked. A function's code may or may not be
written in ECMAScript.
4.4.35 BUILT-IN FUNCTION
built-in object that is a function
Note
Examples of built-in functions include parseInt and Math.exp. A host or
implementation may provide additional built-in functions that are not described
in this specification.
4.4.36 PROPERTY
part of an object that associates a key (either a String value or a Symbol
value) and a value
Note
Depending upon the form of the property the value may be represented either
directly as a data value (a primitive value, an object, or a function object) or
indirectly by a pair of accessor functions.
4.4.37 METHOD
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the
function as its this value.
4.4.38 BUILT-IN METHOD
method that is a built-in function
Note
Standard built-in methods are defined in this specification. A host or
implementation may provide additional built-in methods that are not described in
this specification.
4.4.39 ATTRIBUTE
internal value that defines some characteristic of a property
4.4.40 OWN PROPERTY
property that is directly contained by its object
4.4.41 INHERITED PROPERTY
property of an object that is not an own property but is a property (either own
or inherited) of the object's prototype
4.5 ORGANIZATION OF THIS SPECIFICATION
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used throughout the specification.
Clauses 6 through 10 define the execution environment within which ECMAScript
programs operate.
Clauses 11 through 17 define the actual ECMAScript programming language
including its syntactic encoding and the execution semantics of all language
features.
Clauses 18 through 28 define the ECMAScript standard library. They include the
definitions of all of the standard objects that are available for use by
ECMAScript programs as they execute.
Clause 29 describes the memory consistency model of accesses on
SharedArrayBuffer-backed memory and methods of the Atomics object.
5 NOTATIONAL CONVENTIONS
5.1 SYNTACTIC AND LEXICAL GRAMMARS
5.1.1 CONTEXT-FREE GRAMMARS
A context-free grammar consists of a number of productions. Each production has
an abstract symbol called a nonterminal as its left-hand side, and a sequence of
zero or more nonterminal and terminal symbols as its right-hand side. For each
grammar, the terminal symbols are drawn from a specified alphabet.
A chain production is a production that has exactly one nonterminal symbol on
its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal,
called the goal symbol, a given context-free grammar specifies a language,
namely, the (perhaps infinite) set of possible sequences of terminal symbols
that can result from repeatedly replacing any nonterminal in the sequence with a
right-hand side of a production for which the nonterminal is the left-hand side.
5.1.2 THE LEXICAL AND REGEXP GRAMMARS
A lexical grammar for ECMAScript is given in clause 12. This grammar has as its
terminal symbols Unicode code points that conform to the rules for
SourceCharacter defined in 11.1. It defines a set of productions, starting from
the goal symbol InputElementDiv, InputElementTemplateTail, InputElementRegExp,
InputElementRegExpOrTemplateTail, or InputElementHashbangOrRegExp, that describe
how sequences of such code points are translated into a sequence of input
elements.
Input elements other than white space and comments form the terminal symbols for
the syntactic grammar for ECMAScript and are called ECMAScript tokens. These
tokens are the reserved words, identifiers, literals, and punctuators of the
ECMAScript language. Moreover, line terminators, although not considered to be
tokens, also become part of the stream of input elements and guide the process
of automatic semicolon insertion (12.10). Simple white space and single-line
comments are discarded and do not appear in the stream of input elements for the
syntactic grammar. A MultiLineComment (that is, a comment of the form /*…*/
regardless of whether it spans more than one line) is likewise simply discarded
if it contains no line terminator; but if a MultiLineComment contains one or
more line terminators, then it is replaced by a single line terminator, which
becomes part of the stream of input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its
terminal symbols the code points as defined by SourceCharacter. It defines a set
of productions, starting from the goal symbol Pattern, that describe how
sequences of code points are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two
colons “::” as separating punctuation. The lexical and RegExp grammars share
some productions.
5.1.3 THE NUMERIC STRING GRAMMAR
A numeric string grammar appears in 7.1.4.1. It has as its terminal symbols
SourceCharacter, and is used for translating Strings into numeric values
starting from the goal symbol StringNumericLiteral (which is similar to but
distinct from the lexical grammar for numeric literals).
Productions of the numeric string grammar are distinguished by having three
colons “:::” as punctuation, and are never used for parsing source text.
5.1.4 THE SYNTACTIC GRAMMAR
The syntactic grammar for ECMAScript is given in clauses 13 through 16. This
grammar has ECMAScript tokens defined by the lexical grammar as its terminal
symbols (5.1.2). It defines a set of productions, starting from two alternative
goal symbols Script and Module, that describe how sequences of tokens form
syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript Script or Module,
it is first converted to a stream of input elements by repeated application of
the lexical grammar; this stream of input elements is then parsed by a single
application of the syntactic grammar. The input stream is syntactically in error
if the tokens in the stream of input elements cannot be parsed as a single
instance of the goal nonterminal (Script or Module), with no tokens left over.
When a parse is successful, it constructs a parse tree, a rooted tree structure
in which each node is a Parse Node. Each Parse Node is an instance of a symbol
in the grammar; it represents a span of the source text that can be derived from
that symbol. The root node of the parse tree, representing the whole of the
source text, is an instance of the parse's goal symbol. When a Parse Node is an
instance of a nonterminal, it is also an instance of some production that has
that nonterminal as its left-hand side. Moreover, it has zero or more children,
one for each symbol on the production's right-hand side: each child is a Parse
Node that is an instance of the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser and never
reused between parses even of identical source text. Parse Nodes are considered
the same Parse Node if and only if they represent the same span of source text,
are instances of the same grammar symbol, and resulted from the same parser
invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes. For
example, consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of str into ECMAScript source text and
performs an independent parse that creates its own separate tree of Parse Nodes.
The trees are distinct even though each parse operates upon a source text that
was derived from the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are not required to
use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon
“:” as punctuation.
The syntactic grammar as presented in clauses 13 through 16 is not a complete
account of which token sequences are accepted as a correct ECMAScript Script or
Module. Certain additional token sequences are also accepted, namely, those that
would be described by the grammar if only semicolons were added to the sequence
in certain places (such as before line terminator characters). Furthermore,
certain token sequences that are described by the grammar are not considered
acceptable if a line terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic grammar uses
generalized productions that permit token sequences that do not form a valid
ECMAScript Script or Module. For example, this technique is used for object
literals and object destructuring patterns. In such cases a more restrictive
supplemental grammar is provided that further restricts the acceptable token
sequences. Typically, an early error rule will then state that, in certain
contexts, "P must cover an N", where P is a Parse Node (an instance of the
generalized production) and N is a nonterminal from the supplemental grammar.
This means:
1. The sequence of tokens originally matched by P is parsed again using N as
the goal symbol. If N takes grammatical parameters, then they are set to the
same values used when P was originally parsed.
2. If the sequence of tokens can be parsed as a single instance of N, with no
tokens left over, then:
1. We refer to that instance of N (a Parse Node, unique for a given P) as
"the N that is covered by P".
2. All Early Error rules for N and its derived productions also apply to the
N that is covered by P.
3. Otherwise (if the parse fails), it is an early Syntax Error.
5.1.5 GRAMMAR NOTATION
5.1.5.1 TERMINAL SYMBOLS
In the ECMAScript grammars, some terminal symbols are shown in fixed-width font.
These are to appear in a source text exactly as written. All terminal symbol
code points specified in this way are to be understood as the appropriate
Unicode code points from the Basic Latin block, as opposed to any
similar-looking code points from other Unicode ranges. A code point in a
terminal symbol cannot be expressed by a \ UnicodeEscapeSequence.
In grammars whose terminal symbols are individual Unicode code points (i.e., the
lexical, RegExp, and numeric string grammars), a contiguous run of multiple
fixed-width code points appearing in a production is a simple shorthand for the
same sequence of code points, written as standalone terminal symbols.
For example, the production:
HexIntegerLiteral :: 0x HexDigits
is a shorthand for:
HexIntegerLiteral :: 0 x HexDigits
In contrast, in the syntactic grammar, a contiguous run of fixed-width code
points is a single terminal symbol.
Terminal symbols come in two other forms:
* In the lexical and RegExp grammars, Unicode code points without a
conventional printed representation are instead shown in the form "<ABBREV>"
where "ABBREV" is a mnemonic for the code point or set of code points. These
forms are defined in Unicode Format-Control Characters, White Space, and Line
Terminators.
* In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and
RegularExpressionLiteral) are shown in italics, as they refer to the
nonterminals of the same name in the lexical grammar.
5.1.5.2 NONTERMINAL SYMBOLS AND PRODUCTIONS
Nonterminal symbols are shown in italic type. The definition of a nonterminal
(also called a “production”) is introduced by the name of the nonterminal being
defined followed by one or more colons. (The number of colons indicates to which
grammar the production belongs.) One or more alternative right-hand sides for
the nonterminal then follow on succeeding lines. For example, the syntactic
definition:
WhileStatement : while ( Expression ) Statement
states that the nonterminal WhileStatement represents the token while, followed
by a left parenthesis token, followed by an Expression, followed by a right
parenthesis token, followed by a Statement. The occurrences of Expression and
Statement are themselves nonterminals. As another example, the syntactic
definition:
ArgumentList : AssignmentExpression ArgumentList , AssignmentExpression
states that an ArgumentList may represent either a single AssignmentExpression
or an ArgumentList, followed by a comma, followed by an AssignmentExpression.
This definition of ArgumentList is recursive, that is, it is defined in terms of
itself. The result is that an ArgumentList may contain any positive number of
arguments, separated by commas, where each argument expression is an
AssignmentExpression. Such recursive definitions of nonterminals are common.
5.1.5.3 OPTIONAL SYMBOLS
The subscripted suffix “opt”, which may appear after a terminal or nonterminal,
indicates an optional symbol. The alternative containing the optional symbol
actually specifies two right-hand sides, one that omits the optional element and
one that includes it. This means that:
VariableDeclaration : BindingIdentifier Initializeropt
is a convenient abbreviation for:
VariableDeclaration : BindingIdentifier BindingIdentifier Initializer
and that:
ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt )
Statement
is a convenient abbreviation for:
ForStatement : for ( LexicalDeclaration ; Expressionopt ) Statement for (
LexicalDeclaration Expression ; Expressionopt ) Statement
which in turn is an abbreviation for:
ForStatement : for ( LexicalDeclaration ; ) Statement for ( LexicalDeclaration ;
Expression ) Statement for ( LexicalDeclaration Expression ; ) Statement for (
LexicalDeclaration Expression ; Expression ) Statement
so, in this example, the nonterminal ForStatement actually has four alternative
right-hand sides.
5.1.5.4 GRAMMATICAL PARAMETERS
A production may be parameterized by a subscripted annotation of the form
“[parameters]”, which may appear as a suffix to the nonterminal symbol defined
by the production. “parameters” may be either a single name or a comma separated
list of names. A parameterized production is shorthand for a set of productions
defining all combinations of the parameter names, preceded by an underscore,
appended to the parameterized nonterminal symbol. This means that:
StatementList[Return] : ReturnStatement ExpressionStatement
is a convenient abbreviation for:
StatementList : ReturnStatement ExpressionStatement StatementList_Return :
ReturnStatement ExpressionStatement
and that:
StatementList[Return, In] : ReturnStatement ExpressionStatement
is an abbreviation for:
StatementList : ReturnStatement ExpressionStatement StatementList_Return :
ReturnStatement ExpressionStatement StatementList_In : ReturnStatement
ExpressionStatement StatementList_Return_In : ReturnStatement
ExpressionStatement
Multiple parameters produce a combinatory number of productions, not all of
which are necessarily referenced in a complete grammar.
References to nonterminals on the right-hand side of a production can also be
parameterized. For example:
StatementList : ReturnStatement ExpressionStatement[+In]
is equivalent to saying:
StatementList : ReturnStatement ExpressionStatement_In
and:
StatementList : ReturnStatement ExpressionStatement[~In]
is equivalent to:
StatementList : ReturnStatement ExpressionStatement
A nonterminal reference may have both a parameter list and an “opt” suffix. For
example:
VariableDeclaration : BindingIdentifier Initializer[+In]opt
is an abbreviation for:
VariableDeclaration : BindingIdentifier BindingIdentifier Initializer_In
Prefixing a parameter name with “?” on a right-hand side nonterminal reference
makes that parameter value dependent upon the occurrence of the parameter name
on the reference to the current production's left-hand side symbol. For example:
VariableDeclaration[In] : BindingIdentifier Initializer[?In]
is an abbreviation for:
VariableDeclaration : BindingIdentifier Initializer VariableDeclaration_In :
BindingIdentifier Initializer_In
If a right-hand side alternative is prefixed with “[+parameter]” that
alternative is only available if the named parameter was used in referencing the
production's nonterminal symbol. If a right-hand side alternative is prefixed
with “[~parameter]” that alternative is only available if the named parameter
was not used in referencing the production's nonterminal symbol. This means
that:
StatementList[Return] : [+Return] ReturnStatement ExpressionStatement
is an abbreviation for:
StatementList : ExpressionStatement StatementList_Return : ReturnStatement
ExpressionStatement
and that:
StatementList[Return] : [~Return] ReturnStatement ExpressionStatement
is an abbreviation for:
StatementList : ReturnStatement ExpressionStatement StatementList_Return :
ExpressionStatement
5.1.5.5 ONE OF
When the words “one of” follow the colon(s) in a grammar definition, they
signify that each of the terminal symbols on the following line or lines is an
alternative definition. For example, the lexical grammar for ECMAScript contains
the production:
NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9
which is merely a convenient abbreviation for:
NonZeroDigit :: 1 2 3 4 5 6 7 8 9
5.1.5.6 [EMPTY]
If the phrase “[empty]” appears as the right-hand side of a production, it
indicates that the production's right-hand side contains no terminals or
nonterminals.
5.1.5.7 LOOKAHEAD RESTRICTIONS
If the phrase “[lookahead = seq]” appears in the right-hand side of a
production, it indicates that the production may only be used if the token
sequence seq is a prefix of the immediately following input token sequence.
Similarly, “[lookahead ∈ set]”, where set is a finite non-empty set of token
sequences, indicates that the production may only be used if some element of set
is a prefix of the immediately following token sequence. For convenience, the
set can also be written as a nonterminal, in which case it represents the set of
all token sequences to which that nonterminal could expand. It is considered an
editorial error if the nonterminal could expand to infinitely many distinct
token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the
containing production may only be used if seq is not a prefix of the immediately
following input token sequence, and “[lookahead ∉ set]” indicates that the
production may only be used if no element of set is a prefix of the immediately
following token sequence.
As an example, given the definitions:
DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 DecimalDigits :: DecimalDigit
DecimalDigits DecimalDigit
the definition:
LookaheadExample :: n [lookahead ∉ { 1, 3, 5, 7, 9 }] DecimalDigits DecimalDigit
[lookahead ∉ DecimalDigit]
matches either the letter n followed by one or more decimal digits the first of
which is even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be
possible to unambiguously identify the immediately following token sequence
because determining later tokens requires knowing which lexical goal symbol to
use at later positions. As such, when these are used in the syntactic grammar,
it is considered an editorial error for a token sequence seq to appear in a
lookahead restriction (including as part of a set of sequences) if the choices
of lexical goal symbols to use could change whether or not seq would be a prefix
of the resulting token sequence.
5.1.5.8 [NO LINETERMINATOR HERE]
If the phrase “[no LineTerminator here]” appears in the right-hand side of a
production of the syntactic grammar, it indicates that the production is a
restricted production: it may not be used if a LineTerminator occurs in the
input stream at the indicated position. For example, the production:
ThrowStatement : throw [no LineTerminator here] Expression ;
indicates that the production may not be used if a LineTerminator occurs in the
script between the throw token and the Expression.
Unless the presence of a LineTerminator is forbidden by a restricted production,
any number of occurrences of LineTerminator may appear between any two
consecutive tokens in the stream of input elements without affecting the
syntactic acceptability of the script.
5.1.5.9 BUT NOT
The right-hand side of a production may specify that certain expansions are not
permitted by using the phrase “but not” and then indicating the expansions to be
excluded. For example, the production:
Identifier :: IdentifierName but not ReservedWord
means that the nonterminal Identifier may be replaced by any sequence of code
points that could replace IdentifierName provided that the same sequence of code
points could not replace ReservedWord.
5.1.5.10 DESCRIPTIVE PHRASES
Finally, a few nonterminal symbols are described by a descriptive phrase in
sans-serif type in cases where it would be impractical to list all the
alternatives:
SourceCharacter :: any Unicode code point
5.2 ALGORITHM CONVENTIONS
The specification often uses a numbered list to specify steps in an algorithm.
These algorithms are used to precisely specify the required semantics of
ECMAScript language constructs. The algorithms are not intended to imply the use
of any specific implementation technique. In practice, there may be more
efficient algorithms available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated
sequence of alias names which may be used within the algorithm steps to
reference the argument passed in that position. Optional parameters are denoted
with surrounding brackets ([ , name ]) and are no different from required
parameters within algorithm steps. A rest parameter may appear at the end of a
parameter list, denoted with leading ellipsis (, ...name). The rest parameter
captures all of the arguments provided following the required and optional
parameters into a List. If there are no such additional arguments, that List is
empty.
Algorithm steps may be subdivided into sequential substeps. Substeps are
indented and may themselves be further divided into indented substeps. Outline
numbering conventions are used to identify substeps with the first level of
substeps labelled with lowercase alphabetic characters and the second level of
substeps labelled with lowercase roman numerals. If more than three levels are
required these rules repeat with the fourth level using numeric labels. For
example:
1. 1. 1. Top-level step
1. a. a. Substep.
2. b. b. Substep.
1. i. i. Subsubstep.
1. 1. 1. Subsubsubstep
1. a. a. Subsubsubsubstep
1. i. i. Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its
substeps. In this case, the substeps are only applied if the predicate is true.
If a step or substep begins with the word “else”, it is a predicate that is the
negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its
algorithm. Such assertions are used to make explicit algorithmic invariants that
would otherwise be implicit. Such assertions add no additional semantic
requirements and hence need not be checked by an implementation. They are used
simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be
someValue”. These aliases are reference-like in that both x and someValue refer
to the same underlying data and modifications to either are visible to both.
Algorithm steps that want to avoid this reference-like behaviour should
explicitly make a copy of the right-hand side: “Let x be a copy of someValue”
creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not
be referenced from steps prior to the alias's declaration. Aliases may be
modified using the form “Set x to someOtherValue”.
5.2.1 ABSTRACT OPERATIONS
In order to facilitate their use in multiple parts of this specification, some
algorithms, called abstract operations, are named and written in parameterized
functional form so that they may be referenced by name from within other
algorithms. Abstract operations are typically referenced using a functional
application style such as OperationName(arg1, arg2). Some abstract operations
are treated as polymorphically dispatched methods of class-like specification
abstractions. Such method-like abstract operations are typically referenced
using a method application style such as someValue.OperationName(arg1, arg2).
5.2.2 SYNTAX-DIRECTED OPERATIONS
A syntax-directed operation is a named operation whose definition consists of
algorithms, each of which is associated with one or more productions from one of
the ECMAScript grammars. A production that has multiple alternative definitions
will typically have a distinct algorithm for each alternative. When an algorithm
is associated with a grammar production, it may reference the terminal and
nonterminal symbols of the production alternative as if they were parameters of
the algorithm. When used in this manner, nonterminal symbols refer to the actual
alternative definition that is matched when parsing the source text. The source
text matched by a grammar production or Parse Node derived from it is the
portion of the source text that starts at the beginning of the first terminal
that participated in the match and ends at the end of the last terminal that
participated in the match.
When an algorithm is associated with a production alternative, the alternative
is typically shown without any “[ ]” grammar annotations. Such annotations
should only affect the syntactic recognition of the alternative and have no
effect on the associated semantics for the alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other
parameters by using the conventions on steps 1, 3, and 4 in the following
algorithm:
1. 1. 1. Let status be SyntaxDirectedOperation of SomeNonTerminal.
2. 2. 2. Let someParseNode be the parse of some source text.
3. 3. 3. Perform SyntaxDirectedOperation of someParseNode.
4. 4. 4. Perform SyntaxDirectedOperation of someParseNode with argument
"value".
Unless explicitly specified otherwise, all chain productions have an implicit
definition for every operation that might be applied to that production's
left-hand side nonterminal. The implicit definition simply reapplies the same
operation with the same parameters, if any, to the chain production's sole
right-hand side nonterminal and then returns the result. For example, assume
that some algorithm has a step of the form: “Return Evaluation of Block” and
that there is a production:
Block : { StatementList }
but the Evaluation operation does not associate an algorithm with that
production. In that case, the Evaluation operation implicitly includes an
association of the form:
Runtime Semantics: Evaluation
Block : { StatementList }
1. 1. 1. Return Evaluation of StatementList.
5.2.3 RUNTIME SEMANTICS
Algorithms which specify semantics that must be called at runtime are called
runtime semantics. Runtime semantics are defined by abstract operations or
syntax-directed operations.
5.2.3.1 COMPLETION ( COMPLETIONRECORD )
The abstract operation Completion takes argument completionRecord (a Completion
Record) and returns a Completion Record. It is used to emphasize that a
Completion Record is being returned. It performs the following steps when
called:
1. 1. 1. Assert: completionRecord is a Completion Record.
2. 2. 2. Return completionRecord.
5.2.3.2 THROW AN EXCEPTION
Algorithms steps that say to throw an exception, such as
1. 1. 1. Throw a TypeError exception.
mean the same things as:
1. 1. 1. Return ThrowCompletion(a newly created TypeError object).
5.2.3.3 RETURNIFABRUPT
Algorithms steps that say or are otherwise equivalent to:
1. 1. 1. ReturnIfAbrupt(argument).
mean the same thing as:
1. 1. 1. Assert: argument is a Completion Record.
2. 2. 2. If argument is an abrupt completion, return Completion(argument).
3. 3. 3. Else, set argument to argument.[[Value]].
Algorithms steps that say or are otherwise equivalent to:
1. 1. 1. ReturnIfAbrupt(AbstractOperation()).
mean the same thing as:
1. 1. 1. Let hygienicTemp be AbstractOperation().
2. 2. 2. Assert: hygienicTemp is a Completion Record.
3. 3. 3. If hygienicTemp is an abrupt completion, return
Completion(hygienicTemp).
4. 4. 4. Else, set hygienicTemp to hygienicTemp.[[Value]].
Where hygienicTemp is ephemeral and visible only in the steps pertaining to
ReturnIfAbrupt.
Algorithms steps that say or are otherwise equivalent to:
1. 1. 1. Let result be AbstractOperation(ReturnIfAbrupt(argument)).
mean the same thing as:
1. 1. 1. Assert: argument is a Completion Record.
2. 2. 2. If argument is an abrupt completion, return Completion(argument).
3. 3. 3. Else, set argument to argument.[[Value]].
4. 4. 4. Let result be AbstractOperation(argument).
5.2.3.4 RETURNIFABRUPT SHORTHANDS
Invocations of abstract operations and syntax-directed operations that are
prefixed by ? indicate that ReturnIfAbrupt should be applied to the resulting
Completion Record. For example, the step:
1. 1. 1. ? OperationName().
is equivalent to the following step:
1. 1. 1. ReturnIfAbrupt(OperationName()).
Similarly, for method application style, the step:
1. 1. 1. ? someValue.OperationName().
is equivalent to:
1. 1. 1. ReturnIfAbrupt(someValue.OperationName()).
Similarly, prefix ! is used to indicate that the following invocation of an
abstract or syntax-directed operation will never return an abrupt completion and
that the resulting Completion Record's [[Value]] field should be used in place
of the return value of the operation. For example, the step:
1. 1. 1. Let val be ! OperationName().
is equivalent to the following steps:
1. 1. 1. Let val be OperationName().
2. 2. 2. Assert: val is a normal completion.
3. 3. 3. Set val to val.[[Value]].
Syntax-directed operations for runtime semantics make use of this shorthand by
placing ! or ? before the invocation of the operation:
1. 1. 1. Perform ! SyntaxDirectedOperation of NonTerminal.
5.2.3.5 IMPLICIT NORMAL COMPLETION
In algorithms within abstract operations which are declared to return a
Completion Record, and within all built-in functions, the returned value is
first passed to NormalCompletion, and the result is used instead. This rule does
not apply within the Completion algorithm or when the value being returned is
clearly marked as a Completion Record in that step; these cases are:
* when the result of applying Completion, NormalCompletion, or ThrowCompletion
is directly returned
* when the result of constructing a Completion Record is directly returned
It is an editorial error if a Completion Record is returned from such an
abstract operation through any other means. For example, within these abstract
operations,
1. 1. 1. Return true.
means the same things as any of
1. 1. 1. Return NormalCompletion(true).
or
1. 1. 1. Let completion be NormalCompletion(true).
2. 2. 2. Return Completion(completion).
or
1. 1. 1. Return Completion Record { [[Type]]: normal, [[Value]]: true,
[[Target]]: empty }.
Note that, through the ReturnIfAbrupt expansion, the following example is
allowed, as within the expanded steps, the result of applying Completion is
returned directly in the abrupt case and the implicit NormalCompletion
application occurs after unwrapping in the normal case.
1. 1. 1. Return ? completion.
The following example would be an editorial error because a Completion Record is
being returned without being annotated in that step.
1. 1. 1. Let completion be NormalCompletion(true).
2. 2. 2. Return completion.
5.2.4 STATIC SEMANTICS
Context-free grammars are not sufficiently powerful to express all the rules
that define whether a stream of input elements form a valid ECMAScript Script or
Module that may be evaluated. In some situations additional rules are needed
that may be expressed using either ECMAScript algorithm conventions or prose
requirements. Such rules are always associated with a production of a grammar
and are called the static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm.
Named Static Semantic Rules are associated with grammar productions and a
production that has multiple alternative definitions will typically have for
each alternative a distinct algorithm for each applicable named static semantic
rule.
A special kind of static semantic rule is an Early Error Rule. Early error rules
define early error conditions (see clause 17) that are associated with specific
grammar productions. Evaluation of most early error rules are not explicitly
invoked within the algorithms of this specification. A conforming implementation
must, prior to the first evaluation of a Script or Module, validate all of the
early error rules of the productions used to parse that Script or Module. If any
of the early error rules are violated the Script or Module is invalid and cannot
be evaluated.
5.2.5 MATHEMATICAL OPERATIONS
This specification makes reference to these kinds of numeric values:
* Mathematical values: Arbitrary real numbers, used as the default numeric
type.
* Extended mathematical values: Mathematical values together with +∞ and -∞.
* Numbers: IEEE 754-2019 double-precision floating point values.
* BigInts: ECMAScript language values representing arbitrary integers in a
one-to-one correspondence.
In the language of this specification, numerical values are distinguished among
different numeric kinds using subscript suffixes. The subscript 𝔽 refers to
Numbers, and the subscript ℤ refers to BigInts. Numeric values without a
subscript suffix refer to mathematical values.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined
by the type of the operands. When applied to mathematical values, the operators
refer to the usual mathematical operations. When applied to extended
mathematical values, the operators refer to the usual mathematical operations
over the extended real numbers; indeterminate forms are not defined and their
use in this specification should be considered an editorial error. When applied
to Numbers, the operators refer to the relevant operations within IEEE 754-2019.
When applied to BigInts, the operators refer to the usual mathematical
operations applied to the mathematical value of the BigInt.
In general, when this specification refers to a numerical value, such as in the
phrase, "the length of y" or "the integer represented by the four hexadecimal
digits ...", without explicitly specifying a numeric kind, the phrase refers to
a mathematical value. Phrases which refer to a Number or a BigInt value are
explicitly annotated as such; for example, "the Number value for the number of
code points in …" or "the BigInt value for …".
Numeric operators applied to mixed-type operands (such as a Number and a
mathematical value) are not defined and should be considered an editorial error
in this specification.
This specification denotes most numeric values in base 10; it also uses numeric
values of the form 0x followed by digits 0-9 or A-F as base-16 values.
When the term integer is used in this specification, it refers to a mathematical
value which is in the set of integers, unless otherwise stated. When the term
integral Number is used in this specification, it refers to a Number value whose
mathematical value is in the set of integers.
Conversions between mathematical values and Numbers or BigInts are always
explicit in this document. A conversion from a mathematical value or extended
mathematical value x to a Number is denoted as "the Number value for x" or
𝔽(x), and is defined in 6.1.6.1. A conversion from an integer x to a BigInt is
denoted as "the BigInt value for x" or ℤ(x). A conversion from a Number or
BigInt x to a mathematical value is denoted as "the mathematical value of x", or
ℝ(x). The mathematical value of +0𝔽 and -0𝔽 is the mathematical value 0. The
mathematical value of non-finite values is not defined. The extended
mathematical value of x is the mathematical value of x for finite values, and is
+∞ and -∞ for +∞𝔽 and -∞𝔽 respectively; it is not defined for NaN.
The mathematical function abs(x) produces the absolute value of x, which is -x
if x < 0 and otherwise is x itself.
The mathematical function min(x1, x2, … , xN) produces the mathematically
smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN)
produces the mathematically largest of x1 through xN. The domain and range of
these mathematical functions are the extended mathematical values.
The notation “x modulo y” (y must be finite and non-zero) computes a value k of
the same sign as y (or zero) such that abs(k) < abs(y) and x - k = q × y for
some integer q.
The phrase "the result of clamping x between lower and upper" (where x is an
extended mathematical value and lower and upper are mathematical values such
that lower ≤ upper) produces lower if x < lower, produces upper if x > upper,
and otherwise produces x.
The mathematical function floor(x) produces the largest integer (closest to +∞)
that is not larger than x.
The mathematical function truncate(x) removes the fractional part of x by
rounding towards zero, producing -floor(-x) if x < 0 and otherwise producing
floor(x).
Mathematical functions min, max, abs, floor, and truncate are not defined for
Numbers and BigInts, and any usage of those methods that have non-mathematical
value arguments would be an editorial error in this specification.
Note
floor(x) = x - (x modulo 1).
An interval from lower bound a to upper bound b is a possibly-infinite,
possibly-empty set of numeric values of the same numeric type. Each bound will
be described as either inclusive or exclusive, but not both. There are four
kinds of intervals, as follows:
* An interval from a (inclusive) to b (inclusive), also called an inclusive
interval from a to b, includes all values x of the same numeric type such
that a ≤ x ≤ b, and no others.
* An interval from a (inclusive) to b (exclusive) includes all values x of the
same numeric type such that a ≤ x < b, and no others.
* An interval from a (exclusive) to b (inclusive) includes all values x of the
same numeric type such that a < x ≤ b, and no others.
* An interval from a (exclusive) to b (exclusive) includes all values x of the
same numeric type such that a < x < b, and no others.
For example, the interval from 1 (inclusive) to 2 (exclusive) consists of all
mathematical values between 1 and 2, including 1 and not including 2. For the
purpose of defining intervals, -0𝔽 < +0𝔽, so, for example, an inclusive
interval with a lower bound of +0𝔽 includes +0𝔽 but not -0𝔽. NaN is never
included in an interval.
5.2.6 VALUE NOTATION
In this specification, ECMAScript language values are displayed in bold.
Examples include null, true, or "hello". These are distinguished from ECMAScript
source text such as Function.prototype.apply or let n = 42;.
5.2.7 IDENTITY
In this specification, both specification values and ECMAScript language values
are compared for equality. When comparing for equality, values fall into one of
two categories. Values without identity are equal to other values without
identity if all of their innate characteristics are the same — characteristics
such as the magnitude of an integer or the length of a sequence. Values without
identity may be manifest without prior reference by fully describing their
characteristics. In contrast, each value with identity is unique and therefore
only equal to itself. Values with identity are like values without identity but
with an additional unguessable, unchangeable, universally-unique characteristic
called identity. References to existing values with identity cannot be manifest
simply by describing them, as the identity itself is indescribable; instead,
references to these values must be explicitly passed from one place to another.
Some values with identity are mutable and therefore can have their
characteristics (except their identity) changed in-place, causing all holders of
the value to observe the new characteristics. A value without identity is never
equal to a value with identity.
From the perspective of this specification, the word “is” is used to compare two
values for equality, as in “If bool is true, then ...”, and the word “contains”
is used to search for a value inside lists using equality comparisons, as in "If
list contains a Record r such that r.[[Foo]] is true, then ...". The
specification identity of values determines the result of these comparisons and
is axiomatic in this specification.
From the perspective of the ECMAScript language, language values are compared
for equality using the SameValue abstract operation and the abstract operations
it transitively calls. The algorithms of these comparison abstract operations
determine language identity of ECMAScript language values.
For specification values, examples of values without specification identity
include, but are not limited to: mathematical values and extended mathematical
values; ECMAScript source text, surrogate pairs, Directive Prologues, etc;
UTF-16 code units; Unicode code points; enums; abstract operations, including
syntax-directed operations, host hooks, etc; and ordered pairs. Examples of
specification values with specification identity include, but are not limited
to: any kind of Records, including Property Descriptors, PrivateElements, etc;
Parse Nodes; Lists; Sets and Relations; Abstract Closures; Data Blocks; Private
Names; execution contexts and execution context stacks; agent signifiers; and
WaiterLists.
Specification identity agrees with language identity for all ECMAScript language
values except Symbol values produced by Symbol.for. The ECMAScript language
values without specification identity and without language identity are
undefined, null, Booleans, Strings, Numbers, and BigInts. The ECMAScript
language values with specification identity and language identity are Symbols
not produced by Symbol.for and Objects. Symbol values produced by Symbol.for
have specification identity, but not language identity.
6 ECMASCRIPT DATA TYPES AND VALUES
Algorithms within this specification manipulate values each of which has an
associated type. The possible value types are exactly those defined in this
clause. Types are further subclassified into ECMAScript language types and
specification types.
Within this specification, the notation “Type(x)” is used as shorthand for “the
type of x” where “type” refers to the ECMAScript language and specification
types defined in this clause.
6.1 ECMASCRIPT LANGUAGE TYPES
An ECMAScript language type corresponds to values that are directly manipulated
by an ECMAScript programmer using the ECMAScript language. The ECMAScript
language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and
Object. An ECMAScript language value is a value that is characterized by an
ECMAScript language type.
6.1.1 THE UNDEFINED TYPE
The Undefined type has exactly one value, called undefined. Any variable that
has not been assigned a value has the value undefined.
6.1.2 THE NULL TYPE
The Null type has exactly one value, called null.
6.1.3 THE BOOLEAN TYPE
The Boolean type represents a logical entity having two values, called true and
false.
6.1.4 THE STRING TYPE
The String type is the set of all ordered sequences of zero or more 16-bit
unsigned integer values (“elements”) up to a maximum length of 253 - 1 elements.
The String type is generally used to represent textual data in a running
ECMAScript program, in which case each element in the String is treated as a
UTF-16 code unit value. Each element is regarded as occupying a position within
the sequence. These positions are indexed with non-negative integers. The first
element (if any) is at index 0, the next element (if any) at index 1, and so on.
The length of a String is the number of elements (i.e., 16-bit values) within
it. The empty String has length zero and therefore contains no elements.
ECMAScript operations that do not interpret String contents apply no further
semantics. Operations that do interpret String values treat each element as a
single UTF-16 code unit. However, ECMAScript does not restrict the value of or
relationships between these code units, so operations that further interpret
String contents as sequences of Unicode code points encoded in UTF-16 must
account for ill-formed subsequences. Such operations apply special treatment to
every code unit with a numeric value in the inclusive interval from 0xD800 to
0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally
as a high-surrogate code unit) and every code unit with a numeric value in the
inclusive interval from 0xDC00 to 0xDFFF (defined as a trailing surrogate, or
more formally as a low-surrogate code unit) using the following rules:
* A code unit that is not a leading surrogate and not a trailing surrogate is
interpreted as a code point with the same value.
* A sequence of two code units, where the first code unit c1 is a leading
surrogate and the second code unit c2 a trailing surrogate, is a surrogate
pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400
+ (c2 - 0xDC00) + 0x10000. (See 11.1.3)
* A code unit that is a leading surrogate or trailing surrogate, but is not
part of a surrogate pair, is interpreted as a code point with the same value.
The function String.prototype.normalize (see 22.1.3.14) can be used to
explicitly normalize a String value. String.prototype.localeCompare (see
22.1.3.11) internally normalizes String values, but no other operations
implicitly normalize the strings upon which they operate. Operation results are
not language- and/or locale-sensitive unless stated otherwise.
Note
The rationale behind this design was to keep the implementation of Strings as
simple and high-performing as possible. If ECMAScript source text is in
Normalized Form C, string literals are guaranteed to also be normalized, as long
as they do not contain any Unicode escape sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where
each argument is a String value, a code unit, or a sequence of code units)
denotes the String value whose sequence of code units is the concatenation of
the code units (in order) of each of the arguments (in order).
The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is
a String value or a sequence of code units and inclusiveStart and exclusiveEnd
are integers) denotes the String value consisting of the consecutive code units
of S beginning at index inclusiveStart and ending immediately before index
exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If
the "to" suffix is omitted, the length of S is used as the value of
exclusiveEnd.
The phrase "the ASCII word characters" denotes the following String value, which
consists solely of every letter and number in the Unicode Basic Latin block
along with U+005F (LOW LINE):
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.
6.1.4.1 STRINGINDEXOF ( STRING, SEARCHVALUE, FROMINDEX )
The abstract operation StringIndexOf takes arguments string (a String),
searchValue (a String), and fromIndex (a non-negative integer) and returns an
integer. It performs the following steps when called:
1. 1. 1. Let len be the length of string.
2. 2. 2. If searchValue is the empty String and fromIndex ≤ len, return
fromIndex.
3. 3. 3. Let searchLen be the length of searchValue.
4. 4. 4. For each integer i such that fromIndex ≤ i ≤ len - searchLen, in
ascending order, do
1. a. a. Let candidate be the substring of string from i to i + searchLen.
2. b. b. If candidate is searchValue, return i.
5. 5. 5. Return -1.
Note 1
If searchValue is the empty String and fromIndex ≤ the length of string, this
algorithm returns fromIndex. The empty String is effectively found at every
position within a string, including after the last code unit.
Note 2
This algorithm always returns -1 if fromIndex + the length of searchValue > the
length of string.
6.1.5 THE SYMBOL TYPE
The Symbol type is the set of all non-String values that may be used as the key
of an Object property (6.1.7).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]]
that is either undefined or a String value.
6.1.5.1 WELL-KNOWN SYMBOLS
Well-known symbols are built-in Symbol values that are explicitly referenced by
algorithms of this specification. They are typically used as the keys of
properties whose values serve as extension points of a specification algorithm.
Unless otherwise specified, well-known symbols values are shared by all realms
(9.3).
Within this specification a well-known symbol is referred to by using a notation
of the form @@name, where “name” is one of the values listed in Table 1.
Table 1: Well-known Symbols
Specification Name [[Description]] Value and Purpose @@asyncIterator
"Symbol.asyncIterator" A method that returns the default AsyncIterator for an
object. Called by the semantics of the for-await-of statement. @@hasInstance
"Symbol.hasInstance" A method that determines if a constructor object recognizes
an object as one of the constructor's instances. Called by the semantics of the
instanceof operator. @@isConcatSpreadable "Symbol.isConcatSpreadable" A Boolean
valued property that if true indicates that an object should be flattened to its
array elements by Array.prototype.concat. @@iterator "Symbol.iterator" A method
that returns the default Iterator for an object. Called by the semantics of the
for-of statement. @@match "Symbol.match" A regular expression method that
matches the regular expression against a string. Called by the
String.prototype.match method. @@matchAll "Symbol.matchAll" A regular expression
method that returns an iterator, that yields matches of the regular expression
against a string. Called by the String.prototype.matchAll method. @@replace
"Symbol.replace" A regular expression method that replaces matched substrings of
a string. Called by the String.prototype.replace method. @@search
"Symbol.search" A regular expression method that returns the index within a
string that matches the regular expression. Called by the
String.prototype.search method. @@species "Symbol.species" A function valued
property that is the constructor function that is used to create derived
objects. @@split "Symbol.split" A regular expression method that splits a string
at the indices that match the regular expression. Called by the
String.prototype.split method. @@toPrimitive "Symbol.toPrimitive" A method that
converts an object to a corresponding primitive value. Called by the ToPrimitive
abstract operation. @@toStringTag "Symbol.toStringTag" A String valued property
that is used in the creation of the default string description of an object.
Accessed by the built-in method Object.prototype.toString. @@unscopables
"Symbol.unscopables" An object valued property whose own and inherited property
names are property names that are excluded from the with environment bindings of
the associated object.
6.1.6 NUMERIC TYPES
ECMAScript has two built-in numeric types: Number and BigInt. The following
abstract operations are defined over these numeric types. The "Result" column
shows the return type, along with an indication if it is possible for some
invocations of the operation to return an abrupt completion.
Table 2: Numeric Type Operations
Operation Example source Invoked by the Evaluation semantics of ... Result
Number::unaryMinus -x Unary - Operator Number BigInt::unaryMinus BigInt
Number::bitwiseNOT ~x Bitwise NOT Operator ( ~ ) Number BigInt::bitwiseNOT
BigInt Number::exponentiate x ** y Exponentiation Operator and Math.pow ( base,
exponent ) Number BigInt::exponentiate either a normal completion containing a
BigInt or a throw completion Number::multiply x * y Multiplicative Operators
Number BigInt::multiply BigInt Number::divide x / y Multiplicative Operators
Number BigInt::divide either a normal completion containing a BigInt or a throw
completion Number::remainder x % y Multiplicative Operators Number
BigInt::remainder either a normal completion containing a BigInt or a throw
completion Number::add x ++
++ x
x + y Postfix Increment Operator, Prefix Increment Operator, and The Addition
Operator ( + ) Number BigInt::add BigInt Number::subtract x --
-- x
x - y Postfix Decrement Operator, Prefix Decrement Operator, and The Subtraction
Operator ( - ) Number BigInt::subtract BigInt Number::leftShift x << y The Left
Shift Operator ( << ) Number BigInt::leftShift BigInt Number::signedRightShift
x >> y The Signed Right Shift Operator ( >> ) Number BigInt::signedRightShift
BigInt Number::unsignedRightShift x >>> y The Unsigned Right Shift Operator (
>>> ) Number BigInt::unsignedRightShift a throw completion Number::lessThan
x < y
x > y
x <= y
x >= y Relational Operators, via IsLessThan ( x, y, LeftFirst ) Boolean or
undefined (for unordered inputs) BigInt::lessThan Boolean Number::equal x == y
x != y
x === y
x !== y Equality Operators, via IsStrictlyEqual ( x, y ) Boolean BigInt::equal
Number::sameValue Object.is(x, y) Object internal methods, via SameValue ( x, y
), to test exact value equality Boolean Number::sameValueZero [x].includes(y)
Array, Map, and Set methods, via SameValueZero ( x, y ), to test value equality,
ignoring the difference between +0𝔽 and -0𝔽 Boolean Number::bitwiseAND x & y
Binary Bitwise Operators Number BigInt::bitwiseAND BigInt Number::bitwiseXOR
x ^ y Number BigInt::bitwiseXOR BigInt Number::bitwiseOR x | y Number
BigInt::bitwiseOR BigInt Number::toString String(x) Many expressions and
built-in functions, via ToString ( argument ) String BigInt::toString
Because the numeric types are in general not convertible without loss of
precision or truncation, the ECMAScript language provides no implicit conversion
among these types. Programmers must explicitly call Number and BigInt functions
to convert among types when calling a function which requires another type.
Note
The first and subsequent editions of ECMAScript have provided, for certain
operators, implicit numeric conversions that could lose precision or truncate.
These legacy implicit conversions are maintained for backward compatibility, but
not provided for BigInt in order to minimize opportunity for programmer error,
and to leave open the option of generalized value types in a future edition.
6.1.6.1 THE NUMBER TYPE
The Number type has exactly 18,437,736,874,454,810,627 (that is, 264 - 253 + 3)
values, representing the double-precision 64-bit format IEEE 754-2019 values as
specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that
the 9,007,199,254,740,990 (that is, 253 - 2) distinct “Not-a-Number” values of
the IEEE Standard are represented in ECMAScript as a single special NaN value.
(Note that the NaN value is produced by the program expression NaN.) In some
implementations, external code might be able to detect a difference between
various Not-a-Number values, but such behaviour is implementation-defined; to
ECMAScript code, all NaN values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a
SharedArrayBuffer (see 25.2) after a Number value has been stored into it is not
necessarily the same as the internal representation of that Number value used by
the ECMAScript implementation.
There are two other special values, called positive Infinity and negative
Infinity. For brevity, these values are also referred to for expository purposes
by the symbols +∞𝔽 and -∞𝔽, respectively. (Note that these two infinite Number
values are produced by the program expressions +Infinity (or simply Infinity)
and -Infinity.)
The other 18,437,736,874,454,810,624 (that is, 264 - 253) values are called the
finite numbers. Half of these are positive numbers and half are negative
numbers; for every finite positive Number value there is a corresponding
negative value having the same magnitude.
Note that there is both a positive zero and a negative zero. For brevity, these
values are also referred to for expository purposes by the symbols +0𝔽 and
-0𝔽, respectively. (Note that these two different zero Number values are
produced by the program expressions +0 (or simply 0) and -0.)
The 18,437,736,874,454,810,622 (that is, 264 - 253 - 2) finite non-zero values
are of two kinds:
18,428,729,675,200,069,632 (that is, 264 - 254) of them are normalized, having
the form
s × m × 2e
where s is 1 or -1, m is an integer in the interval from 252 (inclusive) to 253
(exclusive), and e is an integer in the inclusive interval from -1074 to 971.
The remaining 9,007,199,254,740,990 (that is, 253 - 2) values are denormalized,
having the form
s × m × 2e
where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to 252
(exclusive), and e is -1074.
Note that all the positive and negative integers whose magnitude is no greater
than 253 are representable in the Number type. The integer 0 has two
representations in the Number type: +0𝔽 and -0𝔽.
A finite number has an odd significand if it is non-zero and the integer m used
to express it (in one of the two forms shown above) is odd. Otherwise, it has an
even significand.
In this specification, the phrase “the Number value for x” where x represents an
exact real mathematical quantity (which might even be an irrational number such
as π) means a Number value chosen in the following manner. Consider the set of
all finite values of the Number type, with -0𝔽 removed and with two additional
values added to it that are not representable in the Number type, namely 21024
(which is +1 × 253 × 2971) and -21024 (which is -1 × 253 × 2971). Choose the
member of this set that is closest in value to x. If two values of the set are
equally close, then the one with an even significand is chosen; for this
purpose, the two extra values 21024 and -21024 are considered to have even
significands. Finally, if 21024 was chosen, replace it with +∞𝔽; if -21024 was
chosen, replace it with -∞𝔽; if +0𝔽 was chosen, replace it with -0𝔽 if and
only if x < 0; any other chosen value is used unchanged. The result is the
Number value for x. (This procedure corresponds exactly to the behaviour of the
IEEE 754-2019 roundTiesToEven mode.)
The Number value for +∞ is +∞𝔽, and the Number value for -∞ is -∞𝔽.
Some ECMAScript operators deal only with integers in specific ranges such as the
inclusive interval from -231 to 231 - 1 or the inclusive interval from 0 to 216
- 1. These operators accept any value of the Number type but first convert each
such value to an integer value in the expected range. See the descriptions of
the numeric conversion operations in 7.1.
6.1.6.1.1 NUMBER::UNARYMINUS ( X )
The abstract operation Number::unaryMinus takes argument x (a Number) and
returns a Number. It performs the following steps when called:
1. 1. 1. If x is NaN, return NaN.
2. 2. 2. Return the result of negating x; that is, compute a Number with the
same magnitude but opposite sign.
6.1.6.1.2 NUMBER::BITWISENOT ( X )
The abstract operation Number::bitwiseNOT takes argument x (a Number) and
returns an integral Number. It performs the following steps when called:
1. 1. 1. Let oldValue be ! ToInt32(x).
2. 2. 2. Return the result of applying bitwise complement to oldValue. The
mathematical value of the result is exactly representable as a 32-bit two's
complement bit string.
6.1.6.1.3 NUMBER::EXPONENTIATE ( BASE, EXPONENT )
The abstract operation Number::exponentiate takes arguments base (a Number) and
exponent (a Number) and returns a Number. It returns an
implementation-approximated value representing the result of raising base to the
exponent power. It performs the following steps when called:
1. 1. 1. If exponent is NaN, return NaN.
2. 2. 2. If exponent is either +0𝔽 or -0𝔽, return 1𝔽.
3. 3. 3. If base is NaN, return NaN.
4. 4. 4. If base is +∞𝔽, then
1. a. a. If exponent > +0𝔽, return +∞𝔽. Otherwise, return +0𝔽.
5. 5. 5. If base is -∞𝔽, then
1. a. a. If exponent > +0𝔽, then
1. i. i. If exponent is an odd integral Number, return -∞𝔽. Otherwise,
return +∞𝔽.
2. b. b. Else,
1. i. i. If exponent is an odd integral Number, return -0𝔽. Otherwise,
return +0𝔽.
6. 6. 6. If base is +0𝔽, then
1. a. a. If exponent > +0𝔽, return +0𝔽. Otherwise, return +∞𝔽.
7. 7. 7. If base is -0𝔽, then
1. a. a. If exponent > +0𝔽, then
1. i. i. If exponent is an odd integral Number, return -0𝔽. Otherwise,
return +0𝔽.
2. b. b. Else,
1. i. i. If exponent is an odd integral Number, return -∞𝔽. Otherwise,
return +∞𝔽.
8. 8. 8. Assert: base is finite and is neither +0𝔽 nor -0𝔽.
9. 9. 9. If exponent is +∞𝔽, then
1. a. a. If abs(ℝ(base)) > 1, return +∞𝔽.
2. b. b. If abs(ℝ(base)) = 1, return NaN.
3. c. c. If abs(ℝ(base)) < 1, return +0𝔽.
10. 10. 10. If exponent is -∞𝔽, then
1. a. a. If abs(ℝ(base)) > 1, return +0𝔽.
2. b. b. If abs(ℝ(base)) = 1, return NaN.
3. c. c. If abs(ℝ(base)) < 1, return +∞𝔽.
11. 11. 11. Assert: exponent is finite and is neither +0𝔽 nor -0𝔽.
12. 12. 12. If base < -0𝔽 and exponent is not an integral Number, return NaN.
13. 13. 13. Return an implementation-approximated Number value representing the
result of raising ℝ(base) to the ℝ(exponent) power.
Note
The result of base ** exponent when base is 1𝔽 or -1𝔽 and exponent is +∞𝔽 or
-∞𝔽, or when base is 1𝔽 and exponent is NaN, differs from IEEE 754-2019. The
first edition of ECMAScript specified a result of NaN for this operation,
whereas later revisions of IEEE 754 specified 1𝔽. The historical ECMAScript
behaviour is preserved for compatibility reasons.
6.1.6.1.4 NUMBER::MULTIPLY ( X, Y )
The abstract operation Number::multiply takes arguments x (a Number) and y (a
Number) and returns a Number. It performs multiplication according to the rules
of IEEE 754-2019 binary double-precision arithmetic, producing the product of x
and y. It performs the following steps when called:
1. 1. 1. If x is NaN or y is NaN, return NaN.
2. 2. 2. If x is either +∞𝔽 or -∞𝔽, then
1. a. a. If y is either +0𝔽 or -0𝔽, return NaN.
2. b. b. If y > +0𝔽, return x.
3. c. c. Return -x.
3. 3. 3. If y is either +∞𝔽 or -∞𝔽, then
1. a. a. If x is either +0𝔽 or -0𝔽, return NaN.
2. b. b. If x > +0𝔽, return y.
3. c. c. Return -y.
4. 4. 4. If x is -0𝔽, then
1. a. a. If y is -0𝔽 or y < -0𝔽, return +0𝔽.
2. b. b. Else, return -0𝔽.
5. 5. 5. If y is -0𝔽, then
1. a. a. If x < -0𝔽, return +0𝔽.
2. b. b. Else, return -0𝔽.
6. 6. 6. Return 𝔽(ℝ(x) × ℝ(y)).
Note
Finite-precision multiplication is commutative, but not always associative.
6.1.6.1.5 NUMBER::DIVIDE ( X, Y )
The abstract operation Number::divide takes arguments x (a Number) and y (a
Number) and returns a Number. It performs division according to the rules of
IEEE 754-2019 binary double-precision arithmetic, producing the quotient of x
and y where x is the dividend and y is the divisor. It performs the following
steps when called:
1. 1. 1. If x is NaN or y is NaN, return NaN.
2. 2. 2. If x is either +∞𝔽 or -∞𝔽, then
1. a. a. If y is either +∞𝔽 or -∞𝔽, return NaN.
2. b. b. If y is +0𝔽 or y > +0𝔽, return x.
3. c. c. Return -x.
3. 3. 3. If y is +∞𝔽, then
1. a. a. If x is +0𝔽 or x > +0𝔽, return +0𝔽. Otherwise, return -0𝔽.
4. 4. 4. If y is -∞𝔽, then
1. a. a. If x is +0𝔽 or x > +0𝔽, return -0𝔽. Otherwise, return +0𝔽.
5. 5. 5. If x is either +0𝔽 or -0𝔽, then
1. a. a. If y is either +0𝔽 or -0𝔽, return NaN.
2. b. b. If y > +0𝔽, return x.
3. c. c. Return -x.
6. 6. 6. If y is +0𝔽, then
1. a. a. If x > +0𝔽, return +∞𝔽. Otherwise, return -∞𝔽.
7. 7. 7. If y is -0𝔽, then
1. a. a. If x > +0𝔽, return -∞𝔽. Otherwise, return +∞𝔽.
8. 8. 8. Return 𝔽(ℝ(x) / ℝ(y)).
6.1.6.1.6 NUMBER::REMAINDER ( N, D )
The abstract operation Number::remainder takes arguments n (a Number) and d (a
Number) and returns a Number. It yields the remainder from an implied division
of its operands where n is the dividend and d is the divisor. It performs the
following steps when called:
1. 1. 1. If n is NaN or d is NaN, return NaN.
2. 2. 2. If n is either +∞𝔽 or -∞𝔽, return NaN.
3. 3. 3. If d is either +∞𝔽 or -∞𝔽, return n.
4. 4. 4. If d is either +0𝔽 or -0𝔽, return NaN.
5. 5. 5. If n is either +0𝔽 or -0𝔽, return n.
6. 6. 6. Assert: n and d are finite and non-zero.
7. 7. 7. Let quotient be ℝ(n) / ℝ(d).
8. 8. 8. Let q be truncate(quotient).
9. 9. 9. Let r be ℝ(n) - (ℝ(d) × q).
10. 10. 10. If r = 0 and n < -0𝔽, return -0𝔽.
11. 11. 11. Return 𝔽(r).
Note 1
In C and C++, the remainder operator accepts only integral operands; in
ECMAScript, it also accepts floating-point operands.
Note 2
The result of a floating-point remainder operation as computed by the % operator
is not the same as the “remainder” operation defined by IEEE 754-2019. The IEEE
754-2019 “remainder” operation computes the remainder from a rounding division,
not a truncating division, and so its behaviour is not analogous to that of the
usual integer remainder operator. Instead the ECMAScript language defines % on
floating-point operations to behave in a manner analogous to that of the Java
integer remainder operator; this may be compared with the C library function
fmod.
6.1.6.1.7 NUMBER::ADD ( X, Y )
The abstract operation Number::add takes arguments x (a Number) and y (a Number)
and returns a Number. It performs addition according to the rules of IEEE
754-2019 binary double-precision arithmetic, producing the sum of its arguments.
It performs the following steps when called:
1. 1. 1. If x is NaN or y is NaN, return NaN.
2. 2. 2. If x is +∞𝔽 and y is -∞𝔽, return NaN.
3. 3. 3. If x is -∞𝔽 and y is +∞𝔽, return NaN.
4. 4. 4. If x is either +∞𝔽 or -∞𝔽, return x.
5. 5. 5. If y is either +∞𝔽 or -∞𝔽, return y.
6. 6. 6. Assert: x and y are both finite.
7. 7. 7. If x is -0𝔽 and y is -0𝔽, return -0𝔽.
8. 8. 8. Return 𝔽(ℝ(x) + ℝ(y)).
Note
Finite-precision addition is commutative, but not always associative.
6.1.6.1.8 NUMBER::SUBTRACT ( X, Y )
The abstract operation Number::subtract takes arguments x (a Number) and y (a
Number) and returns a Number. It performs subtraction, producing the difference
of its operands; x is the minuend and y is the subtrahend. It performs the
following steps when called:
1. 1. 1. Return Number::add(x, Number::unaryMinus(y)).
Note
It is always the case that x - y produces the same result as x + (-y).
6.1.6.1.9 NUMBER::LEFTSHIFT ( X, Y )
The abstract operation Number::leftShift takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when
called:
1. 1. 1. Let lnum be ! ToInt32(x).
2. 2. 2. Let rnum be ! ToUint32(y).
3. 3. 3. Let shiftCount be ℝ(rnum) modulo 32.
4. 4. 4. Return the result of left shifting lnum by shiftCount bits. The
mathematical value of the result is exactly representable as a 32-bit two's
complement bit string.
6.1.6.1.10 NUMBER::SIGNEDRIGHTSHIFT ( X, Y )
The abstract operation Number::signedRightShift takes arguments x (a Number) and
y (a Number) and returns an integral Number. It performs the following steps
when called:
1. 1. 1. Let lnum be ! ToInt32(x).
2. 2. 2. Let rnum be ! ToUint32(y).
3. 3. 3. Let shiftCount be ℝ(rnum) modulo 32.
4. 4. 4. Return the result of performing a sign-extending right shift of lnum
by shiftCount bits. The most significant bit is propagated. The mathematical
value of the result is exactly representable as a 32-bit two's complement
bit string.
6.1.6.1.11 NUMBER::UNSIGNEDRIGHTSHIFT ( X, Y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number)
and y (a Number) and returns an integral Number. It performs the following steps
when called:
1. 1. 1. Let lnum be ! ToUint32(x).
2. 2. 2. Let rnum be ! ToUint32(y).
3. 3. 3. Let shiftCount be ℝ(rnum) modulo 32.
4. 4. 4. Return the result of performing a zero-filling right shift of lnum by
shiftCount bits. Vacated bits are filled with zero. The mathematical value
of the result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 NUMBER::LESSTHAN ( X, Y )
The abstract operation Number::lessThan takes arguments x (a Number) and y (a
Number) and returns a Boolean or undefined. It performs the following steps when
called:
1. 1. 1. If x is NaN, return undefined.
2. 2. 2. If y is NaN, return undefined.
3. 3. 3. If x is y, return false.
4. 4. 4. If x is +0𝔽 and y is -0𝔽, return false.
5. 5. 5. If x is -0𝔽 and y is +0𝔽, return false.
6. 6. 6. If x is +∞𝔽, return false.
7. 7. 7. If y is +∞𝔽, return true.
8. 8. 8. If y is -∞𝔽, return false.
9. 9. 9. If x is -∞𝔽, return true.
10. 10. 10. Assert: x and y are finite and non-zero.
11. 11. 11. If ℝ(x) < ℝ(y), return true. Otherwise, return false.
6.1.6.1.13 NUMBER::EQUAL ( X, Y )
The abstract operation Number::equal takes arguments x (a Number) and y (a
Number) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If x is NaN, return false.
2. 2. 2. If y is NaN, return false.
3. 3. 3. If x is y, return true.
4. 4. 4. If x is +0𝔽 and y is -0𝔽, return true.
5. 5. 5. If x is -0𝔽 and y is +0𝔽, return true.
6. 6. 6. Return false.
6.1.6.1.14 NUMBER::SAMEVALUE ( X, Y )
The abstract operation Number::sameValue takes arguments x (a Number) and y (a
Number) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If x is NaN and y is NaN, return true.
2. 2. 2. If x is +0𝔽 and y is -0𝔽, return false.
3. 3. 3. If x is -0𝔽 and y is +0𝔽, return false.
4. 4. 4. If x is y, return true.
5. 5. 5. Return false.
6.1.6.1.15 NUMBER::SAMEVALUEZERO ( X, Y )
The abstract operation Number::sameValueZero takes arguments x (a Number) and y
(a Number) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If x is NaN and y is NaN, return true.
2. 2. 2. If x is +0𝔽 and y is -0𝔽, return true.
3. 3. 3. If x is -0𝔽 and y is +0𝔽, return true.
4. 4. 4. If x is y, return true.
5. 5. 5. Return false.
6.1.6.1.16 NUMBERBITWISEOP ( OP, X, Y )
The abstract operation NumberBitwiseOp takes arguments op (&, ^, or |), x (a
Number), and y (a Number) and returns an integral Number. It performs the
following steps when called:
1. 1. 1. Let lnum be ! ToInt32(x).
2. 2. 2. Let rnum be ! ToInt32(y).
3. 3. 3. Let lbits be the 32-bit two's complement bit string representing
ℝ(lnum).
4. 4. 4. Let rbits be the 32-bit two's complement bit string representing
ℝ(rnum).
5. 5. 5. If op is &, let result be the result of applying the bitwise AND
operation to lbits and rbits.
6. 6. 6. Else if op is ^, let result be the result of applying the bitwise
exclusive OR (XOR) operation to lbits and rbits.
7. 7. 7. Else, op is |. Let result be the result of applying the bitwise
inclusive OR operation to lbits and rbits.
8. 8. 8. Return the Number value for the integer represented by the 32-bit
two's complement bit string result.
6.1.6.1.17 NUMBER::BITWISEAND ( X, Y )
The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when
called:
1. 1. 1. Return NumberBitwiseOp(&, x, y).
6.1.6.1.18 NUMBER::BITWISEXOR ( X, Y )
The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when
called:
1. 1. 1. Return NumberBitwiseOp(^, x, y).
6.1.6.1.19 NUMBER::BITWISEOR ( X, Y )
The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when
called:
1. 1. 1. Return NumberBitwiseOp(|, x, y).
6.1.6.1.20 NUMBER::TOSTRING ( X, RADIX )
The abstract operation Number::toString takes arguments x (a Number) and radix
(an integer in the inclusive interval from 2 to 36) and returns a String. It
represents x as a String using a positional numeral system with radix radix. The
digits used in the representation of a number using radix r are taken from the
first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The
representation of numbers with magnitude greater than or equal to 1𝔽 never
includes leading zeroes. It performs the following steps when called:
1. 1. 1. If x is NaN, return "NaN".
2. 2. 2. If x is either +0𝔽 or -0𝔽, return "0".
3. 3. 3. If x < -0𝔽, return the string-concatenation of "-" and
Number::toString(-x, radix).
4. 4. 4. If x is +∞𝔽, return "Infinity".
5. 5. 5. Let n, k, and s be integers such that k ≥ 1, radixk - 1 ≤ s < radixk,
𝔽(s × radixn - k) is x, and k is as small as possible. Note that k is the
number of digits in the representation of s using radix radix, that s is
not divisible by radix, and that the least significant digit of s is not
necessarily uniquely determined by these criteria.
6. 6. 6. If radix ≠ 10 or n is in the inclusive interval from -5 to 21, then
1. a. a. If n ≥ k, then
1. i. i. Return the string-concatenation of:
* the code units of the k digits of the representation of s using
radix radix
* n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
2. b. b. Else if n > 0, then
1. i. i. Return the string-concatenation of:
* the code units of the most significant n digits of the
representation of s using radix radix
* the code unit 0x002E (FULL STOP)
* the code units of the remaining k - n digits of the representation
of s using radix radix
3. c. c. Else,
1. i. i. Assert: n ≤ 0.
2. ii. ii. Return the string-concatenation of:
* the code unit 0x0030 (DIGIT ZERO)
* the code unit 0x002E (FULL STOP)
* -n occurrences of the code unit 0x0030 (DIGIT ZERO)
* the code units of the k digits of the representation of s using
radix radix
7. 7. 7. NOTE: In this case, the input will be represented using scientific E
notation, such as 1.2e+3.
8. 8. 8. Assert: radix is 10.
9. 9. 9. If n < 0, then
1. a. a. Let exponentSign be the code unit 0x002D (HYPHEN-MINUS).
10. 10. 10. Else,
1. a. a. Let exponentSign be the code unit 0x002B (PLUS SIGN).
11. 11. 11. If k = 1, then
1. a. a. Return the string-concatenation of:
* the code unit of the single digit of s
* the code unit 0x0065 (LATIN SMALL LETTER E)
* exponentSign
* the code units of the decimal representation of abs(n - 1)
12. 12. 12. Return the string-concatenation of:
* the code unit of the most significant digit of the decimal representation
of s
* the code unit 0x002E (FULL STOP)
* the code units of the remaining k - 1 digits of the decimal
representation of s
* the code unit 0x0065 (LATIN SMALL LETTER E)
* exponentSign
* the code units of the decimal representation of abs(n - 1)
Note 1
The following observations may be useful as guidelines for implementations, but
are not part of the normative requirements of this Standard:
* If x is any Number value other than -0𝔽, then ToNumber(ToString(x)) is x.
* The least significant digit of s is not always uniquely determined by the
requirements listed in step 5.
Note 2
For implementations that provide more accurate conversions than required by the
rules above, it is recommended that the following alternative version of step 5
be used as a guideline:
5. 5. 5. Let n, k, and s be integers such that k ≥ 1, radixk - 1 ≤ s < radixk,
𝔽(s × radixn - k) is x, and k is as small as possible. If there are
multiple possibilities for s, choose the value of s for which s × radixn - k
is closest in value to ℝ(x). If there are two such possible values of s,
choose the one that is even. Note that k is the number of digits in the
representation of s using radix radix and that s is not divisible by radix.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David
M. Gay for binary-to-decimal conversion of floating-point numbers:
Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions.
Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New
Jersey). 30 November 1990. Available as
http://ampl.com/REFS/abstracts.html#rounding. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib
mirror sites.
6.1.6.2 THE BIGINT TYPE
The BigInt type represents an integer value. The value may be any size and is
not limited to a particular bit-width. Generally, where not otherwise noted,
operations are designed to return exact mathematically-based answers. For binary
operations, BigInts act as two's complement binary strings, with negative
numbers treated as having bits set infinitely to the left.
6.1.6.2.1 BIGINT::UNARYMINUS ( X )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and
returns a BigInt. It performs the following steps when called:
1. 1. 1. If x is 0ℤ, return 0ℤ.
2. 2. 2. Return the BigInt value that represents the negation of ℝ(x).
6.1.6.2.2 BIGINT::BITWISENOT ( X )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and
returns a BigInt. It returns the one's complement of x. It performs the
following steps when called:
1. 1. 1. Return -x - 1ℤ.
6.1.6.2.3 BIGINT::EXPONENTIATE ( BASE, EXPONENT )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and
exponent (a BigInt) and returns either a normal completion containing a BigInt
or a throw completion. It performs the following steps when called:
1. 1. 1. If exponent < 0ℤ, throw a RangeError exception.
2. 2. 2. If base is 0ℤ and exponent is 0ℤ, return 1ℤ.
3. 3. 3. Return the BigInt value that represents ℝ(base) raised to the power
ℝ(exponent).
6.1.6.2.4 BIGINT::MULTIPLY ( X, Y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return the BigInt value that represents the product of x and y.
Note
Even if the result has a much larger bit width than the input, the exact
mathematical answer is given.
6.1.6.2.5 BIGINT::DIVIDE ( X, Y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a
BigInt) and returns either a normal completion containing a BigInt or a throw
completion. It performs the following steps when called:
1. 1. 1. If y is 0ℤ, throw a RangeError exception.
2. 2. 2. Let quotient be ℝ(x) / ℝ(y).
3. 3. 3. Return ℤ(truncate(quotient)).
6.1.6.2.6 BIGINT::REMAINDER ( N, D )
The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a
BigInt) and returns either a normal completion containing a BigInt or a throw
completion. It performs the following steps when called:
1. 1. 1. If d is 0ℤ, throw a RangeError exception.
2. 2. 2. If n is 0ℤ, return 0ℤ.
3. 3. 3. Let quotient be ℝ(n) / ℝ(d).
4. 4. 4. Let q be ℤ(truncate(quotient)).
5. 5. 5. Return n - (d × q).
Note
The sign of the result is the sign of the dividend.
6.1.6.2.7 BIGINT::ADD ( X, Y )
The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt)
and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return the BigInt value that represents the sum of x and y.
6.1.6.2.8 BIGINT::SUBTRACT ( X, Y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return the BigInt value that represents the difference x minus y.
6.1.6.2.9 BIGINT::LEFTSHIFT ( X, Y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. If y < 0ℤ, then
1. a. a. Return the BigInt value that represents ℝ(x) / 2-ℝ(y), rounding
down to the nearest integer, including for negative numbers.
2. 2. 2. Return the BigInt value that represents ℝ(x) × 2ℝ(y).
Note
Semantics here should be equivalent to a bitwise shift, treating the BigInt as
an infinite length string of binary two's complement digits.
6.1.6.2.10 BIGINT::SIGNEDRIGHTSHIFT ( X, Y )
The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return BigInt::leftShift(x, -y).
6.1.6.2.11 BIGINT::UNSIGNEDRIGHTSHIFT ( X, Y )
The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt)
and y (a BigInt) and returns a throw completion. It performs the following steps
when called:
1. 1. 1. Throw a TypeError exception.
6.1.6.2.12 BIGINT::LESSTHAN ( X, Y )
The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a
BigInt) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If ℝ(x) < ℝ(y), return true; otherwise return false.
6.1.6.2.13 BIGINT::EQUAL ( X, Y )
The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a
BigInt) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If ℝ(x) = ℝ(y), return true; otherwise return false.
6.1.6.2.14 BINARYAND ( X, Y )
The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
1. 1. 1. If x = 1 and y = 1, return 1.
2. 2. 2. Else, return 0.
6.1.6.2.15 BINARYOR ( X, Y )
The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
1. 1. 1. If x = 1 or y = 1, return 1.
2. 2. 2. Else, return 0.
6.1.6.2.16 BINARYXOR ( X, Y )
The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
1. 1. 1. If x = 1 and y = 0, return 1.
2. 2. 2. Else if x = 0 and y = 1, return 1.
3. 3. 3. Else, return 0.
6.1.6.2.17 BIGINTBITWISEOP ( OP, X, Y )
The abstract operation BigIntBitwiseOp takes arguments op (&, ^, or |), x (a
BigInt), and y (a BigInt) and returns a BigInt. It performs the following steps
when called:
1. 1. 1. Set x to ℝ(x).
2. 2. 2. Set y to ℝ(y).
3. 3. 3. Let result be 0.
4. 4. 4. Let shift be 0.
5. 5. 5. Repeat, until (x = 0 or x = -1) and (y = 0 or y = -1),
1. a. a. Let xDigit be x modulo 2.
2. b. b. Let yDigit be y modulo 2.
3. c. c. If op is &, set result to result + 2shift × BinaryAnd(xDigit,
yDigit).
4. d. d. Else if op is |, set result to result + 2shift × BinaryOr(xDigit,
yDigit).
5. e. e. Else,
1. i. i. Assert: op is ^.
2. ii. ii. Set result to result + 2shift × BinaryXor(xDigit, yDigit).
6. f. f. Set shift to shift + 1.
7. g. g. Set x to (x - xDigit) / 2.
8. h. h. Set y to (y - yDigit) / 2.
6. 6. 6. If op is &, let tmp be BinaryAnd(x modulo 2, y modulo 2).
7. 7. 7. Else if op is |, let tmp be BinaryOr(x modulo 2, y modulo 2).
8. 8. 8. Else,
1. a. a. Assert: op is ^.
2. b. b. Let tmp be BinaryXor(x modulo 2, y modulo 2).
9. 9. 9. If tmp ≠ 0, then
1. a. a. Set result to result - 2shift.
2. b. b. NOTE: This extends the sign.
10. 10. 10. Return the BigInt value for result.
6.1.6.2.18 BIGINT::BITWISEAND ( X, Y )
The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return BigIntBitwiseOp(&, x, y).
6.1.6.2.19 BIGINT::BITWISEXOR ( X, Y )
The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return BigIntBitwiseOp(^, x, y).
6.1.6.2.20 BIGINT::BITWISEOR ( X, Y )
The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. 1. 1. Return BigIntBitwiseOp(|, x, y).
6.1.6.2.21 BIGINT::TOSTRING ( X, RADIX )
The abstract operation BigInt::toString takes arguments x (a BigInt) and radix
(an integer in the inclusive interval from 2 to 36) and returns a String. It
represents x as a String using a positional numeral system with radix radix. The
digits used in the representation of a BigInt using radix r are taken from the
first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The
representation of BigInts other than 0ℤ never includes leading zeroes. It
performs the following steps when called:
1. 1. 1. If x < 0ℤ, return the string-concatenation of "-" and
BigInt::toString(-x, radix).
2. 2. 2. Return the String value consisting of the representation of x using
radix radix.
6.1.7 THE OBJECT TYPE
An Object is logically a collection of properties. Each property is either a
data property, or an accessor property:
* A data property associates a key value with an ECMAScript language value and
a set of Boolean attributes.
* An accessor property associates a key value with one or two accessor
functions, and a set of Boolean attributes. The accessor functions are used
to store or retrieve an ECMAScript language value that is associated with the
property.
Properties are identified using key values. A property key value is either an
ECMAScript String value or a Symbol value. All String and Symbol values,
including the empty String, are valid as property keys. A property name is a
property key that is a String value.
An integer index is a String-valued property key that is a canonical numeric
string and whose numeric value is either +0𝔽 or a positive integral Number ≤
𝔽(253 - 1). An array index is an integer index whose numeric value i is in the
range +0𝔽 ≤ i < 𝔽(232 - 1).
Property keys are used to access properties and their values. There are two
kinds of access for properties: get and set, corresponding to value retrieval
and assignment, respectively. The properties accessible via get and set access
includes both own properties that are a direct part of an object and inherited
properties which are provided by another associated object via a property
inheritance relationship. Inherited properties may be either own or inherited
properties of the associated object. Each own property of an object must each
have a key value that is distinct from the key values of the other own
properties of that object.
All objects are logically collections of properties, but there are multiple
forms of objects that differ in their semantics for accessing and manipulating
their properties. Please see 6.1.7.2 for definitions of the multiple forms of
objects.
6.1.7.1 PROPERTY ATTRIBUTES
Attributes are used in this specification to define and explain the state of
Object properties as described in Table 3. Unless specified explicitly, the
initial value of each attribute is its Default Value.
Table 3: Attributes of an Object property
Attribute Name Types of property for which it is present Value Domain Default
Value Description [[Value]] data property an ECMAScript language value undefined
The value retrieved by a get access of the property. [[Writable]] data property
a Boolean false If false, attempts by ECMAScript code to change the property's
[[Value]] attribute using [[Set]] will not succeed. [[Get]] accessor property an
Object or undefined undefined If the value is an Object it must be a function
object. The function's [[Call]] internal method (Table 5) is called with an
empty arguments list to retrieve the property value each time a get access of
the property is performed. [[Set]] accessor property an Object or undefined
undefined If the value is an Object it must be a function object. The function's
[[Call]] internal method (Table 5) is called with an arguments list containing
the assigned value as its sole argument each time a set access of the property
is performed. The effect of a property's [[Set]] internal method may, but is not
required to, have an effect on the value returned by subsequent calls to the
property's [[Get]] internal method. [[Enumerable]] data property or accessor
property a Boolean false If true, the property will be enumerated by a for-in
enumeration (see 14.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]] data property or accessor property a Boolean false If false,
attempts to delete the property, change it from a data property to an accessor
property or from an accessor property to a data property, or make any changes to
its attributes (other than replacing an existing [[Value]] or setting
[[Writable]] to false) will fail.
6.1.7.2 OBJECT INTERNAL METHODS AND INTERNAL SLOTS
The actual semantics of objects, in ECMAScript, are specified via algorithms
called internal methods. Each object in an ECMAScript engine is associated with
a set of internal methods that defines its runtime behaviour. These internal
methods are not part of the ECMAScript language. They are defined by this
specification purely for expository purposes. However, each object within an
implementation of ECMAScript must behave as specified by the internal methods
associated with it. The exact manner in which this is accomplished is determined
by the implementation.
Internal method names are polymorphic. This means that different object values
may perform different algorithms when a common internal method name is invoked
upon them. That actual object upon which an internal method is invoked is the
“target” of the invocation. If, at runtime, the implementation of an algorithm
attempts to use an internal method of an object that the object does not
support, a TypeError exception is thrown.
Internal slots correspond to internal state that is associated with objects and
used by various ECMAScript specification algorithms. Internal slots are not
object properties and they are not inherited. Depending upon the specific
internal slot specification, such state may consist of values of any ECMAScript
language type or of specific ECMAScript specification type values. Unless
explicitly specified otherwise, internal slots are allocated as part of the
process of creating an object and may not be dynamically added to an object.
Unless specified otherwise, the initial value of an internal slot is the value
undefined. Various algorithms within this specification create objects that have
internal slots. However, the ECMAScript language provides no direct way to
associate internal slots with an object.
All objects have an internal slot named [[PrivateElements]], which is a List of
PrivateElements. This List represents the values of the private fields, methods,
and accessors for the object. Initially, it is an empty List.
Internal methods and internal slots are identified within this specification
using names enclosed in double square brackets [[ ]].
Table 4 summarizes the essential internal methods used by this specification
that are applicable to all objects created or manipulated by ECMAScript code.
Every object must have algorithms for all of the essential internal methods.
However, all objects do not necessarily use the same algorithms for those
methods.
An ordinary object is an object that satisfies all of the following criteria:
* For the internal methods listed in Table 4, the object uses those defined in
10.1.
* If the object has a [[Call]] internal method, it uses either the one defined
in 10.2.1 or the one defined in 10.3.1.
* If the object has a [[Construct]] internal method, it uses either the one
defined in 10.2.2 or the one defined in 10.3.2.
An exotic object is an object that is not an ordinary object.
This specification recognizes different kinds of exotic objects by those
objects' internal methods. An object that is behaviourally equivalent to a
particular kind of exotic object (such as an Array exotic object or a bound
function exotic object), but does not have the same collection of internal
methods specified for that kind, is not recognized as that kind of exotic
object.
The “Signature” column of Table 4 and other similar tables describes the
invocation pattern for each internal method. The invocation pattern always
includes a parenthesized list of descriptive parameter names. If a parameter
name is the same as an ECMAScript type name then the name describes the required
type of the parameter value. If an internal method explicitly returns a value,
its parameter list is followed by the symbol “→” and the type name of the
returned value. The type names used in signatures refer to the types defined in
clause 6 augmented by the following additional names. “any” means the value may
be any ECMAScript language type.
In addition to its parameters, an internal method always has access to the
object that is the target of the method invocation.
An internal method implicitly returns a Completion Record, either a normal
completion that wraps a value of the return type shown in its invocation
pattern, or a throw completion.
Table 4: Essential Internal Methods
Internal Method Signature Description [[GetPrototypeOf]] ( ) → Object | Null
Determine the object that provides inherited properties for this object. A null
value indicates that there are no inherited properties. [[SetPrototypeOf]]
(Object | Null) → Boolean Associate this object with another object that
provides inherited properties. Passing null indicates that there are no
inherited properties. Returns true indicating that the operation was completed
successfully or false indicating that the operation was not successful.
[[IsExtensible]] ( ) → Boolean Determine whether it is permitted to add
additional properties to this object. [[PreventExtensions]] ( ) → Boolean
Control whether new properties may be added to this object. Returns true if the
operation was successful or false if the operation was unsuccessful.
[[GetOwnProperty]] (propertyKey) → Undefined | Property Descriptor Return a
Property Descriptor for the own property of this object whose key is
propertyKey, or undefined if no such property exists. [[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean Create or alter the own property,
whose key is propertyKey, to have the state described by PropertyDescriptor.
Return true if that property was successfully created/updated or false if the
property could not be created or updated. [[HasProperty]] (propertyKey) →
Boolean Return a Boolean value indicating whether this object already has either
an own or inherited property whose key is propertyKey. [[Get]] (propertyKey,
Receiver) → any Return the value of the property whose key is propertyKey from
this object. If any ECMAScript code must be executed to retrieve the property
value, Receiver is used as the this value when evaluating the code. [[Set]]
(propertyKey, value, Receiver) → Boolean Set the value of the property whose key
is propertyKey to value. If any ECMAScript code must be executed to set the
property value, Receiver is used as the this value when evaluating the code.
Returns true if the property value was set or false if it could not be set.
[[Delete]] (propertyKey) → Boolean Remove the own property whose key is
propertyKey from this object. Return false if the property was not deleted and
is still present. Return true if the property was deleted or is not present.
[[OwnPropertyKeys]] ( ) → List of property keys Return a List whose elements are
all of the own property keys for the object.
Table 5 summarizes additional essential internal methods that are supported by
objects that may be called as functions. A function object is an object that
supports the [[Call]] internal method. A constructor is an object that supports
the [[Construct]] internal method. Every object that supports [[Construct]] must
support [[Call]]; that is, every constructor must be a function object.
Therefore, a constructor may also be referred to as a constructor function or
constructor function object.
Table 5: Additional Essential Internal Methods of Function Objects
Internal Method Signature Description [[Call]] (any, a List of any) → any
Executes code associated with this object. Invoked via a function call
expression. The arguments to the internal method are a this value and a List
whose elements are the arguments passed to the function by a call expression.
Objects that implement this internal method are callable. [[Construct]] (a List
of any, Object) → Object Creates an object. Invoked via the new operator or a
super call. The first argument to the internal method is a List whose elements
are the arguments of the constructor invocation or the super call. The second
argument is the object to which the new operator was initially applied. Objects
that implement this internal method are called constructors. A function object
is not necessarily a constructor and such non-constructor function objects do
not have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and
standard exotic objects are specified in clause 10. If any specified use of an
internal method of an exotic object is not supported by an implementation, that
usage must throw a TypeError exception when attempted.
6.1.7.3 INVARIANTS OF THE ESSENTIAL INTERNAL METHODS
The Internal Methods of Objects of an ECMAScript engine must conform to the list
of invariants specified below. Ordinary ECMAScript Objects as well as all
standard exotic objects in this specification maintain these invariants.
ECMAScript Proxy objects maintain these invariants by means of runtime checks on
the result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these invariants
for those objects. Violation of these invariants may cause ECMAScript code to
have unpredictable behaviour and create security issues. However, violation of
these invariants must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any
manner such as by providing alternative interfaces that implement the
functionality of the essential internal methods without enforcing their
invariants.
DEFINITIONS:
* The target of an internal method is the object upon which the internal method
is called.
* A target is non-extensible if it has been observed to return false from its
[[IsExtensible]] internal method, or true from its [[PreventExtensions]]
internal method.
* A non-existent property is a property that does not exist as an own property
on a non-extensible target.
* All references to SameValue are according to the definition of the SameValue
algorithm.
RETURN VALUE:
The value returned by any internal method must be a Completion Record with
either:
* [[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal
return type" shown below for that internal method, or
* [[Type]] = throw, [[Target]] = empty, and [[Value]] = any ECMAScript language
value.
Note 1
An internal method must not return a continue completion, a break completion, or
a return completion.
[[GETPROTOTYPEOF]] ( )
* The normal return type is either Object or Null.
* If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then
any future calls to [[GetPrototypeOf]] should return the SameValue as V.
Note 2
An object's prototype chain should have finite length (that is, starting from
any object, recursively applying the [[GetPrototypeOf]] internal method to its
result should eventually lead to the value null). However, this requirement is
not enforceable as an object level invariant if the prototype chain includes any
exotic objects that do not use the ordinary object definition of
[[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops
when accessing object properties.
[[SETPROTOTYPEOF]] ( V )
* The normal return type is Boolean.
* If target is non-extensible, [[SetPrototypeOf]] must return false, unless V
is the SameValue as the target's observed [[GetPrototypeOf]] value.
[[ISEXTENSIBLE]] ( )
* The normal return type is Boolean.
* If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on
the target must return false.
[[PREVENTEXTENSIONS]] ( )
* The normal return type is Boolean.
* If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]]
on the target must return false and the target is now considered
non-extensible.
[[GETOWNPROPERTY]] ( P )
* The normal return type is either Property Descriptor or Undefined.
* If the Type of the return value is Property Descriptor, the return value must
be a fully populated Property Descriptor.
* If P is described as a non-configurable, non-writable own data property, all
future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor
whose [[Value]] is SameValue as P's [[Value]] attribute.
* If P's attributes other than [[Writable]] may change over time or if the
property might be deleted, then P's [[Configurable]] attribute must be true.
* If the [[Writable]] attribute may change from false to true, then the
[[Configurable]] attribute must be true.
* If the target is non-extensible and P is non-existent, then all future calls
to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e.
[[GetOwnProperty]] (P) must return undefined).
Note 3
As a consequence of the third invariant, if a property is described as a data
property and it may return different values over time, then either or both of
the [[Writable]] and [[Configurable]] attributes must be true even if no
mechanism to change the value is exposed via the other essential internal
methods.
[[DEFINEOWNPROPERTY]] ( P, DESC )
* The normal return type is Boolean.
* [[DefineOwnProperty]] must return false if P has previously been observed as
a non-configurable own property of the target, unless either:
1. P is a writable data property. A non-configurable writable data property
can be changed into a non-configurable non-writable data property.
2. All attributes of Desc are the SameValue as P's attributes.
* [[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible
and P is a non-existent own property. That is, a non-extensible target object
cannot be extended with new properties.
[[HASPROPERTY]] ( P )
* The normal return type is Boolean.
* If P was previously observed as a non-configurable own data or accessor
property of the target, [[HasProperty]] must return true.
[[GET]] ( P, RECEIVER )
* The normal return type is any ECMAScript language type.
* If P was previously observed as a non-configurable, non-writable own data
property of the target with value V, then [[Get]] must return the SameValue
as V.
* If P was previously observed as a non-configurable own accessor property of
the target whose [[Get]] attribute is undefined, the [[Get]] operation must
return undefined.
[[SET]] ( P, V, RECEIVER )
* The normal return type is Boolean.
* If P was previously observed as a non-configurable, non-writable own data
property of the target, then [[Set]] must return false unless V is the
SameValue as P's [[Value]] attribute.
* If P was previously observed as a non-configurable own accessor property of
the target whose [[Set]] attribute is undefined, the [[Set]] operation must
return false.
[[DELETE]] ( P )
* The normal return type is Boolean.
* If P was previously observed as a non-configurable own data or accessor
property of the target, [[Delete]] must return false.
[[OWNPROPERTYKEYS]] ( )
* The normal return type is List.
* The returned List must not contain any duplicate entries.
* The Type of each element of the returned List is either String or Symbol.
* The returned List must contain at least the keys of all non-configurable own
properties that have previously been observed.
* If the target is non-extensible, the returned List must contain only the keys
of all own properties of the target that are observable using
[[GetOwnProperty]].
[[CALL]] ( )
* The normal return type is any ECMAScript language type.
[[CONSTRUCT]] ( )
* The normal return type is Object.
* The target must also have a [[Call]] internal method.
6.1.7.4 WELL-KNOWN INTRINSIC OBJECTS
Well-known intrinsics are built-in objects that are explicitly referenced by the
algorithms of this specification and which usually have realm-specific
identities. Unless otherwise specified each intrinsic object actually
corresponds to a set of similar objects, one per realm.
Within this specification a reference such as %name% means the intrinsic object,
associated with the current realm, corresponding to the name. A reference such
as %name.a.b% means, as if the "b" property of the value of the "a" property of
the intrinsic object %name% was accessed prior to any ECMAScript code being
evaluated. Determination of the current realm and its intrinsics is described in
9.4. The well-known intrinsics are listed in Table 6.
Table 6: Well-Known Intrinsic Objects
Intrinsic Name Global Name ECMAScript Language Association %AggregateError%
AggregateError The AggregateError constructor (20.5.7.1) %Array% Array The Array
constructor (23.1.1) %ArrayBuffer% ArrayBuffer The ArrayBuffer constructor
(25.1.3) %ArrayIteratorPrototype% The prototype of Array iterator objects
(23.1.5) %AsyncFromSyncIteratorPrototype% The prototype of async-from-sync
iterator objects (27.1.4) %AsyncFunction% The constructor of async function
objects (27.7.1) %AsyncGeneratorFunction% The constructor of async iterator
objects (27.4.1) %AsyncIteratorPrototype% An object that all standard built-in
async iterator objects indirectly inherit from %Atomics% Atomics The Atomics
object (25.4) %BigInt% BigInt The BigInt constructor (21.2.1) %BigInt64Array%
BigInt64Array The BigInt64Array constructor (23.2) %BigUint64Array%
BigUint64Array The BigUint64Array constructor (23.2) %Boolean% Boolean The
Boolean constructor (20.3.1) %DataView% DataView The DataView constructor
(25.3.2) %Date% Date The Date constructor (21.4.2) %decodeURI% decodeURI The
decodeURI function (19.2.6.1) %decodeURIComponent% decodeURIComponent The
decodeURIComponent function (19.2.6.2) %encodeURI% encodeURI The encodeURI
function (19.2.6.3) %encodeURIComponent% encodeURIComponent The
encodeURIComponent function (19.2.6.4) %Error% Error The Error constructor
(20.5.1) %eval% eval The eval function (19.2.1) %EvalError% EvalError The
EvalError constructor (20.5.5.1) %FinalizationRegistry% FinalizationRegistry The
FinalizationRegistry constructor (26.2.1) %Float32Array% Float32Array The
Float32Array constructor (23.2) %Float64Array% Float64Array The Float64Array
constructor (23.2) %ForInIteratorPrototype% The prototype of For-In iterator
objects (14.7.5.10) %Function% Function The Function constructor (20.2.1)
%GeneratorFunction% The constructor of Generators (27.3.1) %Int8Array% Int8Array
The Int8Array constructor (23.2) %Int16Array% Int16Array The Int16Array
constructor (23.2) %Int32Array% Int32Array The Int32Array constructor (23.2)
%isFinite% isFinite The isFinite function (19.2.2) %isNaN% isNaN The isNaN
function (19.2.3) %IteratorPrototype% An object that all standard built-in
iterator objects indirectly inherit from %JSON% JSON The JSON object (25.5)
%Map% Map The Map constructor (24.1.1) %MapIteratorPrototype% The prototype of
Map iterator objects (24.1.5) %Math% Math The Math object (21.3) %Number% Number
The Number constructor (21.1.1) %Object% Object The Object constructor (20.1.1)
%parseFloat% parseFloat The parseFloat function (19.2.4) %parseInt% parseInt The
parseInt function (19.2.5) %Promise% Promise The Promise constructor (27.2.3)
%Proxy% Proxy The Proxy constructor (28.2.1) %RangeError% RangeError The
RangeError constructor (20.5.5.2) %ReferenceError% ReferenceError The
ReferenceError constructor (20.5.5.3) %Reflect% Reflect The Reflect object
(28.1) %RegExp% RegExp The RegExp constructor (22.2.4)
%RegExpStringIteratorPrototype% The prototype of RegExp String Iterator objects
(22.2.9) %Set% Set The Set constructor (24.2.1) %SetIteratorPrototype% The
prototype of Set iterator objects (24.2.5) %SharedArrayBuffer% SharedArrayBuffer
The SharedArrayBuffer constructor (25.2.2) %String% String The String
constructor (22.1.1) %StringIteratorPrototype% The prototype of String iterator
objects (22.1.5) %Symbol% Symbol The Symbol constructor (20.4.1) %SyntaxError%
SyntaxError The SyntaxError constructor (20.5.5.4) %ThrowTypeError% A function
object that unconditionally throws a new instance of %TypeError% %TypedArray%
The super class of all typed Array constructors (23.2.1) %TypeError% TypeError
The TypeError constructor (20.5.5.5) %Uint8Array% Uint8Array The Uint8Array
constructor (23.2) %Uint8ClampedArray% Uint8ClampedArray The Uint8ClampedArray
constructor (23.2) %Uint16Array% Uint16Array The Uint16Array constructor (23.2)
%Uint32Array% Uint32Array The Uint32Array constructor (23.2) %URIError% URIError
The URIError constructor (20.5.5.6) %WeakMap% WeakMap The WeakMap constructor
(24.3.1) %WeakRef% WeakRef The WeakRef constructor (26.1.1) %WeakSet% WeakSet
The WeakSet constructor (24.4.1)
Note
Additional entries in Table 91.
6.2 ECMASCRIPT SPECIFICATION TYPES
A specification type corresponds to meta-values that are used within algorithms
to describe the semantics of ECMAScript language constructs and ECMAScript
language types. The specification types include Reference, List, Completion
Record, Property Descriptor, Environment Record, Abstract Closure, and Data
Block. Specification type values are specification artefacts that do not
necessarily correspond to any specific entity within an ECMAScript
implementation. Specification type values may be used to describe intermediate
results of ECMAScript expression evaluation but such values cannot be stored as
properties of objects or values of ECMAScript language variables.
6.2.1 THE ENUM SPECIFICATION TYPE
Enums are values which are internal to the specification and not directly
observable from ECMAScript code. Enums are denoted using a sans-serif typeface.
For instance, a Completion Record's [[Type]] field takes on values like normal,
return, or throw. Enums have no characteristics other than their name. The name
of an enum serves no purpose other than to distinguish it from other enums, and
implies nothing about its usage or meaning in context.
6.2.2 THE LIST AND RECORD SPECIFICATION TYPES
The List type is used to explain the evaluation of argument lists (see 13.3.8)
in new expressions, in function calls, and in other algorithms where a simple
ordered list of values is needed. Values of the List type are simply ordered
sequences of list elements containing the individual values. These sequences may
be of any length. The elements of a list may be randomly accessed using 0-origin
indices. For notational convenience an array-like syntax can be used to access
List elements. For example, arguments[2] is shorthand for saying the 3rd element
of the List arguments.
When an algorithm iterates over the elements of a List without specifying an
order, the order used is the order of the elements in the List.
For notational convenience within this specification, a literal syntax can be
used to express a new List value. For example, « 1, 2 » defines a List value
that has two elements each of which is initialized to a specific value. A new
empty List can be expressed as « ».
In this specification, the phrase "the list-concatenation of A, B, ..." (where
each argument is a possibly empty List) denotes a new List value whose elements
are the concatenation of the elements (in order) of each of the arguments (in
order).
The Record type is used to describe data aggregations within the algorithms of
this specification. A Record type value consists of one or more named fields.
The value of each field is an ECMAScript language value or specification value.
Field names are always enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like
syntax can be used to express a Record value. For example, { [[Field1]]: 42,
[[Field2]]: false, [[Field3]]: empty } defines a Record value that has three
fields, each of which is initialized to a specific value. Field name order is
not significant. Any fields that are not explicitly listed are considered to be
absent.
In specification text and algorithms, dot notation may be used to refer to a
specific field of a Record value. For example, if R is the record shown in the
previous paragraph then R.[[Field2]] is shorthand for “the field of R named
[[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name
may be used as a prefix to a literal Record value to identify the specific kind
of aggregations that is being described. For example: PropertyDescriptor {
[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true }.
6.2.3 THE SET AND RELATION SPECIFICATION TYPES
The Set type is used to explain a collection of unordered elements for use in
the memory model. It is distinct from the ECMAScript collection type of the same
name. To disambiguate, instances of the ECMAScript collection are consistently
referred to as "Set objects" within this specification. Values of the Set type
are simple collections of elements, where no element appears more than once.
Elements may be added to and removed from Sets. Sets may be unioned,
intersected, or subtracted from each other.
The Relation type is used to explain constraints on Sets. Values of the Relation
type are Sets of ordered pairs of values from its value domain. For example, a
Relation on events is a set of ordered pairs of events. For a Relation R and two
values a and b in the value domain of R, a R b is shorthand for saying the
ordered pair (a, b) is a member of R. A Relation is least with respect to some
conditions when it is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value R that satisfies the following.
* For all a, b, and c in R's domain:
* It is not the case that a R a, and
* If a R b and b R c, then a R c.
Note 1
The two properties above are called irreflexivity and transitivity,
respectively.
A strict total order is a Relation value R that satisfies the following.
* For all a, b, and c in R's domain:
* a is b or a R b or b R a, and
* It is not the case that a R a, and
* If a R b and b R c, then a R c.
Note 2
The three properties above are called totality, irreflexivity, and transitivity,
respectively.
6.2.4 THE COMPLETION RECORD SPECIFICATION TYPE
The Completion Record specification type is used to explain the runtime
propagation of values and control flow such as the behaviour of statements
(break, continue, return and throw) that perform nonlocal transfers of control.
Completion Records have the fields defined in Table 7.
Table 7: Completion Record Fields
Field Name Value Meaning [[Type]] normal, break, continue, return, or throw The
type of completion that occurred. [[Value]] any value except a Completion Record
The value that was produced. [[Target]] a String or empty The target label for
directed control transfers.
The following shorthand terms are sometimes used to refer to Completion Records.
* normal completion refers to any Completion Record with a [[Type]] value of
normal.
* break completion refers to any Completion Record with a [[Type]] value of
break.
* continue completion refers to any Completion Record with a [[Type]] value of
continue.
* return completion refers to any Completion Record with a [[Type]] value of
return.
* throw completion refers to any Completion Record with a [[Type]] value of
throw.
* abrupt completion refers to any Completion Record with a [[Type]] value other
than normal.
* a normal completion containing some type of value refers to a normal
completion that has a value of that type in its [[Value]] field.
Callable objects that are defined in this specification only return a normal
completion or a throw completion. Returning any other kind of Completion Record
is considered an editorial error.
Implementation-defined callable objects must return either a normal completion
or a throw completion.
6.2.4.1 NORMALCOMPLETION ( VALUE )
The abstract operation NormalCompletion takes argument value (any value except a
Completion Record) and returns a normal completion. It performs the following
steps when called:
1. 1. 1. Return Completion Record { [[Type]]: normal, [[Value]]: value,
[[Target]]: empty }.
6.2.4.2 THROWCOMPLETION ( VALUE )
The abstract operation ThrowCompletion takes argument value (an ECMAScript
language value) and returns a throw completion. It performs the following steps
when called:
1. 1. 1. Return Completion Record { [[Type]]: throw, [[Value]]: value,
[[Target]]: empty }.
6.2.4.3 UPDATEEMPTY ( COMPLETIONRECORD, VALUE )
The abstract operation UpdateEmpty takes arguments completionRecord (a
Completion Record) and value (any value except a Completion Record) and returns
a Completion Record. It performs the following steps when called:
1. 1. 1. Assert: If completionRecord.[[Type]] is either return or throw, then
completionRecord.[[Value]] is not empty.
2. 2. 2. If completionRecord.[[Value]] is not empty, return ? completionRecord.
3. 3. 3. Return Completion Record { [[Type]]: completionRecord.[[Type]],
[[Value]]: value, [[Target]]: completionRecord.[[Target]] }.
6.2.5 THE REFERENCE RECORD SPECIFICATION TYPE
The Reference Record type is used to explain the behaviour of such operators as
delete, typeof, the assignment operators, the super keyword and other language
features. For example, the left-hand operand of an assignment is expected to
produce a Reference Record.
A Reference Record is a resolved name or property binding; its fields are
defined by Table 8.
Table 8: Reference Record Fields
Field Name Value Meaning [[Base]] an ECMAScript language value, an Environment
Record, or unresolvable The value or Environment Record which holds the binding.
A [[Base]] of unresolvable indicates that the binding could not be resolved.
[[ReferencedName]] a String, a Symbol, or a Private Name The name of the
binding. Always a String if [[Base]] value is an Environment Record. [[Strict]]
a Boolean true if the Reference Record originated in strict mode code, false
otherwise. [[ThisValue]] an ECMAScript language value or empty If not empty, the
Reference Record represents a property binding that was expressed using the
super keyword; it is called a Super Reference Record and its [[Base]] value will
never be an Environment Record. In that case, the [[ThisValue]] field holds the
this value at the time the Reference Record was created.
The following abstract operations are used in this specification to operate upon
Reference Records:
6.2.5.1 ISPROPERTYREFERENCE ( V )
The abstract operation IsPropertyReference takes argument V (a Reference Record)
and returns a Boolean. It performs the following steps when called:
1. 1. 1. If V.[[Base]] is unresolvable, return false.
2. 2. 2. If V.[[Base]] is an Environment Record, return false; otherwise return
true.
6.2.5.2 ISUNRESOLVABLEREFERENCE ( V )
The abstract operation IsUnresolvableReference takes argument V (a Reference
Record) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If V.[[Base]] is unresolvable, return true; otherwise return false.
6.2.5.3 ISSUPERREFERENCE ( V )
The abstract operation IsSuperReference takes argument V (a Reference Record)
and returns a Boolean. It performs the following steps when called:
1. 1. 1. If V.[[ThisValue]] is not empty, return true; otherwise return false.
6.2.5.4 ISPRIVATEREFERENCE ( V )
The abstract operation IsPrivateReference takes argument V (a Reference Record)
and returns a Boolean. It performs the following steps when called:
1. 1. 1. If V.[[ReferencedName]] is a Private Name, return true; otherwise
return false.
6.2.5.5 GETVALUE ( V )
The abstract operation GetValue takes argument V (a Reference Record or an
ECMAScript language value) and returns either a normal completion containing an
ECMAScript language value or an abrupt completion. It performs the following
steps when called:
1. 1. 1. If V is not a Reference Record, return V.
2. 2. 2. If IsUnresolvableReference(V) is true, throw a ReferenceError
exception.
3. 3. 3. If IsPropertyReference(V) is true, then
1. a. a. Let baseObj be ? ToObject(V.[[Base]]).
2. b. b. If IsPrivateReference(V) is true, then
1. i. i. Return ? PrivateGet(baseObj, V.[[ReferencedName]]).
3. c. c. Return ? baseObj.[[Get]](V.[[ReferencedName]], GetThisValue(V)).
4. 4. 4. Else,
1. a. a. Let base be V.[[Base]].
2. b. b. Assert: base is an Environment Record.
3. c. c. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]])
(see 9.1).
Note
The object that may be created in step 3.a is not accessible outside of the
above abstract operation and the ordinary object [[Get]] internal method. An
implementation might choose to avoid the actual creation of the object.
6.2.5.6 PUTVALUE ( V, W )
The abstract operation PutValue takes arguments V (a Reference Record or an
ECMAScript language value) and W (an ECMAScript language value) and returns
either a normal completion containing unused or an abrupt completion. It
performs the following steps when called:
1. 1. 1. If V is not a Reference Record, throw a ReferenceError exception.
2. 2. 2. If IsUnresolvableReference(V) is true, then
1. a. a. If V.[[Strict]] is true, throw a ReferenceError exception.
2. b. b. Let globalObj be GetGlobalObject().
3. c. c. Perform ? Set(globalObj, V.[[ReferencedName]], W, false).
4. d. d. Return unused.
3. 3. 3. If IsPropertyReference(V) is true, then
1. a. a. Let baseObj be ? ToObject(V.[[Base]]).
2. b. b. If IsPrivateReference(V) is true, then
1. i. i. Return ? PrivateSet(baseObj, V.[[ReferencedName]], W).
3. c. c. Let succeeded be ? baseObj.[[Set]](V.[[ReferencedName]], W,
GetThisValue(V)).
4. d. d. If succeeded is false and V.[[Strict]] is true, throw a TypeError
exception.
5. e. e. Return unused.
4. 4. 4. Else,
1. a. a. Let base be V.[[Base]].
2. b. b. Assert: base is an Environment Record.
3. c. c. Return ? base.SetMutableBinding(V.[[ReferencedName]], W,
V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not accessible outside of the
above abstract operation and the ordinary object [[Set]] internal method. An
implementation might choose to avoid the actual creation of that object.
6.2.5.7 GETTHISVALUE ( V )
The abstract operation GetThisValue takes argument V (a Reference Record) and
returns an ECMAScript language value. It performs the following steps when
called:
1. 1. 1. Assert: IsPropertyReference(V) is true.
2. 2. 2. If IsSuperReference(V) is true, return V.[[ThisValue]]; otherwise
return V.[[Base]].
6.2.5.8 INITIALIZEREFERENCEDBINDING ( V, W )
The abstract operation InitializeReferencedBinding takes arguments V (a
Reference Record) and W (an ECMAScript language value) and returns either a
normal completion containing unused or an abrupt completion. It performs the
following steps when called:
1. 1. 1. Assert: IsUnresolvableReference(V) is false.
2. 2. 2. Let base be V.[[Base]].
3. 3. 3. Assert: base is an Environment Record.
4. 4. 4. Return ? base.InitializeBinding(V.[[ReferencedName]], W).
6.2.5.9 MAKEPRIVATEREFERENCE ( BASEVALUE, PRIVATEIDENTIFIER )
The abstract operation MakePrivateReference takes arguments baseValue (an
ECMAScript language value) and privateIdentifier (a String) and returns a
Reference Record. It performs the following steps when called:
1. 1. 1. Let privEnv be the running execution context's PrivateEnvironment.
2. 2. 2. Assert: privEnv is not null.
3. 3. 3. Let privateName be ResolvePrivateIdentifier(privEnv,
privateIdentifier).
4. 4. 4. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
privateName, [[Strict]]: true, [[ThisValue]]: empty }.
6.2.6 THE PROPERTY DESCRIPTOR SPECIFICATION TYPE
The Property Descriptor type is used to explain the manipulation and reification
of Object property attributes. A Property Descriptor is a Record with zero or
more fields, where each field's name is an attribute name and its value is a
corresponding attribute value as specified in 6.1.7.1. The schema name used
within this specification to tag literal descriptions of Property Descriptor
records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data Property
Descriptors and accessor Property Descriptors based upon the existence or use of
certain fields. A data Property Descriptor is one that includes any fields named
either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that
includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may
have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor
value may not be both a data Property Descriptor and an accessor Property
Descriptor; however, it may be neither (in which case it is a generic Property
Descriptor). A fully populated Property Descriptor is one that is either an
accessor Property Descriptor or a data Property Descriptor and that has all of
the corresponding fields defined in Table 3.
The following abstract operations are used in this specification to operate upon
Property Descriptor values:
6.2.6.1 ISACCESSORDESCRIPTOR ( DESC )
The abstract operation IsAccessorDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns a Boolean. It performs the following steps
when called:
1. 1. 1. If Desc is undefined, return false.
2. 2. 2. If Desc has a [[Get]] field, return true.
3. 3. 3. If Desc has a [[Set]] field, return true.
4. 4. 4. Return false.
6.2.6.2 ISDATADESCRIPTOR ( DESC )
The abstract operation IsDataDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns a Boolean. It performs the following steps
when called:
1. 1. 1. If Desc is undefined, return false.
2. 2. 2. If Desc has a [[Value]] field, return true.
3. 3. 3. If Desc has a [[Writable]] field, return true.
4. 4. 4. Return false.
6.2.6.3 ISGENERICDESCRIPTOR ( DESC )
The abstract operation IsGenericDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns a Boolean. It performs the following steps
when called:
1. 1. 1. If Desc is undefined, return false.
2. 2. 2. If IsAccessorDescriptor(Desc) is true, return false.
3. 3. 3. If IsDataDescriptor(Desc) is true, return false.
4. 4. 4. Return true.
6.2.6.4 FROMPROPERTYDESCRIPTOR ( DESC )
The abstract operation FromPropertyDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns an Object or undefined. It performs the
following steps when called:
1. 1. 1. If Desc is undefined, return undefined.
2. 2. 2. Let obj be OrdinaryObjectCreate(%Object.prototype%).
3. 3. 3. Assert: obj is an extensible ordinary object with no own properties.
4. 4. 4. If Desc has a [[Value]] field, then
1. a. a. Perform ! CreateDataPropertyOrThrow(obj, "value", Desc.[[Value]]).
5. 5. 5. If Desc has a [[Writable]] field, then
1. a. a. Perform ! CreateDataPropertyOrThrow(obj, "writable",
Desc.[[Writable]]).
6. 6. 6. If Desc has a [[Get]] field, then
1. a. a. Perform ! CreateDataPropertyOrThrow(obj, "get", Desc.[[Get]]).
7. 7. 7. If Desc has a [[Set]] field, then
1. a. a. Perform ! CreateDataPropertyOrThrow(obj, "set", Desc.[[Set]]).
8. 8. 8. If Desc has an [[Enumerable]] field, then
1. a. a. Perform ! CreateDataPropertyOrThrow(obj, "enumerable",
Desc.[[Enumerable]]).
9. 9. 9. If Desc has a [[Configurable]] field, then
1. a. a. Perform ! CreateDataPropertyOrThrow(obj, "configurable",
Desc.[[Configurable]]).
10. 10. 10. Return obj.
6.2.6.5 TOPROPERTYDESCRIPTOR ( OBJ )
The abstract operation ToPropertyDescriptor takes argument Obj (an ECMAScript
language value) and returns either a normal completion containing a Property
Descriptor or a throw completion. It performs the following steps when called:
1. 1. 1. If Obj is not an Object, throw a TypeError exception.
2. 2. 2. Let desc be a new Property Descriptor that initially has no fields.
3. 3. 3. Let hasEnumerable be ? HasProperty(Obj, "enumerable").
4. 4. 4. If hasEnumerable is true, then
1. a. a. Let enumerable be ToBoolean(? Get(Obj, "enumerable")).
2. b. b. Set desc.[[Enumerable]] to enumerable.
5. 5. 5. Let hasConfigurable be ? HasProperty(Obj, "configurable").
6. 6. 6. If hasConfigurable is true, then
1. a. a. Let configurable be ToBoolean(? Get(Obj, "configurable")).
2. b. b. Set desc.[[Configurable]] to configurable.
7. 7. 7. Let hasValue be ? HasProperty(Obj, "value").
8. 8. 8. If hasValue is true, then
1. a. a. Let value be ? Get(Obj, "value").
2. b. b. Set desc.[[Value]] to value.
9. 9. 9. Let hasWritable be ? HasProperty(Obj, "writable").
10. 10. 10. If hasWritable is true, then
1. a. a. Let writable be ToBoolean(? Get(Obj, "writable")).
2. b. b. Set desc.[[Writable]] to writable.
11. 11. 11. Let hasGet be ? HasProperty(Obj, "get").
12. 12. 12. If hasGet is true, then
1. a. a. Let getter be ? Get(Obj, "get").
2. b. b. If IsCallable(getter) is false and getter is not undefined, throw
a TypeError exception.
3. c. c. Set desc.[[Get]] to getter.
13. 13. 13. Let hasSet be ? HasProperty(Obj, "set").
14. 14. 14. If hasSet is true, then
1. a. a. Let setter be ? Get(Obj, "set").
2. b. b. If IsCallable(setter) is false and setter is not undefined, throw
a TypeError exception.
3. c. c. Set desc.[[Set]] to setter.
15. 15. 15. If desc has a [[Get]] field or desc has a [[Set]] field, then
1. a. a. If desc has a [[Value]] field or desc has a [[Writable]] field,
throw a TypeError exception.
16. 16. 16. Return desc.
6.2.6.6 COMPLETEPROPERTYDESCRIPTOR ( DESC )
The abstract operation CompletePropertyDescriptor takes argument Desc (a
Property Descriptor) and returns unused. It performs the following steps when
called:
1. 1. 1. Let like be the Record { [[Value]]: undefined, [[Writable]]: false,
[[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false,
[[Configurable]]: false }.
2. 2. 2. If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is
true, then
1. a. a. If Desc does not have a [[Value]] field, set Desc.[[Value]] to
like.[[Value]].
2. b. b. If Desc does not have a [[Writable]] field, set Desc.[[Writable]]
to like.[[Writable]].
3. 3. 3. Else,
1. a. a. If Desc does not have a [[Get]] field, set Desc.[[Get]] to
like.[[Get]].
2. b. b. If Desc does not have a [[Set]] field, set Desc.[[Set]] to
like.[[Set]].
4. 4. 4. If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]]
to like.[[Enumerable]].
5. 5. 5. If Desc does not have a [[Configurable]] field, set
Desc.[[Configurable]] to like.[[Configurable]].
6. 6. 6. Return unused.
6.2.7 THE ENVIRONMENT RECORD SPECIFICATION TYPE
The Environment Record type is used to explain the behaviour of name resolution
in nested functions and blocks. This type and the operations upon it are defined
in 9.1.
6.2.8 THE ABSTRACT CLOSURE SPECIFICATION TYPE
The Abstract Closure specification type is used to refer to algorithm steps
together with a collection of values. Abstract Closures are meta-values and are
invoked using function application style such as closure(arg1, arg2). Like
abstract operations, invocations perform the algorithm steps described by the
Abstract Closure.
In algorithm steps that create an Abstract Closure, values are captured with the
verb "capture" followed by a list of aliases. When an Abstract Closure is
created, it captures the value that is associated with each alias at that time.
In steps that specify the algorithm to be performed when an Abstract Closure is
called, each captured value is referred to by the alias that was used to capture
the value.
If an Abstract Closure returns a Completion Record, that Completion Record's
[[Type]] must be either normal or throw.
Abstract Closures are created inline as part of other algorithms, shown in the
following example.
1. 1. 1. Let addend be 41.
2. 2. 2. Let closure be a new Abstract Closure with parameters (x) that
captures addend and performs the following steps when called:
1. a. a. Return x + addend.
3. 3. 3. Let val be closure(1).
4. 4. 4. Assert: val is 42.
6.2.9 DATA BLOCKS
The Data Block specification type is used to describe a distinct and mutable
sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the
inclusive interval from 0 to 255. A Data Block value is created with a fixed
number of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can
be used to access the individual bytes of a Data Block value. This notation
presents a Data Block value as a 0-origined integer-indexed sequence of bytes.
For example, if db is a 5 byte Data Block value then db[2] can be used to access
its 3rd byte.
A data block that resides in memory that can be referenced from multiple agents
concurrently is designated a Shared Data Block. A Shared Data Block has an
identity (for the purposes of equality testing Shared Data Block values) that is
address-free: it is tied not to the virtual addresses the block is mapped to in
any process, but to the set of locations in memory that the block represents.
Two data blocks are equal only if the sets of the locations they contain are
equal; otherwise, they are not equal and the intersection of the sets of
locations they contain is empty. Finally, Shared Data Blocks can be
distinguished from Data Blocks.
The semantics of Shared Data Blocks is defined using Shared Data Block events by
the memory model. Abstract operations below introduce Shared Data Block events
and act as the interface between evaluation semantics and the event semantics of
the memory model. The events form a candidate execution, on which the memory
model acts as a filter. Please consult the memory model for full semantics.
Shared Data Block events are modeled by Records, defined in the memory model.
The following abstract operations are used in this specification to operate upon
Data Block values:
6.2.9.1 CREATEBYTEDATABLOCK ( SIZE )
The abstract operation CreateByteDataBlock takes argument size (a non-negative
integer) and returns either a normal completion containing a Data Block or a
throw completion. It performs the following steps when called:
1. 1. 1. Let db be a new Data Block value consisting of size bytes. If it is
impossible to create such a Data Block, throw a RangeError exception.
2. 2. 2. Set all of the bytes of db to 0.
3. 3. 3. Return db.
6.2.9.2 CREATESHAREDBYTEDATABLOCK ( SIZE )
The abstract operation CreateSharedByteDataBlock takes argument size (a
non-negative integer) and returns either a normal completion containing a Shared
Data Block or a throw completion. It performs the following steps when called:
1. 1. 1. Let db be a new Shared Data Block value consisting of size bytes. If
it is impossible to create such a Shared Data Block, throw a RangeError
exception.
2. 2. 2. Let execution be the [[CandidateExecution]] field of the surrounding
agent's Agent Record.
3. 3. 3. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
4. 4. 4. Let zero be « 0 ».
5. 5. 5. For each index i of db, do
1. a. a. Append WriteSharedMemory { [[Order]]: Init, [[NoTear]]: true,
[[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero }
to eventsRecord.[[EventList]].
6. 6. 6. Return db.
6.2.9.3 COPYDATABLOCKBYTES ( TOBLOCK, TOINDEX, FROMBLOCK, FROMINDEX, COUNT )
The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data Block
or a Shared Data Block), toIndex (a non-negative integer), fromBlock (a Data
Block or a Shared Data Block), fromIndex (a non-negative integer), and count (a
non-negative integer) and returns unused. It performs the following steps when
called:
1. 1. 1. Assert: fromBlock and toBlock are distinct values.
2. 2. 2. Let fromSize be the number of bytes in fromBlock.
3. 3. 3. Assert: fromIndex + count ≤ fromSize.
4. 4. 4. Let toSize be the number of bytes in toBlock.
5. 5. 5. Assert: toIndex + count ≤ toSize.
6. 6. 6. Repeat, while count > 0,
1. a. a. If fromBlock is a Shared Data Block, then
1. i. i. Let execution be the [[CandidateExecution]] field of the
surrounding agent's Agent Record.
2. ii. ii. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier().
3. iii. iii. Let bytes be a List whose sole element is a
nondeterministically chosen byte value.
4. iv. iv. NOTE: In implementations, bytes is the result of a non-atomic
read instruction on the underlying hardware. The nondeterminism is a
semantic prescription of the memory model to describe observable
behaviour of hardware with weak consistency.
5. v. v. Let readEvent be ReadSharedMemory { [[Order]]: Unordered,
[[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex,
[[ElementSize]]: 1 }.
6. vi. vi. Append readEvent to eventsRecord.[[EventList]].
7. vii. vii. Append Chosen Value Record { [[Event]]: readEvent,
[[ChosenValue]]: bytes } to execution.[[ChosenValues]].
8. viii. viii. If toBlock is a Shared Data Block, then
1. 1. 1. Append WriteSharedMemory { [[Order]]: Unordered, [[NoTear]]:
true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]:
1, [[Payload]]: bytes } to eventsRecord.[[EventList]].
9. ix. ix. Else,
1. 1. 1. Set toBlock[toIndex] to bytes[0].
2. b. b. Else,
1. i. i. Assert: toBlock is not a Shared Data Block.
2. ii. ii. Set toBlock[toIndex] to fromBlock[fromIndex].
3. c. c. Set toIndex to toIndex + 1.
4. d. d. Set fromIndex to fromIndex + 1.
5. e. e. Set count to count - 1.
7. 7. 7. Return unused.
6.2.10 THE PRIVATEELEMENT SPECIFICATION TYPE
The PrivateElement type is a Record used in the specification of private class
fields, methods, and accessors. Although Property Descriptors are not used for
private elements, private fields behave similarly to non-configurable,
non-enumerable, writable data properties, private methods behave similarly to
non-configurable, non-enumerable, non-writable data properties, and private
accessors behave similarly to non-configurable, non-enumerable accessor
properties.
Values of the PrivateElement type are Record values whose fields are defined by
Table 9. Such values are referred to as PrivateElements.
Table 9: PrivateElement Fields
Field Name Values of the [[Kind]] field for which it is present Value Meaning
[[Key]] All a Private Name The name of the field, method, or accessor. [[Kind]]
All field, method, or accessor The kind of the element. [[Value]] field and
method an ECMAScript language value The value of the field. [[Get]] accessor a
function object or undefined The getter for a private accessor. [[Set]] accessor
a function object or undefined The setter for a private accessor.
6.2.11 THE CLASSFIELDDEFINITION RECORD SPECIFICATION TYPE
The ClassFieldDefinition type is a Record used in the specification of class
fields.
Values of the ClassFieldDefinition type are Record values whose fields are
defined by Table 10. Such values are referred to as ClassFieldDefinition
Records.
Table 10: ClassFieldDefinition Record Fields
Field Name Value Meaning [[Name]] a Private Name, a String, or a Symbol The name
of the field. [[Initializer]] a function object or empty The initializer of the
field, if any.
6.2.12 PRIVATE NAMES
The Private Name specification type is used to describe a globally unique value
(one which differs from any other Private Name, even if they are otherwise
indistinguishable) which represents the key of a private class element (field,
method, or accessor). Each Private Name has an associated immutable
[[Description]] which is a String value. A Private Name may be installed on any
ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and then
read or written using PrivateGet and PrivateSet.
6.2.13 THE CLASSSTATICBLOCKDEFINITION RECORD SPECIFICATION TYPE
A ClassStaticBlockDefinition Record is a Record value used to encapsulate the
executable code for a class static initialization block.
ClassStaticBlockDefinition Records have the fields listed in Table 11.
Table 11: ClassStaticBlockDefinition Record Fields
Field Name Value Meaning [[BodyFunction]] a function object The function object
to be called during static initialization of a class.
7 ABSTRACT OPERATIONS
These operations are not a part of the ECMAScript language; they are defined
here solely to aid the specification of the semantics of the ECMAScript
language. Other, more specialized abstract operations are defined throughout
this specification.
7.1 TYPE CONVERSION
The ECMAScript language implicitly performs automatic type conversion as needed.
To clarify the semantics of certain constructs it is useful to define a set of
conversion abstract operations. The conversion abstract operations are
polymorphic; they can accept a value of any ECMAScript language type. But no
other specification types are used with these operations.
The BigInt type has no implicit conversions in the ECMAScript language;
programmers must call BigInt explicitly to convert values from other types.
7.1.1 TOPRIMITIVE ( INPUT [ , PREFERREDTYPE ] )
The abstract operation ToPrimitive takes argument input (an ECMAScript language
value) and optional argument preferredType (string or number) and returns either
a normal completion containing an ECMAScript language value or a throw
completion. It converts its input argument to a non-Object type. If an object is
capable of converting to more than one primitive type, it may use the optional
hint preferredType to favour that type. It performs the following steps when
called:
1. 1. 1. If input is an Object, then
1. a. a. Let exoticToPrim be ? GetMethod(input, @@toPrimitive).
2. b. b. If exoticToPrim is not undefined, then
1. i. i. If preferredType is not present, let hint be "default".
2. ii. ii. Else if preferredType is string, let hint be "string".
3. iii. iii. Else,
1. 1. 1. Assert: preferredType is number.
2. 2. 2. Let hint be "number".
4. iv. iv. Let result be ? Call(exoticToPrim, input, « hint »).
5. v. v. If result is not an Object, return result.
6. vi. vi. Throw a TypeError exception.
3. c. c. If preferredType is not present, let preferredType be number.
4. d. d. Return ? OrdinaryToPrimitive(input, preferredType).
2. 2. 2. Return input.
Note
When ToPrimitive is called without a hint, then it generally behaves as if the
hint were number. However, objects may over-ride this behaviour by defining a
@@toPrimitive method. Of the objects defined in this specification only Dates
(see 21.4.4.45) and Symbol objects (see 20.4.3.5) over-ride the default
ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were
string.
7.1.1.1 ORDINARYTOPRIMITIVE ( O, HINT )
The abstract operation OrdinaryToPrimitive takes arguments O (an Object) and
hint (string or number) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It performs the following steps
when called:
1. 1. 1. If hint is string, then
1. a. a. Let methodNames be « "toString", "valueOf" ».
2. 2. 2. Else,
1. a. a. Let methodNames be « "valueOf", "toString" ».
3. 3. 3. For each element name of methodNames, do
1. a. a. Let method be ? Get(O, name).
2. b. b. If IsCallable(method) is true, then
1. i. i. Let result be ? Call(method, O).
2. ii. ii. If result is not an Object, return result.
4. 4. 4. Throw a TypeError exception.
7.1.2 TOBOOLEAN ( ARGUMENT )
The abstract operation ToBoolean takes argument argument (an ECMAScript language
value) and returns a Boolean. It converts argument to a value of type Boolean.
It performs the following steps when called:
1. 1. 1. If argument is a Boolean, return argument.
2. 2. 2. If argument is one of undefined, null, +0𝔽, -0𝔽, NaN, 0ℤ, or the
empty String, return false.
3. 3. 3. NOTE: This step is replaced in section B.3.6.1.
4. 4. 4. Return true.
7.1.3 TONUMERIC ( VALUE )
The abstract operation ToNumeric takes argument value (an ECMAScript language
value) and returns either a normal completion containing either a Number or a
BigInt, or a throw completion. It returns value converted to a Number or a
BigInt. It performs the following steps when called:
1. 1. 1. Let primValue be ? ToPrimitive(value, number).
2. 2. 2. If primValue is a BigInt, return primValue.
3. 3. 3. Return ? ToNumber(primValue).
7.1.4 TONUMBER ( ARGUMENT )
The abstract operation ToNumber takes argument argument (an ECMAScript language
value) and returns either a normal completion containing a Number or a throw
completion. It converts argument to a value of type Number. It performs the
following steps when called:
1. 1. 1. If argument is a Number, return argument.
2. 2. 2. If argument is either a Symbol or a BigInt, throw a TypeError
exception.
3. 3. 3. If argument is undefined, return NaN.
4. 4. 4. If argument is either null or false, return +0𝔽.
5. 5. 5. If argument is true, return 1𝔽.
6. 6. 6. If argument is a String, return StringToNumber(argument).
7. 7. 7. Assert: argument is an Object.
8. 8. 8. Let primValue be ? ToPrimitive(argument, number).
9. 9. 9. Assert: primValue is not an Object.
10. 10. 10. Return ? ToNumber(primValue).
7.1.4.1 TONUMBER APPLIED TO THE STRING TYPE
The abstract operation StringToNumber specifies how to convert a String value to
a Number value, using the following grammar.
SYNTAX
StringNumericLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrNumericLiteral
StrWhiteSpaceopt StrWhiteSpace ::: StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar ::: WhiteSpace LineTerminator StrNumericLiteral :::
StrDecimalLiteral NonDecimalIntegerLiteral[~Sep] StrDecimalLiteral :::
StrUnsignedDecimalLiteral + StrUnsignedDecimalLiteral -
StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral ::: Infinity
DecimalDigits[~Sep] . DecimalDigits[~Sep]opt ExponentPart[~Sep]opt .
DecimalDigits[~Sep] ExponentPart[~Sep]opt DecimalDigits[~Sep]
ExponentPart[~Sep]opt
All grammar symbols not explicitly defined above have the definitions used in
the Lexical Grammar for numeric literals (12.9.3)
Note
Some differences should be noted between the syntax of a StringNumericLiteral
and a NumericLiteral:
* A StringNumericLiteral may include leading and/or trailing white space and/or
line terminators.
* A StringNumericLiteral that is decimal may have any number of leading 0
digits.
* A StringNumericLiteral that is decimal may include a + or - to indicate its
sign.
* A StringNumericLiteral that is empty or contains only white space is
converted to +0𝔽.
* Infinity and -Infinity are recognized as a StringNumericLiteral but not as a
NumericLiteral.
* A StringNumericLiteral cannot include a BigIntLiteralSuffix.
* A StringNumericLiteral cannot include a NumericLiteralSeparator.
7.1.4.1.1 STRINGTONUMBER ( STR )
The abstract operation StringToNumber takes argument str (a String) and returns
a Number. It performs the following steps when called:
1. 1. 1. Let text be StringToCodePoints(str).
2. 2. 2. Let literal be ParseText(text, StringNumericLiteral).
3. 3. 3. If literal is a List of errors, return NaN.
4. 4. 4. Return StringNumericValue of literal.
7.1.4.1.2 RUNTIME SEMANTICS: STRINGNUMERICVALUE
The syntax-directed operation StringNumericValue takes no arguments and returns
a Number.
Note
The conversion of a StringNumericLiteral to a Number value is similar overall to
the determination of the NumericValue of a NumericLiteral (see 12.9.3), but some
of the details are different.
It is defined piecewise over the following productions:
StringNumericLiteral ::: StrWhiteSpaceopt
1. 1. 1. Return +0𝔽.
StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
1. 1. 1. Return StringNumericValue of StrNumericLiteral.
StrNumericLiteral ::: NonDecimalIntegerLiteral
1. 1. 1. Return 𝔽(MV of NonDecimalIntegerLiteral).
StrDecimalLiteral ::: - StrUnsignedDecimalLiteral
1. 1. 1. Let a be StringNumericValue of StrUnsignedDecimalLiteral.
2. 2. 2. If a is +0𝔽, return -0𝔽.
3. 3. 3. Return -a.
StrUnsignedDecimalLiteral ::: Infinity
1. 1. 1. Return +∞𝔽.
StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigitsopt ExponentPartopt
1. 1. 1. Let a be MV of the first DecimalDigits.
2. 2. 2. If the second DecimalDigits is present, then
1. a. a. Let b be MV of the second DecimalDigits.
2. b. b. Let n be the number of code points in the second DecimalDigits.
3. 3. 3. Else,
1. a. a. Let b be 0.
2. b. b. Let n be 0.
4. 4. 4. If ExponentPart is present, let e be MV of ExponentPart. Otherwise,
let e be 0.
5. 5. 5. Return RoundMVResult((a + (b × 10-n)) × 10e).
StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPartopt
1. 1. 1. Let b be MV of DecimalDigits.
2. 2. 2. If ExponentPart is present, let e be MV of ExponentPart. Otherwise,
let e be 0.
3. 3. 3. Let n be the number of code points in DecimalDigits.
4. 4. 4. Return RoundMVResult(b × 10e - n).
StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPartopt
1. 1. 1. Let a be MV of DecimalDigits.
2. 2. 2. If ExponentPart is present, let e be MV of ExponentPart. Otherwise,
let e be 0.
3. 3. 3. Return RoundMVResult(a × 10e).
7.1.4.1.3 ROUNDMVRESULT ( N )
The abstract operation RoundMVResult takes argument n (a mathematical value) and
returns a Number. It converts n to a Number in an implementation-defined manner.
For the purposes of this abstract operation, a digit is significant if it is not
zero or there is a non-zero digit to its left and there is a non-zero digit to
its right. For the purposes of this abstract operation, "the mathematical value
denoted by" a representation of a mathematical value is the inverse of "the
decimal representation of" a mathematical value. It performs the following steps
when called:
1. 1. 1. If the decimal representation of n has 20 or fewer significant digits,
return 𝔽(n).
2. 2. 2. Let option1 be the mathematical value denoted by the result of
replacing each significant digit in the decimal representation of n after
the 20th with a 0 digit.
3. 3. 3. Let option2 be the mathematical value denoted by the result of
replacing each significant digit in the decimal representation of n after
the 20th with a 0 digit and then incrementing it at the 20th position (with
carrying as necessary).
4. 4. 4. Let chosen be an implementation-defined choice of either option1 or
option2.
5. 5. 5. Return 𝔽(chosen).
7.1.5 TOINTEGERORINFINITY ( ARGUMENT )
The abstract operation ToIntegerOrInfinity takes argument argument (an
ECMAScript language value) and returns either a normal completion containing
either an integer, +∞, or -∞, or a throw completion. It converts argument to an
integer representing its Number value with fractional part truncated, or to +∞
or -∞ when that Number value is infinite. It performs the following steps when
called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is one of NaN, +0𝔽, or -0𝔽, return 0.
3. 3. 3. If number is +∞𝔽, return +∞.
4. 4. 4. If number is -∞𝔽, return -∞.
5. 5. 5. Return truncate(ℝ(number)).
Note
𝔽(ToIntegerOrInfinity(x)) never returns -0𝔽 for any value of x. The truncation
of the fractional part is performed after converting x to a mathematical value.
7.1.6 TOINT32 ( ARGUMENT )
The abstract operation ToInt32 takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It converts argument to one of 232 integral Number values in
the inclusive interval from 𝔽(-231) to 𝔽(231 - 1). It performs the following
steps when called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
3. 3. 3. Let int be truncate(ℝ(number)).
4. 4. 4. Let int32bit be int modulo 232.
5. 5. 5. If int32bit ≥ 231, return 𝔽(int32bit - 232); otherwise return
𝔽(int32bit).
Note
Given the above definition of ToInt32:
* The ToInt32 abstract operation is idempotent: if applied to a result that it
produced, the second application leaves that value unchanged.
* ToInt32(ToUint32(x)) is the same value as ToInt32(x) for all values of x. (It
is to preserve this latter property that +∞𝔽 and -∞𝔽 are mapped to +0𝔽.)
* ToInt32 maps -0𝔽 to +0𝔽.
7.1.7 TOUINT32 ( ARGUMENT )
The abstract operation ToUint32 takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It converts argument to one of 232 integral Number values in
the inclusive interval from +0𝔽 to 𝔽(232 - 1). It performs the following steps
when called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
3. 3. 3. Let int be truncate(ℝ(number)).
4. 4. 4. Let int32bit be int modulo 232.
5. 5. 5. Return 𝔽(int32bit).
Note
Given the above definition of ToUint32:
* Step 5 is the only difference between ToUint32 and ToInt32.
* The ToUint32 abstract operation is idempotent: if applied to a result that it
produced, the second application leaves that value unchanged.
* ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x.
(It is to preserve this latter property that +∞𝔽 and -∞𝔽 are mapped to
+0𝔽.)
* ToUint32 maps -0𝔽 to +0𝔽.
7.1.8 TOINT16 ( ARGUMENT )
The abstract operation ToInt16 takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It converts argument to one of 216 integral Number values in
the inclusive interval from 𝔽(-215) to 𝔽(215 - 1). It performs the following
steps when called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
3. 3. 3. Let int be truncate(ℝ(number)).
4. 4. 4. Let int16bit be int modulo 216.
5. 5. 5. If int16bit ≥ 215, return 𝔽(int16bit - 216); otherwise return
𝔽(int16bit).
7.1.9 TOUINT16 ( ARGUMENT )
The abstract operation ToUint16 takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It converts argument to one of 216 integral Number values in
the inclusive interval from +0𝔽 to 𝔽(216 - 1). It performs the following steps
when called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
3. 3. 3. Let int be truncate(ℝ(number)).
4. 4. 4. Let int16bit be int modulo 216.
5. 5. 5. Return 𝔽(int16bit).
Note
Given the above definition of ToUint16:
* The substitution of 216 for 232 in step 4 is the only difference between
ToUint32 and ToUint16.
* ToUint16 maps -0𝔽 to +0𝔽.
7.1.10 TOINT8 ( ARGUMENT )
The abstract operation ToInt8 takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It converts argument to one of 28 integral Number values in
the inclusive interval from -128𝔽 to 127𝔽. It performs the following steps
when called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
3. 3. 3. Let int be truncate(ℝ(number)).
4. 4. 4. Let int8bit be int modulo 28.
5. 5. 5. If int8bit ≥ 27, return 𝔽(int8bit - 28); otherwise return
𝔽(int8bit).
7.1.11 TOUINT8 ( ARGUMENT )
The abstract operation ToUint8 takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It converts argument to one of 28 integral Number values in
the inclusive interval from +0𝔽 to 255𝔽. It performs the following steps when
called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
3. 3. 3. Let int be truncate(ℝ(number)).
4. 4. 4. Let int8bit be int modulo 28.
5. 5. 5. Return 𝔽(int8bit).
7.1.12 TOUINT8CLAMP ( ARGUMENT )
The abstract operation ToUint8Clamp takes argument argument (an ECMAScript
language value) and returns either a normal completion containing an integral
Number or a throw completion. It converts argument to one of 28 integral Number
values in the inclusive interval from +0𝔽 to 255𝔽. It performs the following
steps when called:
1. 1. 1. Let number be ? ToNumber(argument).
2. 2. 2. If number is NaN, return +0𝔽.
3. 3. 3. If ℝ(number) ≤ 0, return +0𝔽.
4. 4. 4. If ℝ(number) ≥ 255, return 255𝔽.
5. 5. 5. Let f be floor(ℝ(number)).
6. 6. 6. If f + 0.5 < ℝ(number), return 𝔽(f + 1).
7. 7. 7. If ℝ(number) < f + 0.5, return 𝔽(f).
8. 8. 8. If f is odd, return 𝔽(f + 1).
9. 9. 9. Return 𝔽(f).
Note
Unlike the other ECMAScript integer conversion abstract operation, ToUint8Clamp
rounds rather than truncates non-integral values and does not convert +∞𝔽 to
+0𝔽. ToUint8Clamp does “round half to even” tie-breaking. This differs from
Math.round which does “round half up” tie-breaking.
7.1.13 TOBIGINT ( ARGUMENT )
The abstract operation ToBigInt takes argument argument (an ECMAScript language
value) and returns either a normal completion containing a BigInt or a throw
completion. It converts argument to a BigInt value, or throws if an implicit
conversion from Number would be required. It performs the following steps when
called:
1. 1. 1. Let prim be ? ToPrimitive(argument, number).
2. 2. 2. Return the value that prim corresponds to in Table 12.
Table 12: BigInt Conversions
Argument Type Result Undefined Throw a TypeError exception. Null Throw a
TypeError exception. Boolean Return 1n if prim is true and 0n if prim is false.
BigInt Return prim. Number Throw a TypeError exception. String
1. 1. 1. Let n be StringToBigInt(prim).
2. 2. 2. If n is undefined, throw a SyntaxError exception.
3. 3. 3. Return n.
Symbol Throw a TypeError exception.
7.1.14 STRINGTOBIGINT ( STR )
The abstract operation StringToBigInt takes argument str (a String) and returns
a BigInt or undefined. It performs the following steps when called:
1. 1. 1. Let text be StringToCodePoints(str).
2. 2. 2. Let literal be ParseText(text, StringIntegerLiteral).
3. 3. 3. If literal is a List of errors, return undefined.
4. 4. 4. Let mv be the MV of literal.
5. 5. 5. Assert: mv is an integer.
6. 6. 6. Return ℤ(mv).
7.1.14.1 STRINGINTEGERLITERAL GRAMMAR
StringToBigInt uses the following grammar.
SYNTAX
StringIntegerLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrIntegerLiteral
StrWhiteSpaceopt StrIntegerLiteral ::: SignedInteger[~Sep]
NonDecimalIntegerLiteral[~Sep]
7.1.14.2 RUNTIME SEMANTICS: MV
* The MV of StringIntegerLiteral ::: StrWhiteSpaceopt is 0.
* The MV of StringIntegerLiteral ::: StrWhiteSpaceopt StrIntegerLiteral
StrWhiteSpaceopt is the MV of StrIntegerLiteral.
7.1.15 TOBIGINT64 ( ARGUMENT )
The abstract operation ToBigInt64 takes argument argument (an ECMAScript
language value) and returns either a normal completion containing a BigInt or a
throw completion. It converts argument to one of 264 BigInt values in the
inclusive interval from ℤ(-263) to ℤ(263-1). It performs the following steps
when called:
1. 1. 1. Let n be ? ToBigInt(argument).
2. 2. 2. Let int64bit be ℝ(n) modulo 264.
3. 3. 3. If int64bit ≥ 263, return ℤ(int64bit - 264); otherwise return
ℤ(int64bit).
7.1.16 TOBIGUINT64 ( ARGUMENT )
The abstract operation ToBigUint64 takes argument argument (an ECMAScript
language value) and returns either a normal completion containing a BigInt or a
throw completion. It converts argument to one of 264 BigInt values in the
inclusive interval from 0ℤ to ℤ(264-1). It performs the following steps when
called:
1. 1. 1. Let n be ? ToBigInt(argument).
2. 2. 2. Let int64bit be ℝ(n) modulo 264.
3. 3. 3. Return ℤ(int64bit).
7.1.17 TOSTRING ( ARGUMENT )
The abstract operation ToString takes argument argument (an ECMAScript language
value) and returns either a normal completion containing a String or a throw
completion. It converts argument to a value of type String. It performs the
following steps when called:
1. 1. 1. If argument is a String, return argument.
2. 2. 2. If argument is a Symbol, throw a TypeError exception.
3. 3. 3. If argument is undefined, return "undefined".
4. 4. 4. If argument is null, return "null".
5. 5. 5. If argument is true, return "true".
6. 6. 6. If argument is false, return "false".
7. 7. 7. If argument is a Number, return Number::toString(argument, 10).
8. 8. 8. If argument is a BigInt, return BigInt::toString(argument, 10).
9. 9. 9. Assert: argument is an Object.
10. 10. 10. Let primValue be ? ToPrimitive(argument, string).
11. 11. 11. Assert: primValue is not an Object.
12. 12. 12. Return ? ToString(primValue).
7.1.18 TOOBJECT ( ARGUMENT )
The abstract operation ToObject takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an Object or a throw
completion. It converts argument to a value of type Object according to Table
13:
Table 13: ToObject Conversions
Argument Type Result Undefined Throw a TypeError exception. Null Throw a
TypeError exception. Boolean Return a new Boolean object whose [[BooleanData]]
internal slot is set to argument. See 20.3 for a description of Boolean objects.
Number Return a new Number object whose [[NumberData]] internal slot is set to
argument. See 21.1 for a description of Number objects. String Return a new
String object whose [[StringData]] internal slot is set to argument. See 22.1
for a description of String objects. Symbol Return a new Symbol object whose
[[SymbolData]] internal slot is set to argument. See 20.4 for a description of
Symbol objects. BigInt Return a new BigInt object whose [[BigIntData]] internal
slot is set to argument. See 21.2 for a description of BigInt objects. Object
Return argument.
7.1.19 TOPROPERTYKEY ( ARGUMENT )
The abstract operation ToPropertyKey takes argument argument (an ECMAScript
language value) and returns either a normal completion containing a property key
or a throw completion. It converts argument to a value that can be used as a
property key. It performs the following steps when called:
1. 1. 1. Let key be ? ToPrimitive(argument, string).
2. 2. 2. If key is a Symbol, then
1. a. a. Return key.
3. 3. 3. Return ! ToString(key).
7.1.20 TOLENGTH ( ARGUMENT )
The abstract operation ToLength takes argument argument (an ECMAScript language
value) and returns either a normal completion containing an integral Number or a
throw completion. It clamps argument to an integral Number suitable for use as
the length of an array-like object. It performs the following steps when called:
1. 1. 1. Let len be ? ToIntegerOrInfinity(argument).
2. 2. 2. If len ≤ 0, return +0𝔽.
3. 3. 3. Return 𝔽(min(len, 253 - 1)).
7.1.21 CANONICALNUMERICINDEXSTRING ( ARGUMENT )
The abstract operation CanonicalNumericIndexString takes argument argument (a
String) and returns a Number or undefined. If argument is either "-0" or exactly
matches the result of ToString(n) for some Number value n, it returns the
respective Number value. Otherwise, it returns undefined. It performs the
following steps when called:
1. 1. 1. If argument is "-0", return -0𝔽.
2. 2. 2. Let n be ! ToNumber(argument).
3. 3. 3. If ! ToString(n) is argument, return n.
4. 4. 4. Return undefined.
A canonical numeric string is any String value for which the
CanonicalNumericIndexString abstract operation does not return undefined.
7.1.22 TOINDEX ( VALUE )
The abstract operation ToIndex takes argument value (an ECMAScript language
value) and returns either a normal completion containing a non-negative integer
or a throw completion. It converts value to a non-negative integer if the
corresponding decimal representation, as a String, is an integer index. It
performs the following steps when called:
1. 1. 1. If value is undefined, then
1. a. a. Return 0.
2. 2. 2. Else,
1. a. a. Let integer be ? ToIntegerOrInfinity(value).
2. b. b. Let clamped be ! ToLength(𝔽(integer)).
3. c. c. If SameValue(𝔽(integer), clamped) is false, throw a RangeError
exception.
4. d. d. Assert: 0 ≤ integer ≤ 253 - 1.
5. e. e. Return integer.
7.2 TESTING AND COMPARISON OPERATIONS
7.2.1 REQUIREOBJECTCOERCIBLE ( ARGUMENT )
The abstract operation RequireObjectCoercible takes argument argument (an
ECMAScript language value) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It throws an error if argument
is a value that cannot be converted to an Object using ToObject. It is defined
by Table 14:
Table 14: RequireObjectCoercible Results
Argument Type Result Undefined Throw a TypeError exception. Null Throw a
TypeError exception. Boolean Return argument. Number Return argument. String
Return argument. Symbol Return argument. BigInt Return argument. Object Return
argument.
7.2.2 ISARRAY ( ARGUMENT )
The abstract operation IsArray takes argument argument (an ECMAScript language
value) and returns either a normal completion containing a Boolean or a throw
completion. It performs the following steps when called:
1. 1. 1. If argument is not an Object, return false.
2. 2. 2. If argument is an Array exotic object, return true.
3. 3. 3. If argument is a Proxy exotic object, then
1. a. a. Perform ? ValidateNonRevokedProxy(argument).
2. b. b. Let proxyTarget be argument.[[ProxyTarget]].
3. c. c. Return ? IsArray(proxyTarget).
4. 4. 4. Return false.
7.2.3 ISCALLABLE ( ARGUMENT )
The abstract operation IsCallable takes argument argument (an ECMAScript
language value) and returns a Boolean. It determines if argument is a callable
function with a [[Call]] internal method. It performs the following steps when
called:
1. 1. 1. If argument is not an Object, return false.
2. 2. 2. If argument has a [[Call]] internal method, return true.
3. 3. 3. Return false.
7.2.4 ISCONSTRUCTOR ( ARGUMENT )
The abstract operation IsConstructor takes argument argument (an ECMAScript
language value) and returns a Boolean. It determines if argument is a function
object with a [[Construct]] internal method. It performs the following steps
when called:
1. 1. 1. If argument is not an Object, return false.
2. 2. 2. If argument has a [[Construct]] internal method, return true.
3. 3. 3. Return false.
7.2.5 ISEXTENSIBLE ( O )
The abstract operation IsExtensible takes argument O (an Object) and returns
either a normal completion containing a Boolean or a throw completion. It is
used to determine whether additional properties can be added to O. It performs
the following steps when called:
1. 1. 1. Return ? O.[[IsExtensible]]().
7.2.6 ISINTEGRALNUMBER ( ARGUMENT )
The abstract operation IsIntegralNumber takes argument argument (an ECMAScript
language value) and returns a Boolean. It determines if argument is a finite
integral Number value. It performs the following steps when called:
1. 1. 1. If argument is not a Number, return false.
2. 2. 2. If argument is not finite, return false.
3. 3. 3. If truncate(ℝ(argument)) ≠ ℝ(argument), return false.
4. 4. 4. Return true.
7.2.7 ISPROPERTYKEY ( ARGUMENT )
The abstract operation IsPropertyKey takes argument argument (an ECMAScript
language value) and returns a Boolean. It determines if argument is a value that
may be used as a property key. It performs the following steps when called:
1. 1. 1. If argument is a String, return true.
2. 2. 2. If argument is a Symbol, return true.
3. 3. 3. Return false.
7.2.8 ISREGEXP ( ARGUMENT )
The abstract operation IsRegExp takes argument argument (an ECMAScript language
value) and returns either a normal completion containing a Boolean or a throw
completion. It performs the following steps when called:
1. 1. 1. If argument is not an Object, return false.
2. 2. 2. Let matcher be ? Get(argument, @@match).
3. 3. 3. If matcher is not undefined, return ToBoolean(matcher).
4. 4. 4. If argument has a [[RegExpMatcher]] internal slot, return true.
5. 5. 5. Return false.
7.2.9 STATIC SEMANTICS: ISSTRINGWELLFORMEDUNICODE ( STRING )
The abstract operation IsStringWellFormedUnicode takes argument string (a
String) and returns a Boolean. It interprets string as a sequence of UTF-16
encoded code points, as described in 6.1.4, and determines whether it is a well
formed UTF-16 sequence. It performs the following steps when called:
1. 1. 1. Let len be the length of string.
2. 2. 2. Let k be 0.
3. 3. 3. Repeat, while k < len,
1. a. a. Let cp be CodePointAt(string, k).
2. b. b. If cp.[[IsUnpairedSurrogate]] is true, return false.
3. c. c. Set k to k + cp.[[CodeUnitCount]].
4. 4. 4. Return true.
7.2.10 SAMEVALUE ( X, Y )
The abstract operation SameValue takes arguments x (an ECMAScript language
value) and y (an ECMAScript language value) and returns a Boolean. It determines
whether or not the two arguments are the same value. It performs the following
steps when called:
1. 1. 1. If Type(x) is not Type(y), return false.
2. 2. 2. If x is a Number, then
1. a. a. Return Number::sameValue(x, y).
3. 3. 3. Return SameValueNonNumber(x, y).
Note
This algorithm differs from the IsStrictlyEqual Algorithm by treating all NaN
values as equivalent and by differentiating +0𝔽 from -0𝔽.
7.2.11 SAMEVALUEZERO ( X, Y )
The abstract operation SameValueZero takes arguments x (an ECMAScript language
value) and y (an ECMAScript language value) and returns a Boolean. It determines
whether or not the two arguments are the same value (ignoring the difference
between +0𝔽 and -0𝔽). It performs the following steps when called:
1. 1. 1. If Type(x) is not Type(y), return false.
2. 2. 2. If x is a Number, then
1. a. a. Return Number::sameValueZero(x, y).
3. 3. 3. Return SameValueNonNumber(x, y).
Note
SameValueZero differs from SameValue only in that it treats +0𝔽 and -0𝔽 as
equivalent.
7.2.12 SAMEVALUENONNUMBER ( X, Y )
The abstract operation SameValueNonNumber takes arguments x (an ECMAScript
language value, but not a Number) and y (an ECMAScript language value, but not a
Number) and returns a Boolean. It performs the following steps when called:
1. 1. 1. Assert: Type(x) is Type(y).
2. 2. 2. If x is either null or undefined, return true.
3. 3. 3. If x is a BigInt, then
1. a. a. Return BigInt::equal(x, y).
4. 4. 4. If x is a String, then
1. a. a. If x and y have the same length and the same code units in the same
positions, return true; otherwise, return false.
5. 5. 5. If x is a Boolean, then
1. a. a. If x and y are both true or both false, return true; otherwise,
return false.
6. 6. 6. NOTE: All other ECMAScript language values are compared by identity.
7. 7. 7. If x is y, return true; otherwise, return false.
Note 1
For expository purposes, some cases are handled separately within this algorithm
even if it is unnecessary to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.
7.2.13 ISLESSTHAN ( X, Y, LEFTFIRST )
The abstract operation IsLessThan takes arguments x (an ECMAScript language
value), y (an ECMAScript language value), and LeftFirst (a Boolean) and returns
either a normal completion containing either a Boolean or undefined, or a throw
completion. It provides the semantics for the comparison x < y, returning true,
false, or undefined (which indicates that at least one operand is NaN). The
LeftFirst flag is used to control the order in which operations with potentially
visible side-effects are performed upon x and y. It is necessary because
ECMAScript specifies left to right evaluation of expressions. If LeftFirst is
true, the x parameter corresponds to an expression that occurs to the left of
the y parameter's corresponding expression. If LeftFirst is false, the reverse
is the case and operations must be performed upon y before x. It performs the
following steps when called:
1. 1. 1. If LeftFirst is true, then
1. a. a. Let px be ? ToPrimitive(x, number).
2. b. b. Let py be ? ToPrimitive(y, number).
2. 2. 2. Else,
1. a. a. NOTE: The order of evaluation needs to be reversed to preserve left
to right evaluation.
2. b. b. Let py be ? ToPrimitive(y, number).
3. c. c. Let px be ? ToPrimitive(x, number).
3. 3. 3. If px is a String and py is a String, then
1. a. a. Let lx be the length of px.
2. b. b. Let ly be the length of py.
3. c. c. For each integer i such that 0 ≤ i < min(lx, ly), in ascending
order, do
1. i. i. Let cx be the numeric value of the code unit at index i within
px.
2. ii. ii. Let cy be the numeric value of the code unit at index i within
py.
3. iii. iii. If cx < cy, return true.
4. iv. iv. If cx > cy, return false.
4. d. d. If lx < ly, return true. Otherwise, return false.
4. 4. 4. Else,
1. a. a. If px is a BigInt and py is a String, then
1. i. i. Let ny be StringToBigInt(py).
2. ii. ii. If ny is undefined, return undefined.
3. iii. iii. Return BigInt::lessThan(px, ny).
2. b. b. If px is a String and py is a BigInt, then
1. i. i. Let nx be StringToBigInt(px).
2. ii. ii. If nx is undefined, return undefined.
3. iii. iii. Return BigInt::lessThan(nx, py).
3. c. c. NOTE: Because px and py are primitive values, evaluation order is
not important.
4. d. d. Let nx be ? ToNumeric(px).
5. e. e. Let ny be ? ToNumeric(py).
6. f. f. If Type(nx) is Type(ny), then
1. i. i. If nx is a Number, then
1. 1. 1. Return Number::lessThan(nx, ny).
2. ii. ii. Else,
1. 1. 1. Assert: nx is a BigInt.
2. 2. 2. Return BigInt::lessThan(nx, ny).
7. g. g. Assert: nx is a BigInt and ny is a Number, or nx is a Number and
ny is a BigInt.
8. h. h. If nx or ny is NaN, return undefined.
9. i. i. If nx is -∞𝔽 or ny is +∞𝔽, return true.
10. j. j. If nx is +∞𝔽 or ny is -∞𝔽, return false.
11. k. k. If ℝ(nx) < ℝ(ny), return true; otherwise return false.
Note 1
Step 3 differs from step 1.c in the algorithm that handles the addition operator
+ (13.15.3) by using the logical-and operation instead of the logical-or
operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of
UTF-16 code unit values. There is no attempt to use the more complex,
semantically oriented definitions of character or string equality and collating
order defined in the Unicode specification. Therefore String values that are
canonically equal according to the Unicode Standard but not in the same
normalization form could test as unequal. Also note that lexicographic ordering
by code unit differs from ordering by code point for Strings containing
surrogate pairs.
7.2.14 ISLOOSELYEQUAL ( X, Y )
The abstract operation IsLooselyEqual takes arguments x (an ECMAScript language
value) and y (an ECMAScript language value) and returns either a normal
completion containing a Boolean or a throw completion. It provides the semantics
for the == operator. It performs the following steps when called:
1. 1. 1. If Type(x) is Type(y), then
1. a. a. Return IsStrictlyEqual(x, y).
2. 2. 2. If x is null and y is undefined, return true.
3. 3. 3. If x is undefined and y is null, return true.
4. 4. 4. NOTE: This step is replaced in section B.3.6.2.
5. 5. 5. If x is a Number and y is a String, return ! IsLooselyEqual(x,
! ToNumber(y)).
6. 6. 6. If x is a String and y is a Number, return ! IsLooselyEqual(!
ToNumber(x), y).
7. 7. 7. If x is a BigInt and y is a String, then
1. a. a. Let n be StringToBigInt(y).
2. b. b. If n is undefined, return false.
3. c. c. Return ! IsLooselyEqual(x, n).
8. 8. 8. If x is a String and y is a BigInt, return ! IsLooselyEqual(y, x).
9. 9. 9. If x is a Boolean, return ! IsLooselyEqual(! ToNumber(x), y).
10. 10. 10. If y is a Boolean, return ! IsLooselyEqual(x, ! ToNumber(y)).
11. 11. 11. If x is either a String, a Number, a BigInt, or a Symbol and y is
an Object, return ! IsLooselyEqual(x, ? ToPrimitive(y)).
12. 12. 12. If x is an Object and y is either a String, a Number, a BigInt, or
a Symbol, return ! IsLooselyEqual(? ToPrimitive(x), y).
13. 13. 13. If x is a BigInt and y is a Number, or if x is a Number and y is a
BigInt, then
1. a. a. If x is not finite or y is not finite, return false.
2. b. b. If ℝ(x) = ℝ(y), return true; otherwise return false.
14. 14. 14. Return false.
7.2.15 ISSTRICTLYEQUAL ( X, Y )
The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language
value) and y (an ECMAScript language value) and returns a Boolean. It provides
the semantics for the === operator. It performs the following steps when called:
1. 1. 1. If Type(x) is not Type(y), return false.
2. 2. 2. If x is a Number, then
1. a. a. Return Number::equal(x, y).
3. 3. 3. Return SameValueNonNumber(x, y).
Note
This algorithm differs from the SameValue Algorithm in its treatment of signed
zeroes and NaNs.
7.3 OPERATIONS ON OBJECTS
7.3.1 MAKEBASICOBJECT ( INTERNALSLOTSLIST )
The abstract operation MakeBasicObject takes argument internalSlotsList (a List
of internal slot names) and returns an Object. It is the source of all
ECMAScript objects that are created algorithmically, including both ordinary
objects and exotic objects. It factors out common steps used in creating all
objects, and centralizes object creation. It performs the following steps when
called:
1. 1. 1. Let obj be a newly created object with an internal slot for each name
in internalSlotsList.
2. 2. 2. Set obj's essential internal methods to the default ordinary object
definitions specified in 10.1.
3. 3. 3. Assert: If the caller will not be overriding both obj's
[[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then
internalSlotsList contains [[Prototype]].
4. 4. 4. Assert: If the caller will not be overriding all of obj's
[[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential
internal methods, then internalSlotsList contains [[Extensible]].
5. 5. 5. If internalSlotsList contains [[Extensible]], set obj.[[Extensible]]
to true.
6. 6. 6. Return obj.
Note
Within this specification, exotic objects are created in abstract operations
such as ArrayCreate and BoundFunctionCreate by first calling MakeBasicObject to
obtain a basic, foundational object, and then overriding some or all of that
object's internal methods. In order to encapsulate exotic object creation, the
object's essential internal methods are never modified outside those operations.
7.3.2 GET ( O, P )
The abstract operation Get takes arguments O (an Object) and P (a property key)
and returns either a normal completion containing an ECMAScript language value
or a throw completion. It is used to retrieve the value of a specific property
of an object. It performs the following steps when called:
1. 1. 1. Return ? O.[[Get]](P, O).
7.3.3 GETV ( V, P )
The abstract operation GetV takes arguments V (an ECMAScript language value) and
P (a property key) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It is used to retrieve the
value of a specific property of an ECMAScript language value. If the value is
not an object, the property lookup is performed using a wrapper object
appropriate for the type of the value. It performs the following steps when
called:
1. 1. 1. Let O be ? ToObject(V).
2. 2. 2. Return ? O.[[Get]](P, V).
7.3.4 SET ( O, P, V, THROW )
The abstract operation Set takes arguments O (an Object), P (a property key), V
(an ECMAScript language value), and Throw (a Boolean) and returns either a
normal completion containing unused or a throw completion. It is used to set the
value of a specific property of an object. V is the new value for the property.
It performs the following steps when called:
1. 1. 1. Let success be ? O.[[Set]](P, V, O).
2. 2. 2. If success is false and Throw is true, throw a TypeError exception.
3. 3. 3. Return unused.
7.3.5 CREATEDATAPROPERTY ( O, P, V )
The abstract operation CreateDataProperty takes arguments O (an Object), P (a
property key), and V (an ECMAScript language value) and returns either a normal
completion containing a Boolean or a throw completion. It is used to create a
new own property of an object. It performs the following steps when called:
1. 1. 1. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true, [[Enumerable]]: true, [[Configurable]]: true }.
2. 2. 2. Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same
defaults used for properties created by the ECMAScript language assignment
operator. Normally, the property will not already exist. If it does exist and is
not configurable or if O is not extensible, [[DefineOwnProperty]] will return
false.
7.3.6 CREATEMETHODPROPERTY ( O, P, V )
The abstract operation CreateMethodProperty takes arguments O (an Object), P (a
property key), and V (an ECMAScript language value) and returns unused. It is
used to create a new own property of an ordinary object. It performs the
following steps when called:
1. 1. 1. Assert: O is an ordinary, extensible object with no non-configurable
properties.
2. 2. 2. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true, [[Enumerable]]: false, [[Configurable]]: true }.
3. 3. 3. Perform ! DefinePropertyOrThrow(O, P, newDesc).
4. 4. 4. Return unused.
Note
This abstract operation creates a property whose attributes are set to the same
defaults used for built-in methods and methods defined using class declaration
syntax. Normally, the property will not already exist. If it does exist,
DefinePropertyOrThrow is guaranteed to complete normally.
7.3.7 CREATEDATAPROPERTYORTHROW ( O, P, V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object),
P (a property key), and V (an ECMAScript language value) and returns either a
normal completion containing unused or a throw completion. It is used to create
a new own property of an object. It throws a TypeError exception if the
requested property update cannot be performed. It performs the following steps
when called:
1. 1. 1. Let success be ? CreateDataProperty(O, P, V).
2. 2. 2. If success is false, throw a TypeError exception.
3. 3. 3. Return unused.
Note
This abstract operation creates a property whose attributes are set to the same
defaults used for properties created by the ECMAScript language assignment
operator. Normally, the property will not already exist. If it does exist and is
not configurable or if O is not extensible, [[DefineOwnProperty]] will return
false causing this operation to throw a TypeError exception.
7.3.8 CREATENONENUMERABLEDATAPROPERTYORTHROW ( O, P, V )
The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O
(an Object), P (a property key), and V (an ECMAScript language value) and
returns unused. It is used to create a new non-enumerable own property of an
ordinary object. It performs the following steps when called:
1. 1. 1. Assert: O is an ordinary, extensible object with no non-configurable
properties.
2. 2. 2. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true, [[Enumerable]]: false, [[Configurable]]: true }.
3. 3. 3. Perform ! DefinePropertyOrThrow(O, P, newDesc).
4. 4. 4. Return unused.
Note
This abstract operation creates a property whose attributes are set to the same
defaults used for properties created by the ECMAScript language assignment
operator except it is not enumerable. Normally, the property will not already
exist. If it does exist, DefinePropertyOrThrow is guaranteed to complete
normally.
7.3.9 DEFINEPROPERTYORTHROW ( O, P, DESC )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a
property key), and desc (a Property Descriptor) and returns either a normal
completion containing unused or a throw completion. It is used to call the
[[DefineOwnProperty]] internal method of an object in a manner that will throw a
TypeError exception if the requested property update cannot be performed. It
performs the following steps when called:
1. 1. 1. Let success be ? O.[[DefineOwnProperty]](P, desc).
2. 2. 2. If success is false, throw a TypeError exception.
3. 3. 3. Return unused.
7.3.10 DELETEPROPERTYORTHROW ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P
(a property key) and returns either a normal completion containing unused or a
throw completion. It is used to remove a specific own property of an object. It
throws an exception if the property is not configurable. It performs the
following steps when called:
1. 1. 1. Let success be ? O.[[Delete]](P).
2. 2. 2. If success is false, throw a TypeError exception.
3. 3. 3. Return unused.
7.3.11 GETMETHOD ( V, P )
The abstract operation GetMethod takes arguments V (an ECMAScript language
value) and P (a property key) and returns either a normal completion containing
either a function object or undefined, or a throw completion. It is used to get
the value of a specific property of an ECMAScript language value when the value
of the property is expected to be a function. It performs the following steps
when called:
1. 1. 1. Let func be ? GetV(V, P).
2. 2. 2. If func is either undefined or null, return undefined.
3. 3. 3. If IsCallable(func) is false, throw a TypeError exception.
4. 4. 4. Return func.
7.3.12 HASPROPERTY ( O, P )
The abstract operation HasProperty takes arguments O (an Object) and P (a
property key) and returns either a normal completion containing a Boolean or a
throw completion. It is used to determine whether an object has a property with
the specified property key. The property may be either own or inherited. It
performs the following steps when called:
1. 1. 1. Return ? O.[[HasProperty]](P).
7.3.13 HASOWNPROPERTY ( O, P )
The abstract operation HasOwnProperty takes arguments O (an Object) and P (a
property key) and returns either a normal completion containing a Boolean or a
throw completion. It is used to determine whether an object has an own property
with the specified property key. It performs the following steps when called:
1. 1. 1. Let desc be ? O.[[GetOwnProperty]](P).
2. 2. 2. If desc is undefined, return false.
3. 3. 3. Return true.
7.3.14 CALL ( F, V [ , ARGUMENTSLIST ] )
The abstract operation Call takes arguments F (an ECMAScript language value) and
V (an ECMAScript language value) and optional argument argumentsList (a List of
ECMAScript language values) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It is used to call the [[Call]]
internal method of a function object. F is the function object, V is an
ECMAScript language value that is the this value of the [[Call]], and
argumentsList is the value passed to the corresponding argument of the internal
method. If argumentsList is not present, a new empty List is used as its value.
It performs the following steps when called:
1. 1. 1. If argumentsList is not present, set argumentsList to a new empty
List.
2. 2. 2. If IsCallable(F) is false, throw a TypeError exception.
3. 3. 3. Return ? F.[[Call]](V, argumentsList).
7.3.15 CONSTRUCT ( F [ , ARGUMENTSLIST [ , NEWTARGET ] ] )
The abstract operation Construct takes argument F (a constructor) and optional
arguments argumentsList (a List of ECMAScript language values) and newTarget (a
constructor) and returns either a normal completion containing an Object or a
throw completion. It is used to call the [[Construct]] internal method of a
function object. argumentsList and newTarget are the values to be passed as the
corresponding arguments of the internal method. If argumentsList is not present,
a new empty List is used as its value. If newTarget is not present, F is used as
its value. It performs the following steps when called:
1. 1. 1. If newTarget is not present, set newTarget to F.
2. 2. 2. If argumentsList is not present, set argumentsList to a new empty
List.
3. 3. 3. Return ? F.[[Construct]](argumentsList, newTarget).
Note
If newTarget is not present, this operation is equivalent to: new
F(...argumentsList)
7.3.16 SETINTEGRITYLEVEL ( O, LEVEL )
The abstract operation SetIntegrityLevel takes arguments O (an Object) and level
(sealed or frozen) and returns either a normal completion containing a Boolean
or a throw completion. It is used to fix the set of own properties of an object.
It performs the following steps when called:
1. 1. 1. Let status be ? O.[[PreventExtensions]]().
2. 2. 2. If status is false, return false.
3. 3. 3. Let keys be ? O.[[OwnPropertyKeys]]().
4. 4. 4. If level is sealed, then
1. a. a. For each element k of keys, do
1. i. i. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor {
[[Configurable]]: false }).
5. 5. 5. Else,
1. a. a. Assert: level is frozen.
2. b. b. For each element k of keys, do
1. i. i. Let currentDesc be ? O.[[GetOwnProperty]](k).
2. ii. ii. If currentDesc is not undefined, then
1. 1. 1. If IsAccessorDescriptor(currentDesc) is true, then
1. a. a. Let desc be the PropertyDescriptor { [[Configurable]]:
false }.
2. 2. 2. Else,
1. a. a. Let desc be the PropertyDescriptor { [[Configurable]]:
false, [[Writable]]: false }.
3. 3. 3. Perform ? DefinePropertyOrThrow(O, k, desc).
6. 6. 6. Return true.
7.3.17 TESTINTEGRITYLEVEL ( O, LEVEL )
The abstract operation TestIntegrityLevel takes arguments O (an Object) and
level (sealed or frozen) and returns either a normal completion containing a
Boolean or a throw completion. It is used to determine if the set of own
properties of an object are fixed. It performs the following steps when called:
1. 1. 1. Let extensible be ? IsExtensible(O).
2. 2. 2. If extensible is true, return false.
3. 3. 3. NOTE: If the object is extensible, none of its properties are
examined.
4. 4. 4. Let keys be ? O.[[OwnPropertyKeys]]().
5. 5. 5. For each element k of keys, do
1. a. a. Let currentDesc be ? O.[[GetOwnProperty]](k).
2. b. b. If currentDesc is not undefined, then
1. i. i. If currentDesc.[[Configurable]] is true, return false.
2. ii. ii. If level is frozen and IsDataDescriptor(currentDesc) is true,
then
1. 1. 1. If currentDesc.[[Writable]] is true, return false.
6. 6. 6. Return true.
7.3.18 CREATEARRAYFROMLIST ( ELEMENTS )
The abstract operation CreateArrayFromList takes argument elements (a List of
ECMAScript language values) and returns an Array. It is used to create an Array
whose elements are provided by elements. It performs the following steps when
called:
1. 1. 1. Let array be ! ArrayCreate(0).
2. 2. 2. Let n be 0.
3. 3. 3. For each element e of elements, do
1. a. a. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(n)), e).
2. b. b. Set n to n + 1.
4. 4. 4. Return array.
7.3.19 LENGTHOFARRAYLIKE ( OBJ )
The abstract operation LengthOfArrayLike takes argument obj (an Object) and
returns either a normal completion containing a non-negative integer or a throw
completion. It returns the value of the "length" property of an array-like
object. It performs the following steps when called:
1. 1. 1. Return ℝ(? ToLength(? Get(obj, "length"))).
An array-like object is any object for which this operation returns a normal
completion.
Note 1
Typically, an array-like object would also have some properties with integer
index names. However, that is not a requirement of this definition.
Note 2
Arrays and String objects are examples of array-like objects.
7.3.20 CREATELISTFROMARRAYLIKE ( OBJ [ , ELEMENTTYPES ] )
The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript
language value) and optional argument elementTypes (a List of names of
ECMAScript Language Types) and returns either a normal completion containing a
List of ECMAScript language values or a throw completion. It is used to create a
List value whose elements are provided by the indexed properties of obj.
elementTypes contains the names of ECMAScript Language Types that are allowed
for element values of the List that is created. It performs the following steps
when called:
1. 1. 1. If elementTypes is not present, set elementTypes to « Undefined, Null,
Boolean, String, Symbol, Number, BigInt, Object ».
2. 2. 2. If obj is not an Object, throw a TypeError exception.
3. 3. 3. Let len be ? LengthOfArrayLike(obj).
4. 4. 4. Let list be a new empty List.
5. 5. 5. Let index be 0.
6. 6. 6. Repeat, while index < len,
1. a. a. Let indexName be ! ToString(𝔽(index)).
2. b. b. Let next be ? Get(obj, indexName).
3. c. c. If elementTypes does not contain Type(next), throw a TypeError
exception.
4. d. d. Append next to list.
5. e. e. Set index to index + 1.
7. 7. 7. Return list.
7.3.21 INVOKE ( V, P [ , ARGUMENTSLIST ] )
The abstract operation Invoke takes arguments V (an ECMAScript language value)
and P (a property key) and optional argument argumentsList (a List of ECMAScript
language values) and returns either a normal completion containing an ECMAScript
language value or a throw completion. It is used to call a method property of an
ECMAScript language value. V serves as both the lookup point for the property
and the this value of the call. argumentsList is the list of arguments values
passed to the method. If argumentsList is not present, a new empty List is used
as its value. It performs the following steps when called:
1. 1. 1. If argumentsList is not present, set argumentsList to a new empty
List.
2. 2. 2. Let func be ? GetV(V, P).
3. 3. 3. Return ? Call(func, V, argumentsList).
7.3.22 ORDINARYHASINSTANCE ( C, O )
The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript
language value) and O (an ECMAScript language value) and returns either a normal
completion containing a Boolean or a throw completion. It implements the default
algorithm for determining if O inherits from the instance object inheritance
path provided by C. It performs the following steps when called:
1. 1. 1. If IsCallable(C) is false, return false.
2. 2. 2. If C has a [[BoundTargetFunction]] internal slot, then
1. a. a. Let BC be C.[[BoundTargetFunction]].
2. b. b. Return ? InstanceofOperator(O, BC).
3. 3. 3. If O is not an Object, return false.
4. 4. 4. Let P be ? Get(C, "prototype").
5. 5. 5. If P is not an Object, throw a TypeError exception.
6. 6. 6. Repeat,
1. a. a. Set O to ? O.[[GetPrototypeOf]]().
2. b. b. If O is null, return false.
3. c. c. If SameValue(P, O) is true, return true.
7.3.23 SPECIESCONSTRUCTOR ( O, DEFAULTCONSTRUCTOR )
The abstract operation SpeciesConstructor takes arguments O (an Object) and
defaultConstructor (a constructor) and returns either a normal completion
containing a constructor or a throw completion. It is used to retrieve the
constructor that should be used to create new objects that are derived from O.
defaultConstructor is the constructor to use if a constructor @@species property
cannot be found starting from O. It performs the following steps when called:
1. 1. 1. Let C be ? Get(O, "constructor").
2. 2. 2. If C is undefined, return defaultConstructor.
3. 3. 3. If C is not an Object, throw a TypeError exception.
4. 4. 4. Let S be ? Get(C, @@species).
5. 5. 5. If S is either undefined or null, return defaultConstructor.
6. 6. 6. If IsConstructor(S) is true, return S.
7. 7. 7. Throw a TypeError exception.
7.3.24 ENUMERABLEOWNPROPERTIES ( O, KIND )
The abstract operation EnumerableOwnProperties takes arguments O (an Object) and
kind (key, value, or key+value) and returns either a normal completion
containing a List of ECMAScript language values or a throw completion. It
performs the following steps when called:
1. 1. 1. Let ownKeys be ? O.[[OwnPropertyKeys]]().
2. 2. 2. Let results be a new empty List.
3. 3. 3. For each element key of ownKeys, do
1. a. a. If key is a String, then
1. i. i. Let desc be ? O.[[GetOwnProperty]](key).
2. ii. ii. If desc is not undefined and desc.[[Enumerable]] is true, then
1. 1. 1. If kind is key, append key to results.
2. 2. 2. Else,
1. a. a. Let value be ? Get(O, key).
2. b. b. If kind is value, append value to results.
3. c. c. Else,
1. i. i. Assert: kind is key+value.
2. ii. ii. Let entry be CreateArrayFromList(« key, value »).
3. iii. iii. Append entry to results.
4. 4. 4. Return results.
7.3.25 GETFUNCTIONREALM ( OBJ )
The abstract operation GetFunctionRealm takes argument obj (a function object)
and returns either a normal completion containing a Realm Record or a throw
completion. It performs the following steps when called:
1. 1. 1. If obj has a [[Realm]] internal slot, then
1. a. a. Return obj.[[Realm]].
2. 2. 2. If obj is a bound function exotic object, then
1. a. a. Let boundTargetFunction be obj.[[BoundTargetFunction]].
2. b. b. Return ? GetFunctionRealm(boundTargetFunction).
3. 3. 3. If obj is a Proxy exotic object, then
1. a. a. Perform ? ValidateNonRevokedProxy(obj).
2. b. b. Let proxyTarget be obj.[[ProxyTarget]].
3. c. c. Return ? GetFunctionRealm(proxyTarget).
4. 4. 4. Return the current Realm Record.
Note
Step 4 will only be reached if obj is a non-standard function exotic object that
does not have a [[Realm]] internal slot.
7.3.26 COPYDATAPROPERTIES ( TARGET, SOURCE, EXCLUDEDITEMS )
The abstract operation CopyDataProperties takes arguments target (an Object),
source (an ECMAScript language value), and excludedItems (a List of property
keys) and returns either a normal completion containing unused or a throw
completion. It performs the following steps when called:
1. 1. 1. If source is either undefined or null, return unused.
2. 2. 2. Let from be ! ToObject(source).
3. 3. 3. Let keys be ? from.[[OwnPropertyKeys]]().
4. 4. 4. For each element nextKey of keys, do
1. a. a. Let excluded be false.
2. b. b. For each element e of excludedItems, do
1. i. i. If SameValue(e, nextKey) is true, then
1. 1. 1. Set excluded to true.
3. c. c. If excluded is false, then
1. i. i. Let desc be ? from.[[GetOwnProperty]](nextKey).
2. ii. ii. If desc is not undefined and desc.[[Enumerable]] is true, then
1. 1. 1. Let propValue be ? Get(from, nextKey).
2. 2. 2. Perform ! CreateDataPropertyOrThrow(target, nextKey,
propValue).
5. 5. 5. Return unused.
Note
The target passed in here is always a newly created object which is not directly
accessible in case of an error being thrown.
7.3.27 PRIVATEELEMENTFIND ( O, P )
The abstract operation PrivateElementFind takes arguments O (an Object) and P (a
Private Name) and returns a PrivateElement or empty. It performs the following
steps when called:
1. 1. 1. If O.[[PrivateElements]] contains a PrivateElement pe such that
pe.[[Key]] is P, then
1. a. a. Return pe.
2. 2. 2. Return empty.
7.3.28 PRIVATEFIELDADD ( O, P, VALUE )
The abstract operation PrivateFieldAdd takes arguments O (an Object), P (a
Private Name), and value (an ECMAScript language value) and returns either a
normal completion containing unused or a throw completion. It performs the
following steps when called:
1. 1. 1. If the host is a web browser, then
1. a. a. Perform ? HostEnsureCanAddPrivateElement(O).
2. 2. 2. Let entry be PrivateElementFind(O, P).
3. 3. 3. If entry is not empty, throw a TypeError exception.
4. 4. 4. Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value
} to O.[[PrivateElements]].
5. 5. 5. Return unused.
7.3.29 PRIVATEMETHODORACCESSORADD ( O, METHOD )
The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object)
and method (a PrivateElement) and returns either a normal completion containing
unused or a throw completion. It performs the following steps when called:
1. 1. 1. Assert: method.[[Kind]] is either method or accessor.
2. 2. 2. If the host is a web browser, then
1. a. a. Perform ? HostEnsureCanAddPrivateElement(O).
3. 3. 3. Let entry be PrivateElementFind(O, method.[[Key]]).
4. 4. 4. If entry is not empty, throw a TypeError exception.
5. 5. 5. Append method to O.[[PrivateElements]].
6. 6. 6. Return unused.
Note
The values for private methods and accessors are shared across instances. This
operation does not create a new copy of the method or accessor.
7.3.30 HOSTENSURECANADDPRIVATEELEMENT ( O )
The host-defined abstract operation HostEnsureCanAddPrivateElement takes
argument O (an Object) and returns either a normal completion containing unused
or a throw completion. It allows host environments to prevent the addition of
private elements to particular host-defined exotic objects.
An implementation of HostEnsureCanAddPrivateElement must conform to the
following requirements:
* If O is not a host-defined exotic object, this abstract operation must return
NormalCompletion(unused) and perform no other steps.
* Any two calls of this abstract operation with the same argument must return
the same kind of Completion Record.
The default implementation of HostEnsureCanAddPrivateElement is to return
NormalCompletion(unused).
This abstract operation is only invoked by ECMAScript hosts that are web
browsers.
7.3.31 PRIVATEGET ( O, P )
The abstract operation PrivateGet takes arguments O (an Object) and P (a Private
Name) and returns either a normal completion containing an ECMAScript language
value or a throw completion. It performs the following steps when called:
1. 1. 1. Let entry be PrivateElementFind(O, P).
2. 2. 2. If entry is empty, throw a TypeError exception.
3. 3. 3. If entry.[[Kind]] is either field or method, then
1. a. a. Return entry.[[Value]].
4. 4. 4. Assert: entry.[[Kind]] is accessor.
5. 5. 5. If entry.[[Get]] is undefined, throw a TypeError exception.
6. 6. 6. Let getter be entry.[[Get]].
7. 7. 7. Return ? Call(getter, O).
7.3.32 PRIVATESET ( O, P, VALUE )
The abstract operation PrivateSet takes arguments O (an Object), P (a Private
Name), and value (an ECMAScript language value) and returns either a normal
completion containing unused or a throw completion. It performs the following
steps when called:
1. 1. 1. Let entry be PrivateElementFind(O, P).
2. 2. 2. If entry is empty, throw a TypeError exception.
3. 3. 3. If entry.[[Kind]] is field, then
1. a. a. Set entry.[[Value]] to value.
4. 4. 4. Else if entry.[[Kind]] is method, then
1. a. a. Throw a TypeError exception.
5. 5. 5. Else,
1. a. a. Assert: entry.[[Kind]] is accessor.
2. b. b. If entry.[[Set]] is undefined, throw a TypeError exception.
3. c. c. Let setter be entry.[[Set]].
4. d. d. Perform ? Call(setter, O, « value »).
6. 6. 6. Return unused.
7.3.33 DEFINEFIELD ( RECEIVER, FIELDRECORD )
The abstract operation DefineField takes arguments receiver (an Object) and
fieldRecord (a ClassFieldDefinition Record) and returns either a normal
completion containing unused or a throw completion. It performs the following
steps when called:
1. 1. 1. Let fieldName be fieldRecord.[[Name]].
2. 2. 2. Let initializer be fieldRecord.[[Initializer]].
3. 3. 3. If initializer is not empty, then
1. a. a. Let initValue be ? Call(initializer, receiver).
4. 4. 4. Else, let initValue be undefined.
5. 5. 5. If fieldName is a Private Name, then
1. a. a. Perform ? PrivateFieldAdd(receiver, fieldName, initValue).
6. 6. 6. Else,
1. a. a. Assert: IsPropertyKey(fieldName) is true.
2. b. b. Perform ? CreateDataPropertyOrThrow(receiver, fieldName,
initValue).
7. 7. 7. Return unused.
7.3.34 INITIALIZEINSTANCEELEMENTS ( O, CONSTRUCTOR )
The abstract operation InitializeInstanceElements takes arguments O (an Object)
and constructor (an ECMAScript function object) and returns either a normal
completion containing unused or a throw completion. It performs the following
steps when called:
1. 1. 1. Let methods be the value of constructor.[[PrivateMethods]].
2. 2. 2. For each PrivateElement method of methods, do
1. a. a. Perform ? PrivateMethodOrAccessorAdd(O, method).
3. 3. 3. Let fields be the value of constructor.[[Fields]].
4. 4. 4. For each element fieldRecord of fields, do
1. a. a. Perform ? DefineField(O, fieldRecord).
5. 5. 5. Return unused.
7.4 OPERATIONS ON ITERATOR OBJECTS
See Common Iteration Interfaces (27.1).
7.4.1 ITERATOR RECORDS
An Iterator Record is a Record value used to encapsulate an Iterator or
AsyncIterator along with the next method.
Iterator Records have the fields listed in Table 15.
Table 15: Iterator Record Fields
Field Name Value Meaning [[Iterator]] an Object An object that conforms to the
Iterator or AsyncIterator interface. [[NextMethod]] a function object The next
method of the [[Iterator]] object. [[Done]] a Boolean Whether the iterator has
been closed.
7.4.2 GETITERATORFROMMETHOD ( OBJ, METHOD )
The abstract operation GetIteratorFromMethod takes arguments obj (an ECMAScript
language value) and method (a function object) and returns either a normal
completion containing an Iterator Record or a throw completion. It performs the
following steps when called:
1. 1. 1. Let iterator be ? Call(method, obj).
2. 2. 2. If iterator is not an Object, throw a TypeError exception.
3. 3. 3. Let nextMethod be ? GetV(iterator, "next").
4. 4. 4. Let iteratorRecord be the Iterator Record { [[Iterator]]: iterator,
[[NextMethod]]: nextMethod, [[Done]]: false }.
5. 5. 5. Return iteratorRecord.
7.4.3 GETITERATOR ( OBJ, KIND )
The abstract operation GetIterator takes arguments obj (an ECMAScript language
value) and kind (sync or async) and returns either a normal completion
containing an Iterator Record or a throw completion. It performs the following
steps when called:
1. 1. 1. If kind is async, then
1. a. a. Let method be ? GetMethod(obj, @@asyncIterator).
2. b. b. If method is undefined, then
1. i. i. Let syncMethod be ? GetMethod(obj, @@iterator).
2. ii. ii. If syncMethod is undefined, throw a TypeError exception.
3. iii. iii. Let syncIteratorRecord be ? GetIteratorFromMethod(obj,
syncMethod).
4. iv. iv. Return CreateAsyncFromSyncIterator(syncIteratorRecord).
2. 2. 2. Otherwise, let method be ? GetMethod(obj, @@iterator).
3. 3. 3. If method is undefined, throw a TypeError exception.
4. 4. 4. Return ? GetIteratorFromMethod(obj, method).
7.4.4 ITERATORNEXT ( ITERATORRECORD [ , VALUE ] )
The abstract operation IteratorNext takes argument iteratorRecord (an Iterator
Record) and optional argument value (an ECMAScript language value) and returns
either a normal completion containing an Object or a throw completion. It
performs the following steps when called:
1. 1. 1. If value is not present, then
1. a. a. Let result be ? Call(iteratorRecord.[[NextMethod]],
iteratorRecord.[[Iterator]]).
2. 2. 2. Else,
1. a. a. Let result be ? Call(iteratorRecord.[[NextMethod]],
iteratorRecord.[[Iterator]], « value »).
3. 3. 3. If result is not an Object, throw a TypeError exception.
4. 4. 4. Return result.
7.4.5 ITERATORCOMPLETE ( ITERRESULT )
The abstract operation IteratorComplete takes argument iterResult (an Object)
and returns either a normal completion containing a Boolean or a throw
completion. It performs the following steps when called:
1. 1. 1. Return ToBoolean(? Get(iterResult, "done")).
7.4.6 ITERATORVALUE ( ITERRESULT )
The abstract operation IteratorValue takes argument iterResult (an Object) and
returns either a normal completion containing an ECMAScript language value or a
throw completion. It performs the following steps when called:
1. 1. 1. Return ? Get(iterResult, "value").
7.4.7 ITERATORSTEP ( ITERATORRECORD )
The abstract operation IteratorStep takes argument iteratorRecord (an Iterator
Record) and returns either a normal completion containing either an Object or
false, or a throw completion. It requests the next value from
iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns
either false indicating that the iterator has reached its end or the
IteratorResult object if a next value is available. It performs the following
steps when called:
1. 1. 1. Let result be ? IteratorNext(iteratorRecord).
2. 2. 2. Let done be ? IteratorComplete(result).
3. 3. 3. If done is true, return false.
4. 4. 4. Return result.
7.4.8 ITERATORCLOSE ( ITERATORRECORD, COMPLETION )
The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator
Record) and completion (a Completion Record) and returns a Completion Record. It
is used to notify an iterator that it should perform any actions it would
normally perform when it has reached its completed state. It performs the
following steps when called:
1. 1. 1. Assert: iteratorRecord.[[Iterator]] is an Object.
2. 2. 2. Let iterator be iteratorRecord.[[Iterator]].
3. 3. 3. Let innerResult be Completion(GetMethod(iterator, "return")).
4. 4. 4. If innerResult.[[Type]] is normal, then
1. a. a. Let return be innerResult.[[Value]].
2. b. b. If return is undefined, return ? completion.
3. c. c. Set innerResult to Completion(Call(return, iterator)).
5. 5. 5. If completion.[[Type]] is throw, return ? completion.
6. 6. 6. If innerResult.[[Type]] is throw, return ? innerResult.
7. 7. 7. If innerResult.[[Value]] is not an Object, throw a TypeError
exception.
8. 8. 8. Return ? completion.
7.4.9 IFABRUPTCLOSEITERATOR ( VALUE, ITERATORRECORD )
IfAbruptCloseIterator is a shorthand for a sequence of algorithm steps that use
an Iterator Record. An algorithm step of the form:
1. 1. 1. IfAbruptCloseIterator(value, iteratorRecord).
means the same thing as:
1. 1. 1. Assert: value is a Completion Record.
2. 2. 2. If value is an abrupt completion, return
? IteratorClose(iteratorRecord, value).
3. 3. 3. Else, set value to value.[[Value]].
7.4.10 ASYNCITERATORCLOSE ( ITERATORRECORD, COMPLETION )
The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an
Iterator Record) and completion (a Completion Record) and returns a Completion
Record. It is used to notify an async iterator that it should perform any
actions it would normally perform when it has reached its completed state. It
performs the following steps when called:
1. 1. 1. Assert: iteratorRecord.[[Iterator]] is an Object.
2. 2. 2. Let iterator be iteratorRecord.[[Iterator]].
3. 3. 3. Let innerResult be Completion(GetMethod(iterator, "return")).
4. 4. 4. If innerResult.[[Type]] is normal, then
1. a. a. Let return be innerResult.[[Value]].
2. b. b. If return is undefined, return ? completion.
3. c. c. Set innerResult to Completion(Call(return, iterator)).
4. d. d. If innerResult.[[Type]] is normal, set innerResult to
Completion(Await(innerResult.[[Value]])).
5. 5. 5. If completion.[[Type]] is throw, return ? completion.
6. 6. 6. If innerResult.[[Type]] is throw, return ? innerResult.
7. 7. 7. If innerResult.[[Value]] is not an Object, throw a TypeError
exception.
8. 8. 8. Return ? completion.
7.4.11 CREATEITERRESULTOBJECT ( VALUE, DONE )
The abstract operation CreateIterResultObject takes arguments value (an
ECMAScript language value) and done (a Boolean) and returns an Object that
conforms to the IteratorResult interface. It creates an object that conforms to
the IteratorResult interface. It performs the following steps when called:
1. 1. 1. Let obj be OrdinaryObjectCreate(%Object.prototype%).
2. 2. 2. Perform ! CreateDataPropertyOrThrow(obj, "value", value).
3. 3. 3. Perform ! CreateDataPropertyOrThrow(obj, "done", done).
4. 4. 4. Return obj.
7.4.12 CREATELISTITERATORRECORD ( LIST )
The abstract operation CreateListIteratorRecord takes argument list (a List of
ECMAScript language values) and returns an Iterator Record. It creates an
Iterator (27.1.1.2) object record whose next method returns the successive
elements of list. It performs the following steps when called:
1. 1. 1. Let closure be a new Abstract Closure with no parameters that captures
list and performs the following steps when called:
1. a. a. For each element E of list, do
1. i. i. Perform ? GeneratorYield(CreateIterResultObject(E, false)).
2. b. b. Return NormalCompletion(undefined).
2. 2. 2. Let iterator be CreateIteratorFromClosure(closure, empty,
%IteratorPrototype%).
3. 3. 3. Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]:
%GeneratorFunction.prototype.prototype.next%, [[Done]]: false }.
Note
The list iterator object is never directly accessible to ECMAScript code.
7.4.13 ITERATORTOLIST ( ITERATORRECORD )
The abstract operation IteratorToList takes argument iteratorRecord (an Iterator
Record) and returns either a normal completion containing a List of ECMAScript
language values or a throw completion. It performs the following steps when
called:
1. 1. 1. Let values be a new empty List.
2. 2. 2. Let next be true.
3. 3. 3. Repeat, while next is not false,
1. a. a. Set next to ? IteratorStep(iteratorRecord).
2. b. b. If next is not false, then
1. i. i. Let nextValue be ? IteratorValue(next).
2. ii. ii. Append nextValue to values.
4. 4. 4. Return values.
8 SYNTAX-DIRECTED OPERATIONS
In addition to those defined in this section, specialized syntax-directed
operations are defined throughout this specification.
8.1 RUNTIME SEMANTICS: EVALUATION
The syntax-directed operation Evaluation takes no arguments and returns a
Completion Record.
Note
The definitions for this operation are distributed over the "ECMAScript
Language" sections of this specification. Each definition appears after the
defining occurrence of the relevant productions.
8.2 SCOPE ANALYSIS
8.2.1 STATIC SEMANTICS: BOUNDNAMES
The syntax-directed operation BoundNames takes no arguments and returns a List
of Strings.
Note
"*default*" is used within this specification as a synthetic name for a module's
default export when it does not have another name. An entry in the module's
[[Environment]] is created with that name and holds the corresponding value, and
resolving the export named "default" by calling ResolveExport ( exportName [ ,
resolveSet ] ) for the module will return a ResolvedBinding Record whose
[[BindingName]] is "*default*", which will then resolve in the module's
[[Environment]] to the above-mentioned value. This is done only for ease of
specification, so that anonymous default exports can be resolved like any other
export. This "*default*" string is never accessible to ECMAScript code or to the
module linking algorithm.
It is defined piecewise over the following productions:
BindingIdentifier : Identifier
1. 1. 1. Return a List whose sole element is the StringValue of Identifier.
BindingIdentifier : yield
1. 1. 1. Return « "yield" ».
BindingIdentifier : await
1. 1. 1. Return « "await" ».
LexicalDeclaration : LetOrConst BindingList ;
1. 1. 1. Return the BoundNames of BindingList.
BindingList : BindingList , LexicalBinding
1. 1. 1. Let names1 be the BoundNames of BindingList.
2. 2. 2. Let names2 be the BoundNames of LexicalBinding.
3. 3. 3. Return the list-concatenation of names1 and names2.
LexicalBinding : BindingIdentifier Initializeropt
1. 1. 1. Return the BoundNames of BindingIdentifier.
LexicalBinding : BindingPattern Initializer
1. 1. 1. Return the BoundNames of BindingPattern.
VariableDeclarationList : VariableDeclarationList , VariableDeclaration
1. 1. 1. Let names1 be BoundNames of VariableDeclarationList.
2. 2. 2. Let names2 be BoundNames of VariableDeclaration.
3. 3. 3. Return the list-concatenation of names1 and names2.
VariableDeclaration : BindingIdentifier Initializeropt
1. 1. 1. Return the BoundNames of BindingIdentifier.
VariableDeclaration : BindingPattern Initializer
1. 1. 1. Return the BoundNames of BindingPattern.
ObjectBindingPattern : { }
1. 1. 1. Return a new empty List.
ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
1. 1. 1. Let names1 be BoundNames of BindingPropertyList.
2. 2. 2. Let names2 be BoundNames of BindingRestProperty.
3. 3. 3. Return the list-concatenation of names1 and names2.
ArrayBindingPattern : [ Elisionopt ]
1. 1. 1. Return a new empty List.
ArrayBindingPattern : [ Elisionopt BindingRestElement ]
1. 1. 1. Return the BoundNames of BindingRestElement.
ArrayBindingPattern : [ BindingElementList , Elisionopt ]
1. 1. 1. Return the BoundNames of BindingElementList.
ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
1. 1. 1. Let names1 be BoundNames of BindingElementList.
2. 2. 2. Let names2 be BoundNames of BindingRestElement.
3. 3. 3. Return the list-concatenation of names1 and names2.
BindingPropertyList : BindingPropertyList , BindingProperty
1. 1. 1. Let names1 be BoundNames of BindingPropertyList.
2. 2. 2. Let names2 be BoundNames of BindingProperty.
3. 3. 3. Return the list-concatenation of names1 and names2.
BindingElementList : BindingElementList , BindingElisionElement
1. 1. 1. Let names1 be BoundNames of BindingElementList.
2. 2. 2. Let names2 be BoundNames of BindingElisionElement.
3. 3. 3. Return the list-concatenation of names1 and names2.
BindingElisionElement : Elisionopt BindingElement
1. 1. 1. Return BoundNames of BindingElement.
BindingProperty : PropertyName : BindingElement
1. 1. 1. Return the BoundNames of BindingElement.
SingleNameBinding : BindingIdentifier Initializeropt
1. 1. 1. Return the BoundNames of BindingIdentifier.
BindingElement : BindingPattern Initializeropt
1. 1. 1. Return the BoundNames of BindingPattern.
ForDeclaration : LetOrConst ForBinding
1. 1. 1. Return the BoundNames of ForBinding.
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody }
1. 1. 1. Return the BoundNames of BindingIdentifier.
FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
1. 1. 1. Return « "*default*" ».
FormalParameters : [empty]
1. 1. 1. Return a new empty List.
FormalParameters : FormalParameterList , FunctionRestParameter
1. 1. 1. Let names1 be BoundNames of FormalParameterList.
2. 2. 2. Let names2 be BoundNames of FunctionRestParameter.
3. 3. 3. Return the list-concatenation of names1 and names2.
FormalParameterList : FormalParameterList , FormalParameter
1. 1. 1. Let names1 be BoundNames of FormalParameterList.
2. 2. 2. Let names2 be BoundNames of FormalParameter.
3. 3. 3. Return the list-concatenation of names1 and names2.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let formals be the ArrowFormalParameters that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return the BoundNames of formals.
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody }
1. 1. 1. Return the BoundNames of BindingIdentifier.
GeneratorDeclaration : function * ( FormalParameters ) { GeneratorBody }
1. 1. 1. Return « "*default*" ».
AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody }
1. 1. 1. Return the BoundNames of BindingIdentifier.
AsyncGeneratorDeclaration : async function * ( FormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Return « "*default*" ».
ClassDeclaration : class BindingIdentifier ClassTail
1. 1. 1. Return the BoundNames of BindingIdentifier.
ClassDeclaration : class ClassTail
1. 1. 1. Return « "*default*" ».
AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody }
1. 1. 1. Return the BoundNames of BindingIdentifier.
AsyncFunctionDeclaration : async function ( FormalParameters ) {
AsyncFunctionBody }
1. 1. 1. Return « "*default*" ».
CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments
1. 1. 1. Let head be the AsyncArrowHead that is covered by
CoverCallExpressionAndAsyncArrowHead.
2. 2. 2. Return the BoundNames of head.
ImportDeclaration : import ImportClause FromClause ;
1. 1. 1. Return the BoundNames of ImportClause.
ImportDeclaration : import ModuleSpecifier ;
1. 1. 1. Return a new empty List.
ImportClause : ImportedDefaultBinding , NameSpaceImport
1. 1. 1. Let names1 be the BoundNames of ImportedDefaultBinding.
2. 2. 2. Let names2 be the BoundNames of NameSpaceImport.
3. 3. 3. Return the list-concatenation of names1 and names2.
ImportClause : ImportedDefaultBinding , NamedImports
1. 1. 1. Let names1 be the BoundNames of ImportedDefaultBinding.
2. 2. 2. Let names2 be the BoundNames of NamedImports.
3. 3. 3. Return the list-concatenation of names1 and names2.
NamedImports : { }
1. 1. 1. Return a new empty List.
ImportsList : ImportsList , ImportSpecifier
1. 1. 1. Let names1 be the BoundNames of ImportsList.
2. 2. 2. Let names2 be the BoundNames of ImportSpecifier.
3. 3. 3. Return the list-concatenation of names1 and names2.
ImportSpecifier : ModuleExportName as ImportedBinding
1. 1. 1. Return the BoundNames of ImportedBinding.
ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
1. 1. 1. Return a new empty List.
ExportDeclaration : export VariableStatement
1. 1. 1. Return the BoundNames of VariableStatement.
ExportDeclaration : export Declaration
1. 1. 1. Return the BoundNames of Declaration.
ExportDeclaration : export default HoistableDeclaration
1. 1. 1. Let declarationNames be the BoundNames of HoistableDeclaration.
2. 2. 2. If declarationNames does not include the element "*default*", append
"*default*" to declarationNames.
3. 3. 3. Return declarationNames.
ExportDeclaration : export default ClassDeclaration
1. 1. 1. Let declarationNames be the BoundNames of ClassDeclaration.
2. 2. 2. If declarationNames does not include the element "*default*", append
"*default*" to declarationNames.
3. 3. 3. Return declarationNames.
ExportDeclaration : export default AssignmentExpression ;
1. 1. 1. Return « "*default*" ».
8.2.2 STATIC SEMANTICS: DECLARATIONPART
The syntax-directed operation DeclarationPart takes no arguments and returns a
Parse Node. It is defined piecewise over the following productions:
HoistableDeclaration : FunctionDeclaration
1. 1. 1. Return FunctionDeclaration.
HoistableDeclaration : GeneratorDeclaration
1. 1. 1. Return GeneratorDeclaration.
HoistableDeclaration : AsyncFunctionDeclaration
1. 1. 1. Return AsyncFunctionDeclaration.
HoistableDeclaration : AsyncGeneratorDeclaration
1. 1. 1. Return AsyncGeneratorDeclaration.
Declaration : ClassDeclaration
1. 1. 1. Return ClassDeclaration.
Declaration : LexicalDeclaration
1. 1. 1. Return LexicalDeclaration.
8.2.3 STATIC SEMANTICS: ISCONSTANTDECLARATION
The syntax-directed operation IsConstantDeclaration takes no arguments and
returns a Boolean. It is defined piecewise over the following productions:
LexicalDeclaration : LetOrConst BindingList ;
1. 1. 1. Return IsConstantDeclaration of LetOrConst.
LetOrConst : let
1. 1. 1. Return false.
LetOrConst : const
1. 1. 1. Return true.
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody } function ( FormalParameters ) { FunctionBody }
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters )
{ AsyncGeneratorBody } AsyncFunctionDeclaration : async function
BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function (
FormalParameters ) { AsyncFunctionBody }
1. 1. 1. Return false.
ClassDeclaration : class BindingIdentifier ClassTail class ClassTail
1. 1. 1. Return false.
ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
export default AssignmentExpression ;
1. 1. 1. Return false.
Note
It is not necessary to treat export default AssignmentExpression as a constant
declaration because there is no syntax that permits assignment to the internal
bound name used to reference a module's default object.
8.2.4 STATIC SEMANTICS: LEXICALLYDECLAREDNAMES
The syntax-directed operation LexicallyDeclaredNames takes no arguments and
returns a List of Strings. It is defined piecewise over the following
productions:
Block : { }
1. 1. 1. Return a new empty List.
StatementList : StatementList StatementListItem
1. 1. 1. Let names1 be LexicallyDeclaredNames of StatementList.
2. 2. 2. Let names2 be LexicallyDeclaredNames of StatementListItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
StatementListItem : Statement
1. 1. 1. If Statement is Statement : LabelledStatement , return
LexicallyDeclaredNames of LabelledStatement.
2. 2. 2. Return a new empty List.
StatementListItem : Declaration
1. 1. 1. Return the BoundNames of Declaration.
CaseBlock : { }
1. 1. 1. Return a new empty List.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, let names1 be the
LexicallyDeclaredNames of the first CaseClauses.
2. 2. 2. Else, let names1 be a new empty List.
3. 3. 3. Let names2 be LexicallyDeclaredNames of DefaultClause.
4. 4. 4. If the second CaseClauses is present, let names3 be the
LexicallyDeclaredNames of the second CaseClauses.
5. 5. 5. Else, let names3 be a new empty List.
6. 6. 6. Return the list-concatenation of names1, names2, and names3.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let names1 be LexicallyDeclaredNames of CaseClauses.
2. 2. 2. Let names2 be LexicallyDeclaredNames of CaseClause.
3. 3. 3. Return the list-concatenation of names1 and names2.
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return the LexicallyDeclaredNames of
StatementList.
2. 2. 2. Return a new empty List.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return the LexicallyDeclaredNames of
StatementList.
2. 2. 2. Return a new empty List.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return the LexicallyDeclaredNames of LabelledItem.
LabelledItem : Statement
1. 1. 1. Return a new empty List.
LabelledItem : FunctionDeclaration
1. 1. 1. Return BoundNames of FunctionDeclaration.
FunctionStatementList : [empty]
1. 1. 1. Return a new empty List.
FunctionStatementList : StatementList
1. 1. 1. Return TopLevelLexicallyDeclaredNames of StatementList.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return a new empty List.
ClassStaticBlockStatementList : StatementList
1. 1. 1. Return the TopLevelLexicallyDeclaredNames of StatementList.
ConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
AsyncConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
Script : [empty]
1. 1. 1. Return a new empty List.
ScriptBody : StatementList
1. 1. 1. Return TopLevelLexicallyDeclaredNames of StatementList.
Note 1
At the top level of a Script, function declarations are treated like var
declarations rather than like lexical declarations.
Note 2
The LexicallyDeclaredNames of a Module includes the names of all of its imported
bindings.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let names1 be LexicallyDeclaredNames of ModuleItemList.
2. 2. 2. Let names2 be LexicallyDeclaredNames of ModuleItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
ModuleItem : ImportDeclaration
1. 1. 1. Return the BoundNames of ImportDeclaration.
ModuleItem : ExportDeclaration
1. 1. 1. If ExportDeclaration is export VariableStatement, return a new empty
List.
2. 2. 2. Return the BoundNames of ExportDeclaration.
ModuleItem : StatementListItem
1. 1. 1. Return LexicallyDeclaredNames of StatementListItem.
Note 3
At the top level of a Module, function declarations are treated like lexical
declarations rather than like var declarations.
8.2.5 STATIC SEMANTICS: LEXICALLYSCOPEDDECLARATIONS
The syntax-directed operation LexicallyScopedDeclarations takes no arguments and
returns a List of Parse Nodes. It is defined piecewise over the following
productions:
StatementList : StatementList StatementListItem
1. 1. 1. Let declarations1 be LexicallyScopedDeclarations of StatementList.
2. 2. 2. Let declarations2 be LexicallyScopedDeclarations of StatementListItem.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
StatementListItem : Statement
1. 1. 1. If Statement is Statement : LabelledStatement , return
LexicallyScopedDeclarations of LabelledStatement.
2. 2. 2. Return a new empty List.
StatementListItem : Declaration
1. 1. 1. Return a List whose sole element is DeclarationPart of Declaration.
CaseBlock : { }
1. 1. 1. Return a new empty List.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, let declarations1 be the
LexicallyScopedDeclarations of the first CaseClauses.
2. 2. 2. Else, let declarations1 be a new empty List.
3. 3. 3. Let declarations2 be LexicallyScopedDeclarations of DefaultClause.
4. 4. 4. If the second CaseClauses is present, let declarations3 be the
LexicallyScopedDeclarations of the second CaseClauses.
5. 5. 5. Else, let declarations3 be a new empty List.
6. 6. 6. Return the list-concatenation of declarations1, declarations2, and
declarations3.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let declarations1 be LexicallyScopedDeclarations of CaseClauses.
2. 2. 2. Let declarations2 be LexicallyScopedDeclarations of CaseClause.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return the
LexicallyScopedDeclarations of StatementList.
2. 2. 2. Return a new empty List.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return the
LexicallyScopedDeclarations of StatementList.
2. 2. 2. Return a new empty List.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return the LexicallyScopedDeclarations of LabelledItem.
LabelledItem : Statement
1. 1. 1. Return a new empty List.
LabelledItem : FunctionDeclaration
1. 1. 1. Return « FunctionDeclaration ».
FunctionStatementList : [empty]
1. 1. 1. Return a new empty List.
FunctionStatementList : StatementList
1. 1. 1. Return the TopLevelLexicallyScopedDeclarations of StatementList.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return a new empty List.
ClassStaticBlockStatementList : StatementList
1. 1. 1. Return the TopLevelLexicallyScopedDeclarations of StatementList.
ConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
AsyncConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
Script : [empty]
1. 1. 1. Return a new empty List.
ScriptBody : StatementList
1. 1. 1. Return TopLevelLexicallyScopedDeclarations of StatementList.
Module : [empty]
1. 1. 1. Return a new empty List.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let declarations1 be LexicallyScopedDeclarations of ModuleItemList.
2. 2. 2. Let declarations2 be LexicallyScopedDeclarations of ModuleItem.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
ModuleItem : ImportDeclaration
1. 1. 1. Return a new empty List.
ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
export VariableStatement
1. 1. 1. Return a new empty List.
ExportDeclaration : export Declaration
1. 1. 1. Return a List whose sole element is DeclarationPart of Declaration.
ExportDeclaration : export default HoistableDeclaration
1. 1. 1. Return a List whose sole element is DeclarationPart of
HoistableDeclaration.
ExportDeclaration : export default ClassDeclaration
1. 1. 1. Return a List whose sole element is ClassDeclaration.
ExportDeclaration : export default AssignmentExpression ;
1. 1. 1. Return a List whose sole element is this ExportDeclaration.
8.2.6 STATIC SEMANTICS: VARDECLAREDNAMES
The syntax-directed operation VarDeclaredNames takes no arguments and returns a
List of Strings. It is defined piecewise over the following productions:
Statement : EmptyStatement ExpressionStatement ContinueStatement BreakStatement
ReturnStatement ThrowStatement DebuggerStatement
1. 1. 1. Return a new empty List.
Block : { }
1. 1. 1. Return a new empty List.
StatementList : StatementList StatementListItem
1. 1. 1. Let names1 be VarDeclaredNames of StatementList.
2. 2. 2. Let names2 be VarDeclaredNames of StatementListItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
StatementListItem : Declaration
1. 1. 1. Return a new empty List.
VariableStatement : var VariableDeclarationList ;
1. 1. 1. Return BoundNames of VariableDeclarationList.
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let names1 be VarDeclaredNames of the first Statement.
2. 2. 2. Let names2 be VarDeclaredNames of the second Statement.
3. 3. 3. Return the list-concatenation of names1 and names2.
IfStatement : if ( Expression ) Statement
1. 1. 1. Return the VarDeclaredNames of Statement.
DoWhileStatement : do Statement while ( Expression ) ;
1. 1. 1. Return the VarDeclaredNames of Statement.
WhileStatement : while ( Expression ) Statement
1. 1. 1. Return the VarDeclaredNames of Statement.
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
1. 1. 1. Return the VarDeclaredNames of Statement.
ForStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt
) Statement
1. 1. 1. Let names1 be BoundNames of VariableDeclarationList.
2. 2. 2. Let names2 be VarDeclaredNames of Statement.
3. 3. 3. Return the list-concatenation of names1 and names2.
ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt )
Statement
1. 1. 1. Return the VarDeclaredNames of Statement.
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of
AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression )
Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement
for await ( ForDeclaration of AssignmentExpression ) Statement
1. 1. 1. Return the VarDeclaredNames of Statement.
ForInOfStatement : for ( var ForBinding in Expression ) Statement for ( var
ForBinding of AssignmentExpression ) Statement for await ( var ForBinding of
AssignmentExpression ) Statement
1. 1. 1. Let names1 be the BoundNames of ForBinding.
2. 2. 2. Let names2 be the VarDeclaredNames of Statement.
3. 3. 3. Return the list-concatenation of names1 and names2.
Note
This section is extended by Annex B.3.5.
WithStatement : with ( Expression ) Statement
1. 1. 1. Return the VarDeclaredNames of Statement.
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Return the VarDeclaredNames of CaseBlock.
CaseBlock : { }
1. 1. 1. Return a new empty List.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, let names1 be the
VarDeclaredNames of the first CaseClauses.
2. 2. 2. Else, let names1 be a new empty List.
3. 3. 3. Let names2 be VarDeclaredNames of DefaultClause.
4. 4. 4. If the second CaseClauses is present, let names3 be the
VarDeclaredNames of the second CaseClauses.
5. 5. 5. Else, let names3 be a new empty List.
6. 6. 6. Return the list-concatenation of names1, names2, and names3.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let names1 be VarDeclaredNames of CaseClauses.
2. 2. 2. Let names2 be VarDeclaredNames of CaseClause.
3. 3. 3. Return the list-concatenation of names1 and names2.
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return the VarDeclaredNames of
StatementList.
2. 2. 2. Return a new empty List.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return the VarDeclaredNames of
StatementList.
2. 2. 2. Return a new empty List.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return the VarDeclaredNames of LabelledItem.
LabelledItem : FunctionDeclaration
1. 1. 1. Return a new empty List.
TryStatement : try Block Catch
1. 1. 1. Let names1 be VarDeclaredNames of Block.
2. 2. 2. Let names2 be VarDeclaredNames of Catch.
3. 3. 3. Return the list-concatenation of names1 and names2.
TryStatement : try Block Finally
1. 1. 1. Let names1 be VarDeclaredNames of Block.
2. 2. 2. Let names2 be VarDeclaredNames of Finally.
3. 3. 3. Return the list-concatenation of names1 and names2.
TryStatement : try Block Catch Finally
1. 1. 1. Let names1 be VarDeclaredNames of Block.
2. 2. 2. Let names2 be VarDeclaredNames of Catch.
3. 3. 3. Let names3 be VarDeclaredNames of Finally.
4. 4. 4. Return the list-concatenation of names1, names2, and names3.
Catch : catch ( CatchParameter ) Block
1. 1. 1. Return the VarDeclaredNames of Block.
FunctionStatementList : [empty]
1. 1. 1. Return a new empty List.
FunctionStatementList : StatementList
1. 1. 1. Return TopLevelVarDeclaredNames of StatementList.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return a new empty List.
ClassStaticBlockStatementList : StatementList
1. 1. 1. Return the TopLevelVarDeclaredNames of StatementList.
ConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
AsyncConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
Script : [empty]
1. 1. 1. Return a new empty List.
ScriptBody : StatementList
1. 1. 1. Return TopLevelVarDeclaredNames of StatementList.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let names1 be VarDeclaredNames of ModuleItemList.
2. 2. 2. Let names2 be VarDeclaredNames of ModuleItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
ModuleItem : ImportDeclaration
1. 1. 1. Return a new empty List.
ModuleItem : ExportDeclaration
1. 1. 1. If ExportDeclaration is export VariableStatement, return BoundNames of
ExportDeclaration.
2. 2. 2. Return a new empty List.
8.2.7 STATIC SEMANTICS: VARSCOPEDDECLARATIONS
The syntax-directed operation VarScopedDeclarations takes no arguments and
returns a List of Parse Nodes. It is defined piecewise over the following
productions:
Statement : EmptyStatement ExpressionStatement ContinueStatement BreakStatement
ReturnStatement ThrowStatement DebuggerStatement
1. 1. 1. Return a new empty List.
Block : { }
1. 1. 1. Return a new empty List.
StatementList : StatementList StatementListItem
1. 1. 1. Let declarations1 be VarScopedDeclarations of StatementList.
2. 2. 2. Let declarations2 be VarScopedDeclarations of StatementListItem.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
StatementListItem : Declaration
1. 1. 1. Return a new empty List.
VariableDeclarationList : VariableDeclaration
1. 1. 1. Return « VariableDeclaration ».
VariableDeclarationList : VariableDeclarationList , VariableDeclaration
1. 1. 1. Let declarations1 be VarScopedDeclarations of VariableDeclarationList.
2. 2. 2. Return the list-concatenation of declarations1 and «
VariableDeclaration ».
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let declarations1 be VarScopedDeclarations of the first Statement.
2. 2. 2. Let declarations2 be VarScopedDeclarations of the second Statement.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
IfStatement : if ( Expression ) Statement
1. 1. 1. Return the VarScopedDeclarations of Statement.
DoWhileStatement : do Statement while ( Expression ) ;
1. 1. 1. Return the VarScopedDeclarations of Statement.
WhileStatement : while ( Expression ) Statement
1. 1. 1. Return the VarScopedDeclarations of Statement.
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
1. 1. 1. Return the VarScopedDeclarations of Statement.
ForStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt
) Statement
1. 1. 1. Let declarations1 be VarScopedDeclarations of VariableDeclarationList.
2. 2. 2. Let declarations2 be VarScopedDeclarations of Statement.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt )
Statement
1. 1. 1. Return the VarScopedDeclarations of Statement.
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of
AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression )
Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement
for await ( ForDeclaration of AssignmentExpression ) Statement
1. 1. 1. Return the VarScopedDeclarations of Statement.
ForInOfStatement : for ( var ForBinding in Expression ) Statement for ( var
ForBinding of AssignmentExpression ) Statement for await ( var ForBinding of
AssignmentExpression ) Statement
1. 1. 1. Let declarations1 be « ForBinding ».
2. 2. 2. Let declarations2 be VarScopedDeclarations of Statement.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
Note
This section is extended by Annex B.3.5.
WithStatement : with ( Expression ) Statement
1. 1. 1. Return the VarScopedDeclarations of Statement.
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Return the VarScopedDeclarations of CaseBlock.
CaseBlock : { }
1. 1. 1. Return a new empty List.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, let declarations1 be the
VarScopedDeclarations of the first CaseClauses.
2. 2. 2. Else, let declarations1 be a new empty List.
3. 3. 3. Let declarations2 be VarScopedDeclarations of DefaultClause.
4. 4. 4. If the second CaseClauses is present, let declarations3 be the
VarScopedDeclarations of the second CaseClauses.
5. 5. 5. Else, let declarations3 be a new empty List.
6. 6. 6. Return the list-concatenation of declarations1, declarations2, and
declarations3.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let declarations1 be VarScopedDeclarations of CaseClauses.
2. 2. 2. Let declarations2 be VarScopedDeclarations of CaseClause.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return the VarScopedDeclarations of
StatementList.
2. 2. 2. Return a new empty List.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return the VarScopedDeclarations of
StatementList.
2. 2. 2. Return a new empty List.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return the VarScopedDeclarations of LabelledItem.
LabelledItem : FunctionDeclaration
1. 1. 1. Return a new empty List.
TryStatement : try Block Catch
1. 1. 1. Let declarations1 be VarScopedDeclarations of Block.
2. 2. 2. Let declarations2 be VarScopedDeclarations of Catch.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
TryStatement : try Block Finally
1. 1. 1. Let declarations1 be VarScopedDeclarations of Block.
2. 2. 2. Let declarations2 be VarScopedDeclarations of Finally.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
TryStatement : try Block Catch Finally
1. 1. 1. Let declarations1 be VarScopedDeclarations of Block.
2. 2. 2. Let declarations2 be VarScopedDeclarations of Catch.
3. 3. 3. Let declarations3 be VarScopedDeclarations of Finally.
4. 4. 4. Return the list-concatenation of declarations1, declarations2, and
declarations3.
Catch : catch ( CatchParameter ) Block
1. 1. 1. Return the VarScopedDeclarations of Block.
FunctionStatementList : [empty]
1. 1. 1. Return a new empty List.
FunctionStatementList : StatementList
1. 1. 1. Return the TopLevelVarScopedDeclarations of StatementList.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return a new empty List.
ClassStaticBlockStatementList : StatementList
1. 1. 1. Return the TopLevelVarScopedDeclarations of StatementList.
ConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
AsyncConciseBody : ExpressionBody
1. 1. 1. Return a new empty List.
Script : [empty]
1. 1. 1. Return a new empty List.
ScriptBody : StatementList
1. 1. 1. Return TopLevelVarScopedDeclarations of StatementList.
Module : [empty]
1. 1. 1. Return a new empty List.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let declarations1 be VarScopedDeclarations of ModuleItemList.
2. 2. 2. Let declarations2 be VarScopedDeclarations of ModuleItem.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
ModuleItem : ImportDeclaration
1. 1. 1. Return a new empty List.
ModuleItem : ExportDeclaration
1. 1. 1. If ExportDeclaration is export VariableStatement, return
VarScopedDeclarations of VariableStatement.
2. 2. 2. Return a new empty List.
8.2.8 STATIC SEMANTICS: TOPLEVELLEXICALLYDECLAREDNAMES
The syntax-directed operation TopLevelLexicallyDeclaredNames takes no arguments
and returns a List of Strings. It is defined piecewise over the following
productions:
StatementList : StatementList StatementListItem
1. 1. 1. Let names1 be TopLevelLexicallyDeclaredNames of StatementList.
2. 2. 2. Let names2 be TopLevelLexicallyDeclaredNames of StatementListItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
StatementListItem : Statement
1. 1. 1. Return a new empty List.
StatementListItem : Declaration
1. 1. 1. If Declaration is Declaration : HoistableDeclaration , then
1. a. a. Return a new empty List.
2. 2. 2. Return the BoundNames of Declaration.
Note
At the top level of a function, or script, function declarations are treated
like var declarations rather than like lexical declarations.
8.2.9 STATIC SEMANTICS: TOPLEVELLEXICALLYSCOPEDDECLARATIONS
The syntax-directed operation TopLevelLexicallyScopedDeclarations takes no
arguments and returns a List of Parse Nodes. It is defined piecewise over the
following productions:
StatementList : StatementList StatementListItem
1. 1. 1. Let declarations1 be TopLevelLexicallyScopedDeclarations of
StatementList.
2. 2. 2. Let declarations2 be TopLevelLexicallyScopedDeclarations of
StatementListItem.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
StatementListItem : Statement
1. 1. 1. Return a new empty List.
StatementListItem : Declaration
1. 1. 1. If Declaration is Declaration : HoistableDeclaration , then
1. a. a. Return a new empty List.
2. 2. 2. Return « Declaration ».
8.2.10 STATIC SEMANTICS: TOPLEVELVARDECLAREDNAMES
The syntax-directed operation TopLevelVarDeclaredNames takes no arguments and
returns a List of Strings. It is defined piecewise over the following
productions:
StatementList : StatementList StatementListItem
1. 1. 1. Let names1 be TopLevelVarDeclaredNames of StatementList.
2. 2. 2. Let names2 be TopLevelVarDeclaredNames of StatementListItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
StatementListItem : Declaration
1. 1. 1. If Declaration is Declaration : HoistableDeclaration , then
1. a. a. Return the BoundNames of HoistableDeclaration.
2. 2. 2. Return a new empty List.
StatementListItem : Statement
1. 1. 1. If Statement is Statement : LabelledStatement , return
TopLevelVarDeclaredNames of Statement.
2. 2. 2. Return VarDeclaredNames of Statement.
Note
At the top level of a function or script, inner function declarations are
treated like var declarations.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return the TopLevelVarDeclaredNames of LabelledItem.
LabelledItem : Statement
1. 1. 1. If Statement is Statement : LabelledStatement , return
TopLevelVarDeclaredNames of Statement.
2. 2. 2. Return VarDeclaredNames of Statement.
LabelledItem : FunctionDeclaration
1. 1. 1. Return BoundNames of FunctionDeclaration.
8.2.11 STATIC SEMANTICS: TOPLEVELVARSCOPEDDECLARATIONS
The syntax-directed operation TopLevelVarScopedDeclarations takes no arguments
and returns a List of Parse Nodes. It is defined piecewise over the following
productions:
StatementList : StatementList StatementListItem
1. 1. 1. Let declarations1 be TopLevelVarScopedDeclarations of StatementList.
2. 2. 2. Let declarations2 be TopLevelVarScopedDeclarations of
StatementListItem.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
StatementListItem : Statement
1. 1. 1. If Statement is Statement : LabelledStatement , return
TopLevelVarScopedDeclarations of Statement.
2. 2. 2. Return VarScopedDeclarations of Statement.
StatementListItem : Declaration
1. 1. 1. If Declaration is Declaration : HoistableDeclaration , then
1. a. a. Let declaration be DeclarationPart of HoistableDeclaration.
2. b. b. Return « declaration ».
2. 2. 2. Return a new empty List.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return the TopLevelVarScopedDeclarations of LabelledItem.
LabelledItem : Statement
1. 1. 1. If Statement is Statement : LabelledStatement , return
TopLevelVarScopedDeclarations of Statement.
2. 2. 2. Return VarScopedDeclarations of Statement.
LabelledItem : FunctionDeclaration
1. 1. 1. Return « FunctionDeclaration ».
8.3 LABELS
8.3.1 STATIC SEMANTICS: CONTAINSDUPLICATELABELS
The syntax-directed operation ContainsDuplicateLabels takes argument labelSet (a
List of Strings) and returns a Boolean. It is defined piecewise over the
following productions:
Statement : VariableStatement EmptyStatement ExpressionStatement
ContinueStatement BreakStatement ReturnStatement ThrowStatement
DebuggerStatement Block : { } StatementListItem : Declaration
1. 1. 1. Return false.
StatementList : StatementList StatementListItem
1. 1. 1. Let hasDuplicates be ContainsDuplicateLabels of StatementList with
argument labelSet.
2. 2. 2. If hasDuplicates is true, return true.
3. 3. 3. Return ContainsDuplicateLabels of StatementListItem with argument
labelSet.
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let hasDuplicate be ContainsDuplicateLabels of the first Statement
with argument labelSet.
2. 2. 2. If hasDuplicate is true, return true.
3. 3. 3. Return ContainsDuplicateLabels of the second Statement with argument
labelSet.
IfStatement : if ( Expression ) Statement
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
DoWhileStatement : do Statement while ( Expression ) ;
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
WhileStatement : while ( Expression ) Statement
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
var ForBinding in Expression ) Statement for ( ForDeclaration in Expression )
Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for (
var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of
AssignmentExpression ) Statement for await ( LeftHandSideExpression of
AssignmentExpression ) Statement for await ( var ForBinding of
AssignmentExpression ) Statement for await ( ForDeclaration of
AssignmentExpression ) Statement
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
Note
This section is extended by Annex B.3.5.
WithStatement : with ( Expression ) Statement
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Return ContainsDuplicateLabels of CaseBlock with argument labelSet.
CaseBlock : { }
1. 1. 1. Return false.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, then
1. a. a. If ContainsDuplicateLabels of the first CaseClauses with argument
labelSet is true, return true.
2. 2. 2. If ContainsDuplicateLabels of DefaultClause with argument labelSet is
true, return true.
3. 3. 3. If the second CaseClauses is not present, return false.
4. 4. 4. Return ContainsDuplicateLabels of the second CaseClauses with argument
labelSet.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let hasDuplicates be ContainsDuplicateLabels of CaseClauses with
argument labelSet.
2. 2. 2. If hasDuplicates is true, return true.
3. 3. 3. Return ContainsDuplicateLabels of CaseClause with argument labelSet.
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return ContainsDuplicateLabels of
StatementList with argument labelSet.
2. 2. 2. Return false.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return ContainsDuplicateLabels of
StatementList with argument labelSet.
2. 2. 2. Return false.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Let label be the StringValue of LabelIdentifier.
2. 2. 2. If labelSet contains label, return true.
3. 3. 3. Let newLabelSet be the list-concatenation of labelSet and « label ».
4. 4. 4. Return ContainsDuplicateLabels of LabelledItem with argument
newLabelSet.
LabelledItem : FunctionDeclaration
1. 1. 1. Return false.
TryStatement : try Block Catch
1. 1. 1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument
labelSet.
2. 2. 2. If hasDuplicates is true, return true.
3. 3. 3. Return ContainsDuplicateLabels of Catch with argument labelSet.
TryStatement : try Block Finally
1. 1. 1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument
labelSet.
2. 2. 2. If hasDuplicates is true, return true.
3. 3. 3. Return ContainsDuplicateLabels of Finally with argument labelSet.
TryStatement : try Block Catch Finally
1. 1. 1. If ContainsDuplicateLabels of Block with argument labelSet is true,
return true.
2. 2. 2. If ContainsDuplicateLabels of Catch with argument labelSet is true,
return true.
3. 3. 3. Return ContainsDuplicateLabels of Finally with argument labelSet.
Catch : catch ( CatchParameter ) Block
1. 1. 1. Return ContainsDuplicateLabels of Block with argument labelSet.
FunctionStatementList : [empty]
1. 1. 1. Return false.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return false.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let hasDuplicates be ContainsDuplicateLabels of ModuleItemList with
argument labelSet.
2. 2. 2. If hasDuplicates is true, return true.
3. 3. 3. Return ContainsDuplicateLabels of ModuleItem with argument labelSet.
ModuleItem : ImportDeclaration ExportDeclaration
1. 1. 1. Return false.
8.3.2 STATIC SEMANTICS: CONTAINSUNDEFINEDBREAKTARGET
The syntax-directed operation ContainsUndefinedBreakTarget takes argument
labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over
the following productions:
Statement : VariableStatement EmptyStatement ExpressionStatement
ContinueStatement ReturnStatement ThrowStatement DebuggerStatement Block : { }
StatementListItem : Declaration
1. 1. 1. Return false.
StatementList : StatementList StatementListItem
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of
StatementList with argument labelSet.
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of StatementListItem with argument
labelSet.
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first
Statement with argument labelSet.
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of the second Statement with
argument labelSet.
IfStatement : if ( Expression ) Statement
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
DoWhileStatement : do Statement while ( Expression ) ;
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
WhileStatement : while ( Expression ) Statement
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
var ForBinding in Expression ) Statement for ( ForDeclaration in Expression )
Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for (
var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of
AssignmentExpression ) Statement for await ( LeftHandSideExpression of
AssignmentExpression ) Statement for await ( var ForBinding of
AssignmentExpression ) Statement for await ( ForDeclaration of
AssignmentExpression ) Statement
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
Note
This section is extended by Annex B.3.5.
BreakStatement : break ;
1. 1. 1. Return false.
BreakStatement : break LabelIdentifier ;
1. 1. 1. If labelSet does not contain the StringValue of LabelIdentifier,
return true.
2. 2. 2. Return false.
WithStatement : with ( Expression ) Statement
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Return ContainsUndefinedBreakTarget of CaseBlock with argument
labelSet.
CaseBlock : { }
1. 1. 1. Return false.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, then
1. a. a. If ContainsUndefinedBreakTarget of the first CaseClauses with
argument labelSet is true, return true.
2. 2. 2. If ContainsUndefinedBreakTarget of DefaultClause with argument
labelSet is true, return true.
3. 3. 3. If the second CaseClauses is not present, return false.
4. 4. 4. Return ContainsUndefinedBreakTarget of the second CaseClauses with
argument labelSet.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of CaseClauses
with argument labelSet.
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of CaseClause with argument
labelSet.
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return ContainsUndefinedBreakTarget
of StatementList with argument labelSet.
2. 2. 2. Return false.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return ContainsUndefinedBreakTarget
of StatementList with argument labelSet.
2. 2. 2. Return false.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Let label be the StringValue of LabelIdentifier.
2. 2. 2. Let newLabelSet be the list-concatenation of labelSet and « label ».
3. 3. 3. Return ContainsUndefinedBreakTarget of LabelledItem with argument
newLabelSet.
LabelledItem : FunctionDeclaration
1. 1. 1. Return false.
TryStatement : try Block Catch
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with
argument labelSet.
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of Catch with argument labelSet.
TryStatement : try Block Finally
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with
argument labelSet.
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of Finally with argument labelSet.
TryStatement : try Block Catch Finally
1. 1. 1. If ContainsUndefinedBreakTarget of Block with argument labelSet is
true, return true.
2. 2. 2. If ContainsUndefinedBreakTarget of Catch with argument labelSet is
true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of Finally with argument labelSet.
Catch : catch ( CatchParameter ) Block
1. 1. 1. Return ContainsUndefinedBreakTarget of Block with argument labelSet.
FunctionStatementList : [empty]
1. 1. 1. Return false.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return false.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of
ModuleItemList with argument labelSet.
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedBreakTarget of ModuleItem with argument
labelSet.
ModuleItem : ImportDeclaration ExportDeclaration
1. 1. 1. Return false.
8.3.3 STATIC SEMANTICS: CONTAINSUNDEFINEDCONTINUETARGET
The syntax-directed operation ContainsUndefinedContinueTarget takes arguments
iterationSet (a List of Strings) and labelSet (a List of Strings) and returns a
Boolean. It is defined piecewise over the following productions:
Statement : VariableStatement EmptyStatement ExpressionStatement BreakStatement
ReturnStatement ThrowStatement DebuggerStatement Block : { } StatementListItem :
Declaration
1. 1. 1. Return false.
Statement : BlockStatement
1. 1. 1. Return ContainsUndefinedContinueTarget of BlockStatement with
arguments iterationSet and « ».
BreakableStatement : IterationStatement
1. 1. 1. Let newIterationSet be the list-concatenation of iterationSet and
labelSet.
2. 2. 2. Return ContainsUndefinedContinueTarget of IterationStatement with
arguments newIterationSet and « ».
StatementList : StatementList StatementListItem
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of
StatementList with arguments iterationSet and « ».
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of StatementListItem with
arguments iterationSet and « ».
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first
Statement with arguments iterationSet and « ».
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of the second Statement with
arguments iterationSet and « ».
IfStatement : if ( Expression ) Statement
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
DoWhileStatement : do Statement while ( Expression ) ;
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
WhileStatement : while ( Expression ) Statement
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
var ForBinding in Expression ) Statement for ( ForDeclaration in Expression )
Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for (
var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of
AssignmentExpression ) Statement for await ( LeftHandSideExpression of
AssignmentExpression ) Statement for await ( var ForBinding of
AssignmentExpression ) Statement for await ( ForDeclaration of
AssignmentExpression ) Statement
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
Note
This section is extended by Annex B.3.5.
ContinueStatement : continue ;
1. 1. 1. Return false.
ContinueStatement : continue LabelIdentifier ;
1. 1. 1. If iterationSet does not contain the StringValue of LabelIdentifier,
return true.
2. 2. 2. Return false.
WithStatement : with ( Expression ) Statement
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Return ContainsUndefinedContinueTarget of CaseBlock with arguments
iterationSet and « ».
CaseBlock : { }
1. 1. 1. Return false.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. If the first CaseClauses is present, then
1. a. a. If ContainsUndefinedContinueTarget of the first CaseClauses with
arguments iterationSet and « » is true, return true.
2. 2. 2. If ContainsUndefinedContinueTarget of DefaultClause with arguments
iterationSet and « » is true, return true.
3. 3. 3. If the second CaseClauses is not present, return false.
4. 4. 4. Return ContainsUndefinedContinueTarget of the second CaseClauses with
arguments iterationSet and « ».
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of
CaseClauses with arguments iterationSet and « ».
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of CaseClause with arguments
iterationSet and « ».
CaseClause : case Expression : StatementListopt
1. 1. 1. If the StatementList is present, return
ContainsUndefinedContinueTarget of StatementList with arguments iterationSet
and « ».
2. 2. 2. Return false.
DefaultClause : default : StatementListopt
1. 1. 1. If the StatementList is present, return
ContainsUndefinedContinueTarget of StatementList with arguments iterationSet
and « ».
2. 2. 2. Return false.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Let label be the StringValue of LabelIdentifier.
2. 2. 2. Let newLabelSet be the list-concatenation of labelSet and « label ».
3. 3. 3. Return ContainsUndefinedContinueTarget of LabelledItem with arguments
iterationSet and newLabelSet.
LabelledItem : FunctionDeclaration
1. 1. 1. Return false.
TryStatement : try Block Catch
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block
with arguments iterationSet and « ».
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of Catch with arguments
iterationSet and « ».
TryStatement : try Block Finally
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block
with arguments iterationSet and « ».
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of Finally with arguments
iterationSet and « ».
TryStatement : try Block Catch Finally
1. 1. 1. If ContainsUndefinedContinueTarget of Block with arguments
iterationSet and « » is true, return true.
2. 2. 2. If ContainsUndefinedContinueTarget of Catch with arguments
iterationSet and « » is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of Finally with arguments
iterationSet and « ».
Catch : catch ( CatchParameter ) Block
1. 1. 1. Return ContainsUndefinedContinueTarget of Block with arguments
iterationSet and « ».
FunctionStatementList : [empty]
1. 1. 1. Return false.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return false.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of
ModuleItemList with arguments iterationSet and « ».
2. 2. 2. If hasUndefinedLabels is true, return true.
3. 3. 3. Return ContainsUndefinedContinueTarget of ModuleItem with arguments
iterationSet and « ».
ModuleItem : ImportDeclaration ExportDeclaration
1. 1. 1. Return false.
8.4 FUNCTION NAME INFERENCE
8.4.1 STATIC SEMANTICS: HASNAME
The syntax-directed operation HasName takes no arguments and returns a Boolean.
It is defined piecewise over the following productions:
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let expr be the ParenthesizedExpression that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. If IsFunctionDefinition of expr is false, return false.
3. 3. 3. Return HasName of expr.
FunctionExpression : function ( FormalParameters ) { FunctionBody }
GeneratorExpression : function * ( FormalParameters ) { GeneratorBody }
AsyncGeneratorExpression : async function * ( FormalParameters ) {
AsyncGeneratorBody } AsyncFunctionExpression : async function ( FormalParameters
) { AsyncFunctionBody } ArrowFunction : ArrowParameters => ConciseBody
AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody ClassExpression : class
ClassTail
1. 1. 1. Return false.
FunctionExpression : function BindingIdentifier ( FormalParameters ) {
FunctionBody } GeneratorExpression : function * BindingIdentifier (
FormalParameters ) { GeneratorBody } AsyncGeneratorExpression : async function *
BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }
AsyncFunctionExpression : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody } ClassExpression : class BindingIdentifier ClassTail
1. 1. 1. Return true.
8.4.2 STATIC SEMANTICS: ISFUNCTIONDEFINITION
The syntax-directed operation IsFunctionDefinition takes no arguments and
returns a Boolean. It is defined piecewise over the following productions:
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let expr be the ParenthesizedExpression that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return IsFunctionDefinition of expr.
PrimaryExpression : this IdentifierReference Literal ArrayLiteral ObjectLiteral
RegularExpressionLiteral TemplateLiteral MemberExpression : MemberExpression [
Expression ] MemberExpression . IdentifierName MemberExpression TemplateLiteral
SuperProperty MetaProperty new MemberExpression Arguments MemberExpression .
PrivateIdentifier NewExpression : new NewExpression LeftHandSideExpression :
CallExpression OptionalExpression UpdateExpression : LeftHandSideExpression ++
LeftHandSideExpression -- ++ UnaryExpression -- UnaryExpression UnaryExpression
: delete UnaryExpression void UnaryExpression typeof UnaryExpression +
UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression
AwaitExpression ExponentiationExpression : UpdateExpression **
ExponentiationExpression MultiplicativeExpression : MultiplicativeExpression
MultiplicativeOperator ExponentiationExpression AdditiveExpression :
AdditiveExpression + MultiplicativeExpression AdditiveExpression -
MultiplicativeExpression ShiftExpression : ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression
RelationalExpression : RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression RelationalExpression instanceof
ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in
ShiftExpression EqualityExpression : EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression EqualityExpression ===
RelationalExpression EqualityExpression !== RelationalExpression
BitwiseANDExpression : BitwiseANDExpression & EqualityExpression
BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression
BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression
LogicalANDExpression : LogicalANDExpression && BitwiseORExpression
LogicalORExpression : LogicalORExpression || LogicalANDExpression
CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression
ConditionalExpression : ShortCircuitExpression ? AssignmentExpression :
AssignmentExpression AssignmentExpression : YieldExpression
LeftHandSideExpression = AssignmentExpression LeftHandSideExpression
AssignmentOperator AssignmentExpression LeftHandSideExpression &&=
AssignmentExpression LeftHandSideExpression ||= AssignmentExpression
LeftHandSideExpression ??= AssignmentExpression Expression : Expression ,
AssignmentExpression
1. 1. 1. Return false.
AssignmentExpression : ArrowFunction AsyncArrowFunction FunctionExpression :
function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) {
GeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt
( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionExpression : async
function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }
ClassExpression : class BindingIdentifieropt ClassTail
1. 1. 1. Return true.
8.4.3 STATIC SEMANTICS: ISANONYMOUSFUNCTIONDEFINITION ( EXPR )
The abstract operation IsAnonymousFunctionDefinition takes argument expr (an
AssignmentExpression Parse Node or an Initializer Parse Node) and returns a
Boolean. It determines if its argument is a function definition that does not
bind a name. It performs the following steps when called:
1. 1. 1. If IsFunctionDefinition of expr is false, return false.
2. 2. 2. Let hasName be HasName of expr.
3. 3. 3. If hasName is true, return false.
4. 4. 4. Return true.
8.4.4 STATIC SEMANTICS: ISIDENTIFIERREF
The syntax-directed operation IsIdentifierRef takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
PrimaryExpression : IdentifierReference
1. 1. 1. Return true.
PrimaryExpression : this Literal ArrayLiteral ObjectLiteral FunctionExpression
ClassExpression GeneratorExpression AsyncFunctionExpression
AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral
CoverParenthesizedExpressionAndArrowParameterList MemberExpression :
MemberExpression [ Expression ] MemberExpression . IdentifierName
MemberExpression TemplateLiteral SuperProperty MetaProperty new MemberExpression
Arguments MemberExpression . PrivateIdentifier NewExpression : new NewExpression
LeftHandSideExpression : CallExpression OptionalExpression
1. 1. 1. Return false.
8.4.5 RUNTIME SEMANTICS: NAMEDEVALUATION
The syntax-directed operation NamedEvaluation takes argument name (a property
key or a Private Name) and returns either a normal completion containing a
function object or an abrupt completion. It is defined piecewise over the
following productions:
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let expr be the ParenthesizedExpression that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return ? NamedEvaluation of expr with argument name.
ParenthesizedExpression : ( Expression )
1. 1. 1. Assert: IsAnonymousFunctionDefinition(Expression) is true.
2. 2. 2. Return ? NamedEvaluation of Expression with argument name.
FunctionExpression : function ( FormalParameters ) { FunctionBody }
1. 1. 1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression
with argument name.
GeneratorExpression : function * ( FormalParameters ) { GeneratorBody }
1. 1. 1. Return InstantiateGeneratorFunctionExpression of GeneratorExpression
with argument name.
AsyncGeneratorExpression : async function * ( FormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Return InstantiateAsyncGeneratorFunctionExpression of
AsyncGeneratorExpression with argument name.
AsyncFunctionExpression : async function ( FormalParameters ) {
AsyncFunctionBody }
1. 1. 1. Return InstantiateAsyncFunctionExpression of AsyncFunctionExpression
with argument name.
ArrowFunction : ArrowParameters => ConciseBody
1. 1. 1. Return InstantiateArrowFunctionExpression of ArrowFunction with
argument name.
AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
1. 1. 1. Return InstantiateAsyncArrowFunctionExpression of AsyncArrowFunction
with argument name.
ClassExpression : class ClassTail
1. 1. 1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments
undefined and name.
2. 2. 2. Set value.[[SourceText]] to the source text matched by
ClassExpression.
3. 3. 3. Return value.
8.5 CONTAINS
8.5.1 STATIC SEMANTICS: CONTAINS
The syntax-directed operation Contains takes argument symbol (a grammar symbol)
and returns a Boolean.
Every grammar production alternative in this specification which is not listed
below implicitly has the following default definition of Contains:
1. 1. 1. For each child node child of this Parse Node, do
1. a. a. If child is an instance of symbol, return true.
2. b. b. If child is an instance of a nonterminal, then
1. i. i. Let contained be the result of child Contains symbol.
2. ii. ii. If contained is true, return true.
2. 2. 2. Return false.
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression
: function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) {
GeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters )
{ AsyncGeneratorBody } AsyncGeneratorExpression : async function *
BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }
AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters
) { AsyncFunctionBody }
1. 1. 1. Return false.
Note 1
Static semantic rules that depend upon substructure generally do not look into
function definitions.
ClassTail : ClassHeritageopt { ClassBody }
1. 1. 1. If symbol is ClassBody, return true.
2. 2. 2. If symbol is ClassHeritage, then
1. a. a. If ClassHeritage is present, return true; otherwise return false.
3. 3. 3. If ClassHeritage is present, then
1. a. a. If ClassHeritage Contains symbol is true, return true.
4. 4. 4. Return the result of ComputedPropertyContains of ClassBody with
argument symbol.
Note 2
Static semantic rules that depend upon substructure generally do not look into
class bodies except for PropertyNames.
ClassStaticBlock : static { ClassStaticBlockBody }
1. 1. 1. Return false.
Note 3
Static semantic rules that depend upon substructure generally do not look into
static initialization blocks.
ArrowFunction : ArrowParameters => ConciseBody
1. 1. 1. If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or
this, return false.
2. 2. 2. If ArrowParameters Contains symbol is true, return true.
3. 3. 3. Return ConciseBody Contains symbol.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let formals be the ArrowFormalParameters that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return formals Contains symbol.
AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
1. 1. 1. If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or
this, return false.
2. 2. 2. Return AsyncConciseBody Contains symbol.
AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
1. 1. 1. If symbol is not one of NewTarget, SuperProperty, SuperCall, super, or
this, return false.
2. 2. 2. Let head be the AsyncArrowHead that is covered by
CoverCallExpressionAndAsyncArrowHead.
3. 3. 3. If head Contains symbol is true, return true.
4. 4. 4. Return AsyncConciseBody Contains symbol.
Note 4
Contains is used to detect new.target, this, and super usage within an
ArrowFunction or AsyncArrowFunction.
PropertyDefinition : MethodDefinition
1. 1. 1. If symbol is MethodDefinition, return true.
2. 2. 2. Return the result of ComputedPropertyContains of MethodDefinition with
argument symbol.
LiteralPropertyName : IdentifierName
1. 1. 1. Return false.
MemberExpression : MemberExpression . IdentifierName
1. 1. 1. If MemberExpression Contains symbol is true, return true.
2. 2. 2. Return false.
SuperProperty : super . IdentifierName
1. 1. 1. If symbol is the ReservedWord super, return true.
2. 2. 2. Return false.
CallExpression : CallExpression . IdentifierName
1. 1. 1. If CallExpression Contains symbol is true, return true.
2. 2. 2. Return false.
OptionalChain : ?. IdentifierName
1. 1. 1. Return false.
OptionalChain : OptionalChain . IdentifierName
1. 1. 1. If OptionalChain Contains symbol is true, return true.
2. 2. 2. Return false.
8.5.2 STATIC SEMANTICS: COMPUTEDPROPERTYCONTAINS
The syntax-directed operation ComputedPropertyContains takes argument symbol (a
grammar symbol) and returns a Boolean. It is defined piecewise over the
following productions:
ClassElementName : PrivateIdentifier PropertyName : LiteralPropertyName
1. 1. 1. Return false.
PropertyName : ComputedPropertyName
1. 1. 1. Return the result of ComputedPropertyName Contains symbol.
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
get ClassElementName ( ) { FunctionBody } set ClassElementName (
PropertySetParameterList ) { FunctionBody }
1. 1. 1. Return the result of ComputedPropertyContains of ClassElementName with
argument symbol.
GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody
}
1. 1. 1. Return the result of ComputedPropertyContains of ClassElementName with
argument symbol.
AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Return the result of ComputedPropertyContains of ClassElementName with
argument symbol.
ClassElementList : ClassElementList ClassElement
1. 1. 1. Let inList be ComputedPropertyContains of ClassElementList with
argument symbol.
2. 2. 2. If inList is true, return true.
3. 3. 3. Return the result of ComputedPropertyContains of ClassElement with
argument symbol.
ClassElement : ClassStaticBlock
1. 1. 1. Return false.
ClassElement : ;
1. 1. 1. Return false.
AsyncMethod : async ClassElementName ( UniqueFormalParameters ) {
AsyncFunctionBody }
1. 1. 1. Return the result of ComputedPropertyContains of ClassElementName with
argument symbol.
FieldDefinition : ClassElementName Initializeropt
1. 1. 1. Return the result of ComputedPropertyContains of ClassElementName with
argument symbol.
8.6 MISCELLANEOUS
These operations are used in multiple places throughout the specification.
8.6.1 RUNTIME SEMANTICS: INSTANTIATEFUNCTIONOBJECT
The syntax-directed operation InstantiateFunctionObject takes arguments env (an
Environment Record) and privateEnv (a PrivateEnvironment Record or null) and
returns a function object. It is defined piecewise over the following
productions:
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody } function ( FormalParameters ) { FunctionBody }
1. 1. 1. Return InstantiateOrdinaryFunctionObject of FunctionDeclaration with
arguments env and privateEnv.
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
1. 1. 1. Return InstantiateGeneratorFunctionObject of GeneratorDeclaration with
arguments env and privateEnv.
AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters )
{ AsyncGeneratorBody }
1. 1. 1. Return InstantiateAsyncGeneratorFunctionObject of
AsyncGeneratorDeclaration with arguments env and privateEnv.
AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
1. 1. 1. Return InstantiateAsyncFunctionObject of AsyncFunctionDeclaration with
arguments env and privateEnv.
8.6.2 RUNTIME SEMANTICS: BINDINGINITIALIZATION
The syntax-directed operation BindingInitialization takes arguments value (an
ECMAScript language value) and environment (an Environment Record or undefined)
and returns either a normal completion containing unused or an abrupt
completion.
Note
undefined is passed for environment to indicate that a PutValue operation should
be used to assign the initialization value. This is the case for var statements
and formal parameter lists of some non-strict functions (See 10.2.11). In those
cases a lexical binding is hoisted and preinitialized prior to evaluation of its
initializer.
It is defined piecewise over the following productions:
BindingIdentifier : Identifier
1. 1. 1. Let name be StringValue of Identifier.
2. 2. 2. Return ? InitializeBoundName(name, value, environment).
BindingIdentifier : yield
1. 1. 1. Return ? InitializeBoundName("yield", value, environment).
BindingIdentifier : await
1. 1. 1. Return ? InitializeBoundName("await", value, environment).
BindingPattern : ObjectBindingPattern
1. 1. 1. Perform ? RequireObjectCoercible(value).
2. 2. 2. Return ? BindingInitialization of ObjectBindingPattern with arguments
value and environment.
BindingPattern : ArrayBindingPattern
1. 1. 1. Let iteratorRecord be ? GetIterator(value, sync).
2. 2. 2. Let result be Completion(IteratorBindingInitialization of
ArrayBindingPattern with arguments iteratorRecord and environment).
3. 3. 3. If iteratorRecord.[[Done]] is false, return
? IteratorClose(iteratorRecord, result).
4. 4. 4. Return ? result.
ObjectBindingPattern : { }
1. 1. 1. Return unused.
ObjectBindingPattern : { BindingPropertyList } { BindingPropertyList , }
1. 1. 1. Perform ? PropertyBindingInitialization of BindingPropertyList with
arguments value and environment.
2. 2. 2. Return unused.
ObjectBindingPattern : { BindingRestProperty }
1. 1. 1. Let excludedNames be a new empty List.
2. 2. 2. Return ? RestBindingInitialization of BindingRestProperty with
arguments value, environment, and excludedNames.
ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
1. 1. 1. Let excludedNames be ? PropertyBindingInitialization of
BindingPropertyList with arguments value and environment.
2. 2. 2. Return ? RestBindingInitialization of BindingRestProperty with
arguments value, environment, and excludedNames.
8.6.2.1 INITIALIZEBOUNDNAME ( NAME, VALUE, ENVIRONMENT )
The abstract operation InitializeBoundName takes arguments name (a String),
value (an ECMAScript language value), and environment (an Environment Record or
undefined) and returns either a normal completion containing unused or an abrupt
completion. It performs the following steps when called:
1. 1. 1. If environment is not undefined, then
1. a. a. Perform ! environment.InitializeBinding(name, value).
2. b. b. Return unused.
2. 2. 2. Else,
1. a. a. Let lhs be ? ResolveBinding(name).
2. b. b. Return ? PutValue(lhs, value).
8.6.3 RUNTIME SEMANTICS: ITERATORBINDINGINITIALIZATION
The syntax-directed operation IteratorBindingInitialization takes arguments
iteratorRecord (an Iterator Record) and environment (an Environment Record or
undefined) and returns either a normal completion containing unused or an abrupt
completion.
Note
When undefined is passed for environment it indicates that a PutValue operation
should be used to assign the initialization value. This is the case for formal
parameter lists of non-strict functions. In that case the formal parameter
bindings are preinitialized in order to deal with the possibility of multiple
parameters with the same name.
It is defined piecewise over the following productions:
ArrayBindingPattern : [ ]
1. 1. 1. Return unused.
ArrayBindingPattern : [ Elision ]
1. 1. 1. Return ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
ArrayBindingPattern : [ Elisionopt BindingRestElement ]
1. 1. 1. If Elision is present, then
1. a. a. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
2. 2. 2. Return ? IteratorBindingInitialization of BindingRestElement with
arguments iteratorRecord and environment.
ArrayBindingPattern : [ BindingElementList , Elision ]
1. 1. 1. Perform ? IteratorBindingInitialization of BindingElementList with
arguments iteratorRecord and environment.
2. 2. 2. Return ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
1. 1. 1. Perform ? IteratorBindingInitialization of BindingElementList with
arguments iteratorRecord and environment.
2. 2. 2. If Elision is present, then
1. a. a. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
3. 3. 3. Return ? IteratorBindingInitialization of BindingRestElement with
arguments iteratorRecord and environment.
BindingElementList : BindingElementList , BindingElisionElement
1. 1. 1. Perform ? IteratorBindingInitialization of BindingElementList with
arguments iteratorRecord and environment.
2. 2. 2. Return ? IteratorBindingInitialization of BindingElisionElement with
arguments iteratorRecord and environment.
BindingElisionElement : Elision BindingElement
1. 1. 1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
2. 2. 2. Return ? IteratorBindingInitialization of BindingElement with
arguments iteratorRecord and environment.
SingleNameBinding : BindingIdentifier Initializeropt
1. 1. 1. Let bindingId be StringValue of BindingIdentifier.
2. 2. 2. Let lhs be ? ResolveBinding(bindingId, environment).
3. 3. 3. Let v be undefined.
4. 4. 4. If iteratorRecord.[[Done]] is false, then
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, set iteratorRecord.[[Done]] to true.
5. e. e. Else,
1. i. i. Set v to Completion(IteratorValue(next)).
2. ii. ii. If v is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. iii. iii. ReturnIfAbrupt(v).
5. 5. 5. If Initializer is present and v is undefined, then
1. a. a. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. i. i. Set v to ? NamedEvaluation of Initializer with argument
bindingId.
2. b. b. Else,
1. i. i. Let defaultValue be ? Evaluation of Initializer.
2. ii. ii. Set v to ? GetValue(defaultValue).
6. 6. 6. If environment is undefined, return ? PutValue(lhs, v).
7. 7. 7. Return ? InitializeReferencedBinding(lhs, v).
BindingElement : BindingPattern Initializeropt
1. 1. 1. Let v be undefined.
2. 2. 2. If iteratorRecord.[[Done]] is false, then
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, set iteratorRecord.[[Done]] to true.
5. e. e. Else,
1. i. i. Set v to Completion(IteratorValue(next)).
2. ii. ii. If v is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. iii. iii. ReturnIfAbrupt(v).
3. 3. 3. If Initializer is present and v is undefined, then
1. a. a. Let defaultValue be ? Evaluation of Initializer.
2. b. b. Set v to ? GetValue(defaultValue).
4. 4. 4. Return ? BindingInitialization of BindingPattern with arguments v and
environment.
BindingRestElement : ... BindingIdentifier
1. 1. 1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier,
environment).
2. 2. 2. Let A be ! ArrayCreate(0).
3. 3. 3. Let n be 0.
4. 4. 4. Repeat,
1. a. a. If iteratorRecord.[[Done]] is false, then
1. i. i. Let next be Completion(IteratorStep(iteratorRecord)).
2. ii. ii. If next is an abrupt completion, set iteratorRecord.[[Done]]
to true.
3. iii. iii. ReturnIfAbrupt(next).
4. iv. iv. If next is false, set iteratorRecord.[[Done]] to true.
2. b. b. If iteratorRecord.[[Done]] is true, then
1. i. i. If environment is undefined, return ? PutValue(lhs, A).
2. ii. ii. Return ? InitializeReferencedBinding(lhs, A).
3. c. c. Let nextValue be Completion(IteratorValue(next)).
4. d. d. If nextValue is an abrupt completion, set iteratorRecord.[[Done]]
to true.
5. e. e. ReturnIfAbrupt(nextValue).
6. f. f. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
nextValue).
7. g. g. Set n to n + 1.
BindingRestElement : ... BindingPattern
1. 1. 1. Let A be ! ArrayCreate(0).
2. 2. 2. Let n be 0.
3. 3. 3. Repeat,
1. a. a. If iteratorRecord.[[Done]] is false, then
1. i. i. Let next be Completion(IteratorStep(iteratorRecord)).
2. ii. ii. If next is an abrupt completion, set iteratorRecord.[[Done]]
to true.
3. iii. iii. ReturnIfAbrupt(next).
4. iv. iv. If next is false, set iteratorRecord.[[Done]] to true.
2. b. b. If iteratorRecord.[[Done]] is true, then
1. i. i. Return ? BindingInitialization of BindingPattern with arguments
A and environment.
3. c. c. Let nextValue be Completion(IteratorValue(next)).
4. d. d. If nextValue is an abrupt completion, set iteratorRecord.[[Done]]
to true.
5. e. e. ReturnIfAbrupt(nextValue).
6. f. f. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
nextValue).
7. g. g. Set n to n + 1.
FormalParameters : [empty]
1. 1. 1. Return unused.
FormalParameters : FormalParameterList , FunctionRestParameter
1. 1. 1. Perform ? IteratorBindingInitialization of FormalParameterList with
arguments iteratorRecord and environment.
2. 2. 2. Return ? IteratorBindingInitialization of FunctionRestParameter with
arguments iteratorRecord and environment.
FormalParameterList : FormalParameterList , FormalParameter
1. 1. 1. Perform ? IteratorBindingInitialization of FormalParameterList with
arguments iteratorRecord and environment.
2. 2. 2. Return ? IteratorBindingInitialization of FormalParameter with
arguments iteratorRecord and environment.
ArrowParameters : BindingIdentifier
1. 1. 1. Let v be undefined.
2. 2. 2. Assert: iteratorRecord.[[Done]] is false.
3. 3. 3. Let next be Completion(IteratorStep(iteratorRecord)).
4. 4. 4. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
5. 5. 5. ReturnIfAbrupt(next).
6. 6. 6. If next is false, set iteratorRecord.[[Done]] to true.
7. 7. 7. Else,
1. a. a. Set v to Completion(IteratorValue(next)).
2. b. b. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. c. c. ReturnIfAbrupt(v).
8. 8. 8. Return ? BindingInitialization of BindingIdentifier with arguments v
and environment.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let formals be the ArrowFormalParameters that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return ? IteratorBindingInitialization of formals with arguments
iteratorRecord and environment.
AsyncArrowBindingIdentifier : BindingIdentifier
1. 1. 1. Let v be undefined.
2. 2. 2. Assert: iteratorRecord.[[Done]] is false.
3. 3. 3. Let next be Completion(IteratorStep(iteratorRecord)).
4. 4. 4. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
5. 5. 5. ReturnIfAbrupt(next).
6. 6. 6. If next is false, set iteratorRecord.[[Done]] to true.
7. 7. 7. Else,
1. a. a. Set v to Completion(IteratorValue(next)).
2. b. b. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
3. c. c. ReturnIfAbrupt(v).
8. 8. 8. Return ? BindingInitialization of BindingIdentifier with arguments v
and environment.
8.6.4 STATIC SEMANTICS: ASSIGNMENTTARGETTYPE
The syntax-directed operation AssignmentTargetType takes no arguments and
returns simple or invalid. It is defined piecewise over the following
productions:
IdentifierReference : Identifier
1. 1. 1. If this IdentifierReference is contained in strict mode code and
StringValue of Identifier is either "eval" or "arguments", return invalid.
2. 2. 2. Return simple.
IdentifierReference : yield await CallExpression : CallExpression [ Expression ]
CallExpression . IdentifierName CallExpression . PrivateIdentifier
MemberExpression : MemberExpression [ Expression ] MemberExpression .
IdentifierName SuperProperty MemberExpression . PrivateIdentifier
1. 1. 1. Return simple.
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let expr be the ParenthesizedExpression that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return AssignmentTargetType of expr.
PrimaryExpression : this Literal ArrayLiteral ObjectLiteral FunctionExpression
ClassExpression GeneratorExpression AsyncFunctionExpression
AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral CallExpression
: CoverCallExpressionAndAsyncArrowHead SuperCall ImportCall CallExpression
Arguments CallExpression TemplateLiteral NewExpression : new NewExpression
MemberExpression : MemberExpression TemplateLiteral new MemberExpression
Arguments NewTarget : new . target ImportMeta : import . meta
LeftHandSideExpression : OptionalExpression UpdateExpression :
LeftHandSideExpression ++ LeftHandSideExpression -- ++ UnaryExpression --
UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression
typeof UnaryExpression + UnaryExpression - UnaryExpression ~ UnaryExpression !
UnaryExpression AwaitExpression ExponentiationExpression : UpdateExpression **
ExponentiationExpression MultiplicativeExpression : MultiplicativeExpression
MultiplicativeOperator ExponentiationExpression AdditiveExpression :
AdditiveExpression + MultiplicativeExpression AdditiveExpression -
MultiplicativeExpression ShiftExpression : ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression
RelationalExpression : RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression RelationalExpression instanceof
ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in
ShiftExpression EqualityExpression : EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression EqualityExpression ===
RelationalExpression EqualityExpression !== RelationalExpression
BitwiseANDExpression : BitwiseANDExpression & EqualityExpression
BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression
BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression
LogicalANDExpression : LogicalANDExpression && BitwiseORExpression
LogicalORExpression : LogicalORExpression || LogicalANDExpression
CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression
ConditionalExpression : ShortCircuitExpression ? AssignmentExpression :
AssignmentExpression AssignmentExpression : YieldExpression ArrowFunction
AsyncArrowFunction LeftHandSideExpression = AssignmentExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression
LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||=
AssignmentExpression LeftHandSideExpression ??= AssignmentExpression Expression
: Expression , AssignmentExpression
1. 1. 1. Return invalid.
8.6.5 STATIC SEMANTICS: PROPNAME
The syntax-directed operation PropName takes no arguments and returns a String
or empty. It is defined piecewise over the following productions:
PropertyDefinition : IdentifierReference
1. 1. 1. Return StringValue of IdentifierReference.
PropertyDefinition : ... AssignmentExpression
1. 1. 1. Return empty.
PropertyDefinition : PropertyName : AssignmentExpression
1. 1. 1. Return PropName of PropertyName.
LiteralPropertyName : IdentifierName
1. 1. 1. Return StringValue of IdentifierName.
LiteralPropertyName : StringLiteral
1. 1. 1. Return the SV of StringLiteral.
LiteralPropertyName : NumericLiteral
1. 1. 1. Let nbr be the NumericValue of NumericLiteral.
2. 2. 2. Return ! ToString(nbr).
ComputedPropertyName : [ AssignmentExpression ]
1. 1. 1. Return empty.
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
get ClassElementName ( ) { FunctionBody } set ClassElementName (
PropertySetParameterList ) { FunctionBody }
1. 1. 1. Return PropName of ClassElementName.
GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody
}
1. 1. 1. Return PropName of ClassElementName.
AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Return PropName of ClassElementName.
ClassElement : ClassStaticBlock
1. 1. 1. Return empty.
ClassElement : ;
1. 1. 1. Return empty.
AsyncMethod : async ClassElementName ( UniqueFormalParameters ) {
AsyncFunctionBody }
1. 1. 1. Return PropName of ClassElementName.
FieldDefinition : ClassElementName Initializeropt
1. 1. 1. Return PropName of ClassElementName.
ClassElementName : PrivateIdentifier
1. 1. 1. Return empty.
9 EXECUTABLE CODE AND EXECUTION CONTEXTS
9.1 ENVIRONMENT RECORDS
Environment Record is a specification type used to define the association of
Identifiers to specific variables and functions, based upon the lexical nesting
structure of ECMAScript code. Usually an Environment Record is associated with
some specific syntactic structure of ECMAScript code such as a
FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each
time such code is evaluated, a new Environment Record is created to record the
identifier bindings that are created by that code.
Every Environment Record has an [[OuterEnv]] field, which is either null or a
reference to an outer Environment Record. This is used to model the logical
nesting of Environment Record values. The outer reference of an (inner)
Environment Record is a reference to the Environment Record that logically
surrounds the inner Environment Record. An outer Environment Record may, of
course, have its own outer Environment Record. An Environment Record may serve
as the outer environment for multiple inner Environment Records. For example, if
a FunctionDeclaration contains two nested FunctionDeclarations then the
Environment Records of each of the nested functions will have as their outer
Environment Record the Environment Record of the current evaluation of the
surrounding function.
Environment Records are purely specification mechanisms and need not correspond
to any specific artefact of an ECMAScript implementation. It is impossible for
an ECMAScript program to directly access or manipulate such values.
9.1.1 THE ENVIRONMENT RECORD TYPE HIERARCHY
Environment Records can be thought of as existing in a simple object-oriented
hierarchy where Environment Record is an abstract class with three concrete
subclasses: Declarative Environment Record, Object Environment Record, and
Global Environment Record. Function Environment Records and Module Environment
Records are subclasses of Declarative Environment Record.
* Environment Record (abstract)
* A Declarative Environment Record is used to define the effect of ECMAScript
language syntactic elements such as FunctionDeclarations,
VariableDeclarations, and Catch clauses that directly associate identifier
bindings with ECMAScript language values.
* A Function Environment Record corresponds to the invocation of an
ECMAScript function object, and contains bindings for the top-level
declarations within that function. It may establish a new this binding.
It also captures the state necessary to support super method invocations.
* A Module Environment Record contains the bindings for the top-level
declarations of a Module. It also contains the bindings that are
explicitly imported by the Module. Its [[OuterEnv]] is a Global
Environment Record.
* An Object Environment Record is used to define the effect of ECMAScript
elements such as WithStatement that associate identifier bindings with the
properties of some object.
* A Global Environment Record is used for Script global declarations. It does
not have an outer environment; its [[OuterEnv]] is null. It may be
prepopulated with identifier bindings and it includes an associated global
object whose properties provide some of the global environment's identifier
bindings. As ECMAScript code is executed, additional properties may be
added to the global object and the initial properties may be modified.
The Environment Record abstract class includes the abstract specification
methods defined in Table 16. These abstract methods have distinct concrete
algorithms for each of the concrete subclasses.
Table 16: Abstract Methods of Environment Records
Method Purpose HasBinding(N) Determine if an Environment Record has a binding
for the String value N. Return true if it does and false if it does not.
CreateMutableBinding(N, D) Create a new but uninitialized mutable binding in an
Environment Record. The String value N is the text of the bound name. If the
Boolean argument D is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S) Create a new but uninitialized immutable binding in
an Environment Record. The String value N is the text of the bound name. If S is
true then attempts to set it after it has been initialized will always throw an
exception, regardless of the strict mode setting of operations that reference
that binding. InitializeBinding(N, V) Set the value of an already existing but
uninitialized binding in an Environment Record. The String value N is the text
of the bound name. V is the value for the binding and is a value of any
ECMAScript language type. SetMutableBinding(N, V, S) Set the value of an already
existing mutable binding in an Environment Record. The String value N is the
text of the bound name. V is the value for the binding and may be a value of any
ECMAScript language type. S is a Boolean flag. If S is true and the binding
cannot be set throw a TypeError exception. GetBindingValue(N, S) Returns the
value of an already existing binding from an Environment Record. The String
value N is the text of the bound name. S is used to identify references
originating in strict mode code or that otherwise require strict mode reference
semantics. If S is true and the binding does not exist throw a ReferenceError
exception. If the binding exists but is uninitialized a ReferenceError is
thrown, regardless of the value of S. DeleteBinding(N) Delete a binding from an
Environment Record. The String value N is the text of the bound name. If a
binding for N exists, remove the binding and return true. If the binding exists
but cannot be removed return false. If the binding does not exist return true.
HasThisBinding() Determine if an Environment Record establishes a this binding.
Return true if it does and false if it does not. HasSuperBinding() Determine if
an Environment Record establishes a super method binding. Return true if it does
and false if it does not. WithBaseObject() If this Environment Record is
associated with a with statement, return the with object. Otherwise, return
undefined.
9.1.1.1 DECLARATIVE ENVIRONMENT RECORDS
Each Declarative Environment Record is associated with an ECMAScript program
scope containing variable, constant, let, class, module, import, and/or function
declarations. A Declarative Environment Record binds the set of identifiers
defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for Declarative Environment
Records is defined by the following algorithms.
9.1.1.1.1 HASBINDING ( N )
The HasBinding concrete method of a Declarative Environment Record envRec takes
argument N (a String) and returns a normal completion containing a Boolean. It
determines if the argument identifier is one of the identifiers bound by the
record. It performs the following steps when called:
1. 1. 1. If envRec has a binding for N, return true.
2. 2. 2. Return false.
9.1.1.1.2 CREATEMUTABLEBINDING ( N, D )
The CreateMutableBinding concrete method of a Declarative Environment Record
envRec takes arguments N (a String) and D (a Boolean) and returns a normal
completion containing unused. It creates a new mutable binding for the name N
that is uninitialized. A binding must not already exist in this Environment
Record for N. If D is true, the new binding is marked as being subject to
deletion. It performs the following steps when called:
1. 1. 1. Assert: envRec does not already have a binding for N.
2. 2. 2. Create a mutable binding in envRec for N and record that it is
uninitialized. If D is true, record that the newly created binding may be
deleted by a subsequent DeleteBinding call.
3. 3. 3. Return unused.
9.1.1.1.3 CREATEIMMUTABLEBINDING ( N, S )
The CreateImmutableBinding concrete method of a Declarative Environment Record
envRec takes arguments N (a String) and S (a Boolean) and returns a normal
completion containing unused. It creates a new immutable binding for the name N
that is uninitialized. A binding must not already exist in this Environment
Record for N. If S is true, the new binding is marked as a strict binding. It
performs the following steps when called:
1. 1. 1. Assert: envRec does not already have a binding for N.
2. 2. 2. Create an immutable binding in envRec for N and record that it is
uninitialized. If S is true, record that the newly created binding is a
strict binding.
3. 3. 3. Return unused.
9.1.1.1.4 INITIALIZEBINDING ( N, V )
The InitializeBinding concrete method of a Declarative Environment Record envRec
takes arguments N (a String) and V (an ECMAScript language value) and returns a
normal completion containing unused. It is used to set the bound value of the
current binding of the identifier whose name is N to the value V. An
uninitialized binding for N must already exist. It performs the following steps
when called:
1. 1. 1. Assert: envRec must have an uninitialized binding for N.
2. 2. 2. Set the bound value for N in envRec to V.
3. 3. 3. Record that the binding for N in envRec has been initialized.
4. 4. 4. Return unused.
9.1.1.1.5 SETMUTABLEBINDING ( N, V, S )
The SetMutableBinding concrete method of a Declarative Environment Record envRec
takes arguments N (a String), V (an ECMAScript language value), and S (a
Boolean) and returns either a normal completion containing unused or a throw
completion. It attempts to change the bound value of the current binding of the
identifier whose name is N to the value V. A binding for N normally already
exists, but in rare cases it may not. If the binding is an immutable binding, a
TypeError is thrown if S is true. It performs the following steps when called:
1. 1. 1. If envRec does not have a binding for N, then
1. a. a. If S is true, throw a ReferenceError exception.
2. b. b. Perform ! envRec.CreateMutableBinding(N, true).
3. c. c. Perform ! envRec.InitializeBinding(N, V).
4. d. d. Return unused.
2. 2. 2. If the binding for N in envRec is a strict binding, set S to true.
3. 3. 3. If the binding for N in envRec has not yet been initialized, throw a
ReferenceError exception.
4. 4. 4. Else if the binding for N in envRec is a mutable binding, change its
bound value to V.
5. 5. 5. Else,
1. a. a. Assert: This is an attempt to change the value of an immutable
binding.
2. b. b. If S is true, throw a TypeError exception.
6. 6. 6. Return unused.
Note
An example of ECMAScript code that results in a missing binding at step 1 is:
function f() { eval("var x; x = (delete x, 0);"); }
9.1.1.1.6 GETBINDINGVALUE ( N, S )
The GetBindingValue concrete method of a Declarative Environment Record envRec
takes arguments N (a String) and S (a Boolean) and returns either a normal
completion containing an ECMAScript language value or a throw completion. It
returns the value of its bound identifier whose name is N. If the binding exists
but is uninitialized a ReferenceError is thrown, regardless of the value of S.
It performs the following steps when called:
1. 1. 1. Assert: envRec has a binding for N.
2. 2. 2. If the binding for N in envRec is an uninitialized binding, throw a
ReferenceError exception.
3. 3. 3. Return the value currently bound to N in envRec.
9.1.1.1.7 DELETEBINDING ( N )
The DeleteBinding concrete method of a Declarative Environment Record envRec
takes argument N (a String) and returns a normal completion containing a
Boolean. It can only delete bindings that have been explicitly designated as
being subject to deletion. It performs the following steps when called:
1. 1. 1. Assert: envRec has a binding for N.
2. 2. 2. If the binding for N in envRec cannot be deleted, return false.
3. 3. 3. Remove the binding for N from envRec.
4. 4. 4. Return true.
9.1.1.1.8 HASTHISBINDING ( )
The HasThisBinding concrete method of a Declarative Environment Record envRec
takes no arguments and returns false. It performs the following steps when
called:
1. 1. 1. Return false.
Note
A regular Declarative Environment Record (i.e., one that is neither a Function
Environment Record nor a Module Environment Record) does not provide a this
binding.
9.1.1.1.9 HASSUPERBINDING ( )
The HasSuperBinding concrete method of a Declarative Environment Record envRec
takes no arguments and returns false. It performs the following steps when
called:
1. 1. 1. Return false.
Note
A regular Declarative Environment Record (i.e., one that is neither a Function
Environment Record nor a Module Environment Record) does not provide a super
binding.
9.1.1.1.10 WITHBASEOBJECT ( )
The WithBaseObject concrete method of a Declarative Environment Record envRec
takes no arguments and returns undefined. It performs the following steps when
called:
1. 1. 1. Return undefined.
9.1.1.2 OBJECT ENVIRONMENT RECORDS
Each Object Environment Record is associated with an object called its binding
object. An Object Environment Record binds the set of string identifier names
that directly correspond to the property names of its binding object. Property
keys that are not strings in the form of an IdentifierName are not included in
the set of bound identifiers. Both own and inherited properties are included in
the set regardless of the setting of their [[Enumerable]] attribute. Because
properties can be dynamically added and deleted from objects, the set of
identifiers bound by an Object Environment Record may potentially change as a
side-effect of any operation that adds or deletes properties. Any bindings that
are created as a result of such a side-effect are considered to be a mutable
binding even if the Writable attribute of the corresponding property is false.
Immutable bindings do not exist for Object Environment Records.
Object Environment Records created for with statements (14.11) can provide their
binding object as an implicit this value for use in function calls. The
capability is controlled by a Boolean [[IsWithEnvironment]] field.
Object Environment Records have the additional state fields listed in Table 17.
Table 17: Additional Fields of Object Environment Records
Field Name Value Meaning [[BindingObject]] an Object The binding object of this
Environment Record. [[IsWithEnvironment]] a Boolean Indicates whether this
Environment Record is created for a with statement.
The behaviour of the concrete specification methods for Object Environment
Records is defined by the following algorithms.
9.1.1.2.1 HASBINDING ( N )
The HasBinding concrete method of an Object Environment Record envRec takes
argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if its associated binding object
has a property whose name is N. It performs the following steps when called:
1. 1. 1. Let bindingObject be envRec.[[BindingObject]].
2. 2. 2. Let foundBinding be ? HasProperty(bindingObject, N).
3. 3. 3. If foundBinding is false, return false.
4. 4. 4. If envRec.[[IsWithEnvironment]] is false, return true.
5. 5. 5. Let unscopables be ? Get(bindingObject, @@unscopables).
6. 6. 6. If unscopables is an Object, then
1. a. a. Let blocked be ToBoolean(? Get(unscopables, N)).
2. b. b. If blocked is true, return false.
7. 7. 7. Return true.
9.1.1.2.2 CREATEMUTABLEBINDING ( N, D )
The CreateMutableBinding concrete method of an Object Environment Record envRec
takes arguments N (a String) and D (a Boolean) and returns either a normal
completion containing unused or a throw completion. It creates in an Environment
Record's associated binding object a property whose name is N and initializes it
to the value undefined. If D is true, the new property's [[Configurable]]
attribute is set to true; otherwise it is set to false. It performs the
following steps when called:
1. 1. 1. Let bindingObject be envRec.[[BindingObject]].
2. 2. 2. Perform ? DefinePropertyOrThrow(bindingObject, N, PropertyDescriptor {
[[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true,
[[Configurable]]: D }).
3. 3. 3. Return unused.
Note
Normally envRec will not have a binding for N but if it does, the semantics of
DefinePropertyOrThrow may result in an existing binding being replaced or
shadowed or cause an abrupt completion to be returned.
9.1.1.2.3 CREATEIMMUTABLEBINDING ( N, S )
The CreateImmutableBinding concrete method of an Object Environment Record is
never used within this specification.
9.1.1.2.4 INITIALIZEBINDING ( N, V )
The InitializeBinding concrete method of an Object Environment Record envRec
takes arguments N (a String) and V (an ECMAScript language value) and returns
either a normal completion containing unused or a throw completion. It is used
to set the bound value of the current binding of the identifier whose name is N
to the value V. It performs the following steps when called:
1. 1. 1. Perform ? envRec.SetMutableBinding(N, V, false).
2. 2. 2. Return unused.
Note
In this specification, all uses of CreateMutableBinding for Object Environment
Records are immediately followed by a call to InitializeBinding for the same
name. Hence, this specification does not explicitly track the initialization
state of bindings in Object Environment Records.
9.1.1.2.5 SETMUTABLEBINDING ( N, V, S )
The SetMutableBinding concrete method of an Object Environment Record envRec
takes arguments N (a String), V (an ECMAScript language value), and S (a
Boolean) and returns either a normal completion containing unused or a throw
completion. It attempts to set the value of the Environment Record's associated
binding object's property whose name is N to the value V. A property named N
normally already exists but if it does not or is not currently writable, error
handling is determined by S. It performs the following steps when called:
1. 1. 1. Let bindingObject be envRec.[[BindingObject]].
2. 2. 2. Let stillExists be ? HasProperty(bindingObject, N).
3. 3. 3. If stillExists is false and S is true, throw a ReferenceError
exception.
4. 4. 4. Perform ? Set(bindingObject, N, V, S).
5. 5. 5. Return unused.
9.1.1.2.6 GETBINDINGVALUE ( N, S )
The GetBindingValue concrete method of an Object Environment Record envRec takes
arguments N (a String) and S (a Boolean) and returns either a normal completion
containing an ECMAScript language value or a throw completion. It returns the
value of its associated binding object's property whose name is N. The property
should already exist but if it does not the result depends upon S. It performs
the following steps when called:
1. 1. 1. Let bindingObject be envRec.[[BindingObject]].
2. 2. 2. Let value be ? HasProperty(bindingObject, N).
3. 3. 3. If value is false, then
1. a. a. If S is false, return undefined; otherwise throw a ReferenceError
exception.
4. 4. 4. Return ? Get(bindingObject, N).
9.1.1.2.7 DELETEBINDING ( N )
The DeleteBinding concrete method of an Object Environment Record envRec takes
argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It can only delete bindings that correspond to
properties of the environment object whose [[Configurable]] attribute have the
value true. It performs the following steps when called:
1. 1. 1. Let bindingObject be envRec.[[BindingObject]].
2. 2. 2. Return ? bindingObject.[[Delete]](N).
9.1.1.2.8 HASTHISBINDING ( )
The HasThisBinding concrete method of an Object Environment Record envRec takes
no arguments and returns false. It performs the following steps when called:
1. 1. 1. Return false.
Note
Object Environment Records do not provide a this binding.
9.1.1.2.9 HASSUPERBINDING ( )
The HasSuperBinding concrete method of an Object Environment Record envRec takes
no arguments and returns false. It performs the following steps when called:
1. 1. 1. Return false.
Note
Object Environment Records do not provide a super binding.
9.1.1.2.10 WITHBASEOBJECT ( )
The WithBaseObject concrete method of an Object Environment Record envRec takes
no arguments and returns an Object or undefined. It performs the following steps
when called:
1. 1. 1. If envRec.[[IsWithEnvironment]] is true, return
envRec.[[BindingObject]].
2. 2. 2. Otherwise, return undefined.
9.1.1.3 FUNCTION ENVIRONMENT RECORDS
A Function Environment Record is a Declarative Environment Record that is used
to represent the top-level scope of a function and, if the function is not an
ArrowFunction, provides a this binding. If a function is not an ArrowFunction
function and references super, its Function Environment Record also contains the
state that is used to perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in Table
18.
Table 18: Additional Fields of Function Environment Records
Field Name Value Meaning [[ThisValue]] an ECMAScript language value This is the
this value used for this invocation of the function. [[ThisBindingStatus]]
lexical, initialized, or uninitialized If the value is lexical, this is an
ArrowFunction and does not have a local this value. [[FunctionObject]] an
ECMAScript function object The function object whose invocation caused this
Environment Record to be created. [[NewTarget]] an Object or undefined If this
Environment Record was created by the [[Construct]] internal method,
[[NewTarget]] is the value of the [[Construct]] newTarget parameter. Otherwise,
its value is undefined.
Function Environment Records support all of the Declarative Environment Record
methods listed in Table 16 and share the same specifications for all of those
methods except for HasThisBinding and HasSuperBinding. In addition, Function
Environment Records support the methods listed in Table 19:
Table 19: Additional Methods of Function Environment Records
Method Purpose BindThisValue(V) Set the [[ThisValue]] and record that it has
been initialized. GetThisBinding() Return the value of this Environment Record's
this binding. Throws a ReferenceError if the this binding has not been
initialized. GetSuperBase() Return the object that is the base for super
property accesses bound in this Environment Record. The value undefined
indicates that such accesses will produce runtime errors.
The behaviour of the additional concrete specification methods for Function
Environment Records is defined by the following algorithms:
9.1.1.3.1 BINDTHISVALUE ( V )
The BindThisValue concrete method of a Function Environment Record envRec takes
argument V (an ECMAScript language value) and returns either a normal completion
containing an ECMAScript language value or a throw completion. It performs the
following steps when called:
1. 1. 1. Assert: envRec.[[ThisBindingStatus]] is not lexical.
2. 2. 2. If envRec.[[ThisBindingStatus]] is initialized, throw a ReferenceError
exception.
3. 3. 3. Set envRec.[[ThisValue]] to V.
4. 4. 4. Set envRec.[[ThisBindingStatus]] to initialized.
5. 5. 5. Return V.
9.1.1.3.2 HASTHISBINDING ( )
The HasThisBinding concrete method of a Function Environment Record envRec takes
no arguments and returns a Boolean. It performs the following steps when called:
1. 1. 1. If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise,
return true.
9.1.1.3.3 HASSUPERBINDING ( )
The HasSuperBinding concrete method of a Function Environment Record envRec
takes no arguments and returns a Boolean. It performs the following steps when
called:
1. 1. 1. If envRec.[[ThisBindingStatus]] is lexical, return false.
2. 2. 2. If envRec.[[FunctionObject]].[[HomeObject]] is undefined, return
false; otherwise, return true.
9.1.1.3.4 GETTHISBINDING ( )
The GetThisBinding concrete method of a Function Environment Record envRec takes
no arguments and returns either a normal completion containing an ECMAScript
language value or a throw completion. It performs the following steps when
called:
1. 1. 1. Assert: envRec.[[ThisBindingStatus]] is not lexical.
2. 2. 2. If envRec.[[ThisBindingStatus]] is uninitialized, throw a
ReferenceError exception.
3. 3. 3. Return envRec.[[ThisValue]].
9.1.1.3.5 GETSUPERBASE ( )
The GetSuperBase concrete method of a Function Environment Record envRec takes
no arguments and returns either a normal completion containing either an Object,
null, or undefined, or a throw completion. It performs the following steps when
called:
1. 1. 1. Let home be envRec.[[FunctionObject]].[[HomeObject]].
2. 2. 2. If home is undefined, return undefined.
3. 3. 3. Assert: home is an Object.
4. 4. 4. Return ? home.[[GetPrototypeOf]]().
9.1.1.4 GLOBAL ENVIRONMENT RECORDS
A Global Environment Record is used to represent the outer most scope that is
shared by all of the ECMAScript Script elements that are processed in a common
realm. A Global Environment Record provides the bindings for built-in globals
(clause 19), properties of the global object, and for all top-level declarations
(8.2.9, 8.2.11) that occur within a Script.
A Global Environment Record is logically a single record but it is specified as
a composite encapsulating an Object Environment Record and a Declarative
Environment Record. The Object Environment Record has as its base object the
global object of the associated Realm Record. This global object is the value
returned by the Global Environment Record's GetThisBinding concrete method. The
Object Environment Record component of a Global Environment Record contains the
bindings for all built-in globals (clause 19) and all bindings introduced by a
FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration,
AsyncGeneratorDeclaration, or VariableStatement contained in global code. The
bindings for all other ECMAScript declarations in global code are contained in
the Declarative Environment Record component of the Global Environment Record.
Properties may be created directly on a global object. Hence, the Object
Environment Record component of a Global Environment Record may contain both
bindings created explicitly by FunctionDeclaration, GeneratorDeclaration,
AsyncFunctionDeclaration, AsyncGeneratorDeclaration, or VariableDeclaration
declarations and bindings created implicitly as properties of the global object.
In order to identify which bindings were explicitly created using declarations,
a Global Environment Record maintains a list of the names bound using its
CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.
Global Environment Records have the additional fields listed in Table 20 and the
additional methods listed in Table 21.
Table 20: Additional Fields of Global Environment Records
Field Name Value Meaning [[ObjectRecord]] an Object Environment Record Binding
object is the global object. It contains global built-in bindings as well as
FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration,
AsyncGeneratorDeclaration, and VariableDeclaration bindings in global code for
the associated realm. [[GlobalThisValue]] an Object The value returned by this
in global scope. Hosts may provide any ECMAScript Object value.
[[DeclarativeRecord]] a Declarative Environment Record Contains bindings for all
declarations in global code for the associated realm code except for
FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration,
AsyncGeneratorDeclaration, and VariableDeclaration bindings. [[VarNames]] a List
of Strings The string names bound by FunctionDeclaration, GeneratorDeclaration,
AsyncFunctionDeclaration, AsyncGeneratorDeclaration, and VariableDeclaration
declarations in global code for the associated realm.
Table 21: Additional Methods of Global Environment Records
Method Purpose GetThisBinding() Return the value of this Environment Record's
this binding. HasVarDeclaration (N) Determines if the argument identifier has a
binding in this Environment Record that was created using a VariableDeclaration,
FunctionDeclaration, GeneratorDeclaration, AsyncFunctionDeclaration, or
AsyncGeneratorDeclaration. HasLexicalDeclaration (N) Determines if the argument
identifier has a binding in this Environment Record that was created using a
lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
HasRestrictedGlobalProperty (N) Determines if the argument is the name of a
global object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N) Determines if a corresponding CreateGlobalVarBinding
call would succeed if called for the same argument N. CanDeclareGlobalFunction
(N) Determines if a corresponding CreateGlobalFunctionBinding call would succeed
if called for the same argument N. CreateGlobalVarBinding(N, D) Used to create
and initialize to undefined a global var binding in the [[ObjectRecord]]
component of a Global Environment Record. The binding will be a mutable binding.
The corresponding global object property will have attribute values appropriate
for a var. The String value N is the bound name. If D is true, the binding may
be deleted. Logically equivalent to CreateMutableBinding followed by a
SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D) Create and initialize a global function
binding in the [[ObjectRecord]] component of a Global Environment Record. The
binding will be a mutable binding. The corresponding global object property will
have attribute values appropriate for a function. The String value N is the
bound name. V is the initialization value. If the Boolean argument D is true,
the binding may be deleted. Logically equivalent to CreateMutableBinding
followed by a SetMutableBinding but it allows function declarations to receive
special treatment.
The behaviour of the concrete specification methods for Global Environment
Records is defined by the following algorithms.
9.1.1.4.1 HASBINDING ( N )
The HasBinding concrete method of a Global Environment Record envRec takes
argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if the argument identifier is one
of the identifiers bound by the record. It performs the following steps when
called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, return true.
3. 3. 3. Let ObjRec be envRec.[[ObjectRecord]].
4. 4. 4. Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CREATEMUTABLEBINDING ( N, D )
The CreateMutableBinding concrete method of a Global Environment Record envRec
takes arguments N (a String) and D (a Boolean) and returns either a normal
completion containing unused or a throw completion. It creates a new mutable
binding for the name N that is uninitialized. The binding is created in the
associated DeclarativeRecord. A binding for N must not already exist in the
DeclarativeRecord. If D is true, the new binding is marked as being subject to
deletion. It performs the following steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
3. 3. 3. Return ! DclRec.CreateMutableBinding(N, D).
9.1.1.4.3 CREATEIMMUTABLEBINDING ( N, S )
The CreateImmutableBinding concrete method of a Global Environment Record envRec
takes arguments N (a String) and S (a Boolean) and returns either a normal
completion containing unused or a throw completion. It creates a new immutable
binding for the name N that is uninitialized. A binding must not already exist
in this Environment Record for N. If S is true, the new binding is marked as a
strict binding. It performs the following steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
3. 3. 3. Return ! DclRec.CreateImmutableBinding(N, S).
9.1.1.4.4 INITIALIZEBINDING ( N, V )
The InitializeBinding concrete method of a Global Environment Record envRec
takes arguments N (a String) and V (an ECMAScript language value) and returns
either a normal completion containing unused or a throw completion. It is used
to set the bound value of the current binding of the identifier whose name is N
to the value V. An uninitialized binding for N must already exist. It performs
the following steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, then
1. a. a. Return ! DclRec.InitializeBinding(N, V).
3. 3. 3. Assert: If the binding exists, it must be in the Object Environment
Record.
4. 4. 4. Let ObjRec be envRec.[[ObjectRecord]].
5. 5. 5. Return ? ObjRec.InitializeBinding(N, V).
9.1.1.4.5 SETMUTABLEBINDING ( N, V, S )
The SetMutableBinding concrete method of a Global Environment Record envRec
takes arguments N (a String), V (an ECMAScript language value), and S (a
Boolean) and returns either a normal completion containing unused or a throw
completion. It attempts to change the bound value of the current binding of the
identifier whose name is N to the value V. If the binding is an immutable
binding and S is true, a TypeError is thrown. A property named N normally
already exists but if it does not or is not currently writable, error handling
is determined by S. It performs the following steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, then
1. a. a. Return ? DclRec.SetMutableBinding(N, V, S).
3. 3. 3. Let ObjRec be envRec.[[ObjectRecord]].
4. 4. 4. Return ? ObjRec.SetMutableBinding(N, V, S).
9.1.1.4.6 GETBINDINGVALUE ( N, S )
The GetBindingValue concrete method of a Global Environment Record envRec takes
arguments N (a String) and S (a Boolean) and returns either a normal completion
containing an ECMAScript language value or a throw completion. It returns the
value of its bound identifier whose name is N. If the binding is an
uninitialized binding throw a ReferenceError exception. A property named N
normally already exists but if it does not or is not currently writable, error
handling is determined by S. It performs the following steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, then
1. a. a. Return ? DclRec.GetBindingValue(N, S).
3. 3. 3. Let ObjRec be envRec.[[ObjectRecord]].
4. 4. 4. Return ? ObjRec.GetBindingValue(N, S).
9.1.1.4.7 DELETEBINDING ( N )
The DeleteBinding concrete method of a Global Environment Record envRec takes
argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It can only delete bindings that have been
explicitly designated as being subject to deletion. It performs the following
steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. If ! DclRec.HasBinding(N) is true, then
1. a. a. Return ! DclRec.DeleteBinding(N).
3. 3. 3. Let ObjRec be envRec.[[ObjectRecord]].
4. 4. 4. Let globalObject be ObjRec.[[BindingObject]].
5. 5. 5. Let existingProp be ? HasOwnProperty(globalObject, N).
6. 6. 6. If existingProp is true, then
1. a. a. Let status be ? ObjRec.DeleteBinding(N).
2. b. b. If status is true and envRec.[[VarNames]] contains N, then
1. i. i. Remove N from envRec.[[VarNames]].
3. c. c. Return status.
7. 7. 7. Return true.
9.1.1.4.8 HASTHISBINDING ( )
The HasThisBinding concrete method of a Global Environment Record envRec takes
no arguments and returns true. It performs the following steps when called:
1. 1. 1. Return true.
Note
Global Environment Records always provide a this binding.
9.1.1.4.9 HASSUPERBINDING ( )
The HasSuperBinding concrete method of a Global Environment Record envRec takes
no arguments and returns false. It performs the following steps when called:
1. 1. 1. Return false.
Note
Global Environment Records do not provide a super binding.
9.1.1.4.10 WITHBASEOBJECT ( )
The WithBaseObject concrete method of a Global Environment Record envRec takes
no arguments and returns undefined. It performs the following steps when called:
1. 1. 1. Return undefined.
9.1.1.4.11 GETTHISBINDING ( )
The GetThisBinding concrete method of a Global Environment Record envRec takes
no arguments and returns a normal completion containing an Object. It performs
the following steps when called:
1. 1. 1. Return envRec.[[GlobalThisValue]].
9.1.1.4.12 HASVARDECLARATION ( N )
The HasVarDeclaration concrete method of a Global Environment Record envRec
takes argument N (a String) and returns a Boolean. It determines if the argument
identifier has a binding in this record that was created using a
VariableStatement or a FunctionDeclaration. It performs the following steps when
called:
1. 1. 1. Let varDeclaredNames be envRec.[[VarNames]].
2. 2. 2. If varDeclaredNames contains N, return true.
3. 3. 3. Return false.
9.1.1.4.13 HASLEXICALDECLARATION ( N )
The HasLexicalDeclaration concrete method of a Global Environment Record envRec
takes argument N (a String) and returns a Boolean. It determines if the argument
identifier has a binding in this record that was created using a lexical
declaration such as a LexicalDeclaration or a ClassDeclaration. It performs the
following steps when called:
1. 1. 1. Let DclRec be envRec.[[DeclarativeRecord]].
2. 2. 2. Return ! DclRec.HasBinding(N).
9.1.1.4.14 HASRESTRICTEDGLOBALPROPERTY ( N )
The HasRestrictedGlobalProperty concrete method of a Global Environment Record
envRec takes argument N (a String) and returns either a normal completion
containing a Boolean or a throw completion. It determines if the argument
identifier is the name of a property of the global object that must not be
shadowed by a global lexical binding. It performs the following steps when
called:
1. 1. 1. Let ObjRec be envRec.[[ObjectRecord]].
2. 2. 2. Let globalObject be ObjRec.[[BindingObject]].
3. 3. 3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
4. 4. 4. If existingProp is undefined, return false.
5. 5. 5. If existingProp.[[Configurable]] is true, return false.
6. 6. 6. Return true.
Note
Properties may exist upon a global object that were directly created rather than
being declared using a var or function declaration. A global lexical binding may
not be created that has the same name as a non-configurable property of the
global object. The global property "undefined" is an example of such a property.
9.1.1.4.15 CANDECLAREGLOBALVAR ( N )
The CanDeclareGlobalVar concrete method of a Global Environment Record envRec
takes argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if a corresponding
CreateGlobalVarBinding call would succeed if called for the same argument N.
Redundant var declarations and var declarations for pre-existing global object
properties are allowed. It performs the following steps when called:
1. 1. 1. Let ObjRec be envRec.[[ObjectRecord]].
2. 2. 2. Let globalObject be ObjRec.[[BindingObject]].
3. 3. 3. Let hasProperty be ? HasOwnProperty(globalObject, N).
4. 4. 4. If hasProperty is true, return true.
5. 5. 5. Return ? IsExtensible(globalObject).
9.1.1.4.16 CANDECLAREGLOBALFUNCTION ( N )
The CanDeclareGlobalFunction concrete method of a Global Environment Record
envRec takes argument N (a String) and returns either a normal completion
containing a Boolean or a throw completion. It determines if a corresponding
CreateGlobalFunctionBinding call would succeed if called for the same argument
N. It performs the following steps when called:
1. 1. 1. Let ObjRec be envRec.[[ObjectRecord]].
2. 2. 2. Let globalObject be ObjRec.[[BindingObject]].
3. 3. 3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
4. 4. 4. If existingProp is undefined, return ? IsExtensible(globalObject).
5. 5. 5. If existingProp.[[Configurable]] is true, return true.
6. 6. 6. If IsDataDescriptor(existingProp) is true and existingProp has
attribute values { [[Writable]]: true, [[Enumerable]]: true }, return true.
7. 7. 7. Return false.
9.1.1.4.17 CREATEGLOBALVARBINDING ( N, D )
The CreateGlobalVarBinding concrete method of a Global Environment Record envRec
takes arguments N (a String) and D (a Boolean) and returns either a normal
completion containing unused or a throw completion. It creates and initializes a
mutable binding in the associated Object Environment Record and records the
bound name in the associated [[VarNames]] List. If a binding already exists, it
is reused and assumed to be initialized. It performs the following steps when
called:
1. 1. 1. Let ObjRec be envRec.[[ObjectRecord]].
2. 2. 2. Let globalObject be ObjRec.[[BindingObject]].
3. 3. 3. Let hasProperty be ? HasOwnProperty(globalObject, N).
4. 4. 4. Let extensible be ? IsExtensible(globalObject).
5. 5. 5. If hasProperty is false and extensible is true, then
1. a. a. Perform ? ObjRec.CreateMutableBinding(N, D).
2. b. b. Perform ? ObjRec.InitializeBinding(N, undefined).
6. 6. 6. If envRec.[[VarNames]] does not contain N, then
1. a. a. Append N to envRec.[[VarNames]].
7. 7. 7. Return unused.
9.1.1.4.18 CREATEGLOBALFUNCTIONBINDING ( N, V, D )
The CreateGlobalFunctionBinding concrete method of a Global Environment Record
envRec takes arguments N (a String), V (an ECMAScript language value), and D (a
Boolean) and returns either a normal completion containing unused or a throw
completion. It creates and initializes a mutable binding in the associated
Object Environment Record and records the bound name in the associated
[[VarNames]] List. If a binding already exists, it is replaced. It performs the
following steps when called:
1. 1. 1. Let ObjRec be envRec.[[ObjectRecord]].
2. 2. 2. Let globalObject be ObjRec.[[BindingObject]].
3. 3. 3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
4. 4. 4. If existingProp is undefined or existingProp.[[Configurable]] is true,
then
1. a. a. Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true, [[Enumerable]]: true, [[Configurable]]: D }.
5. 5. 5. Else,
1. a. a. Let desc be the PropertyDescriptor { [[Value]]: V }.
6. 6. 6. Perform ? DefinePropertyOrThrow(globalObject, N, desc).
7. 7. 7. Perform ? Set(globalObject, N, V, false).
8. 8. 8. If envRec.[[VarNames]] does not contain N, then
1. a. a. Append N to envRec.[[VarNames]].
9. 9. 9. Return unused.
Note
Global function declarations are always represented as own properties of the
global object. If possible, an existing own property is reconfigured to have a
standard set of attribute values. Step 7 is equivalent to what calling the
InitializeBinding concrete method would do and if globalObject is a Proxy will
produce the same sequence of Proxy trap calls.
9.1.1.5 MODULE ENVIRONMENT RECORDS
A Module Environment Record is a Declarative Environment Record that is used to
represent the outer scope of an ECMAScript Module. In additional to normal
mutable and immutable bindings, Module Environment Records also provide
immutable import bindings which are bindings that provide indirect access to a
target binding that exists in another Environment Record.
Module Environment Records support all of the Declarative Environment Record
methods listed in Table 16 and share the same specifications for all of those
methods except for GetBindingValue, DeleteBinding, HasThisBinding and
GetThisBinding. In addition, Module Environment Records support the methods
listed in Table 22:
Table 22: Additional Methods of Module Environment Records
Method Purpose CreateImportBinding(N, M, N2) Create an immutable indirect
binding in a Module Environment Record. The String value N is the text of the
bound name. M is a Module Record, and N2 is a binding that exists in M's Module
Environment Record. GetThisBinding() Return the value of this Environment
Record's this binding.
The behaviour of the additional concrete specification methods for Module
Environment Records are defined by the following algorithms:
9.1.1.5.1 GETBINDINGVALUE ( N, S )
The GetBindingValue concrete method of a Module Environment Record envRec takes
arguments N (a String) and S (a Boolean) and returns either a normal completion
containing an ECMAScript language value or a throw completion. It returns the
value of its bound identifier whose name is N. However, if the binding is an
indirect binding the value of the target binding is returned. If the binding
exists but is uninitialized a ReferenceError is thrown. It performs the
following steps when called:
1. 1. 1. Assert: S is true.
2. 2. 2. Assert: envRec has a binding for N.
3. 3. 3. If the binding for N is an indirect binding, then
1. a. a. Let M and N2 be the indirection values provided when this binding
for N was created.
2. b. b. Let targetEnv be M.[[Environment]].
3. c. c. If targetEnv is empty, throw a ReferenceError exception.
4. d. d. Return ? targetEnv.GetBindingValue(N2, true).
4. 4. 4. If the binding for N in envRec is an uninitialized binding, throw a
ReferenceError exception.
5. 5. 5. Return the value currently bound to N in envRec.
Note
S will always be true because a Module is always strict mode code.
9.1.1.5.2 DELETEBINDING ( N )
The DeleteBinding concrete method of a Module Environment Record is never used
within this specification.
Note
Module Environment Records are only used within strict code and an early error
rule prevents the delete operator, in strict code, from being applied to a
Reference Record that would resolve to a Module Environment Record binding. See
13.5.1.1.
9.1.1.5.3 HASTHISBINDING ( )
The HasThisBinding concrete method of a Module Environment Record envRec takes
no arguments and returns true. It performs the following steps when called:
1. 1. 1. Return true.
Note
Module Environment Records always provide a this binding.
9.1.1.5.4 GETTHISBINDING ( )
The GetThisBinding concrete method of a Module Environment Record envRec takes
no arguments and returns a normal completion containing undefined. It performs
the following steps when called:
1. 1. 1. Return undefined.
9.1.1.5.5 CREATEIMPORTBINDING ( N, M, N2 )
The CreateImportBinding concrete method of a Module Environment Record envRec
takes arguments N (a String), M (a Module Record), and N2 (a String) and returns
unused. It creates a new initialized immutable indirect binding for the name N.
A binding must not already exist in this Environment Record for N. N2 is the
name of a binding that exists in M's Module Environment Record. Accesses to the
value of the new binding will indirectly access the bound value of the target
binding. It performs the following steps when called:
1. 1. 1. Assert: envRec does not already have a binding for N.
2. 2. 2. Assert: When M.[[Environment]] is instantiated, it will have a direct
binding for N2.
3. 3. 3. Create an immutable indirect binding in envRec for N that references M
and N2 as its target binding and record that the binding is initialized.
4. 4. 4. Return unused.
9.1.2 ENVIRONMENT RECORD OPERATIONS
The following abstract operations are used in this specification to operate upon
Environment Records:
9.1.2.1 GETIDENTIFIERREFERENCE ( ENV, NAME, STRICT )
The abstract operation GetIdentifierReference takes arguments env (an
Environment Record or null), name (a String), and strict (a Boolean) and returns
either a normal completion containing a Reference Record or a throw completion.
It performs the following steps when called:
1. 1. 1. If env is null, then
1. a. a. Return the Reference Record { [[Base]]: unresolvable,
[[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
2. 2. 2. Let exists be ? env.HasBinding(name).
3. 3. 3. If exists is true, then
1. a. a. Return the Reference Record { [[Base]]: env, [[ReferencedName]]:
name, [[Strict]]: strict, [[ThisValue]]: empty }.
4. 4. 4. Else,
1. a. a. Let outer be env.[[OuterEnv]].
2. b. b. Return ? GetIdentifierReference(outer, name, strict).
9.1.2.2 NEWDECLARATIVEENVIRONMENT ( E )
The abstract operation NewDeclarativeEnvironment takes argument E (an
Environment Record or null) and returns a Declarative Environment Record. It
performs the following steps when called:
1. 1. 1. Let env be a new Declarative Environment Record containing no
bindings.
2. 2. 2. Set env.[[OuterEnv]] to E.
3. 3. 3. Return env.
9.1.2.3 NEWOBJECTENVIRONMENT ( O, W, E )
The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a
Boolean), and E (an Environment Record or null) and returns an Object
Environment Record. It performs the following steps when called:
1. 1. 1. Let env be a new Object Environment Record.
2. 2. 2. Set env.[[BindingObject]] to O.
3. 3. 3. Set env.[[IsWithEnvironment]] to W.
4. 4. 4. Set env.[[OuterEnv]] to E.
5. 5. 5. Return env.
9.1.2.4 NEWFUNCTIONENVIRONMENT ( F, NEWTARGET )
The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript
function) and newTarget (an Object or undefined) and returns a Function
Environment Record. It performs the following steps when called:
1. 1. 1. Let env be a new Function Environment Record containing no bindings.
2. 2. 2. Set env.[[FunctionObject]] to F.
3. 3. 3. If F.[[ThisMode]] is lexical, set env.[[ThisBindingStatus]] to
lexical.
4. 4. 4. Else, set env.[[ThisBindingStatus]] to uninitialized.
5. 5. 5. Set env.[[NewTarget]] to newTarget.
6. 6. 6. Set env.[[OuterEnv]] to F.[[Environment]].
7. 7. 7. Return env.
9.1.2.5 NEWGLOBALENVIRONMENT ( G, THISVALUE )
The abstract operation NewGlobalEnvironment takes arguments G (an Object) and
thisValue (an Object) and returns a Global Environment Record. It performs the
following steps when called:
1. 1. 1. Let objRec be NewObjectEnvironment(G, false, null).
2. 2. 2. Let dclRec be NewDeclarativeEnvironment(null).
3. 3. 3. Let env be a new Global Environment Record.
4. 4. 4. Set env.[[ObjectRecord]] to objRec.
5. 5. 5. Set env.[[GlobalThisValue]] to thisValue.
6. 6. 6. Set env.[[DeclarativeRecord]] to dclRec.
7. 7. 7. Set env.[[VarNames]] to a new empty List.
8. 8. 8. Set env.[[OuterEnv]] to null.
9. 9. 9. Return env.
9.1.2.6 NEWMODULEENVIRONMENT ( E )
The abstract operation NewModuleEnvironment takes argument E (an Environment
Record) and returns a Module Environment Record. It performs the following steps
when called:
1. 1. 1. Let env be a new Module Environment Record containing no bindings.
2. 2. 2. Set env.[[OuterEnv]] to E.
3. 3. 3. Return env.
9.2 PRIVATEENVIRONMENT RECORDS
A PrivateEnvironment Record is a specification mechanism used to track Private
Names based upon the lexical nesting structure of ClassDeclarations and
ClassExpressions in ECMAScript code. They are similar to, but distinct from,
Environment Records. Each PrivateEnvironment Record is associated with a
ClassDeclaration or ClassExpression. Each time such a class is evaluated, a new
PrivateEnvironment Record is created to record the Private Names declared by
that class.
Each PrivateEnvironment Record has the fields defined in Table 23.
Table 23: PrivateEnvironment Record Fields
Field Name Value Type Meaning [[OuterPrivateEnvironment]] a PrivateEnvironment
Record or null The PrivateEnvironment Record of the nearest containing class.
null if the class with which this PrivateEnvironment Record is associated is not
contained in any other class. [[Names]] a List of Private Names The Private
Names declared by this class.
9.2.1 PRIVATEENVIRONMENT RECORD OPERATIONS
The following abstract operations are used in this specification to operate upon
PrivateEnvironment Records:
9.2.1.1 NEWPRIVATEENVIRONMENT ( OUTERPRIVENV )
The abstract operation NewPrivateEnvironment takes argument outerPrivEnv (a
PrivateEnvironment Record or null) and returns a PrivateEnvironment Record. It
performs the following steps when called:
1. 1. 1. Let names be a new empty List.
2. 2. 2. Return the PrivateEnvironment Record { [[OuterPrivateEnvironment]]:
outerPrivEnv, [[Names]]: names }.
9.2.1.2 RESOLVEPRIVATEIDENTIFIER ( PRIVENV, IDENTIFIER )
The abstract operation ResolvePrivateIdentifier takes arguments privEnv (a
PrivateEnvironment Record) and identifier (a String) and returns a Private Name.
It performs the following steps when called:
1. 1. 1. Let names be privEnv.[[Names]].
2. 2. 2. For each Private Name pn of names, do
1. a. a. If pn.[[Description]] is identifier, then
1. i. i. Return pn.
3. 3. 3. Let outerPrivEnv be privEnv.[[OuterPrivateEnvironment]].
4. 4. 4. Assert: outerPrivEnv is not null.
5. 5. 5. Return ResolvePrivateIdentifier(outerPrivEnv, identifier).
9.3 REALMS
Before it is evaluated, all ECMAScript code must be associated with a realm.
Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript
global environment, all of the ECMAScript code that is loaded within the scope
of that global environment, and other associated state and resources.
A realm is represented in this specification as a Realm Record with the fields
specified in Table 24:
Table 24: Realm Record Fields
Field Name Value Meaning [[Intrinsics]] a Record whose field names are intrinsic
keys and whose values are objects The intrinsic values used by code associated
with this realm [[GlobalObject]] an Object or undefined The global object for
this realm [[GlobalEnv]] a Global Environment Record The global environment for
this realm [[TemplateMap]] a List of Records with fields [[Site]] (a
TemplateLiteral Parse Node) and [[Array]] (an Array)
Template objects are canonicalized separately for each realm using its Realm
Record's [[TemplateMap]]. Each [[Site]] value is a Parse Node that is a
TemplateLiteral. The associated [[Array]] value is the corresponding template
object that is passed to a tag function.
Note 1
Once a Parse Node becomes unreachable, the corresponding [[Array]] is also
unreachable, and it would be unobservable if an implementation removed the pair
from the [[TemplateMap]] list.
[[LoadedModules]] a List of Records with fields [[Specifier]] (a String) and
[[Module]] (a Module Record)
A map from the specifier strings imported by this realm to the resolved Module
Record. The list does not contain two different Records with the same
[[Specifier]].
Note 2
As mentioned in HostLoadImportedModule (16.2.1.8 Note 1), [[LoadedModules]] in
Realm Records is only used when running an import() expression in a context
where there is no active script or module.
[[HostDefined]] anything (default value is undefined) Field reserved for use by
hosts that need to associate additional information with a Realm Record.
9.3.1 CREATEREALM ( )
The abstract operation CreateRealm takes no arguments and returns a Realm
Record. It performs the following steps when called:
1. 1. 1. Let realmRec be a new Realm Record.
2. 2. 2. Perform CreateIntrinsics(realmRec).
3. 3. 3. Set realmRec.[[GlobalObject]] to undefined.
4. 4. 4. Set realmRec.[[GlobalEnv]] to undefined.
5. 5. 5. Set realmRec.[[TemplateMap]] to a new empty List.
6. 6. 6. Return realmRec.
9.3.2 CREATEINTRINSICS ( REALMREC )
The abstract operation CreateIntrinsics takes argument realmRec (a Realm Record)
and returns unused. It performs the following steps when called:
1. 1. 1. Set realmRec.[[Intrinsics]] to a new Record.
2. 2. 2. Set fields of realmRec.[[Intrinsics]] with the values listed in Table
6. The field names are the names listed in column one of the table. The
value of each field is a new object value fully and recursively populated
with property values as defined by the specification of each object in
clauses 19 through 28. All object property values are newly created object
values. All values that are built-in function objects are created by
performing CreateBuiltinFunction(steps, length, name, slots, realmRec,
prototype) where steps is the definition of that function provided by this
specification, name is the initial value of the function's "name" property,
length is the initial value of the function's "length" property, slots is a
list of the names, if any, of the function's specified internal slots, and
prototype is the specified value of the function's [[Prototype]] internal
slot. The creation of the intrinsics and their properties must be ordered to
avoid any dependencies upon objects that have not yet been created.
3. 3. 3. Perform
AddRestrictedFunctionProperties(realmRec.[[Intrinsics]].[[%Function.prototype%]],
realmRec).
4. 4. 4. Return unused.
9.3.3 SETREALMGLOBALOBJECT ( REALMREC, GLOBALOBJ, THISVALUE )
The abstract operation SetRealmGlobalObject takes arguments realmRec (a Realm
Record), globalObj (an Object or undefined), and thisValue (an Object or
undefined) and returns unused. It performs the following steps when called:
1. 1. 1. If globalObj is undefined, then
1. a. a. Let intrinsics be realmRec.[[Intrinsics]].
2. b. b. Set globalObj to
OrdinaryObjectCreate(intrinsics.[[%Object.prototype%]]).
2. 2. 2. Assert: globalObj is an Object.
3. 3. 3. If thisValue is undefined, set thisValue to globalObj.
4. 4. 4. Set realmRec.[[GlobalObject]] to globalObj.
5. 5. 5. Let newGlobalEnv be NewGlobalEnvironment(globalObj, thisValue).
6. 6. 6. Set realmRec.[[GlobalEnv]] to newGlobalEnv.
7. 7. 7. Return unused.
9.3.4 SETDEFAULTGLOBALBINDINGS ( REALMREC )
The abstract operation SetDefaultGlobalBindings takes argument realmRec (a Realm
Record) and returns either a normal completion containing an Object or a throw
completion. It performs the following steps when called:
1. 1. 1. Let global be realmRec.[[GlobalObject]].
2. 2. 2. For each property of the Global Object specified in clause 19, do
1. a. a. Let name be the String value of the property name.
2. b. b. Let desc be the fully populated data Property Descriptor for the
property, containing the specified attributes for the property. For
properties listed in 19.2, 19.3, or 19.4 the value of the [[Value]]
attribute is the corresponding intrinsic object from realmRec.
3. c. c. Perform ? DefinePropertyOrThrow(global, name, desc).
3. 3. 3. Return global.
9.4 EXECUTION CONTEXTS
An execution context is a specification device that is used to track the runtime
evaluation of code by an ECMAScript implementation. At any point in time, there
is at most one execution context per agent that is actually executing code. This
is known as the agent's running execution context. All references to the running
execution context in this specification denote the running execution context of
the surrounding agent.
The execution context stack is used to track execution contexts. The running
execution context is always the top element of this stack. A new execution
context is created whenever control is transferred from the executable code
associated with the currently running execution context to executable code that
is not associated with that execution context. The newly created execution
context is pushed onto the stack and becomes the running execution context.
An execution context contains whatever implementation specific state is
necessary to track the execution progress of its associated code. Each execution
context has at least the state components listed in Table 25.
Table 25: State Components for All Execution Contexts
Component Purpose code evaluation state Any state needed to perform, suspend,
and resume evaluation of the code associated with this execution context.
Function If this execution context is evaluating the code of a function object,
then the value of this component is that function object. If the context is
evaluating the code of a Script or Module, the value is null. Realm The Realm
Record from which associated code accesses ECMAScript resources. ScriptOrModule
The Module Record or Script Record from which associated code originates. If
there is no originating script or module, as is the case for the original
execution context created in InitializeHostDefinedRealm, the value is null.
Evaluation of code by the running execution context may be suspended at various
points defined within this specification. Once the running execution context has
been suspended a different execution context may become the running execution
context and commence evaluating its code. At some later time a suspended
execution context may again become the running execution context and continue
evaluating its code at the point where it had previously been suspended.
Transition of the running execution context status among execution contexts
usually occurs in stack-like last-in/first-out manner. However, some ECMAScript
features require non-LIFO transitions of the running execution context.
The value of the Realm component of the running execution context is also called
the current Realm Record. The value of the Function component of the running
execution context is also called the active function object.
ECMAScript code execution contexts have the additional state components listed
in Table 26.
Table 26: Additional State Components for ECMAScript Code Execution Contexts
Component Purpose LexicalEnvironment Identifies the Environment Record used to
resolve identifier references made by code within this execution context.
VariableEnvironment Identifies the Environment Record that holds bindings
created by VariableStatements within this execution context. PrivateEnvironment
Identifies the PrivateEnvironment Record that holds Private Names created by
ClassElements in the nearest containing class. null if there is no containing
class.
The LexicalEnvironment and VariableEnvironment components of an execution
context are always Environment Records.
Execution contexts representing the evaluation of Generators have the additional
state components listed in Table 27.
Table 27: Additional State Components for Generator Execution Contexts
Component Purpose Generator The Generator that this execution context is
evaluating.
In most situations only the running execution context (the top of the execution
context stack) is directly manipulated by algorithms within this specification.
Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used
without qualification they are in reference to those components of the running
execution context.
An execution context is purely a specification mechanism and need not correspond
to any particular artefact of an ECMAScript implementation. It is impossible for
ECMAScript code to directly access or observe an execution context.
9.4.1 GETACTIVESCRIPTORMODULE ( )
The abstract operation GetActiveScriptOrModule takes no arguments and returns a
Script Record, a Module Record, or null. It is used to determine the running
script or module, based on the running execution context. It performs the
following steps when called:
1. 1. 1. If the execution context stack is empty, return null.
2. 2. 2. Let ec be the topmost execution context on the execution context stack
whose ScriptOrModule component is not null.
3. 3. 3. If no such execution context exists, return null. Otherwise, return
ec's ScriptOrModule.
9.4.2 RESOLVEBINDING ( NAME [ , ENV ] )
The abstract operation ResolveBinding takes argument name (a String) and
optional argument env (an Environment Record or undefined) and returns either a
normal completion containing a Reference Record or a throw completion. It is
used to determine the binding of name. env can be used to explicitly provide the
Environment Record that is to be searched for the binding. It performs the
following steps when called:
1. 1. 1. If env is not present or env is undefined, then
1. a. a. Set env to the running execution context's LexicalEnvironment.
2. 2. 2. Assert: env is an Environment Record.
3. 3. 3. If the source text matched by the syntactic production that is being
evaluated is contained in strict mode code, let strict be true; else let
strict be false.
4. 4. 4. Return ? GetIdentifierReference(env, name, strict).
Note
The result of ResolveBinding is always a Reference Record whose
[[ReferencedName]] field is name.
9.4.3 GETTHISENVIRONMENT ( )
The abstract operation GetThisEnvironment takes no arguments and returns an
Environment Record. It finds the Environment Record that currently supplies the
binding of the keyword this. It performs the following steps when called:
1. 1. 1. Let env be the running execution context's LexicalEnvironment.
2. 2. 2. Repeat,
1. a. a. Let exists be env.HasThisBinding().
2. b. b. If exists is true, return env.
3. c. c. Let outer be env.[[OuterEnv]].
4. d. d. Assert: outer is not null.
5. e. e. Set env to outer.
Note
The loop in step 2 will always terminate because the list of environments always
ends with the global environment which has a this binding.
9.4.4 RESOLVETHISBINDING ( )
The abstract operation ResolveThisBinding takes no arguments and returns either
a normal completion containing an ECMAScript language value or a throw
completion. It determines the binding of the keyword this using the
LexicalEnvironment of the running execution context. It performs the following
steps when called:
1. 1. 1. Let envRec be GetThisEnvironment().
2. 2. 2. Return ? envRec.GetThisBinding().
9.4.5 GETNEWTARGET ( )
The abstract operation GetNewTarget takes no arguments and returns an Object or
undefined. It determines the NewTarget value using the LexicalEnvironment of the
running execution context. It performs the following steps when called:
1. 1. 1. Let envRec be GetThisEnvironment().
2. 2. 2. Assert: envRec has a [[NewTarget]] field.
3. 3. 3. Return envRec.[[NewTarget]].
9.4.6 GETGLOBALOBJECT ( )
The abstract operation GetGlobalObject takes no arguments and returns an Object.
It returns the global object used by the currently running execution context. It
performs the following steps when called:
1. 1. 1. Let currentRealm be the current Realm Record.
2. 2. 2. Return currentRealm.[[GlobalObject]].
9.5 JOBS AND HOST OPERATIONS TO ENQUEUE JOBS
A Job is an Abstract Closure with no parameters that initiates an ECMAScript
computation when no other ECMAScript computation is currently in progress.
Jobs are scheduled for execution by ECMAScript host environments. This
specification describes the host hook HostEnqueuePromiseJob to schedule one kind
of job; hosts may define additional abstract operations which schedule jobs.
Such operations accept a Job Abstract Closure as the parameter and schedule it
to be performed at some future time. Their implementations must conform to the
following requirements:
* At some future point in time, when there is no running execution context and
the execution context stack is empty, the implementation must:
1. Perform any host-defined preparation steps.
2. Invoke the Job Abstract Closure.
3. Perform any host-defined cleanup steps, after which the execution context
stack must be empty.
* Only one Job may be actively undergoing evaluation at any point in time.
* Once evaluation of a Job starts, it must run to completion before evaluation
of any other Job starts.
* The Abstract Closure must return a normal completion, implementing its own
handling of errors.
Note 1
Host environments are not required to treat Jobs uniformly with respect to
scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as
a higher priority than other work; future features may add Jobs that are not
treated at such a high priority.
At any particular time, scriptOrModule (a Script Record, a Module Record, or
null) is the active script or module if all of the following conditions are
true:
* GetActiveScriptOrModule() is scriptOrModule.
* If scriptOrModule is a Script Record or Module Record, let ec be the topmost
execution context on the execution context stack whose ScriptOrModule
component is scriptOrModule. The Realm component of ec is
scriptOrModule.[[Realm]].
At any particular time, an execution is prepared to evaluate ECMAScript code if
all of the following conditions are true:
* The execution context stack is not empty.
* The Realm component of the topmost execution context on the execution context
stack is a Realm Record.
Note 2
Host environments may prepare an execution to evaluate code by pushing execution
contexts onto the execution context stack. The specific steps are
implementation-defined.
The specific choice of Realm is up to the host environment. This initial
execution context and Realm is only in use before any callback function is
invoked. When a callback function related to a Job, like a Promise handler, is
invoked, the invocation pushes its own execution context and Realm.
Particular kinds of Jobs have additional conformance requirements.
9.5.1 JOBCALLBACK RECORDS
A JobCallback Record is a Record value used to store a function object and a
host-defined value. Function objects that are invoked via a Job enqueued by the
host may have additional host-defined context. To propagate the state, Job
Abstract Closures should not capture and call function objects directly.
Instead, use HostMakeJobCallback and HostCallJobCallback.
Note
The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses
the host-defined value to propagate the incumbent settings object for Promise
callbacks.
JobCallback Records have the fields listed in Table 28.
Table 28: JobCallback Record Fields
Field Name Value Meaning [[Callback]] a function object The function to invoke
when the Job is invoked. [[HostDefined]] anything (default value is empty) Field
reserved for use by hosts.
9.5.2 HOSTMAKEJOBCALLBACK ( CALLBACK )
The host-defined abstract operation HostMakeJobCallback takes argument callback
(a function object) and returns a JobCallback Record.
An implementation of HostMakeJobCallback must conform to the following
requirements:
* It must return a JobCallback Record whose [[Callback]] field is callback.
The default implementation of HostMakeJobCallback performs the following steps
when called:
1. 1. 1. Return the JobCallback Record { [[Callback]]: callback,
[[HostDefined]]: empty }.
ECMAScript hosts that are not web browsers must use the default implementation
of HostMakeJobCallback.
Note
This is called at the time that the callback is passed to the function that is
responsible for its being eventually scheduled and run. For example,
promise.then(thenAction) calls MakeJobCallback on thenAction at the time of
invoking Promise.prototype.then, not at the time of scheduling the reaction Job.
9.5.3 HOSTCALLJOBCALLBACK ( JOBCALLBACK, V, ARGUMENTSLIST )
The host-defined abstract operation HostCallJobCallback takes arguments
jobCallback (a JobCallback Record), V (an ECMAScript language value), and
argumentsList (a List of ECMAScript language values) and returns either a normal
completion containing an ECMAScript language value or a throw completion.
An implementation of HostCallJobCallback must conform to the following
requirements:
* It must perform and return the result of Call(jobCallback.[[Callback]], V,
argumentsList).
Note
This requirement means that hosts cannot change the [[Call]] behaviour of
function objects defined in this specification.
The default implementation of HostCallJobCallback performs the following steps
when called:
1. 1. 1. Assert: IsCallable(jobCallback.[[Callback]]) is true.
2. 2. 2. Return ? Call(jobCallback.[[Callback]], V, argumentsList).
ECMAScript hosts that are not web browsers must use the default implementation
of HostCallJobCallback.
9.5.4 HOSTENQUEUEPROMISEJOB ( JOB, REALM )
The host-defined abstract operation HostEnqueuePromiseJob takes arguments job (a
Job Abstract Closure) and realm (a Realm Record or null) and returns unused. It
schedules job to be performed at some future time. The Abstract Closures used
with this algorithm are intended to be related to the handling of Promises, or
otherwise, to be scheduled with equal priority to Promise handling operations.
An implementation of HostEnqueuePromiseJob must conform to the requirements in
9.5 as well as the following:
* If realm is not null, each time job is invoked the implementation must
perform implementation-defined steps such that execution is prepared to
evaluate ECMAScript code at the time of job's invocation.
* Let scriptOrModule be GetActiveScriptOrModule() at the time
HostEnqueuePromiseJob is invoked. If realm is not null, each time job is
invoked the implementation must perform implementation-defined steps such
that scriptOrModule is the active script or module at the time of job's
invocation.
* Jobs must run in the same order as the HostEnqueuePromiseJob invocations that
scheduled them.
Note
The realm for Jobs returned by NewPromiseResolveThenableJob is usually the
result of calling GetFunctionRealm on the then function object. The realm for
Jobs returned by NewPromiseReactionJob is usually the result of calling
GetFunctionRealm on the handler if the handler is not undefined. If the handler
is undefined, realm is null. For both kinds of Jobs, when GetFunctionRealm
completes abnormally (i.e. called on a revoked Proxy), realm is the current
Realm at the time of the GetFunctionRealm call. When the realm is null, no user
ECMAScript code will be evaluated and no new ECMAScript objects (e.g. Error
objects) will be created. The WHATWG HTML specification
(https://html.spec.whatwg.org/), for example, uses realm to check for the
ability to run script and for the entry concept.
9.6 INITIALIZEHOSTDEFINEDREALM ( )
The abstract operation InitializeHostDefinedRealm takes no arguments and returns
either a normal completion containing unused or a throw completion. It performs
the following steps when called:
1. 1. 1. Let realm be CreateRealm().
2. 2. 2. Let newContext be a new execution context.
3. 3. 3. Set the Function of newContext to null.
4. 4. 4. Set the Realm of newContext to realm.
5. 5. 5. Set the ScriptOrModule of newContext to null.
6. 6. 6. Push newContext onto the execution context stack; newContext is now
the running execution context.
7. 7. 7. If the host requires use of an exotic object to serve as realm's
global object, let global be such an object created in a host-defined
manner. Otherwise, let global be undefined, indicating that an ordinary
object should be created as the global object.
8. 8. 8. If the host requires that the this binding in realm's global scope
return an object other than the global object, let thisValue be such an
object created in a host-defined manner. Otherwise, let thisValue be
undefined, indicating that realm's global this binding should be the global
object.
9. 9. 9. Perform SetRealmGlobalObject(realm, global, thisValue).
10. 10. 10. Let globalObj be ? SetDefaultGlobalBindings(realm).
11. 11. 11. Create any host-defined global object properties on globalObj.
12. 12. 12. Return unused.
9.7 AGENTS
An agent comprises a set of ECMAScript execution contexts, an execution context
stack, a running execution context, an Agent Record, and an executing thread.
Except for the executing thread, the constituents of an agent belong exclusively
to that agent.
An agent's executing thread executes a job on the agent's execution contexts
independently of other agents, except that an executing thread may be used as
the executing thread by multiple agents, provided none of the agents sharing the
thread have an Agent Record whose [[CanBlock]] field is true.
Note 1
Some web browsers share a single executing thread across multiple unrelated tabs
of a browser window, for example.
While an agent's executing thread executes jobs, the agent is the surrounding
agent for the code in those jobs. The code uses the surrounding agent to access
the specification-level execution objects held within the agent: the running
execution context, the execution context stack, and the Agent Record's fields.
An agent signifier is a globally-unique opaque value used to identify an Agent.
Table 29: Agent Record Fields
Field Name Value Meaning [[LittleEndian]] a Boolean The default value computed
for the isLittleEndian parameter when it is needed by the algorithms
GetValueFromBuffer and SetValueInBuffer. The choice is implementation-defined
and should be the alternative that is most efficient for the implementation.
Once the value has been observed it cannot change. [[CanBlock]] a Boolean
Determines whether the agent can block or not. [[Signifier]] an agent signifier
Uniquely identifies the agent within its agent cluster. [[IsLockFree1]] a
Boolean true if atomic operations on one-byte values are lock-free, false
otherwise. [[IsLockFree2]] a Boolean true if atomic operations on two-byte
values are lock-free, false otherwise. [[IsLockFree8]] a Boolean true if atomic
operations on eight-byte values are lock-free, false otherwise.
[[CandidateExecution]] a candidate execution Record See the memory model.
[[KeptAlive]] a List of either Objects or Symbols Initially a new empty List,
representing the list of objects and/or symbols to be kept alive until the end
of the current Job
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been
observed by any agent in the agent cluster they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined
by the hardware, but may also reflect implementation choices that can vary over
time and between ECMAScript implementations.
There is no [[IsLockFree4]] field: 4-byte atomic operations are always
lock-free.
In practice, if an atomic operation is implemented with any type of lock the
operation is not lock-free. Lock-free does not imply wait-free: there is no
upper bound on how many machine steps may be required to complete a lock-free
atomic operation.
That an atomic access of size n is lock-free does not imply anything about the
(perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic
accesses may still be performed as a sequence of several separate memory
accesses. See ReadSharedMemory and WriteSharedMemory for details.
Note 3
An agent is a specification mechanism and need not correspond to any particular
artefact of an ECMAScript implementation.
9.7.1 AGENTSIGNIFIER ( )
The abstract operation AgentSignifier takes no arguments and returns an agent
signifier. It performs the following steps when called:
1. 1. 1. Let AR be the Agent Record of the surrounding agent.
2. 2. 2. Return AR.[[Signifier]].
9.7.2 AGENTCANSUSPEND ( )
The abstract operation AgentCanSuspend takes no arguments and returns a Boolean.
It performs the following steps when called:
1. 1. 1. Let AR be the Agent Record of the surrounding agent.
2. 2. 2. Return AR.[[CanBlock]].
Note
In some environments it may not be reasonable for a given agent to suspend. For
example, in a web browser environment, it may be reasonable to disallow
suspending a document's main event handling thread, while still allowing
workers' event handling threads to suspend.
9.8 AGENT CLUSTERS
An agent cluster is a maximal set of agents that can communicate by operating on
shared memory.
Note 1
Programs within different agents may share memory by unspecified means. At a
minimum, the backing memory for SharedArrayBuffers can be shared among the
agents in the cluster.
There may be agents that can communicate by message passing that cannot share
memory; they are never in the same agent cluster.
Every agent belongs to exactly one agent cluster.
Note 2
The agents in a cluster need not all be alive at some particular point in time.
If agent A creates another agent B, after which A terminates and B creates agent
C, the three agents are in the same cluster if A could share some memory with B
and B could share some memory with C.
All agents within a cluster must have the same value for the [[LittleEndian]]
field in their respective Agent Records.
Note 3
If different agents within an agent cluster have different values of
[[LittleEndian]] it becomes hard to use shared memory for multi-byte data.
All agents within a cluster must have the same values for the [[IsLockFree1]]
field in their respective Agent Records; similarly for the [[IsLockFree2]]
field.
All agents within a cluster must have different values for the [[Signifier]]
field in their respective Agent Records.
An embedding may deactivate (stop forward progress) or activate (resume forward
progress) an agent without the agent's knowledge or cooperation. If the
embedding does so, it must not leave some agents in the cluster active while
other agents in the cluster are deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where an agent
deadlocks or starves because another agent has been deactivated. For example, if
an HTML shared worker that has a lifetime independent of documents in any
windows were allowed to share memory with the dedicated worker of such an
independent document, and the document and its dedicated worker were to be
deactivated while the dedicated worker holds a lock (say, the document is pushed
into its window's history), and the shared worker then tries to acquire the
lock, then the shared worker will be blocked until the dedicated worker is
activated again, if ever. Meanwhile other workers trying to access the shared
worker from other windows will starve.
The implication of the restriction is that it will not be possible to share
memory between agents that don't belong to the same suspend/wake collective
within the embedding.
An embedding may terminate an agent without any of the agent's cluster's other
agents' prior knowledge or cooperation. If an agent is terminated not by
programmatic action of its own or of another agent in the cluster but by forces
external to the cluster, then the embedding must choose one of two strategies:
Either terminate all the agents in the cluster, or provide reliable APIs that
allow the agents in the cluster to coordinate so that at least one remaining
member of the cluster will be able to detect the termination, with the
termination data containing enough information to identify the agent that was
terminated.
Note 5
Examples of that type of termination are: operating systems or users terminating
agents that are running in separate processes; the embedding itself terminating
an agent that is running in-process with the other agents when per-agent
resource accounting indicates that the agent is runaway.
Prior to any evaluation of any ECMAScript code by any agent in a cluster, the
[[CandidateExecution]] field of the Agent Record for all agents in the cluster
is set to the initial candidate execution. The initial candidate execution is an
empty candidate execution whose [[EventsRecords]] field is a List containing,
for each agent, an Agent Events Record whose [[AgentSignifier]] field is that
agent's agent signifier, and whose [[EventList]] and [[AgentSynchronizesWith]]
fields are empty Lists.
Note 6
All agents in an agent cluster share the same candidate execution in its Agent
Record's [[CandidateExecution]] field. The candidate execution is a
specification mechanism used by the memory model.
Note 7
An agent cluster is a specification mechanism and need not correspond to any
particular artefact of an ECMAScript implementation.
9.9 FORWARD PROGRESS
For an agent to make forward progress is for it to perform an evaluation step
according to this specification.
An agent becomes blocked when its running execution context waits synchronously
and indefinitely for an external event. Only agents whose Agent Record's
[[CanBlock]] field is true can become blocked in this sense. An unblocked agent
is one that is not blocked.
Implementations must ensure that:
* every unblocked agent with a dedicated executing thread eventually makes
forward progress
* in a set of agents that share an executing thread, one agent eventually makes
forward progress
* an agent does not cause another agent to become blocked except via explicit
APIs that provide blocking.
Note
This, along with the liveness guarantee in the memory model, ensures that all
SeqCst writes eventually become observable to all agents.
9.10 PROCESSING MODEL OF WEAKREF AND FINALIZATIONREGISTRY TARGETS
9.10.1 OBJECTIVES
This specification does not make any guarantees that any object or symbol will
be garbage collected. Objects or symbols which are not live may be released
after long periods of time, or never at all. For this reason, this specification
uses the term "may" when describing behaviour triggered by garbage collection.
The semantics of WeakRefs and FinalizationRegistrys is based on two operations
which happen at particular points in time:
* When WeakRef.prototype.deref is called, the referent (if undefined is not
returned) is kept alive so that subsequent, synchronous accesses also return
the same value. This list is reset when synchronous work is done using the
ClearKeptObjects abstract operation.
* When an object or symbol which is registered with a FinalizationRegistry
becomes unreachable, a call of the FinalizationRegistry's cleanup callback
may eventually be made, after synchronous ECMAScript execution completes. The
FinalizationRegistry cleanup is performed with the
CleanupFinalizationRegistry abstract operation.
Neither of these actions (ClearKeptObjects or CleanupFinalizationRegistry) may
interrupt synchronous ECMAScript execution. Because hosts may assemble longer,
synchronous ECMAScript execution runs, this specification defers the scheduling
of ClearKeptObjects and CleanupFinalizationRegistry to the host environment.
Some ECMAScript implementations include garbage collector implementations which
run in the background, including when ECMAScript is idle. Letting the host
environment schedule CleanupFinalizationRegistry allows it to resume ECMAScript
execution in order to run finalizer work, which may free up held values,
reducing overall memory usage.
9.10.2 LIVENESS
For some set of objects and/or symbols S a hypothetical WeakRef-oblivious
execution with respect to S is an execution whereby the abstract operation
WeakRefDeref of a WeakRef whose referent is an element of S always returns
undefined.
Note 1
WeakRef-obliviousness, together with liveness, capture two notions. One, that a
WeakRef itself does not keep its referent alive. Two, that cycles in liveness
does not imply that a value is live. To be concrete, if determining v's liveness
depends on determining the liveness of a WeakRef referent, r, r's liveness
cannot assume v's liveness, which would be circular reasoning.
Note 2
WeakRef-obliviousness is defined on sets of objects or symbols instead of
individual values to account for cycles. If it were defined on individual
values, then a WeakRef referent in a cycle will be considered live even though
its identity is only observed via other WeakRef referents in the cycle.
Note 3
Colloquially, we say that an individual object or symbol is live if every set
containing it is live.
At any point during evaluation, a set of objects and/or symbols S is considered
live if either of the following conditions is met:
* Any element in S is included in any agent's [[KeptAlive]] List.
* There exists a valid future hypothetical WeakRef-oblivious execution with
respect to S that observes the identity of any value in S.
Note 4
The second condition above intends to capture the intuition that a value is live
if its identity is observable via non-WeakRef means. A value's identity may be
observed by observing a strict equality comparison or observing the value being
used as key in a Map.
Note 5
Presence of an object or a symbol in a field, an internal slot, or a property
does not imply that the value is live. For example if the value in question is
never passed back to the program, then it cannot be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the
[[WeakRefTarget]] and [[UnregisterToken]] fields of a FinalizationRegistry Cell
record.
The above definition implies that, if a key in a WeakMap is not live, then its
corresponding value is not necessarily live either.
Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not
empty. Liveness as defined here is undecidable. In practice, engines use
conservative approximations such as reachability. There is expected to be
significant implementation leeway.
9.10.3 EXECUTION
At any time, if a set of objects and/or symbols S is not live, an ECMAScript
implementation may perform the following steps atomically:
1. 1. 1. For each element value of S, do
1. a. a. For each WeakRef ref such that ref.[[WeakRefTarget]] is value, do
1. i. i. Set ref.[[WeakRefTarget]] to empty.
2. b. b. For each FinalizationRegistry fg such that fg.[[Cells]] contains a
Record cell such that cell.[[WeakRefTarget]] is value, do
1. i. i. Set cell.[[WeakRefTarget]] to empty.
2. ii. ii. Optionally, perform
HostEnqueueFinalizationRegistryCleanupJob(fg).
3. c. c. For each WeakMap map such that map.[[WeakMapData]] contains a
Record r such that r.[[Key]] is value, do
1. i. i. Set r.[[Key]] to empty.
2. ii. ii. Set r.[[Value]] to empty.
4. d. d. For each WeakSet set such that set.[[WeakSetData]] contains value,
do
1. i. i. Replace the element of set.[[WeakSetData]] whose value is value
with an element whose value is empty.
Note 1
Together with the definition of liveness, this clause prescribes optimizations
that an implementation may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations
such as dead variable elimination and scalar replacement on properties of
non-escaping objects whose identity is not observed are allowed. These
optimizations are thus allowed to observably empty WeakRefs that point to such
objects.
On the other hand, if an object's identity is observable, and that object is in
the [[WeakRefTarget]] internal slot of a WeakRef, optimizations such as
rematerialization that observably empty the WeakRef are prohibited.
Because calling HostEnqueueFinalizationRegistryCleanupJob is optional,
registered objects in a FinalizationRegistry do not necessarily hold that
FinalizationRegistry live. Implementations may omit FinalizationRegistry
callbacks for any reason, e.g., if the FinalizationRegistry itself becomes dead,
or if the application is shutting down.
Note 2
Implementations are not obligated to empty WeakRefs for maximal sets of non-live
objects or symbols.
If an implementation chooses a non-live set S in which to empty WeakRefs, this
definition requires that it empties WeakRefs for all values in S simultaneously.
In other words, it is not conformant for an implementation to empty a WeakRef
pointing to a value v without emptying out other WeakRefs that, if not emptied,
could result in an execution that observes the value of v.
9.10.4 HOST HOOKS
9.10.4.1 HOSTENQUEUEFINALIZATIONREGISTRYCLEANUPJOB ( FINALIZATIONREGISTRY )
The host-defined abstract operation HostEnqueueFinalizationRegistryCleanupJob
takes argument finalizationRegistry (a FinalizationRegistry) and returns unused.
Let cleanupJob be a new Job Abstract Closure with no parameters that captures
finalizationRegistry and performs the following steps when called:
1. 1. 1. Let cleanupResult be
Completion(CleanupFinalizationRegistry(finalizationRegistry)).
2. 2. 2. If cleanupResult is an abrupt completion, perform any host-defined
steps for reporting the error.
3. 3. 3. Return unused.
An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules
cleanupJob to be performed at some future time, if possible. It must also
conform to the requirements in 9.5.
9.11 CLEARKEPTOBJECTS ( )
The abstract operation ClearKeptObjects takes no arguments and returns unused.
ECMAScript implementations are expected to call ClearKeptObjects when a
synchronous sequence of ECMAScript executions completes. It performs the
following steps when called:
1. 1. 1. Let agentRecord be the surrounding agent's Agent Record.
2. 2. 2. Set agentRecord.[[KeptAlive]] to a new empty List.
3. 3. 3. Return unused.
9.12 ADDTOKEPTOBJECTS ( VALUE )
The abstract operation AddToKeptObjects takes argument value (an Object or a
Symbol) and returns unused. It performs the following steps when called:
1. 1. 1. Let agentRecord be the surrounding agent's Agent Record.
2. 2. 2. Append value to agentRecord.[[KeptAlive]].
3. 3. 3. Return unused.
Note
When the abstract operation AddToKeptObjects is called with a target object or
symbol, it adds the target to a list that will point strongly at the target
until ClearKeptObjects is called.
9.13 CLEANUPFINALIZATIONREGISTRY ( FINALIZATIONREGISTRY )
The abstract operation CleanupFinalizationRegistry takes argument
finalizationRegistry (a FinalizationRegistry) and returns either a normal
completion containing unused or a throw completion. It performs the following
steps when called:
1. 1. 1. Assert: finalizationRegistry has [[Cells]] and [[CleanupCallback]]
internal slots.
2. 2. 2. Let callback be finalizationRegistry.[[CleanupCallback]].
3. 3. 3. While finalizationRegistry.[[Cells]] contains a Record cell such that
cell.[[WeakRefTarget]] is empty, an implementation may perform the following
steps:
1. a. a. Choose any such cell.
2. b. b. Remove cell from finalizationRegistry.[[Cells]].
3. c. c. Perform ? HostCallJobCallback(callback, undefined, «
cell.[[HeldValue]] »).
4. 4. 4. Return unused.
9.14 CANBEHELDWEAKLY ( V )
The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript language
value) and returns a Boolean. It returns true if and only if v is suitable for
use as a weak reference. Only values that are suitable for use as a weak
reference may be a key of a WeakMap, an element of a WeakSet, the target of a
WeakRef, or one of the targets of a FinalizationRegistry. It performs the
following steps when called:
1. 1. 1. If v is an Object, return true.
2. 2. 2. If v is a Symbol and KeyForSymbol(v) is undefined, return true.
3. 3. 3. Return false.
Note
A language value without language identity can be manifested without prior
reference and is unsuitable for use as a weak reference. A Symbol value produced
by Symbol.for, unlike other Symbol values, does not have language identity and
is unsuitable for use as a weak reference. Well-known symbols are likely to
never be collected, but are nonetheless treated as suitable for use as a weak
reference because they are limited in number and therefore manageable by a
variety of implementation approaches. However, any value associated to a
well-known symbol in a live WeakMap is unlikely to be collected and could “leak”
memory resources in implementations.
10 ORDINARY AND EXOTIC OBJECTS BEHAVIOURS
10.1 ORDINARY OBJECT INTERNAL METHODS AND INTERNAL SLOTS
All ordinary objects have an internal slot called [[Prototype]]. The value of
this internal slot is either null or an object and is used for implementing
inheritance. Assume a property named P is missing from an ordinary object O but
exists on its [[Prototype]] object. If P refers to a data property on the
[[Prototype]] object, O inherits it for get access, making it behave as if P was
a property of O. If P refers to a writable data property on the [[Prototype]]
object, set access of P on O creates a new data property named P on O. If P
refers to a non-writable data property on the [[Prototype]] object, set access
of P on O fails. If P refers to an accessor property on the [[Prototype]]
object, the accessor is inherited by O for both get access and set access.
Every ordinary object has a Boolean-valued [[Extensible]] internal slot which is
used to fulfill the extensibility-related internal method invariants specified
in 6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot
has been set to false, it is no longer possible to add properties to the object,
to modify the value of the object's [[Prototype]] internal slot, or to
subsequently change the value of [[Extensible]] to true.
In the following algorithm descriptions, assume O is an ordinary object, P is a
property key value, V is any ECMAScript language value, and Desc is a Property
Descriptor record.
Each ordinary object internal method delegates to a similarly-named abstract
operation. If such an abstract operation depends on another internal method,
then the internal method is invoked on O rather than calling the similarly-named
abstract operation directly. These semantics ensure that exotic objects have
their overridden internal methods invoked when ordinary object internal methods
are applied to them.
10.1.1 [[GETPROTOTYPEOF]] ( )
The [[GetPrototypeOf]] internal method of an ordinary object O takes no
arguments and returns a normal completion containing either an Object or null.
It performs the following steps when called:
1. 1. 1. Return OrdinaryGetPrototypeOf(O).
10.1.1.1 ORDINARYGETPROTOTYPEOF ( O )
The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and
returns an Object or null. It performs the following steps when called:
1. 1. 1. Return O.[[Prototype]].
10.1.2 [[SETPROTOTYPEOF]] ( V )
The [[SetPrototypeOf]] internal method of an ordinary object O takes argument V
(an Object or null) and returns a normal completion containing a Boolean. It
performs the following steps when called:
1. 1. 1. Return OrdinarySetPrototypeOf(O, V).
10.1.2.1 ORDINARYSETPROTOTYPEOF ( O, V )
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and
V (an Object or null) and returns a Boolean. It performs the following steps
when called:
1. 1. 1. Let current be O.[[Prototype]].
2. 2. 2. If SameValue(V, current) is true, return true.
3. 3. 3. Let extensible be O.[[Extensible]].
4. 4. 4. If extensible is false, return false.
5. 5. 5. Let p be V.
6. 6. 6. Let done be false.
7. 7. 7. Repeat, while done is false,
1. a. a. If p is null, set done to true.
2. b. b. Else if SameValue(p, O) is true, return false.
3. c. c. Else,
1. i. i. If p.[[GetPrototypeOf]] is not the ordinary object internal
method defined in 10.1.1, set done to true.
2. ii. ii. Else, set p to p.[[Prototype]].
8. 8. 8. Set O.[[Prototype]] to V.
9. 9. 9. Return true.
Note
The loop in step 7 guarantees that there will be no circularities in any
prototype chain that only includes objects that use the ordinary object
definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
10.1.3 [[ISEXTENSIBLE]] ( )
The [[IsExtensible]] internal method of an ordinary object O takes no arguments
and returns a normal completion containing a Boolean. It performs the following
steps when called:
1. 1. 1. Return OrdinaryIsExtensible(O).
10.1.3.1 ORDINARYISEXTENSIBLE ( O )
The abstract operation OrdinaryIsExtensible takes argument O (an Object) and
returns a Boolean. It performs the following steps when called:
1. 1. 1. Return O.[[Extensible]].
10.1.4 [[PREVENTEXTENSIONS]] ( )
The [[PreventExtensions]] internal method of an ordinary object O takes no
arguments and returns a normal completion containing true. It performs the
following steps when called:
1. 1. 1. Return OrdinaryPreventExtensions(O).
10.1.4.1 ORDINARYPREVENTEXTENSIONS ( O )
The abstract operation OrdinaryPreventExtensions takes argument O (an Object)
and returns true. It performs the following steps when called:
1. 1. 1. Set O.[[Extensible]] to false.
2. 2. 2. Return true.
10.1.5 [[GETOWNPROPERTY]] ( P )
The [[GetOwnProperty]] internal method of an ordinary object O takes argument P
(a property key) and returns a normal completion containing either a Property
Descriptor or undefined. It performs the following steps when called:
1. 1. 1. Return OrdinaryGetOwnProperty(O, P).
10.1.5.1 ORDINARYGETOWNPROPERTY ( O, P )
The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and
P (a property key) and returns a Property Descriptor or undefined. It performs
the following steps when called:
1. 1. 1. If O does not have an own property with key P, return undefined.
2. 2. 2. Let D be a newly created Property Descriptor with no fields.
3. 3. 3. Let X be O's own property whose key is P.
4. 4. 4. If X is a data property, then
1. a. a. Set D.[[Value]] to the value of X's [[Value]] attribute.
2. b. b. Set D.[[Writable]] to the value of X's [[Writable]] attribute.
5. 5. 5. Else,
1. a. a. Assert: X is an accessor property.
2. b. b. Set D.[[Get]] to the value of X's [[Get]] attribute.
3. c. c. Set D.[[Set]] to the value of X's [[Set]] attribute.
6. 6. 6. Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
7. 7. 7. Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
8. 8. 8. Return D.
10.1.6 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of an ordinary object O takes
arguments P (a property key) and Desc (a Property Descriptor) and returns either
a normal completion containing a Boolean or a throw completion. It performs the
following steps when called:
1. 1. 1. Return ? OrdinaryDefineOwnProperty(O, P, Desc).
10.1.6.1 ORDINARYDEFINEOWNPROPERTY ( O, P, DESC )
The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object),
P (a property key), and Desc (a Property Descriptor) and returns either a normal
completion containing a Boolean or a throw completion. It performs the following
steps when called:
1. 1. 1. Let current be ? O.[[GetOwnProperty]](P).
2. 2. 2. Let extensible be ? IsExtensible(O).
3. 3. 3. Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc,
current).
10.1.6.2 ISCOMPATIBLEPROPERTYDESCRIPTOR ( EXTENSIBLE, DESC, CURRENT )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible
(a Boolean), Desc (a Property Descriptor), and Current (a Property Descriptor)
and returns a Boolean. It performs the following steps when called:
1. 1. 1. Return ValidateAndApplyPropertyDescriptor(undefined, "", Extensible,
Desc, Current).
10.1.6.3 VALIDATEANDAPPLYPROPERTYDESCRIPTOR ( O, P, EXTENSIBLE, DESC, CURRENT )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an
Object or undefined), P (a property key), extensible (a Boolean), Desc (a
Property Descriptor), and current (a Property Descriptor or undefined) and
returns a Boolean. It returns true if and only if Desc can be applied as the
property of an object with specified extensibility and current property current
while upholding invariants. When such application is possible and O is not
undefined, it is performed for the property named P (which is created if
necessary). It performs the following steps when called:
1. 1. 1. Assert: IsPropertyKey(P) is true.
2. 2. 2. If current is undefined, then
1. a. a. If extensible is false, return false.
2. b. b. If O is undefined, return true.
3. c. c. If IsAccessorDescriptor(Desc) is true, then
1. i. i. Create an own accessor property named P of object O whose
[[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attributes are
set to the value of the corresponding field in Desc if Desc has that
field, or to the attribute's default value otherwise.
4. d. d. Else,
1. i. i. Create an own data property named P of object O whose [[Value]],
[[Writable]], [[Enumerable]], and [[Configurable]] attributes are set
to the value of the corresponding field in Desc if Desc has that
field, or to the attribute's default value otherwise.
5. e. e. Return true.
3. 3. 3. Assert: current is a fully populated Property Descriptor.
4. 4. 4. If Desc does not have any fields, return true.
5. 5. 5. If current.[[Configurable]] is false, then
1. a. a. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is
true, return false.
2. b. b. If Desc has an [[Enumerable]] field and
SameValue(Desc.[[Enumerable]], current.[[Enumerable]]) is false, return
false.
3. c. c. If IsGenericDescriptor(Desc) is false and
SameValue(IsAccessorDescriptor(Desc), IsAccessorDescriptor(current)) is
false, return false.
4. d. d. If IsAccessorDescriptor(current) is true, then
1. i. i. If Desc has a [[Get]] field and SameValue(Desc.[[Get]],
current.[[Get]]) is false, return false.
2. ii. ii. If Desc has a [[Set]] field and SameValue(Desc.[[Set]],
current.[[Set]]) is false, return false.
5. e. e. Else if current.[[Writable]] is false, then
1. i. i. If Desc has a [[Writable]] field and Desc.[[Writable]] is true,
return false.
2. ii. ii. If Desc has a [[Value]] field and SameValue(Desc.[[Value]],
current.[[Value]]) is false, return false.
6. 6. 6. If O is not undefined, then
1. a. a. If IsDataDescriptor(current) is true and IsAccessorDescriptor(Desc)
is true, then
1. i. i. If Desc has a [[Configurable]] field, let configurable be
Desc.[[Configurable]]; else let configurable be
current.[[Configurable]].
2. ii. ii. If Desc has a [[Enumerable]] field, let enumerable be
Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
3. iii. iii. Replace the property named P of object O with an accessor
property whose [[Configurable]] and [[Enumerable]] attributes are set
to configurable and enumerable, respectively, and whose [[Get]] and
[[Set]] attributes are set to the value of the corresponding field in
Desc if Desc has that field, or to the attribute's default value
otherwise.
2. b. b. Else if IsAccessorDescriptor(current) is true and
IsDataDescriptor(Desc) is true, then
1. i. i. If Desc has a [[Configurable]] field, let configurable be
Desc.[[Configurable]]; else let configurable be
current.[[Configurable]].
2. ii. ii. If Desc has a [[Enumerable]] field, let enumerable be
Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
3. iii. iii. Replace the property named P of object O with a data
property whose [[Configurable]] and [[Enumerable]] attributes are set
to configurable and enumerable, respectively, and whose [[Value]] and
[[Writable]] attributes are set to the value of the corresponding
field in Desc if Desc has that field, or to the attribute's default
value otherwise.
3. c. c. Else,
1. i. i. For each field of Desc, set the corresponding attribute of the
property named P of object O to the value of the field.
7. 7. 7. Return true.
10.1.7 [[HASPROPERTY]] ( P )
The [[HasProperty]] internal method of an ordinary object O takes argument P (a
property key) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. Return ? OrdinaryHasProperty(O, P).
10.1.7.1 ORDINARYHASPROPERTY ( O, P )
The abstract operation OrdinaryHasProperty takes arguments O (an Object) and P
(a property key) and returns either a normal completion containing a Boolean or
a throw completion. It performs the following steps when called:
1. 1. 1. Let hasOwn be ? O.[[GetOwnProperty]](P).
2. 2. 2. If hasOwn is not undefined, return true.
3. 3. 3. Let parent be ? O.[[GetPrototypeOf]]().
4. 4. 4. If parent is not null, then
1. a. a. Return ? parent.[[HasProperty]](P).
5. 5. 5. Return false.
10.1.8 [[GET]] ( P, RECEIVER )
The [[Get]] internal method of an ordinary object O takes arguments P (a
property key) and Receiver (an ECMAScript language value) and returns either a
normal completion containing an ECMAScript language value or a throw completion.
It performs the following steps when called:
1. 1. 1. Return ? OrdinaryGet(O, P, Receiver).
10.1.8.1 ORDINARYGET ( O, P, RECEIVER )
The abstract operation OrdinaryGet takes arguments O (an Object), P (a property
key), and Receiver (an ECMAScript language value) and returns either a normal
completion containing an ECMAScript language value or a throw completion. It
performs the following steps when called:
1. 1. 1. Let desc be ? O.[[GetOwnProperty]](P).
2. 2. 2. If desc is undefined, then
1. a. a. Let parent be ? O.[[GetPrototypeOf]]().
2. b. b. If parent is null, return undefined.
3. c. c. Return ? parent.[[Get]](P, Receiver).
3. 3. 3. If IsDataDescriptor(desc) is true, return desc.[[Value]].
4. 4. 4. Assert: IsAccessorDescriptor(desc) is true.
5. 5. 5. Let getter be desc.[[Get]].
6. 6. 6. If getter is undefined, return undefined.
7. 7. 7. Return ? Call(getter, Receiver).
10.1.9 [[SET]] ( P, V, RECEIVER )
The [[Set]] internal method of an ordinary object O takes arguments P (a
property key), V (an ECMAScript language value), and Receiver (an ECMAScript
language value) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. Return ? OrdinarySet(O, P, V, Receiver).
10.1.9.1 ORDINARYSET ( O, P, V, RECEIVER )
The abstract operation OrdinarySet takes arguments O (an Object), P (a property
key), V (an ECMAScript language value), and Receiver (an ECMAScript language
value) and returns either a normal completion containing a Boolean or a throw
completion. It performs the following steps when called:
1. 1. 1. Let ownDesc be ? O.[[GetOwnProperty]](P).
2. 2. 2. Return ? OrdinarySetWithOwnDescriptor(O, P, V, Receiver, ownDesc).
10.1.9.2 ORDINARYSETWITHOWNDESCRIPTOR ( O, P, V, RECEIVER, OWNDESC )
The abstract operation OrdinarySetWithOwnDescriptor takes arguments O (an
Object), P (a property key), V (an ECMAScript language value), Receiver (an
ECMAScript language value), and ownDesc (a Property Descriptor or undefined) and
returns either a normal completion containing a Boolean or a throw completion.
It performs the following steps when called:
1. 1. 1. If ownDesc is undefined, then
1. a. a. Let parent be ? O.[[GetPrototypeOf]]().
2. b. b. If parent is not null, then
1. i. i. Return ? parent.[[Set]](P, V, Receiver).
3. c. c. Else,
1. i. i. Set ownDesc to the PropertyDescriptor { [[Value]]: undefined,
[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
2. 2. 2. If IsDataDescriptor(ownDesc) is true, then
1. a. a. If ownDesc.[[Writable]] is false, return false.
2. b. b. If Receiver is not an Object, return false.
3. c. c. Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
4. d. d. If existingDescriptor is not undefined, then
1. i. i. If IsAccessorDescriptor(existingDescriptor) is true, return
false.
2. ii. ii. If existingDescriptor.[[Writable]] is false, return false.
3. iii. iii. Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
4. iv. iv. Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
5. e. e. Else,
1. i. i. Assert: Receiver does not currently have a property P.
2. ii. ii. Return ? CreateDataProperty(Receiver, P, V).
3. 3. 3. Assert: IsAccessorDescriptor(ownDesc) is true.
4. 4. 4. Let setter be ownDesc.[[Set]].
5. 5. 5. If setter is undefined, return false.
6. 6. 6. Perform ? Call(setter, Receiver, « V »).
7. 7. 7. Return true.
10.1.10 [[DELETE]] ( P )
The [[Delete]] internal method of an ordinary object O takes argument P (a
property key) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. Return ? OrdinaryDelete(O, P).
10.1.10.1 ORDINARYDELETE ( O, P )
The abstract operation OrdinaryDelete takes arguments O (an Object) and P (a
property key) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. Let desc be ? O.[[GetOwnProperty]](P).
2. 2. 2. If desc is undefined, return true.
3. 3. 3. If desc.[[Configurable]] is true, then
1. a. a. Remove the own property with name P from O.
2. b. b. Return true.
4. 4. 4. Return false.
10.1.11 [[OWNPROPERTYKEYS]] ( )
The [[OwnPropertyKeys]] internal method of an ordinary object O takes no
arguments and returns a normal completion containing a List of property keys. It
performs the following steps when called:
1. 1. 1. Return OrdinaryOwnPropertyKeys(O).
10.1.11.1 ORDINARYOWNPROPERTYKEYS ( O )
The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and
returns a List of property keys. It performs the following steps when called:
1. 1. 1. Let keys be a new empty List.
2. 2. 2. For each own property key P of O such that P is an array index, in
ascending numeric index order, do
1. a. a. Append P to keys.
3. 3. 3. For each own property key P of O such that P is a String and P is not
an array index, in ascending chronological order of property creation, do
1. a. a. Append P to keys.
4. 4. 4. For each own property key P of O such that P is a Symbol, in ascending
chronological order of property creation, do
1. a. a. Append P to keys.
5. 5. 5. Return keys.
10.1.12 ORDINARYOBJECTCREATE ( PROTO [ , ADDITIONALINTERNALSLOTSLIST ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object or
null) and optional argument additionalInternalSlotsList (a List of names of
internal slots) and returns an Object. It is used to specify the runtime
creation of new ordinary objects. additionalInternalSlotsList contains the names
of additional internal slots that must be defined as part of the object, beyond
[[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not
provided, a new empty List is used. It performs the following steps when called:
1. 1. 1. Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
2. 2. 2. If additionalInternalSlotsList is present, set internalSlotsList to
the list-concatenation of internalSlotsList and additionalInternalSlotsList.
3. 3. 3. Let O be MakeBasicObject(internalSlotsList).
4. 4. 4. Set O.[[Prototype]] to proto.
5. 5. 5. Return O.
Note
Although OrdinaryObjectCreate does little more than call MakeBasicObject, its
use communicates the intention to create an ordinary object, and not an exotic
one. Thus, within this specification, it is not called by any algorithm that
subsequently modifies the internal methods of the object in ways that would make
the result non-ordinary. Operations that create exotic objects invoke
MakeBasicObject directly.
10.1.13 ORDINARYCREATEFROMCONSTRUCTOR ( CONSTRUCTOR, INTRINSICDEFAULTPROTO [ ,
INTERNALSLOTSLIST ] )
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor
(a constructor) and intrinsicDefaultProto (a String) and optional argument
internalSlotsList (a List of names of internal slots) and returns either a
normal completion containing an Object or a throw completion. It creates an
ordinary object whose [[Prototype]] value is retrieved from a constructor's
"prototype" property, if it exists. Otherwise the intrinsic named by
intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the
names of additional internal slots that must be defined as part of the object.
If internalSlotsList is not provided, a new empty List is used. It performs the
following steps when called:
1. 1. 1. Assert: intrinsicDefaultProto is this specification's name of an
intrinsic object. The corresponding object must be an intrinsic that is
intended to be used as the [[Prototype]] value of an object.
2. 2. 2. Let proto be ? GetPrototypeFromConstructor(constructor,
intrinsicDefaultProto).
3. 3. 3. If internalSlotsList is present, let slotsList be internalSlotsList.
4. 4. 4. Else, let slotsList be a new empty List.
5. 5. 5. Return OrdinaryObjectCreate(proto, slotsList).
10.1.14 GETPROTOTYPEFROMCONSTRUCTOR ( CONSTRUCTOR, INTRINSICDEFAULTPROTO )
The abstract operation GetPrototypeFromConstructor takes arguments constructor
(a function object) and intrinsicDefaultProto (a String) and returns either a
normal completion containing an Object or a throw completion. It determines the
[[Prototype]] value that should be used to create an object corresponding to a
specific constructor. The value is retrieved from the constructor's "prototype"
property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto
is used for [[Prototype]]. It performs the following steps when called:
1. 1. 1. Assert: intrinsicDefaultProto is this specification's name of an
intrinsic object. The corresponding object must be an intrinsic that is
intended to be used as the [[Prototype]] value of an object.
2. 2. 2. Let proto be ? Get(constructor, "prototype").
3. 3. 3. If proto is not an Object, then
1. a. a. Let realm be ? GetFunctionRealm(constructor).
2. b. b. Set proto to realm's intrinsic object named intrinsicDefaultProto.
4. 4. 4. Return proto.
Note
If constructor does not supply a [[Prototype]] value, the default value that is
used is obtained from the realm of the constructor function rather than from the
running execution context.
10.1.15 REQUIREINTERNALSLOT ( O, INTERNALSLOT )
The abstract operation RequireInternalSlot takes arguments O (an ECMAScript
language value) and internalSlot (an internal slot name) and returns either a
normal completion containing unused or a throw completion. It throws an
exception unless O is an Object and has the given internal slot. It performs the
following steps when called:
1. 1. 1. If O is not an Object, throw a TypeError exception.
2. 2. 2. If O does not have an internalSlot internal slot, throw a TypeError
exception.
3. 3. 3. Return unused.
10.2 ECMASCRIPT FUNCTION OBJECTS
ECMAScript function objects encapsulate parameterized ECMAScript code closed
over a lexical environment and support the dynamic evaluation of that code. An
ECMAScript function object is an ordinary object and has the same internal slots
and the same internal methods as other ordinary objects. The code of an
ECMAScript function object may be either strict mode code (11.2.2) or non-strict
code. An ECMAScript function object whose code is strict mode code is called a
strict function. One whose code is not strict mode code is called a non-strict
function.
In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects
also have the internal slots listed in Table 30.
Table 30: Internal Slots of ECMAScript Function Objects
Internal Slot Type Description [[Environment]] an Environment Record The
Environment Record that the function was closed over. Used as the outer
environment when evaluating the code of the function. [[PrivateEnvironment]] a
PrivateEnvironment Record or null The PrivateEnvironment Record for Private
Names that the function was closed over. null if this function is not
syntactically contained within a class. Used as the outer PrivateEnvironment for
inner classes when evaluating the code of the function. [[FormalParameters]] a
Parse Node The root parse node of the source text that defines the function's
formal parameter list. [[ECMAScriptCode]] a Parse Node The root parse node of
the source text that defines the function's body. [[ConstructorKind]] base or
derived Whether or not the function is a derived class constructor. [[Realm]] a
Realm Record The realm in which the function was created and which provides any
intrinsic objects that are accessed when evaluating the function.
[[ScriptOrModule]] a Script Record or a Module Record The script or module in
which the function was created. [[ThisMode]] lexical, strict, or global Defines
how this references are interpreted within the formal parameters and code body
of the function. lexical means that this refers to the this value of a lexically
enclosing function. strict means that the this value is used exactly as provided
by an invocation of the function. global means that a this value of undefined or
null is interpreted as a reference to the global object, and any other this
value is first passed to ToObject. [[Strict]] a Boolean true if this is a strict
function, false if this is a non-strict function. [[HomeObject]] an Object If
the function uses super, this is the object whose [[GetPrototypeOf]] provides
the object where super property lookups begin. [[SourceText]] a sequence of
Unicode code points The source text that defines the function. [[Fields]] a List
of ClassFieldDefinition Records If the function is a class, this is a list of
Records representing the non-static fields and corresponding initializers of the
class. [[PrivateMethods]] a List of PrivateElements If the function is a class,
this is a list representing the non-static private methods and accessors of the
class. [[ClassFieldInitializerName]] a String, a Symbol, a Private Name, or
empty If the function is created as the initializer of a class field, the name
to use for NamedEvaluation of the field; empty otherwise. [[IsClassConstructor]]
a Boolean Indicates whether the function is a class constructor. (If true,
invoking the function's [[Call]] will immediately throw a TypeError exception.)
All ECMAScript function objects have the [[Call]] internal method defined here.
ECMAScript functions that are also constructors in addition have the
[[Construct]] internal method.
10.2.1 [[CALL]] ( THISARGUMENT, ARGUMENTSLIST )
The [[Call]] internal method of an ECMAScript function object F takes arguments
thisArgument (an ECMAScript language value) and argumentsList (a List of
ECMAScript language values) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It performs the following steps
when called:
1. 1. 1. Let callerContext be the running execution context.
2. 2. 2. Let calleeContext be PrepareForOrdinaryCall(F, undefined).
3. 3. 3. Assert: calleeContext is now the running execution context.
4. 4. 4. If F.[[IsClassConstructor]] is true, then
1. a. a. Let error be a newly created TypeError object.
2. b. b. NOTE: error is created in calleeContext with F's associated Realm
Record.
3. c. c. Remove calleeContext from the execution context stack and restore
callerContext as the running execution context.
4. d. d. Return ThrowCompletion(error).
5. 5. 5. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
6. 6. 6. Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
7. 7. 7. Remove calleeContext from the execution context stack and restore
callerContext as the running execution context.
8. 8. 8. If result.[[Type]] is return, return result.[[Value]].
9. 9. 9. ReturnIfAbrupt(result).
10. 10. 10. Return undefined.
Note
When calleeContext is removed from the execution context stack in step 7 it must
not be destroyed if it is suspended and retained for later resumption by an
accessible Generator.
10.2.1.1 PREPAREFORORDINARYCALL ( F, NEWTARGET )
The abstract operation PrepareForOrdinaryCall takes arguments F (a function
object) and newTarget (an Object or undefined) and returns an execution context.
It performs the following steps when called:
1. 1. 1. Let callerContext be the running execution context.
2. 2. 2. Let calleeContext be a new ECMAScript code execution context.
3. 3. 3. Set the Function of calleeContext to F.
4. 4. 4. Let calleeRealm be F.[[Realm]].
5. 5. 5. Set the Realm of calleeContext to calleeRealm.
6. 6. 6. Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
7. 7. 7. Let localEnv be NewFunctionEnvironment(F, newTarget).
8. 8. 8. Set the LexicalEnvironment of calleeContext to localEnv.
9. 9. 9. Set the VariableEnvironment of calleeContext to localEnv.
10. 10. 10. Set the PrivateEnvironment of calleeContext to
F.[[PrivateEnvironment]].
11. 11. 11. If callerContext is not already suspended, suspend callerContext.
12. 12. 12. Push calleeContext onto the execution context stack; calleeContext
is now the running execution context.
13. 13. 13. NOTE: Any exception objects produced after this point are
associated with calleeRealm.
14. 14. 14. Return calleeContext.
10.2.1.2 ORDINARYCALLBINDTHIS ( F, CALLEECONTEXT, THISARGUMENT )
The abstract operation OrdinaryCallBindThis takes arguments F (a function
object), calleeContext (an execution context), and thisArgument (an ECMAScript
language value) and returns unused. It performs the following steps when called:
1. 1. 1. Let thisMode be F.[[ThisMode]].
2. 2. 2. If thisMode is lexical, return unused.
3. 3. 3. Let calleeRealm be F.[[Realm]].
4. 4. 4. Let localEnv be the LexicalEnvironment of calleeContext.
5. 5. 5. If thisMode is strict, let thisValue be thisArgument.
6. 6. 6. Else,
1. a. a. If thisArgument is either undefined or null, then
1. i. i. Let globalEnv be calleeRealm.[[GlobalEnv]].
2. ii. ii. Assert: globalEnv is a Global Environment Record.
3. iii. iii. Let thisValue be globalEnv.[[GlobalThisValue]].
2. b. b. Else,
1. i. i. Let thisValue be ! ToObject(thisArgument).
2. ii. ii. NOTE: ToObject produces wrapper objects using calleeRealm.
7. 7. 7. Assert: localEnv is a Function Environment Record.
8. 8. 8. Assert: The next step never returns an abrupt completion because
localEnv.[[ThisBindingStatus]] is not initialized.
9. 9. 9. Perform ! localEnv.BindThisValue(thisValue).
10. 10. 10. Return unused.
10.2.1.3 RUNTIME SEMANTICS: EVALUATEBODY
The syntax-directed operation EvaluateBody takes arguments functionObject (a
function object) and argumentsList (a List of ECMAScript language values) and
returns either a normal completion containing an ECMAScript language value or an
abrupt completion. It is defined piecewise over the following productions:
FunctionBody : FunctionStatementList
1. 1. 1. Return ? EvaluateFunctionBody of FunctionBody with arguments
functionObject and argumentsList.
ConciseBody : ExpressionBody
1. 1. 1. Return ? EvaluateConciseBody of ConciseBody with arguments
functionObject and argumentsList.
GeneratorBody : FunctionBody
1. 1. 1. Return ? EvaluateGeneratorBody of GeneratorBody with arguments
functionObject and argumentsList.
AsyncGeneratorBody : FunctionBody
1. 1. 1. Return ? EvaluateAsyncGeneratorBody of AsyncGeneratorBody with
arguments functionObject and argumentsList.
AsyncFunctionBody : FunctionBody
1. 1. 1. Return ? EvaluateAsyncFunctionBody of AsyncFunctionBody with arguments
functionObject and argumentsList.
AsyncConciseBody : ExpressionBody
1. 1. 1. Return ? EvaluateAsyncConciseBody of AsyncConciseBody with arguments
functionObject and argumentsList.
Initializer : = AssignmentExpression
1. 1. 1. Assert: argumentsList is empty.
2. 2. 2. Assert: functionObject.[[ClassFieldInitializerName]] is not empty.
3. 3. 3. If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. a. a. Let value be ? NamedEvaluation of Initializer with argument
functionObject.[[ClassFieldInitializerName]].
4. 4. 4. Else,
1. a. a. Let rhs be ? Evaluation of AssignmentExpression.
2. b. b. Let value be ? GetValue(rhs).
5. 5. 5. Return Completion Record { [[Type]]: return, [[Value]]: value,
[[Target]]: empty }.
Note
Even though field initializers constitute a function boundary, calling
FunctionDeclarationInstantiation does not have any observable effect and so is
omitted.
ClassStaticBlockBody : ClassStaticBlockStatementList
1. 1. 1. Assert: argumentsList is empty.
2. 2. 2. Return ? EvaluateClassStaticBlockBody of ClassStaticBlockBody with
argument functionObject.
10.2.1.4 ORDINARYCALLEVALUATEBODY ( F, ARGUMENTSLIST )
The abstract operation OrdinaryCallEvaluateBody takes arguments F (a function
object) and argumentsList (a List of ECMAScript language values) and returns
either a normal completion containing an ECMAScript language value or an abrupt
completion. It performs the following steps when called:
1. 1. 1. Return ? EvaluateBody of F.[[ECMAScriptCode]] with arguments F and
argumentsList.
10.2.2 [[CONSTRUCT]] ( ARGUMENTSLIST, NEWTARGET )
The [[Construct]] internal method of an ECMAScript function object F takes
arguments argumentsList (a List of ECMAScript language values) and newTarget (a
constructor) and returns either a normal completion containing an Object or a
throw completion. It performs the following steps when called:
1. 1. 1. Let callerContext be the running execution context.
2. 2. 2. Let kind be F.[[ConstructorKind]].
3. 3. 3. If kind is base, then
1. a. a. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget,
"%Object.prototype%").
4. 4. 4. Let calleeContext be PrepareForOrdinaryCall(F, newTarget).
5. 5. 5. Assert: calleeContext is now the running execution context.
6. 6. 6. If kind is base, then
1. a. a. Perform OrdinaryCallBindThis(F, calleeContext, thisArgument).
2. b. b. Let initializeResult be
Completion(InitializeInstanceElements(thisArgument, F)).
3. c. c. If initializeResult is an abrupt completion, then
1. i. i. Remove calleeContext from the execution context stack and
restore callerContext as the running execution context.
2. ii. ii. Return ? initializeResult.
7. 7. 7. Let constructorEnv be the LexicalEnvironment of calleeContext.
8. 8. 8. Let result be Completion(OrdinaryCallEvaluateBody(F, argumentsList)).
9. 9. 9. Remove calleeContext from the execution context stack and restore
callerContext as the running execution context.
10. 10. 10. If result.[[Type]] is return, then
1. a. a. If result.[[Value]] is an Object, return result.[[Value]].
2. b. b. If kind is base, return thisArgument.
3. c. c. If result.[[Value]] is not undefined, throw a TypeError exception.
11. 11. 11. Else, ReturnIfAbrupt(result).
12. 12. 12. Let thisBinding be ? constructorEnv.GetThisBinding().
13. 13. 13. Assert: thisBinding is an Object.
14. 14. 14. Return thisBinding.
10.2.3 ORDINARYFUNCTIONCREATE ( FUNCTIONPROTOTYPE, SOURCETEXT, PARAMETERLIST,
BODY, THISMODE, ENV, PRIVATEENV )
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype
(an Object), sourceText (a sequence of Unicode code points), ParameterList (a
Parse Node), Body (a Parse Node), thisMode (lexical-this or non-lexical-this),
env (an Environment Record), and privateEnv (a PrivateEnvironment Record or
null) and returns a function object. It is used to specify the runtime creation
of a new function with a default [[Call]] internal method and no [[Construct]]
internal method (although one may be subsequently added by an operation such as
MakeConstructor). sourceText is the source text of the syntactic definition of
the function to be created. It performs the following steps when called:
1. 1. 1. Let internalSlotsList be the internal slots listed in Table 30.
2. 2. 2. Let F be OrdinaryObjectCreate(functionPrototype, internalSlotsList).
3. 3. 3. Set F.[[Call]] to the definition specified in 10.2.1.
4. 4. 4. Set F.[[SourceText]] to sourceText.
5. 5. 5. Set F.[[FormalParameters]] to ParameterList.
6. 6. 6. Set F.[[ECMAScriptCode]] to Body.
7. 7. 7. If the source text matched by Body is strict mode code, let Strict be
true; else let Strict be false.
8. 8. 8. Set F.[[Strict]] to Strict.
9. 9. 9. If thisMode is lexical-this, set F.[[ThisMode]] to lexical.
10. 10. 10. Else if Strict is true, set F.[[ThisMode]] to strict.
11. 11. 11. Else, set F.[[ThisMode]] to global.
12. 12. 12. Set F.[[IsClassConstructor]] to false.
13. 13. 13. Set F.[[Environment]] to env.
14. 14. 14. Set F.[[PrivateEnvironment]] to privateEnv.
15. 15. 15. Set F.[[ScriptOrModule]] to GetActiveScriptOrModule().
16. 16. 16. Set F.[[Realm]] to the current Realm Record.
17. 17. 17. Set F.[[HomeObject]] to undefined.
18. 18. 18. Set F.[[Fields]] to a new empty List.
19. 19. 19. Set F.[[PrivateMethods]] to a new empty List.
20. 20. 20. Set F.[[ClassFieldInitializerName]] to empty.
21. 21. 21. Let len be the ExpectedArgumentCount of ParameterList.
22. 22. 22. Perform SetFunctionLength(F, len).
23. 23. 23. Return F.
10.2.4 ADDRESTRICTEDFUNCTIONPROPERTIES ( F, REALM )
The abstract operation AddRestrictedFunctionProperties takes arguments F (a
function object) and realm (a Realm Record) and returns unused. It performs the
following steps when called:
1. 1. 1. Assert: realm.[[Intrinsics]].[[%ThrowTypeError%]] exists and has been
initialized.
2. 2. 2. Let thrower be realm.[[Intrinsics]].[[%ThrowTypeError%]].
3. 3. 3. Perform ! DefinePropertyOrThrow(F, "caller", PropertyDescriptor {
[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]:
true }).
4. 4. 4. Perform ! DefinePropertyOrThrow(F, "arguments", PropertyDescriptor {
[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]:
true }).
5. 5. 5. Return unused.
10.2.4.1 %THROWTYPEERROR% ( )
This function is the %ThrowTypeError% intrinsic object.
It is an anonymous built-in function object that is defined once for each realm.
It performs the following steps when called:
1. 1. 1. Throw a TypeError exception.
The value of the [[Extensible]] internal slot of this function is false.
The "length" property of this function has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
The "name" property of this function has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
10.2.5 MAKECONSTRUCTOR ( F [ , WRITABLEPROTOTYPE [ , PROTOTYPE ] ] )
The abstract operation MakeConstructor takes argument F (an ECMAScript function
object or a built-in function object) and optional arguments writablePrototype
(a Boolean) and prototype (an Object) and returns unused. It converts F into a
constructor. It performs the following steps when called:
1. 1. 1. If F is an ECMAScript function object, then
1. a. a. Assert: IsConstructor(F) is false.
2. b. b. Assert: F is an extensible object that does not have a "prototype"
own property.
3. c. c. Set F.[[Construct]] to the definition specified in 10.2.2.
2. 2. 2. Else,
1. a. a. Set F.[[Construct]] to the definition specified in 10.3.2.
3. 3. 3. Set F.[[ConstructorKind]] to base.
4. 4. 4. If writablePrototype is not present, set writablePrototype to true.
5. 5. 5. If prototype is not present, then
1. a. a. Set prototype to OrdinaryObjectCreate(%Object.prototype%).
2. b. b. Perform ! DefinePropertyOrThrow(prototype, "constructor",
PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype,
[[Enumerable]]: false, [[Configurable]]: true }).
6. 6. 6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor {
[[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]:
false, [[Configurable]]: false }).
7. 7. 7. Return unused.
10.2.6 MAKECLASSCONSTRUCTOR ( F )
The abstract operation MakeClassConstructor takes argument F (an ECMAScript
function object) and returns unused. It performs the following steps when
called:
1. 1. 1. Assert: F.[[IsClassConstructor]] is false.
2. 2. 2. Set F.[[IsClassConstructor]] to true.
3. 3. 3. Return unused.
10.2.7 MAKEMETHOD ( F, HOMEOBJECT )
The abstract operation MakeMethod takes arguments F (an ECMAScript function
object) and homeObject (an Object) and returns unused. It configures F as a
method. It performs the following steps when called:
1. 1. 1. Set F.[[HomeObject]] to homeObject.
2. 2. 2. Return unused.
10.2.8 DEFINEMETHODPROPERTY ( HOMEOBJECT, KEY, CLOSURE, ENUMERABLE )
The abstract operation DefineMethodProperty takes arguments homeObject (an
Object), key (a property key or Private Name), closure (a function object), and
enumerable (a Boolean) and returns a PrivateElement or unused. It performs the
following steps when called:
1. 1. 1. Assert: homeObject is an ordinary, extensible object with no
non-configurable properties.
2. 2. 2. If key is a Private Name, then
1. a. a. Return PrivateElement { [[Key]]: key, [[Kind]]: method, [[Value]]:
closure }.
3. 3. 3. Else,
1. a. a. Let desc be the PropertyDescriptor { [[Value]]: closure,
[[Writable]]: true, [[Enumerable]]: enumerable, [[Configurable]]: true }.
2. b. b. Perform ! DefinePropertyOrThrow(homeObject, key, desc).
3. c. c. Return unused.
10.2.9 SETFUNCTIONNAME ( F, NAME [ , PREFIX ] )
The abstract operation SetFunctionName takes arguments F (a function object) and
name (a property key or Private Name) and optional argument prefix (a String)
and returns unused. It adds a "name" property to F. It performs the following
steps when called:
1. 1. 1. Assert: F is an extensible object that does not have a "name" own
property.
2. 2. 2. If name is a Symbol, then
1. a. a. Let description be name's [[Description]] value.
2. b. b. If description is undefined, set name to the empty String.
3. c. c. Else, set name to the string-concatenation of "[", description, and
"]".
3. 3. 3. Else if name is a Private Name, then
1. a. a. Set name to name.[[Description]].
4. 4. 4. If F has an [[InitialName]] internal slot, then
1. a. a. Set F.[[InitialName]] to name.
5. 5. 5. If prefix is present, then
1. a. a. Set name to the string-concatenation of prefix, the code unit
0x0020 (SPACE), and name.
2. b. b. If F has an [[InitialName]] internal slot, then
1. i. i. Optionally, set F.[[InitialName]] to name.
6. 6. 6. Perform ! DefinePropertyOrThrow(F, "name", PropertyDescriptor {
[[Value]]: name, [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }).
7. 7. 7. Return unused.
10.2.10 SETFUNCTIONLENGTH ( F, LENGTH )
The abstract operation SetFunctionLength takes arguments F (a function object)
and length (a non-negative integer or +∞) and returns unused. It adds a "length"
property to F. It performs the following steps when called:
1. 1. 1. Assert: F is an extensible object that does not have a "length" own
property.
2. 2. 2. Perform ! DefinePropertyOrThrow(F, "length", PropertyDescriptor {
[[Value]]: 𝔽(length), [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }).
3. 3. 3. Return unused.
10.2.11 FUNCTIONDECLARATIONINSTANTIATION ( FUNC, ARGUMENTSLIST )
The abstract operation FunctionDeclarationInstantiation takes arguments func (a
function object) and argumentsList (a List of ECMAScript language values) and
returns either a normal completion containing unused or an abrupt completion.
func is the function object for which the execution context is being
established.
Note 1
When an execution context is established for evaluating an ECMAScript function a
new Function Environment Record is created and bindings for each formal
parameter are instantiated in that Environment Record. Each declaration in the
function body is also instantiated. If the function's formal parameters do not
include any default value initializers then the body declarations are
instantiated in the same Environment Record as the parameters. If default value
parameter initializers exist, a second Environment Record is created for the
body declarations. Formal parameters and functions are initialized as part of
FunctionDeclarationInstantiation. All other bindings are initialized during
evaluation of the function body.
It performs the following steps when called:
1. 1. 1. Let calleeContext be the running execution context.
2. 2. 2. Let code be func.[[ECMAScriptCode]].
3. 3. 3. Let strict be func.[[Strict]].
4. 4. 4. Let formals be func.[[FormalParameters]].
5. 5. 5. Let parameterNames be the BoundNames of formals.
6. 6. 6. If parameterNames has any duplicate entries, let hasDuplicates be
true. Otherwise, let hasDuplicates be false.
7. 7. 7. Let simpleParameterList be IsSimpleParameterList of formals.
8. 8. 8. Let hasParameterExpressions be ContainsExpression of formals.
9. 9. 9. Let varNames be the VarDeclaredNames of code.
10. 10. 10. Let varDeclarations be the VarScopedDeclarations of code.
11. 11. 11. Let lexicalNames be the LexicallyDeclaredNames of code.
12. 12. 12. Let functionNames be a new empty List.
13. 13. 13. Let functionsToInitialize be a new empty List.
14. 14. 14. For each element d of varDeclarations, in reverse List order, do
1. a. a. If d is neither a VariableDeclaration nor a ForBinding nor a
BindingIdentifier, then
1. i. i. Assert: d is either a FunctionDeclaration, a
GeneratorDeclaration, an AsyncFunctionDeclaration, or an
AsyncGeneratorDeclaration.
2. ii. ii. Let fn be the sole element of the BoundNames of d.
3. iii. iii. If functionNames does not contain fn, then
1. 1. 1. Insert fn as the first element of functionNames.
2. 2. 2. NOTE: If there are multiple function declarations for the
same name, the last declaration is used.
3. 3. 3. Insert d as the first element of functionsToInitialize.
15. 15. 15. Let argumentsObjectNeeded be true.
16. 16. 16. If func.[[ThisMode]] is lexical, then
1. a. a. NOTE: Arrow functions never have an arguments object.
2. b. b. Set argumentsObjectNeeded to false.
17. 17. 17. Else if parameterNames contains "arguments", then
1. a. a. Set argumentsObjectNeeded to false.
18. 18. 18. Else if hasParameterExpressions is false, then
1. a. a. If functionNames contains "arguments" or lexicalNames contains
"arguments", then
1. i. i. Set argumentsObjectNeeded to false.
19. 19. 19. If strict is true or hasParameterExpressions is false, then
1. a. a. NOTE: Only a single Environment Record is needed for the
parameters, since calls to eval in strict mode code cannot create new
bindings which are visible outside of the eval.
2. b. b. Let env be the LexicalEnvironment of calleeContext.
20. 20. 20. Else,
1. a. a. NOTE: A separate Environment Record is needed to ensure that
bindings created by direct eval calls in the formal parameter list are
outside the environment where parameters are declared.
2. b. b. Let calleeEnv be the LexicalEnvironment of calleeContext.
3. c. c. Let env be NewDeclarativeEnvironment(calleeEnv).
4. d. d. Assert: The VariableEnvironment of calleeContext is calleeEnv.
5. e. e. Set the LexicalEnvironment of calleeContext to env.
21. 21. 21. For each String paramName of parameterNames, do
1. a. a. Let alreadyDeclared be ! env.HasBinding(paramName).
2. b. b. NOTE: Early errors ensure that duplicate parameter names can only
occur in non-strict functions that do not have parameter default values
or rest parameters.
3. c. c. If alreadyDeclared is false, then
1. i. i. Perform ! env.CreateMutableBinding(paramName, false).
2. ii. ii. If hasDuplicates is true, then
1. 1. 1. Perform ! env.InitializeBinding(paramName, undefined).
22. 22. 22. If argumentsObjectNeeded is true, then
1. a. a. If strict is true or simpleParameterList is false, then
1. i. i. Let ao be CreateUnmappedArgumentsObject(argumentsList).
2. b. b. Else,
1. i. i. NOTE: A mapped argument object is only provided for non-strict
functions that don't have a rest parameter, any parameter default
value initializers, or any destructured parameters.
2. ii. ii. Let ao be CreateMappedArgumentsObject(func, formals,
argumentsList, env).
3. c. c. If strict is true, then
1. i. i. Perform ! env.CreateImmutableBinding("arguments", false).
2. ii. ii. NOTE: In strict mode code early errors prevent attempting to
assign to this binding, so its mutability is not observable.
4. d. d. Else,
1. i. i. Perform ! env.CreateMutableBinding("arguments", false).
5. e. e. Perform ! env.InitializeBinding("arguments", ao).
6. f. f. Let parameterBindings be the list-concatenation of parameterNames
and « "arguments" ».
23. 23. 23. Else,
1. a. a. Let parameterBindings be parameterNames.
24. 24. 24. Let iteratorRecord be CreateListIteratorRecord(argumentsList).
25. 25. 25. If hasDuplicates is true, then
1. a. a. Perform ? IteratorBindingInitialization of formals with arguments
iteratorRecord and undefined.
26. 26. 26. Else,
1. a. a. Perform ? IteratorBindingInitialization of formals with arguments
iteratorRecord and env.
27. 27. 27. If hasParameterExpressions is false, then
1. a. a. NOTE: Only a single Environment Record is needed for the
parameters and top-level vars.
2. b. b. Let instantiatedVarNames be a copy of the List parameterBindings.
3. c. c. For each element n of varNames, do
1. i. i. If instantiatedVarNames does not contain n, then
1. 1. 1. Append n to instantiatedVarNames.
2. 2. 2. Perform ! env.CreateMutableBinding(n, false).
3. 3. 3. Perform ! env.InitializeBinding(n, undefined).
4. d. d. Let varEnv be env.
28. 28. 28. Else,
1. a. a. NOTE: A separate Environment Record is needed to ensure that
closures created by expressions in the formal parameter list do not have
visibility of declarations in the function body.
2. b. b. Let varEnv be NewDeclarativeEnvironment(env).
3. c. c. Set the VariableEnvironment of calleeContext to varEnv.
4. d. d. Let instantiatedVarNames be a new empty List.
5. e. e. For each element n of varNames, do
1. i. i. If instantiatedVarNames does not contain n, then
1. 1. 1. Append n to instantiatedVarNames.
2. 2. 2. Perform ! varEnv.CreateMutableBinding(n, false).
3. 3. 3. If parameterBindings does not contain n, or if functionNames
contains n, let initialValue be undefined.
4. 4. 4. Else,
1. a. a. Let initialValue be ! env.GetBindingValue(n, false).
5. 5. 5. Perform ! varEnv.InitializeBinding(n, initialValue).
6. 6. 6. NOTE: A var with the same name as a formal parameter
initially has the same value as the corresponding initialized
parameter.
29. 29. 29. NOTE: Annex B.3.2.1 adds additional steps at this point.
30. 30. 30. If strict is false, then
1. a. a. Let lexEnv be NewDeclarativeEnvironment(varEnv).
2. b. b. NOTE: Non-strict functions use a separate Environment Record for
top-level lexical declarations so that a direct eval can determine
whether any var scoped declarations introduced by the eval code conflict
with pre-existing top-level lexically scoped declarations. This is not
needed for strict functions because a strict direct eval always places
all declarations into a new Environment Record.
31. 31. 31. Else, let lexEnv be varEnv.
32. 32. 32. Set the LexicalEnvironment of calleeContext to lexEnv.
33. 33. 33. Let lexDeclarations be the LexicallyScopedDeclarations of code.
34. 34. 34. For each element d of lexDeclarations, do
1. a. a. NOTE: A lexically declared name cannot be the same as a
function/generator declaration, formal parameter, or a var name.
Lexically declared names are only instantiated here but not initialized.
2. b. b. For each element dn of the BoundNames of d, do
1. i. i. If IsConstantDeclaration of d is true, then
1. 1. 1. Perform ! lexEnv.CreateImmutableBinding(dn, true).
2. ii. ii. Else,
1. 1. 1. Perform ! lexEnv.CreateMutableBinding(dn, false).
35. 35. 35. Let privateEnv be the PrivateEnvironment of calleeContext.
36. 36. 36. For each Parse Node f of functionsToInitialize, do
1. a. a. Let fn be the sole element of the BoundNames of f.
2. b. b. Let fo be InstantiateFunctionObject of f with arguments lexEnv and
privateEnv.
3. c. c. Perform ! varEnv.SetMutableBinding(fn, fo, false).
37. 37. 37. Return unused.
Note 2
B.3.2 provides an extension to the above algorithm that is necessary for
backwards compatibility with web browser implementations of ECMAScript that
predate ECMAScript 2015.
10.3 BUILT-IN FUNCTION OBJECTS
The built-in function objects defined in this specification may be implemented
as either ECMAScript function objects (10.2) whose behaviour is provided using
ECMAScript code or as function objects whose behaviour is provided in some other
manner. In either case, the effect of calling such functions must conform to
their specifications. An implementation may also provide additional built-in
function objects that are not defined in this specification.
If a built-in function object is implemented as an ECMAScript function object,
it must have all the internal slots described in 10.2 ([[Prototype]],
[[Extensible]], and the slots listed in Table 30), and [[InitialName]]. The
value of the [[InitialName]] internal slot is a String value that is the initial
name of the function. It is used by 20.2.3.5.
Built-in function objects must have the ordinary object behaviour specified in
10.1. All such function objects have [[Prototype]], [[Extensible]], [[Realm]],
and [[InitialName]] internal slots, with the same meanings as above.
Unless otherwise specified every built-in function object has the
%Function.prototype% object as the initial value of its [[Prototype]] internal
slot.
The behaviour specified for each built-in function via algorithm steps or other
means is the specification of the function body behaviour for both [[Call]] and
[[Construct]] invocations of the function. However, [[Construct]] invocation is
not supported by all built-in functions. For each built-in function, when
invoked with [[Call]], the [[Call]] thisArgument provides the this value, the
[[Call]] argumentsList provides the named parameters, and the NewTarget value is
undefined. When invoked with [[Construct]], the this value is uninitialized, the
[[Construct]] argumentsList provides the named parameters, and the [[Construct]]
newTarget parameter provides the NewTarget value. If the built-in function is
implemented as an ECMAScript function object then this specified behaviour must
be implemented by the ECMAScript code that is the body of the function. Built-in
functions that are ECMAScript function objects must be strict functions. If a
built-in constructor has any [[Call]] behaviour other than throwing a TypeError
exception, an ECMAScript implementation of the function must be done in a manner
that does not cause the function's [[IsClassConstructor]] internal slot to have
the value true.
Built-in function objects that are not identified as constructors do not
implement the [[Construct]] internal method unless otherwise specified in the
description of a particular function. When a built-in constructor is called as
part of a new expression the argumentsList parameter of the invoked
[[Construct]] internal method provides the values for the built-in constructor's
named parameters.
Built-in functions that are not constructors do not have a "prototype" property
unless otherwise specified in the description of a particular function.
If a built-in function object is not implemented as an ECMAScript function it
must provide [[Call]] and [[Construct]] internal methods that conform to the
following definitions:
10.3.1 [[CALL]] ( THISARGUMENT, ARGUMENTSLIST )
The [[Call]] internal method of a built-in function object F takes arguments
thisArgument (an ECMAScript language value) and argumentsList (a List of
ECMAScript language values) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It performs the following steps
when called:
1. 1. 1. Let callerContext be the running execution context.
2. 2. 2. If callerContext is not already suspended, suspend callerContext.
3. 3. 3. Let calleeContext be a new execution context.
4. 4. 4. Set the Function of calleeContext to F.
5. 5. 5. Let calleeRealm be F.[[Realm]].
6. 6. 6. Set the Realm of calleeContext to calleeRealm.
7. 7. 7. Set the ScriptOrModule of calleeContext to null.
8. 8. 8. Perform any necessary implementation-defined initialization of
calleeContext.
9. 9. 9. Push calleeContext onto the execution context stack; calleeContext is
now the running execution context.
10. 10. 10. Let result be the Completion Record that is the result of
evaluating F in a manner that conforms to the specification of F.
thisArgument is the this value, argumentsList provides the named
parameters, and the NewTarget value is undefined.
11. 11. 11. Remove calleeContext from the execution context stack and restore
callerContext as the running execution context.
12. 12. 12. Return ? result.
Note
When calleeContext is removed from the execution context stack it must not be
destroyed if it has been suspended and retained by an accessible Generator for
later resumption.
10.3.2 [[CONSTRUCT]] ( ARGUMENTSLIST, NEWTARGET )
The [[Construct]] internal method of a built-in function object F (when the
method is present) takes arguments argumentsList (a List of ECMAScript language
values) and newTarget (a constructor) and returns either a normal completion
containing an Object or a throw completion. The steps performed are the same as
[[Call]] (see 10.3.1) except that step 10 is replaced by:
10. 10. 10. Let result be the Completion Record that is the result of
evaluating F in a manner that conforms to the specification of F. The this
value is uninitialized, argumentsList provides the named parameters, and
newTarget provides the NewTarget value.
10.3.3 CREATEBUILTINFUNCTION ( BEHAVIOUR, LENGTH, NAME,
ADDITIONALINTERNALSLOTSLIST [ , REALM [ , PROTOTYPE [ , PREFIX ] ] ] )
The abstract operation CreateBuiltinFunction takes arguments behaviour (an
Abstract Closure, a set of algorithm steps, or some other definition of a
function's behaviour provided in this specification), length (a non-negative
integer or +∞), name (a property key or a Private Name), and
additionalInternalSlotsList (a List of names of internal slots) and optional
arguments realm (a Realm Record), prototype (an Object or null), and prefix (a
String) and returns a function object. additionalInternalSlotsList contains the
names of additional internal slots that must be defined as part of the object.
This operation creates a built-in function object. It performs the following
steps when called:
1. 1. 1. If realm is not present, set realm to the current Realm Record.
2. 2. 2. If prototype is not present, set prototype to
realm.[[Intrinsics]].[[%Function.prototype%]].
3. 3. 3. Let internalSlotsList be a List containing the names of all the
internal slots that 10.3 requires for the built-in function object that is
about to be created.
4. 4. 4. Append to internalSlotsList the elements of
additionalInternalSlotsList.
5. 5. 5. Let func be a new built-in function object that, when called,
performs the action described by behaviour using the provided arguments as
the values of the corresponding parameters specified by behaviour. The new
function object has internal slots whose names are the elements of
internalSlotsList, and an [[InitialName]] internal slot.
6. 6. 6. Set func.[[Prototype]] to prototype.
7. 7. 7. Set func.[[Extensible]] to true.
8. 8. 8. Set func.[[Realm]] to realm.
9. 9. 9. Set func.[[InitialName]] to null.
10. 10. 10. Perform SetFunctionLength(func, length).
11. 11. 11. If prefix is not present, then
1. a. a. Perform SetFunctionName(func, name).
12. 12. 12. Else,
1. a. a. Perform SetFunctionName(func, name, prefix).
13. 13. 13. Return func.
Each built-in function defined in this specification is created by calling the
CreateBuiltinFunction abstract operation.
10.4 BUILT-IN EXOTIC OBJECT INTERNAL METHODS AND SLOTS
This specification defines several kinds of built-in exotic objects. These
objects generally behave similar to ordinary objects except for a few specific
situations. The following exotic objects use the ordinary object internal
methods except where it is explicitly specified otherwise below:
10.4.1 BOUND FUNCTION EXOTIC OBJECTS
A bound function exotic object is an exotic object that wraps another function
object. A bound function exotic object is callable (it has a [[Call]] internal
method and may have a [[Construct]] internal method). Calling a bound function
exotic object generally results in a call of its wrapped function.
An object is a bound function exotic object if its [[Call]] and (if applicable)
[[Construct]] internal methods use the following implementations, and its other
essential internal methods use the definitions found in 10.1. These methods are
installed in BoundFunctionCreate.
Bound function exotic objects do not have the internal slots of ECMAScript
function objects listed in Table 30. Instead they have the internal slots listed
in Table 31, in addition to [[Prototype]] and [[Extensible]].
Table 31: Internal Slots of Bound Function Exotic Objects
Internal Slot Type Description [[BoundTargetFunction]] a callable Object The
wrapped function object. [[BoundThis]] an ECMAScript language value The value
that is always passed as the this value when calling the wrapped function.
[[BoundArguments]] a List of ECMAScript language values A list of values whose
elements are used as the first arguments to any call to the wrapped function.
10.4.1.1 [[CALL]] ( THISARGUMENT, ARGUMENTSLIST )
The [[Call]] internal method of a bound function exotic object F takes arguments
thisArgument (an ECMAScript language value) and argumentsList (a List of
ECMAScript language values) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It performs the following steps
when called:
1. 1. 1. Let target be F.[[BoundTargetFunction]].
2. 2. 2. Let boundThis be F.[[BoundThis]].
3. 3. 3. Let boundArgs be F.[[BoundArguments]].
4. 4. 4. Let args be the list-concatenation of boundArgs and argumentsList.
5. 5. 5. Return ? Call(target, boundThis, args).
10.4.1.2 [[CONSTRUCT]] ( ARGUMENTSLIST, NEWTARGET )
The [[Construct]] internal method of a bound function exotic object F takes
arguments argumentsList (a List of ECMAScript language values) and newTarget (a
constructor) and returns either a normal completion containing an Object or a
throw completion. It performs the following steps when called:
1. 1. 1. Let target be F.[[BoundTargetFunction]].
2. 2. 2. Assert: IsConstructor(target) is true.
3. 3. 3. Let boundArgs be F.[[BoundArguments]].
4. 4. 4. Let args be the list-concatenation of boundArgs and argumentsList.
5. 5. 5. If SameValue(F, newTarget) is true, set newTarget to target.
6. 6. 6. Return ? Construct(target, args, newTarget).
10.4.1.3 BOUNDFUNCTIONCREATE ( TARGETFUNCTION, BOUNDTHIS, BOUNDARGS )
The abstract operation BoundFunctionCreate takes arguments targetFunction (a
function object), boundThis (an ECMAScript language value), and boundArgs (a
List of ECMAScript language values) and returns either a normal completion
containing a function object or a throw completion. It is used to specify the
creation of new bound function exotic objects. It performs the following steps
when called:
1. 1. 1. Let proto be ? targetFunction.[[GetPrototypeOf]]().
2. 2. 2. Let internalSlotsList be the list-concatenation of « [[Prototype]],
[[Extensible]] » and the internal slots listed in Table 31.
3. 3. 3. Let obj be MakeBasicObject(internalSlotsList).
4. 4. 4. Set obj.[[Prototype]] to proto.
5. 5. 5. Set obj.[[Call]] as described in 10.4.1.1.
6. 6. 6. If IsConstructor(targetFunction) is true, then
1. a. a. Set obj.[[Construct]] as described in 10.4.1.2.
7. 7. 7. Set obj.[[BoundTargetFunction]] to targetFunction.
8. 8. 8. Set obj.[[BoundThis]] to boundThis.
9. 9. 9. Set obj.[[BoundArguments]] to boundArgs.
10. 10. 10. Return obj.
10.4.2 ARRAY EXOTIC OBJECTS
An Array is an exotic object that gives special treatment to array index
property keys (see 6.1.7). A property whose property name is an array index is
also called an element. Every Array has a non-configurable "length" property
whose value is always a non-negative integral Number whose mathematical value is
strictly less than 232. The value of the "length" property is numerically
greater than the name of every own property whose name is an array index;
whenever an own property of an Array is created or changed, other properties are
adjusted as necessary to maintain this invariant. Specifically, whenever an own
property is added whose name is an array index, the value of the "length"
property is changed, if necessary, to be one more than the numeric value of that
array index; and whenever the value of the "length" property is changed, every
own property whose name is an array index whose value is not smaller than the
new length is deleted. This constraint applies only to own properties of an
Array and is unaffected by "length" or array index properties that may be
inherited from its prototypes.
Note
A String property name P is an array index if and only if ToString(ToUint32(P))
is P and ToUint32(P) is not 𝔽(232 - 1).
An object is an Array exotic object (or simply, an Array) if its
[[DefineOwnProperty]] internal method uses the following implementation, and its
other essential internal methods use the definitions found in 10.1. These
methods are installed in ArrayCreate.
10.4.2.1 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of an Array exotic object A takes
arguments P (a property key) and Desc (a Property Descriptor) and returns either
a normal completion containing a Boolean or a throw completion. It performs the
following steps when called:
1. 1. 1. If P is "length", then
1. a. a. Return ? ArraySetLength(A, Desc).
2. 2. 2. Else if P is an array index, then
1. a. a. Let lengthDesc be OrdinaryGetOwnProperty(A, "length").
2. b. b. Assert: IsDataDescriptor(lengthDesc) is true.
3. c. c. Assert: lengthDesc.[[Configurable]] is false.
4. d. d. Let length be lengthDesc.[[Value]].
5. e. e. Assert: length is a non-negative integral Number.
6. f. f. Let index be ! ToUint32(P).
7. g. g. If index ≥ length and lengthDesc.[[Writable]] is false, return
false.
8. h. h. Let succeeded be ! OrdinaryDefineOwnProperty(A, P, Desc).
9. i. i. If succeeded is false, return false.
10. j. j. If index ≥ length, then
1. i. i. Set lengthDesc.[[Value]] to index + 1𝔽.
2. ii. ii. Set succeeded to ! OrdinaryDefineOwnProperty(A, "length",
lengthDesc).
3. iii. iii. Assert: succeeded is true.
11. k. k. Return true.
3. 3. 3. Return ? OrdinaryDefineOwnProperty(A, P, Desc).
10.4.2.2 ARRAYCREATE ( LENGTH [ , PROTO ] )
The abstract operation ArrayCreate takes argument length (a non-negative
integer) and optional argument proto (an Object) and returns either a normal
completion containing an Array exotic object or a throw completion. It is used
to specify the creation of new Arrays. It performs the following steps when
called:
1. 1. 1. If length > 232 - 1, throw a RangeError exception.
2. 2. 2. If proto is not present, set proto to %Array.prototype%.
3. 3. 3. Let A be MakeBasicObject(« [[Prototype]], [[Extensible]] »).
4. 4. 4. Set A.[[Prototype]] to proto.
5. 5. 5. Set A.[[DefineOwnProperty]] as specified in 10.4.2.1.
6. 6. 6. Perform ! OrdinaryDefineOwnProperty(A, "length", PropertyDescriptor {
[[Value]]: 𝔽(length), [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
7. 7. 7. Return A.
10.4.2.3 ARRAYSPECIESCREATE ( ORIGINALARRAY, LENGTH )
The abstract operation ArraySpeciesCreate takes arguments originalArray (an
Object) and length (a non-negative integer) and returns either a normal
completion containing an Object or a throw completion. It is used to specify the
creation of a new Array or similar object using a constructor function that is
derived from originalArray. It does not enforce that the constructor function
returns an Array. It performs the following steps when called:
1. 1. 1. Let isArray be ? IsArray(originalArray).
2. 2. 2. If isArray is false, return ? ArrayCreate(length).
3. 3. 3. Let C be ? Get(originalArray, "constructor").
4. 4. 4. If IsConstructor(C) is true, then
1. a. a. Let thisRealm be the current Realm Record.
2. b. b. Let realmC be ? GetFunctionRealm(C).
3. c. c. If thisRealm and realmC are not the same Realm Record, then
1. i. i. If SameValue(C, realmC.[[Intrinsics]].[[%Array%]]) is true, set
C to undefined.
5. 5. 5. If C is an Object, then
1. a. a. Set C to ? Get(C, @@species).
2. b. b. If C is null, set C to undefined.
6. 6. 6. If C is undefined, return ? ArrayCreate(length).
7. 7. 7. If IsConstructor(C) is false, throw a TypeError exception.
8. 8. 8. Return ? Construct(C, « 𝔽(length) »).
Note
If originalArray was created using the standard built-in Array constructor for a
realm that is not the realm of the running execution context, then a new Array
is created using the realm of the running execution context. This maintains
compatibility with Web browsers that have historically had that behaviour for
the Array.prototype methods that now are defined using ArraySpeciesCreate.
10.4.2.4 ARRAYSETLENGTH ( A, DESC )
The abstract operation ArraySetLength takes arguments A (an Array) and Desc (a
Property Descriptor) and returns either a normal completion containing a Boolean
or a throw completion. It performs the following steps when called:
1. 1. 1. If Desc does not have a [[Value]] field, then
1. a. a. Return ! OrdinaryDefineOwnProperty(A, "length", Desc).
2. 2. 2. Let newLenDesc be a copy of Desc.
3. 3. 3. Let newLen be ? ToUint32(Desc.[[Value]]).
4. 4. 4. Let numberLen be ? ToNumber(Desc.[[Value]]).
5. 5. 5. If SameValueZero(newLen, numberLen) is false, throw a RangeError
exception.
6. 6. 6. Set newLenDesc.[[Value]] to newLen.
7. 7. 7. Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
8. 8. 8. Assert: IsDataDescriptor(oldLenDesc) is true.
9. 9. 9. Assert: oldLenDesc.[[Configurable]] is false.
10. 10. 10. Let oldLen be oldLenDesc.[[Value]].
11. 11. 11. If newLen ≥ oldLen, then
1. a. a. Return ! OrdinaryDefineOwnProperty(A, "length", newLenDesc).
12. 12. 12. If oldLenDesc.[[Writable]] is false, return false.
13. 13. 13. If newLenDesc does not have a [[Writable]] field or
newLenDesc.[[Writable]] is true, let newWritable be true.
14. 14. 14. Else,
1. a. a. NOTE: Setting the [[Writable]] attribute to false is deferred in
case any elements cannot be deleted.
2. b. b. Let newWritable be false.
3. c. c. Set newLenDesc.[[Writable]] to true.
15. 15. 15. Let succeeded be ! OrdinaryDefineOwnProperty(A, "length",
newLenDesc).
16. 16. 16. If succeeded is false, return false.
17. 17. 17. For each own property key P of A such that P is an array index and
! ToUint32(P) ≥ newLen, in descending numeric index order, do
1. a. a. Let deleteSucceeded be ! A.[[Delete]](P).
2. b. b. If deleteSucceeded is false, then
1. i. i. Set newLenDesc.[[Value]] to ! ToUint32(P) + 1𝔽.
2. ii. ii. If newWritable is false, set newLenDesc.[[Writable]] to
false.
3. iii. iii. Perform ! OrdinaryDefineOwnProperty(A, "length",
newLenDesc).
4. iv. iv. Return false.
18. 18. 18. If newWritable is false, then
1. a. a. Set succeeded to ! OrdinaryDefineOwnProperty(A, "length",
PropertyDescriptor { [[Writable]]: false }).
2. b. b. Assert: succeeded is true.
19. 19. 19. Return true.
Note
In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is
called twice. This is legacy behaviour that was specified with this effect
starting with the 2nd Edition of this specification.
10.4.3 STRING EXOTIC OBJECTS
A String object is an exotic object that encapsulates a String value and exposes
virtual integer-indexed data properties corresponding to the individual code
unit elements of the String value. String exotic objects always have a data
property named "length" whose value is the length of the encapsulated String
value. Both the code unit data properties and the "length" property are
non-writable and non-configurable.
An object is a String exotic object (or simply, a String object) if its
[[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal
methods use the following implementations, and its other essential internal
methods use the definitions found in 10.1. These methods are installed in
StringCreate.
String exotic objects have the same internal slots as ordinary objects. They
also have a [[StringData]] internal slot.
10.4.3.1 [[GETOWNPROPERTY]] ( P )
The [[GetOwnProperty]] internal method of a String exotic object S takes
argument P (a property key) and returns a normal completion containing either a
Property Descriptor or undefined. It performs the following steps when called:
1. 1. 1. Let desc be OrdinaryGetOwnProperty(S, P).
2. 2. 2. If desc is not undefined, return desc.
3. 3. 3. Return StringGetOwnProperty(S, P).
10.4.3.2 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of a String exotic object S takes
arguments P (a property key) and Desc (a Property Descriptor) and returns a
normal completion containing a Boolean. It performs the following steps when
called:
1. 1. 1. Let stringDesc be StringGetOwnProperty(S, P).
2. 2. 2. If stringDesc is not undefined, then
1. a. a. Let extensible be S.[[Extensible]].
2. b. b. Return IsCompatiblePropertyDescriptor(extensible, Desc,
stringDesc).
3. 3. 3. Return ! OrdinaryDefineOwnProperty(S, P, Desc).
10.4.3.3 [[OWNPROPERTYKEYS]] ( )
The [[OwnPropertyKeys]] internal method of a String exotic object O takes no
arguments and returns a normal completion containing a List of property keys. It
performs the following steps when called:
1. 1. 1. Let keys be a new empty List.
2. 2. 2. Let str be O.[[StringData]].
3. 3. 3. Assert: str is a String.
4. 4. 4. Let len be the length of str.
5. 5. 5. For each integer i such that 0 ≤ i < len, in ascending order, do
1. a. a. Append ! ToString(𝔽(i)) to keys.
6. 6. 6. For each own property key P of O such that P is an array index and
! ToIntegerOrInfinity(P) ≥ len, in ascending numeric index order, do
1. a. a. Append P to keys.
7. 7. 7. For each own property key P of O such that P is a String and P is not
an array index, in ascending chronological order of property creation, do
1. a. a. Append P to keys.
8. 8. 8. For each own property key P of O such that P is a Symbol, in ascending
chronological order of property creation, do
1. a. a. Append P to keys.
9. 9. 9. Return keys.
10.4.3.4 STRINGCREATE ( VALUE, PROTOTYPE )
The abstract operation StringCreate takes arguments value (a String) and
prototype (an Object) and returns a String exotic object. It is used to specify
the creation of new String exotic objects. It performs the following steps when
called:
1. 1. 1. Let S be MakeBasicObject(« [[Prototype]], [[Extensible]],
[[StringData]] »).
2. 2. 2. Set S.[[Prototype]] to prototype.
3. 3. 3. Set S.[[StringData]] to value.
4. 4. 4. Set S.[[GetOwnProperty]] as specified in 10.4.3.1.
5. 5. 5. Set S.[[DefineOwnProperty]] as specified in 10.4.3.2.
6. 6. 6. Set S.[[OwnPropertyKeys]] as specified in 10.4.3.3.
7. 7. 7. Let length be the length of value.
8. 8. 8. Perform ! DefinePropertyOrThrow(S, "length", PropertyDescriptor {
[[Value]]: 𝔽(length), [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }).
9. 9. 9. Return S.
10.4.3.5 STRINGGETOWNPROPERTY ( S, P )
The abstract operation StringGetOwnProperty takes arguments S (an Object that
has a [[StringData]] internal slot) and P (a property key) and returns a
Property Descriptor or undefined. It performs the following steps when called:
1. 1. 1. If P is not a String, return undefined.
2. 2. 2. Let index be CanonicalNumericIndexString(P).
3. 3. 3. If index is undefined, return undefined.
4. 4. 4. If IsIntegralNumber(index) is false, return undefined.
5. 5. 5. If index is -0𝔽, return undefined.
6. 6. 6. Let str be S.[[StringData]].
7. 7. 7. Assert: str is a String.
8. 8. 8. Let len be the length of str.
9. 9. 9. If ℝ(index) < 0 or len ≤ ℝ(index), return undefined.
10. 10. 10. Let resultStr be the substring of str from ℝ(index) to ℝ(index) +
1.
11. 11. 11. Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]:
false, [[Enumerable]]: true, [[Configurable]]: false }.
10.4.4 ARGUMENTS EXOTIC OBJECTS
Most ECMAScript functions make an arguments object available to their code.
Depending upon the characteristics of the function definition, its arguments
object is either an ordinary object or an arguments exotic object. An arguments
exotic object is an exotic object whose array index properties map to the formal
parameters bindings of an invocation of its associated ECMAScript function.
An object is an arguments exotic object if its internal methods use the
following implementations, with the ones not specified here using those found in
10.1. These methods are installed in CreateMappedArgumentsObject.
Note 1
While CreateUnmappedArgumentsObject is grouped into this clause, it creates an
ordinary object, not an arguments exotic object.
Arguments exotic objects have the same internal slots as ordinary objects. They
also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have
a [[ParameterMap]] internal slot whose value is always undefined. For ordinary
argument objects the [[ParameterMap]] internal slot is only used by
Object.prototype.toString (20.1.3.6) to identify them as such.
Note 2
The integer-indexed data properties of an arguments exotic object whose numeric
name values are less than the number of formal parameters of the corresponding
function object initially share their values with the corresponding argument
bindings in the function's execution context. This means that changing the
property changes the corresponding value of the argument binding and vice-versa.
This correspondence is broken if such a property is deleted and then redefined
or if the property is changed into an accessor property. If the arguments object
is an ordinary object, the values of its properties are simply a copy of the
arguments passed to the function and there is no dynamic linkage between the
property values and the formal parameter values.
Note 3
The ParameterMap object and its property values are used as a device for
specifying the arguments object correspondence to argument bindings. The
ParameterMap object and the objects that are the values of its properties are
not directly observable from ECMAScript code. An ECMAScript implementation does
not need to actually create or use such objects to implement the specified
semantics.
Note 4
Ordinary arguments objects define a non-configurable accessor property named
"callee" which throws a TypeError exception on access. The "callee" property has
a more specific meaning for arguments exotic objects, which are created only for
some class of non-strict functions. The definition of this property in the
ordinary variant exists to ensure that it is not defined in any other manner by
conforming ECMAScript implementations.
Note 5
ECMAScript implementations of arguments exotic objects have historically
contained an accessor property named "caller". Prior to ECMAScript 2017, this
specification included the definition of a throwing "caller" property on
ordinary arguments objects. Since implementations do not contain this extension
any longer, ECMAScript 2017 dropped the requirement for a throwing "caller"
accessor.
10.4.4.1 [[GETOWNPROPERTY]] ( P )
The [[GetOwnProperty]] internal method of an arguments exotic object args takes
argument P (a property key) and returns a normal completion containing either a
Property Descriptor or undefined. It performs the following steps when called:
1. 1. 1. Let desc be OrdinaryGetOwnProperty(args, P).
2. 2. 2. If desc is undefined, return desc.
3. 3. 3. Let map be args.[[ParameterMap]].
4. 4. 4. Let isMapped be ! HasOwnProperty(map, P).
5. 5. 5. If isMapped is true, then
1. a. a. Set desc.[[Value]] to ! Get(map, P).
6. 6. 6. Return desc.
10.4.4.2 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of an arguments exotic object args
takes arguments P (a property key) and Desc (a Property Descriptor) and returns
a normal completion containing a Boolean. It performs the following steps when
called:
1. 1. 1. Let map be args.[[ParameterMap]].
2. 2. 2. Let isMapped be ! HasOwnProperty(map, P).
3. 3. 3. Let newArgDesc be Desc.
4. 4. 4. If isMapped is true and IsDataDescriptor(Desc) is true, then
1. a. a. If Desc does not have a [[Value]] field, Desc has a [[Writable]]
field, and Desc.[[Writable]] is false, then
1. i. i. Set newArgDesc to a copy of Desc.
2. ii. ii. Set newArgDesc.[[Value]] to ! Get(map, P).
5. 5. 5. Let allowed be ! OrdinaryDefineOwnProperty(args, P, newArgDesc).
6. 6. 6. If allowed is false, return false.
7. 7. 7. If isMapped is true, then
1. a. a. If IsAccessorDescriptor(Desc) is true, then
1. i. i. Perform ! map.[[Delete]](P).
2. b. b. Else,
1. i. i. If Desc has a [[Value]] field, then
1. 1. 1. Assert: The following Set will succeed, since formal
parameters mapped by arguments objects are always writable.
2. 2. 2. Perform ! Set(map, P, Desc.[[Value]], false).
2. ii. ii. If Desc has a [[Writable]] field and Desc.[[Writable]] is
false, then
1. 1. 1. Perform ! map.[[Delete]](P).
8. 8. 8. Return true.
10.4.4.3 [[GET]] ( P, RECEIVER )
The [[Get]] internal method of an arguments exotic object args takes arguments P
(a property key) and Receiver (an ECMAScript language value) and returns either
a normal completion containing an ECMAScript language value or a throw
completion. It performs the following steps when called:
1. 1. 1. Let map be args.[[ParameterMap]].
2. 2. 2. Let isMapped be ! HasOwnProperty(map, P).
3. 3. 3. If isMapped is false, then
1. a. a. Return ? OrdinaryGet(args, P, Receiver).
4. 4. 4. Else,
1. a. a. Assert: map contains a formal parameter mapping for P.
2. b. b. Return ! Get(map, P).
10.4.4.4 [[SET]] ( P, V, RECEIVER )
The [[Set]] internal method of an arguments exotic object args takes arguments P
(a property key), V (an ECMAScript language value), and Receiver (an ECMAScript
language value) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. If SameValue(args, Receiver) is false, then
1. a. a. Let isMapped be false.
2. 2. 2. Else,
1. a. a. Let map be args.[[ParameterMap]].
2. b. b. Let isMapped be ! HasOwnProperty(map, P).
3. 3. 3. If isMapped is true, then
1. a. a. Assert: The following Set will succeed, since formal parameters
mapped by arguments objects are always writable.
2. b. b. Perform ! Set(map, P, V, false).
4. 4. 4. Return ? OrdinarySet(args, P, V, Receiver).
10.4.4.5 [[DELETE]] ( P )
The [[Delete]] internal method of an arguments exotic object args takes argument
P (a property key) and returns either a normal completion containing a Boolean
or a throw completion. It performs the following steps when called:
1. 1. 1. Let map be args.[[ParameterMap]].
2. 2. 2. Let isMapped be ! HasOwnProperty(map, P).
3. 3. 3. Let result be ? OrdinaryDelete(args, P).
4. 4. 4. If result is true and isMapped is true, then
1. a. a. Perform ! map.[[Delete]](P).
5. 5. 5. Return result.
10.4.4.6 CREATEUNMAPPEDARGUMENTSOBJECT ( ARGUMENTSLIST )
The abstract operation CreateUnmappedArgumentsObject takes argument
argumentsList (a List of ECMAScript language values) and returns an ordinary
object. It performs the following steps when called:
1. 1. 1. Let len be the number of elements in argumentsList.
2. 2. 2. Let obj be OrdinaryObjectCreate(%Object.prototype%, « [[ParameterMap]]
»).
3. 3. 3. Set obj.[[ParameterMap]] to undefined.
4. 4. 4. Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor {
[[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true }).
5. 5. 5. Let index be 0.
6. 6. 6. Repeat, while index < len,
1. a. a. Let val be argumentsList[index].
2. b. b. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)),
val).
3. c. c. Set index to index + 1.
7. 7. 7. Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor {
[[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: true }).
8. 8. 8. Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor {
[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false,
[[Configurable]]: false }).
9. 9. 9. Return obj.
10.4.4.7 CREATEMAPPEDARGUMENTSOBJECT ( FUNC, FORMALS, ARGUMENTSLIST, ENV )
The abstract operation CreateMappedArgumentsObject takes arguments func (an
Object), formals (a Parse Node), argumentsList (a List of ECMAScript language
values), and env (an Environment Record) and returns an arguments exotic object.
It performs the following steps when called:
1. 1. 1. Assert: formals does not contain a rest parameter, any binding
patterns, or any initializers. It may contain duplicate identifiers.
2. 2. 2. Let len be the number of elements in argumentsList.
3. 3. 3. Let obj be MakeBasicObject(« [[Prototype]], [[Extensible]],
[[ParameterMap]] »).
4. 4. 4. Set obj.[[GetOwnProperty]] as specified in 10.4.4.1.
5. 5. 5. Set obj.[[DefineOwnProperty]] as specified in 10.4.4.2.
6. 6. 6. Set obj.[[Get]] as specified in 10.4.4.3.
7. 7. 7. Set obj.[[Set]] as specified in 10.4.4.4.
8. 8. 8. Set obj.[[Delete]] as specified in 10.4.4.5.
9. 9. 9. Set obj.[[Prototype]] to %Object.prototype%.
10. 10. 10. Let map be OrdinaryObjectCreate(null).
11. 11. 11. Set obj.[[ParameterMap]] to map.
12. 12. 12. Let parameterNames be the BoundNames of formals.
13. 13. 13. Let numberOfParameters be the number of elements in parameterNames.
14. 14. 14. Let index be 0.
15. 15. 15. Repeat, while index < len,
1. a. a. Let val be argumentsList[index].
2. b. b. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)),
val).
3. c. c. Set index to index + 1.
16. 16. 16. Perform ! DefinePropertyOrThrow(obj, "length", PropertyDescriptor {
[[Value]]: 𝔽(len), [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true }).
17. 17. 17. Let mappedNames be a new empty List.
18. 18. 18. Set index to numberOfParameters - 1.
19. 19. 19. Repeat, while index ≥ 0,
1. a. a. Let name be parameterNames[index].
2. b. b. If mappedNames does not contain name, then
1. i. i. Append name to mappedNames.
2. ii. ii. If index < len, then
1. 1. 1. Let g be MakeArgGetter(name, env).
2. 2. 2. Let p be MakeArgSetter(name, env).
3. 3. 3. Perform ! map.[[DefineOwnProperty]](! ToString(𝔽(index)),
PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]:
false, [[Configurable]]: true }).
3. c. c. Set index to index - 1.
20. 20. 20. Perform ! DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor
{ [[Value]]: %Array.prototype.values%, [[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: true }).
21. 21. 21. Perform ! DefinePropertyOrThrow(obj, "callee", PropertyDescriptor {
[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true }).
22. 22. 22. Return obj.
10.4.4.7.1 MAKEARGGETTER ( NAME, ENV )
The abstract operation MakeArgGetter takes arguments name (a String) and env (an
Environment Record) and returns a function object. It creates a built-in
function object that when executed returns the value bound for name in env. It
performs the following steps when called:
1. 1. 1. Let getterClosure be a new Abstract Closure with no parameters that
captures name and env and performs the following steps when called:
1. a. a. Return env.GetBindingValue(name, false).
2. 2. 2. Let getter be CreateBuiltinFunction(getterClosure, 0, "", « »).
3. 3. 3. NOTE: getter is never directly accessible to ECMAScript code.
4. 4. 4. Return getter.
10.4.4.7.2 MAKEARGSETTER ( NAME, ENV )
The abstract operation MakeArgSetter takes arguments name (a String) and env (an
Environment Record) and returns a function object. It creates a built-in
function object that when executed sets the value bound for name in env. It
performs the following steps when called:
1. 1. 1. Let setterClosure be a new Abstract Closure with parameters (value)
that captures name and env and performs the following steps when called:
1. a. a. Return ! env.SetMutableBinding(name, value, false).
2. 2. 2. Let setter be CreateBuiltinFunction(setterClosure, 1, "", « »).
3. 3. 3. NOTE: setter is never directly accessible to ECMAScript code.
4. 4. 4. Return setter.
10.4.5 INTEGER-INDEXED EXOTIC OBJECTS
An Integer-Indexed exotic object is an exotic object that performs special
handling of integer index property keys.
Integer-Indexed exotic objects have the same internal slots as ordinary objects
and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]],
[[ContentType]], and [[TypedArrayName]] internal slots.
An object is an Integer-Indexed exotic object if its [[GetOwnProperty]],
[[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and
[[OwnPropertyKeys]] internal methods use the definitions in this section, and
its other essential internal methods use the definitions found in 10.1. These
methods are installed by IntegerIndexedObjectCreate.
10.4.5.1 [[GETOWNPROPERTY]] ( P )
The [[GetOwnProperty]] internal method of an Integer-Indexed exotic object O
takes argument P (a property key) and returns a normal completion containing
either a Property Descriptor or undefined. It performs the following steps when
called:
1. 1. 1. If P is a String, then
1. a. a. Let numericIndex be CanonicalNumericIndexString(P).
2. b. b. If numericIndex is not undefined, then
1. i. i. Let value be IntegerIndexedElementGet(O, numericIndex).
2. ii. ii. If value is undefined, return undefined.
3. iii. iii. Return the PropertyDescriptor { [[Value]]: value,
[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
2. 2. 2. Return OrdinaryGetOwnProperty(O, P).
10.4.5.2 [[HASPROPERTY]] ( P )
The [[HasProperty]] internal method of an Integer-Indexed exotic object O takes
argument P (a property key) and returns either a normal completion containing a
Boolean or a throw completion. It performs the following steps when called:
1. 1. 1. If P is a String, then
1. a. a. Let numericIndex be CanonicalNumericIndexString(P).
2. b. b. If numericIndex is not undefined, return IsValidIntegerIndex(O,
numericIndex).
2. 2. 2. Return ? OrdinaryHasProperty(O, P).
10.4.5.3 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of an Integer-Indexed exotic object O
takes arguments P (a property key) and Desc (a Property Descriptor) and returns
either a normal completion containing a Boolean or a throw completion. It
performs the following steps when called:
1. 1. 1. If P is a String, then
1. a. a. Let numericIndex be CanonicalNumericIndexString(P).
2. b. b. If numericIndex is not undefined, then
1. i. i. If IsValidIntegerIndex(O, numericIndex) is false, return false.
2. ii. ii. If Desc has a [[Configurable]] field and Desc.[[Configurable]]
is false, return false.
3. iii. iii. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]]
is false, return false.
4. iv. iv. If IsAccessorDescriptor(Desc) is true, return false.
5. v. v. If Desc has a [[Writable]] field and Desc.[[Writable]] is false,
return false.
6. vi. vi. If Desc has a [[Value]] field, perform
? IntegerIndexedElementSet(O, numericIndex, Desc.[[Value]]).
7. vii. vii. Return true.
2. 2. 2. Return ! OrdinaryDefineOwnProperty(O, P, Desc).
10.4.5.4 [[GET]] ( P, RECEIVER )
The [[Get]] internal method of an Integer-Indexed exotic object O takes
arguments P (a property key) and Receiver (an ECMAScript language value) and
returns either a normal completion containing an ECMAScript language value or a
throw completion. It performs the following steps when called:
1. 1. 1. If P is a String, then
1. a. a. Let numericIndex be CanonicalNumericIndexString(P).
2. b. b. If numericIndex is not undefined, then
1. i. i. Return IntegerIndexedElementGet(O, numericIndex).
2. 2. 2. Return ? OrdinaryGet(O, P, Receiver).
10.4.5.5 [[SET]] ( P, V, RECEIVER )
The [[Set]] internal method of an Integer-Indexed exotic object O takes
arguments P (a property key), V (an ECMAScript language value), and Receiver (an
ECMAScript language value) and returns either a normal completion containing a
Boolean or a throw completion. It performs the following steps when called:
1. 1. 1. If P is a String, then
1. a. a. Let numericIndex be CanonicalNumericIndexString(P).
2. b. b. If numericIndex is not undefined, then
1. i. i. If SameValue(O, Receiver) is true, then
1. 1. 1. Perform ? IntegerIndexedElementSet(O, numericIndex, V).
2. 2. 2. Return true.
2. ii. ii. If IsValidIntegerIndex(O, numericIndex) is false, return true.
2. 2. 2. Return ? OrdinarySet(O, P, V, Receiver).
10.4.5.6 [[DELETE]] ( P )
The [[Delete]] internal method of an Integer-Indexed exotic object O takes
argument P (a property key) and returns a normal completion containing a
Boolean. It performs the following steps when called:
1. 1. 1. If P is a String, then
1. a. a. Let numericIndex be CanonicalNumericIndexString(P).
2. b. b. If numericIndex is not undefined, then
1. i. i. If IsValidIntegerIndex(O, numericIndex) is false, return true;
else return false.
2. 2. 2. Return ! OrdinaryDelete(O, P).
10.4.5.7 [[OWNPROPERTYKEYS]] ( )
The [[OwnPropertyKeys]] internal method of an Integer-Indexed exotic object O
takes no arguments and returns a normal completion containing a List of property
keys. It performs the following steps when called:
1. 1. 1. Let keys be a new empty List.
2. 2. 2. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is false, then
1. a. a. For each integer i such that 0 ≤ i < O.[[ArrayLength]], in
ascending order, do
1. i. i. Append ! ToString(𝔽(i)) to keys.
3. 3. 3. For each own property key P of O such that P is a String and P is not
an integer index, in ascending chronological order of property creation, do
1. a. a. Append P to keys.
4. 4. 4. For each own property key P of O such that P is a Symbol, in ascending
chronological order of property creation, do
1. a. a. Append P to keys.
5. 5. 5. Return keys.
10.4.5.8 INTEGERINDEXEDOBJECTCREATE ( PROTOTYPE )
The abstract operation IntegerIndexedObjectCreate takes argument prototype (an
Object) and returns an Integer-Indexed exotic object. It is used to specify the
creation of new Integer-Indexed exotic objects. It performs the following steps
when called:
1. 1. 1. Let internalSlotsList be « [[Prototype]], [[Extensible]],
[[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]],
[[ByteOffset]], [[ArrayLength]] ».
2. 2. 2. Let A be MakeBasicObject(internalSlotsList).
3. 3. 3. Set A.[[GetOwnProperty]] as specified in 10.4.5.1.
4. 4. 4. Set A.[[HasProperty]] as specified in 10.4.5.2.
5. 5. 5. Set A.[[DefineOwnProperty]] as specified in 10.4.5.3.
6. 6. 6. Set A.[[Get]] as specified in 10.4.5.4.
7. 7. 7. Set A.[[Set]] as specified in 10.4.5.5.
8. 8. 8. Set A.[[Delete]] as specified in 10.4.5.6.
9. 9. 9. Set A.[[OwnPropertyKeys]] as specified in 10.4.5.7.
10. 10. 10. Set A.[[Prototype]] to prototype.
11. 11. 11. Return A.
10.4.5.9 ISVALIDINTEGERINDEX ( O, INDEX )
The abstract operation IsValidIntegerIndex takes arguments O (an Integer-Indexed
exotic object) and index (a Number) and returns a Boolean. It performs the
following steps when called:
1. 1. 1. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return false.
2. 2. 2. If IsIntegralNumber(index) is false, return false.
3. 3. 3. If index is -0𝔽, return false.
4. 4. 4. If ℝ(index) < 0 or ℝ(index) ≥ O.[[ArrayLength]], return false.
5. 5. 5. Return true.
10.4.5.10 INTEGERINDEXEDELEMENTGET ( O, INDEX )
The abstract operation IntegerIndexedElementGet takes arguments O (an
Integer-Indexed exotic object) and index (a Number) and returns a Number, a
BigInt, or undefined. It performs the following steps when called:
1. 1. 1. If IsValidIntegerIndex(O, index) is false, return undefined.
2. 2. 2. Let offset be O.[[ByteOffset]].
3. 3. 3. Let elementSize be TypedArrayElementSize(O).
4. 4. 4. Let indexedPosition be (ℝ(index) × elementSize) + offset.
5. 5. 5. Let elementType be TypedArrayElementType(O).
6. 6. 6. Return GetValueFromBuffer(O.[[ViewedArrayBuffer]], indexedPosition,
elementType, true, Unordered).
10.4.5.11 INTEGERINDEXEDELEMENTSET ( O, INDEX, VALUE )
The abstract operation IntegerIndexedElementSet takes arguments O (an
Integer-Indexed exotic object), index (a Number), and value (an ECMAScript
language value) and returns either a normal completion containing unused or a
throw completion. It performs the following steps when called:
1. 1. 1. If O.[[ContentType]] is BigInt, let numValue be ? ToBigInt(value).
2. 2. 2. Otherwise, let numValue be ? ToNumber(value).
3. 3. 3. If IsValidIntegerIndex(O, index) is true, then
1. a. a. Let offset be O.[[ByteOffset]].
2. b. b. Let elementSize be TypedArrayElementSize(O).
3. c. c. Let indexedPosition be (ℝ(index) × elementSize) + offset.
4. d. d. Let elementType be TypedArrayElementType(O).
5. e. e. Perform SetValueInBuffer(O.[[ViewedArrayBuffer]], indexedPosition,
elementType, numValue, true, Unordered).
4. 4. 4. Return unused.
Note
This operation always appears to succeed, but it has no effect when attempting
to write past the end of a TypedArray or to a TypedArray which is backed by a
detached ArrayBuffer.
10.4.6 MODULE NAMESPACE EXOTIC OBJECTS
A module namespace exotic object is an exotic object that exposes the bindings
exported from an ECMAScript Module (See 16.2.3). There is a one-to-one
correspondence between the String-keyed own properties of a module namespace
exotic object and the binding names exported by the Module. The exported
bindings include any bindings that are indirectly exported using export * export
items. Each String-valued own property key is the StringValue of the
corresponding exported binding name. These are the only String-keyed properties
of a module namespace exotic object. Each such property has the attributes {
[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module
namespace exotic objects are not extensible.
An object is a module namespace exotic object if its [[GetPrototypeOf]],
[[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]],
[[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and
[[OwnPropertyKeys]] internal methods use the definitions in this section, and
its other essential internal methods use the definitions found in 10.1. These
methods are installed by ModuleNamespaceCreate.
Module namespace exotic objects have the internal slots defined in Table 32.
Table 32: Internal Slots of Module Namespace Exotic Objects
Internal Slot Type Description [[Module]] a Module Record The Module Record
whose exports this namespace exposes. [[Exports]] a List of Strings A List whose
elements are the String values of the exported names exposed as own properties
of this object. The list is ordered as if an Array of those String values had
been sorted using %Array.prototype.sort% using undefined as comparefn.
10.4.6.1 [[GETPROTOTYPEOF]] ( )
The [[GetPrototypeOf]] internal method of a module namespace exotic object takes
no arguments and returns a normal completion containing null. It performs the
following steps when called:
1. 1. 1. Return null.
10.4.6.2 [[SETPROTOTYPEOF]] ( V )
The [[SetPrototypeOf]] internal method of a module namespace exotic object O
takes argument V (an Object or null) and returns a normal completion containing
a Boolean. It performs the following steps when called:
1. 1. 1. Return ! SetImmutablePrototype(O, V).
10.4.6.3 [[ISEXTENSIBLE]] ( )
The [[IsExtensible]] internal method of a module namespace exotic object takes
no arguments and returns a normal completion containing false. It performs the
following steps when called:
1. 1. 1. Return false.
10.4.6.4 [[PREVENTEXTENSIONS]] ( )
The [[PreventExtensions]] internal method of a module namespace exotic object
takes no arguments and returns a normal completion containing true. It performs
the following steps when called:
1. 1. 1. Return true.
10.4.6.5 [[GETOWNPROPERTY]] ( P )
The [[GetOwnProperty]] internal method of a module namespace exotic object O
takes argument P (a property key) and returns either a normal completion
containing either a Property Descriptor or undefined, or a throw completion. It
performs the following steps when called:
1. 1. 1. If P is a Symbol, return OrdinaryGetOwnProperty(O, P).
2. 2. 2. Let exports be O.[[Exports]].
3. 3. 3. If exports does not contain P, return undefined.
4. 4. 4. Let value be ? O.[[Get]](P, O).
5. 5. 5. Return PropertyDescriptor { [[Value]]: value, [[Writable]]: true,
[[Enumerable]]: true, [[Configurable]]: false }.
10.4.6.6 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of a module namespace exotic object O
takes arguments P (a property key) and Desc (a Property Descriptor) and returns
either a normal completion containing a Boolean or a throw completion. It
performs the following steps when called:
1. 1. 1. If P is a Symbol, return ! OrdinaryDefineOwnProperty(O, P, Desc).
2. 2. 2. Let current be ? O.[[GetOwnProperty]](P).
3. 3. 3. If current is undefined, return false.
4. 4. 4. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is
true, return false.
5. 5. 5. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is false,
return false.
6. 6. 6. If IsAccessorDescriptor(Desc) is true, return false.
7. 7. 7. If Desc has a [[Writable]] field and Desc.[[Writable]] is false,
return false.
8. 8. 8. If Desc has a [[Value]] field, return SameValue(Desc.[[Value]],
current.[[Value]]).
9. 9. 9. Return true.
10.4.6.7 [[HASPROPERTY]] ( P )
The [[HasProperty]] internal method of a module namespace exotic object O takes
argument P (a property key) and returns a normal completion containing a
Boolean. It performs the following steps when called:
1. 1. 1. If P is a Symbol, return ! OrdinaryHasProperty(O, P).
2. 2. 2. Let exports be O.[[Exports]].
3. 3. 3. If exports contains P, return true.
4. 4. 4. Return false.
10.4.6.8 [[GET]] ( P, RECEIVER )
The [[Get]] internal method of a module namespace exotic object O takes
arguments P (a property key) and Receiver (an ECMAScript language value) and
returns either a normal completion containing an ECMAScript language value or a
throw completion. It performs the following steps when called:
1. 1. 1. If P is a Symbol, then
1. a. a. Return ! OrdinaryGet(O, P, Receiver).
2. 2. 2. Let exports be O.[[Exports]].
3. 3. 3. If exports does not contain P, return undefined.
4. 4. 4. Let m be O.[[Module]].
5. 5. 5. Let binding be m.ResolveExport(P).
6. 6. 6. Assert: binding is a ResolvedBinding Record.
7. 7. 7. Let targetModule be binding.[[Module]].
8. 8. 8. Assert: targetModule is not undefined.
9. 9. 9. If binding.[[BindingName]] is namespace, then
1. a. a. Return GetModuleNamespace(targetModule).
10. 10. 10. Let targetEnv be targetModule.[[Environment]].
11. 11. 11. If targetEnv is empty, throw a ReferenceError exception.
12. 12. 12. Return ? targetEnv.GetBindingValue(binding.[[BindingName]], true).
Note
ResolveExport is side-effect free. Each time this operation is called with a
specific exportName, resolveSet pair as arguments it must return the same
result. An implementation might choose to pre-compute or cache the ResolveExport
results for the [[Exports]] of each module namespace exotic object.
10.4.6.9 [[SET]] ( P, V, RECEIVER )
The [[Set]] internal method of a module namespace exotic object takes arguments
P (a property key), V (an ECMAScript language value), and Receiver (an
ECMAScript language value) and returns a normal completion containing false. It
performs the following steps when called:
1. 1. 1. Return false.
10.4.6.10 [[DELETE]] ( P )
The [[Delete]] internal method of a module namespace exotic object O takes
argument P (a property key) and returns a normal completion containing a
Boolean. It performs the following steps when called:
1. 1. 1. If P is a Symbol, then
1. a. a. Return ! OrdinaryDelete(O, P).
2. 2. 2. Let exports be O.[[Exports]].
3. 3. 3. If exports contains P, return false.
4. 4. 4. Return true.
10.4.6.11 [[OWNPROPERTYKEYS]] ( )
The [[OwnPropertyKeys]] internal method of a module namespace exotic object O
takes no arguments and returns a normal completion containing a List of property
keys. It performs the following steps when called:
1. 1. 1. Let exports be O.[[Exports]].
2. 2. 2. Let symbolKeys be OrdinaryOwnPropertyKeys(O).
3. 3. 3. Return the list-concatenation of exports and symbolKeys.
10.4.6.12 MODULENAMESPACECREATE ( MODULE, EXPORTS )
The abstract operation ModuleNamespaceCreate takes arguments module (a Module
Record) and exports (a List of Strings) and returns a module namespace exotic
object. It is used to specify the creation of new module namespace exotic
objects. It performs the following steps when called:
1. 1. 1. Assert: module.[[Namespace]] is empty.
2. 2. 2. Let internalSlotsList be the internal slots listed in Table 32.
3. 3. 3. Let M be MakeBasicObject(internalSlotsList).
4. 4. 4. Set M's essential internal methods to the definitions specified in
10.4.6.
5. 5. 5. Set M.[[Module]] to module.
6. 6. 6. Let sortedExports be a List whose elements are the elements of
exports ordered as if an Array of the same values had been sorted using
%Array.prototype.sort% using undefined as comparefn.
7. 7. 7. Set M.[[Exports]] to sortedExports.
8. 8. 8. Create own properties of M corresponding to the definitions in 28.3.
9. 9. 9. Set module.[[Namespace]] to M.
10. 10. 10. Return M.
10.4.7 IMMUTABLE PROTOTYPE EXOTIC OBJECTS
An immutable prototype exotic object is an exotic object that has a
[[Prototype]] internal slot that will not change once it is initialized.
An object is an immutable prototype exotic object if its [[SetPrototypeOf]]
internal method uses the following implementation. (Its other essential internal
methods may use any implementation, depending on the specific immutable
prototype exotic object in question.)
Note
Unlike other exotic objects, there is not a dedicated creation abstract
operation provided for immutable prototype exotic objects. This is because they
are only used by %Object.prototype% and by host environments, and in host
environments, the relevant objects are potentially exotic in other ways and thus
need their own dedicated creation operation.
10.4.7.1 [[SETPROTOTYPEOF]] ( V )
The [[SetPrototypeOf]] internal method of an immutable prototype exotic object O
takes argument V (an Object or null) and returns either a normal completion
containing a Boolean or a throw completion. It performs the following steps when
called:
1. 1. 1. Return ? SetImmutablePrototype(O, V).
10.4.7.2 SETIMMUTABLEPROTOTYPE ( O, V )
The abstract operation SetImmutablePrototype takes arguments O (an Object) and V
(an Object or null) and returns either a normal completion containing a Boolean
or a throw completion. It performs the following steps when called:
1. 1. 1. Let current be ? O.[[GetPrototypeOf]]().
2. 2. 2. If SameValue(V, current) is true, return true.
3. 3. 3. Return false.
10.5 PROXY OBJECT INTERNAL METHODS AND INTERNAL SLOTS
A Proxy object is an exotic object whose essential internal methods are
partially implemented using ECMAScript code. Every Proxy object has an internal
slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called
the proxy's handler object, or null. Methods (see Table 33) of a handler object
may be used to augment the implementation for one or more of the Proxy object's
internal methods. Every Proxy object also has an internal slot called
[[ProxyTarget]] whose value is either an object or the null value. This object
is called the proxy's target object.
An object is a Proxy exotic object if its essential internal methods (including
[[Call]] and [[Construct]], if applicable) use the definitions in this section.
These internal methods are installed in ProxyCreate.
Table 33: Proxy Handler Methods
Internal Method Handler Method [[GetPrototypeOf]] getPrototypeOf
[[SetPrototypeOf]] setPrototypeOf [[IsExtensible]] isExtensible
[[PreventExtensions]] preventExtensions [[GetOwnProperty]]
getOwnPropertyDescriptor [[DefineOwnProperty]] defineProperty [[HasProperty]]
has [[Get]] get [[Set]] set [[Delete]] deleteProperty [[OwnPropertyKeys]]
ownKeys [[Call]] apply [[Construct]] construct
When a handler method is called to provide the implementation of a Proxy object
internal method, the handler method is passed the proxy's target object as a
parameter. A proxy's handler object does not necessarily have a method
corresponding to every essential internal method. Invoking an internal method on
the proxy results in the invocation of the corresponding internal method on the
proxy's target object if the handler object does not have a method corresponding
to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a Proxy object are
always initialized when the object is created and typically may not be modified.
Some Proxy objects are created in a manner that permits them to be subsequently
revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]]
internal slots are set to null causing subsequent invocations of internal
methods on that Proxy object to throw a TypeError exception.
Because Proxy objects permit the implementation of internal methods to be
provided by arbitrary ECMAScript code, it is possible to define a Proxy object
whose handler methods violates the invariants defined in 6.1.7.3. Some of the
internal method invariants defined in 6.1.7.3 are essential integrity
invariants. These invariants are explicitly enforced by the Proxy object
internal methods specified in this section. An ECMAScript implementation must be
robust in the presence of all possible invariant violations.
In the following algorithm descriptions, assume O is an ECMAScript Proxy object,
P is a property key value, V is any ECMAScript language value and Desc is a
Property Descriptor record.
10.5.1 [[GETPROTOTYPEOF]] ( )
The [[GetPrototypeOf]] internal method of a Proxy exotic object O takes no
arguments and returns either a normal completion containing either an Object or
null, or a throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "getPrototypeOf").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[GetPrototypeOf]]().
7. 7. 7. Let handlerProto be ? Call(trap, handler, « target »).
8. 8. 8. If handlerProto is not an Object and handlerProto is not null, throw
a TypeError exception.
9. 9. 9. Let extensibleTarget be ? IsExtensible(target).
10. 10. 10. If extensibleTarget is true, return handlerProto.
11. 11. 11. Let targetProto be ? target.[[GetPrototypeOf]]().
12. 12. 12. If SameValue(handlerProto, targetProto) is false, throw a TypeError
exception.
13. 13. 13. Return handlerProto.
Note
[[GetPrototypeOf]] for Proxy objects enforces the following invariants:
* The result of [[GetPrototypeOf]] must be either an Object or null.
* If the target object is not extensible, [[GetPrototypeOf]] applied to the
Proxy object must return the same value as [[GetPrototypeOf]] applied to the
Proxy object's target object.
10.5.2 [[SETPROTOTYPEOF]] ( V )
The [[SetPrototypeOf]] internal method of a Proxy exotic object O takes argument
V (an Object or null) and returns either a normal completion containing a
Boolean or a throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "setPrototypeOf").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[SetPrototypeOf]](V).
7. 7. 7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, V
»)).
8. 8. 8. If booleanTrapResult is false, return false.
9. 9. 9. Let extensibleTarget be ? IsExtensible(target).
10. 10. 10. If extensibleTarget is true, return true.
11. 11. 11. Let targetProto be ? target.[[GetPrototypeOf]]().
12. 12. 12. If SameValue(V, targetProto) is false, throw a TypeError exception.
13. 13. 13. Return true.
Note
[[SetPrototypeOf]] for Proxy objects enforces the following invariants:
* The result of [[SetPrototypeOf]] is a Boolean value.
* If the target object is not extensible, the argument value must be the same
as the result of [[GetPrototypeOf]] applied to target object.
10.5.3 [[ISEXTENSIBLE]] ( )
The [[IsExtensible]] internal method of a Proxy exotic object O takes no
arguments and returns either a normal completion containing a Boolean or a throw
completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "isExtensible").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? IsExtensible(target).
7. 7. 7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target
»)).
8. 8. 8. Let targetResult be ? IsExtensible(target).
9. 9. 9. If SameValue(booleanTrapResult, targetResult) is false, throw a
TypeError exception.
10. 10. 10. Return booleanTrapResult.
Note
[[IsExtensible]] for Proxy objects enforces the following invariants:
* The result of [[IsExtensible]] is a Boolean value.
* [[IsExtensible]] applied to the Proxy object must return the same value as
[[IsExtensible]] applied to the Proxy object's target object with the same
argument.
10.5.4 [[PREVENTEXTENSIONS]] ( )
The [[PreventExtensions]] internal method of a Proxy exotic object O takes no
arguments and returns either a normal completion containing a Boolean or a throw
completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "preventExtensions").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[PreventExtensions]]().
7. 7. 7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
8. 8. 8. If booleanTrapResult is true, then
1. a. a. Let extensibleTarget be ? IsExtensible(target).
2. b. b. If extensibleTarget is true, throw a TypeError exception.
9. 9. 9. Return booleanTrapResult.
Note
[[PreventExtensions]] for Proxy objects enforces the following invariants:
* The result of [[PreventExtensions]] is a Boolean value.
* [[PreventExtensions]] applied to the Proxy object only returns true if
[[IsExtensible]] applied to the Proxy object's target object is false.
10.5.5 [[GETOWNPROPERTY]] ( P )
The [[GetOwnProperty]] internal method of a Proxy exotic object O takes argument
P (a property key) and returns either a normal completion containing either a
Property Descriptor or undefined, or a throw completion. It performs the
following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "getOwnPropertyDescriptor").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[GetOwnProperty]](P).
7. 7. 7. Let trapResultObj be ? Call(trap, handler, « target, P »).
8. 8. 8. If trapResultObj is not an Object and trapResultObj is not undefined,
throw a TypeError exception.
9. 9. 9. Let targetDesc be ? target.[[GetOwnProperty]](P).
10. 10. 10. If trapResultObj is undefined, then
1. a. a. If targetDesc is undefined, return undefined.
2. b. b. If targetDesc.[[Configurable]] is false, throw a TypeError
exception.
3. c. c. Let extensibleTarget be ? IsExtensible(target).
4. d. d. If extensibleTarget is false, throw a TypeError exception.
5. e. e. Return undefined.
11. 11. 11. Let extensibleTarget be ? IsExtensible(target).
12. 12. 12. Let resultDesc be ? ToPropertyDescriptor(trapResultObj).
13. 13. 13. Perform CompletePropertyDescriptor(resultDesc).
14. 14. 14. Let valid be IsCompatiblePropertyDescriptor(extensibleTarget,
resultDesc, targetDesc).
15. 15. 15. If valid is false, throw a TypeError exception.
16. 16. 16. If resultDesc.[[Configurable]] is false, then
1. a. a. If targetDesc is undefined or targetDesc.[[Configurable]] is true,
then
1. i. i. Throw a TypeError exception.
2. b. b. If resultDesc has a [[Writable]] field and resultDesc.[[Writable]]
is false, then
1. i. i. Assert: targetDesc has a [[Writable]] field.
2. ii. ii. If targetDesc.[[Writable]] is true, throw a TypeError
exception.
17. 17. 17. Return resultDesc.
Note
[[GetOwnProperty]] for Proxy objects enforces the following invariants:
* The result of [[GetOwnProperty]] must be either an Object or undefined.
* A property cannot be reported as non-existent, if it exists as a
non-configurable own property of the target object.
* A property cannot be reported as non-existent, if it exists as an own
property of a non-extensible target object.
* A property cannot be reported as existent, if it does not exist as an own
property of the target object and the target object is not extensible.
* A property cannot be reported as non-configurable, unless it exists as a
non-configurable own property of the target object.
* A property cannot be reported as both non-configurable and non-writable,
unless it exists as a non-configurable, non-writable own property of the
target object.
10.5.6 [[DEFINEOWNPROPERTY]] ( P, DESC )
The [[DefineOwnProperty]] internal method of a Proxy exotic object O takes
arguments P (a property key) and Desc (a Property Descriptor) and returns either
a normal completion containing a Boolean or a throw completion. It performs the
following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "defineProperty").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[DefineOwnProperty]](P, Desc).
7. 7. 7. Let descObj be FromPropertyDescriptor(Desc).
8. 8. 8. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P,
descObj »)).
9. 9. 9. If booleanTrapResult is false, return false.
10. 10. 10. Let targetDesc be ? target.[[GetOwnProperty]](P).
11. 11. 11. Let extensibleTarget be ? IsExtensible(target).
12. 12. 12. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is
false, then
1. a. a. Let settingConfigFalse be true.
13. 13. 13. Else, let settingConfigFalse be false.
14. 14. 14. If targetDesc is undefined, then
1. a. a. If extensibleTarget is false, throw a TypeError exception.
2. b. b. If settingConfigFalse is true, throw a TypeError exception.
15. 15. 15. Else,
1. a. a. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc,
targetDesc) is false, throw a TypeError exception.
2. b. b. If settingConfigFalse is true and targetDesc.[[Configurable]] is
true, throw a TypeError exception.
3. c. c. If IsDataDescriptor(targetDesc) is true,
targetDesc.[[Configurable]] is false, and targetDesc.[[Writable]] is
true, then
1. i. i. If Desc has a [[Writable]] field and Desc.[[Writable]] is
false, throw a TypeError exception.
16. 16. 16. Return true.
Note
[[DefineOwnProperty]] for Proxy objects enforces the following invariants:
* The result of [[DefineOwnProperty]] is a Boolean value.
* A property cannot be added, if the target object is not extensible.
* A property cannot be non-configurable, unless there exists a corresponding
non-configurable own property of the target object.
* A non-configurable property cannot be non-writable, unless there exists a
corresponding non-configurable, non-writable own property of the target
object.
* If a property has a corresponding target object property then applying the
Property Descriptor of the property to the target object using
[[DefineOwnProperty]] will not throw an exception.
10.5.7 [[HASPROPERTY]] ( P )
The [[HasProperty]] internal method of a Proxy exotic object O takes argument P
(a property key) and returns either a normal completion containing a Boolean or
a throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "has").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[HasProperty]](P).
7. 7. 7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P
»)).
8. 8. 8. If booleanTrapResult is false, then
1. a. a. Let targetDesc be ? target.[[GetOwnProperty]](P).
2. b. b. If targetDesc is not undefined, then
1. i. i. If targetDesc.[[Configurable]] is false, throw a TypeError
exception.
2. ii. ii. Let extensibleTarget be ? IsExtensible(target).
3. iii. iii. If extensibleTarget is false, throw a TypeError exception.
9. 9. 9. Return booleanTrapResult.
Note
[[HasProperty]] for Proxy objects enforces the following invariants:
* The result of [[HasProperty]] is a Boolean value.
* A property cannot be reported as non-existent, if it exists as a
non-configurable own property of the target object.
* A property cannot be reported as non-existent, if it exists as an own
property of the target object and the target object is not extensible.
10.5.8 [[GET]] ( P, RECEIVER )
The [[Get]] internal method of a Proxy exotic object O takes arguments P (a
property key) and Receiver (an ECMAScript language value) and returns either a
normal completion containing an ECMAScript language value or a throw completion.
It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "get").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[Get]](P, Receiver).
7. 7. 7. Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
8. 8. 8. Let targetDesc be ? target.[[GetOwnProperty]](P).
9. 9. 9. If targetDesc is not undefined and targetDesc.[[Configurable]] is
false, then
1. a. a. If IsDataDescriptor(targetDesc) is true and
targetDesc.[[Writable]] is false, then
1. i. i. If SameValue(trapResult, targetDesc.[[Value]]) is false, throw
a TypeError exception.
2. b. b. If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Get]]
is undefined, then
1. i. i. If trapResult is not undefined, throw a TypeError exception.
10. 10. 10. Return trapResult.
Note
[[Get]] for Proxy objects enforces the following invariants:
* The value reported for a property must be the same as the value of the
corresponding target object property if the target object property is a
non-writable, non-configurable own data property.
* The value reported for a property must be undefined if the corresponding
target object property is a non-configurable own accessor property that has
undefined as its [[Get]] attribute.
10.5.9 [[SET]] ( P, V, RECEIVER )
The [[Set]] internal method of a Proxy exotic object O takes arguments P (a
property key), V (an ECMAScript language value), and Receiver (an ECMAScript
language value) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "set").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[Set]](P, V, Receiver).
7. 7. 7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P,
V, Receiver »)).
8. 8. 8. If booleanTrapResult is false, return false.
9. 9. 9. Let targetDesc be ? target.[[GetOwnProperty]](P).
10. 10. 10. If targetDesc is not undefined and targetDesc.[[Configurable]] is
false, then
1. a. a. If IsDataDescriptor(targetDesc) is true and
targetDesc.[[Writable]] is false, then
1. i. i. If SameValue(V, targetDesc.[[Value]]) is false, throw a
TypeError exception.
2. b. b. If IsAccessorDescriptor(targetDesc) is true, then
1. i. i. If targetDesc.[[Set]] is undefined, throw a TypeError
exception.
11. 11. 11. Return true.
Note
[[Set]] for Proxy objects enforces the following invariants:
* The result of [[Set]] is a Boolean value.
* Cannot change the value of a property to be different from the value of the
corresponding target object property if the corresponding target object
property is a non-writable, non-configurable own data property.
* Cannot set the value of a property if the corresponding target object
property is a non-configurable own accessor property that has undefined as
its [[Set]] attribute.
10.5.10 [[DELETE]] ( P )
The [[Delete]] internal method of a Proxy exotic object O takes argument P (a
property key) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "deleteProperty").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[Delete]](P).
7. 7. 7. Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P
»)).
8. 8. 8. If booleanTrapResult is false, return false.
9. 9. 9. Let targetDesc be ? target.[[GetOwnProperty]](P).
10. 10. 10. If targetDesc is undefined, return true.
11. 11. 11. If targetDesc.[[Configurable]] is false, throw a TypeError
exception.
12. 12. 12. Let extensibleTarget be ? IsExtensible(target).
13. 13. 13. If extensibleTarget is false, throw a TypeError exception.
14. 14. 14. Return true.
Note
[[Delete]] for Proxy objects enforces the following invariants:
* The result of [[Delete]] is a Boolean value.
* A property cannot be reported as deleted, if it exists as a non-configurable
own property of the target object.
* A property cannot be reported as deleted, if it exists as an own property of
the target object and the target object is non-extensible.
10.5.11 [[OWNPROPERTYKEYS]] ( )
The [[OwnPropertyKeys]] internal method of a Proxy exotic object O takes no
arguments and returns either a normal completion containing a List of property
keys or a throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "ownKeys").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? target.[[OwnPropertyKeys]]().
7. 7. 7. Let trapResultArray be ? Call(trap, handler, « target »).
8. 8. 8. Let trapResult be ? CreateListFromArrayLike(trapResultArray, «
String, Symbol »).
9. 9. 9. If trapResult contains any duplicate entries, throw a TypeError
exception.
10. 10. 10. Let extensibleTarget be ? IsExtensible(target).
11. 11. 11. Let targetKeys be ? target.[[OwnPropertyKeys]]().
12. 12. 12. Assert: targetKeys is a List of property keys.
13. 13. 13. Assert: targetKeys contains no duplicate entries.
14. 14. 14. Let targetConfigurableKeys be a new empty List.
15. 15. 15. Let targetNonconfigurableKeys be a new empty List.
16. 16. 16. For each element key of targetKeys, do
1. a. a. Let desc be ? target.[[GetOwnProperty]](key).
2. b. b. If desc is not undefined and desc.[[Configurable]] is false, then
1. i. i. Append key to targetNonconfigurableKeys.
3. c. c. Else,
1. i. i. Append key to targetConfigurableKeys.
17. 17. 17. If extensibleTarget is true and targetNonconfigurableKeys is empty,
then
1. a. a. Return trapResult.
18. 18. 18. Let uncheckedResultKeys be a List whose elements are the elements
of trapResult.
19. 19. 19. For each element key of targetNonconfigurableKeys, do
1. a. a. If uncheckedResultKeys does not contain key, throw a TypeError
exception.
2. b. b. Remove key from uncheckedResultKeys.
20. 20. 20. If extensibleTarget is true, return trapResult.
21. 21. 21. For each element key of targetConfigurableKeys, do
1. a. a. If uncheckedResultKeys does not contain key, throw a TypeError
exception.
2. b. b. Remove key from uncheckedResultKeys.
22. 22. 22. If uncheckedResultKeys is not empty, throw a TypeError exception.
23. 23. 23. Return trapResult.
Note
[[OwnPropertyKeys]] for Proxy objects enforces the following invariants:
* The result of [[OwnPropertyKeys]] is a List.
* The returned List contains no duplicate entries.
* The Type of each result List element is either String or Symbol.
* The result List must contain the keys of all non-configurable own properties
of the target object.
* If the target object is not extensible, then the result List must contain all
the keys of the own properties of the target object and no other values.
10.5.12 [[CALL]] ( THISARGUMENT, ARGUMENTSLIST )
The [[Call]] internal method of a Proxy exotic object O takes arguments
thisArgument (an ECMAScript language value) and argumentsList (a List of
ECMAScript language values) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It performs the following steps
when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Let handler be O.[[ProxyHandler]].
4. 4. 4. Assert: handler is an Object.
5. 5. 5. Let trap be ? GetMethod(handler, "apply").
6. 6. 6. If trap is undefined, then
1. a. a. Return ? Call(target, thisArgument, argumentsList).
7. 7. 7. Let argArray be CreateArrayFromList(argumentsList).
8. 8. 8. Return ? Call(trap, handler, « target, thisArgument, argArray »).
Note
A Proxy exotic object only has a [[Call]] internal method if the initial value
of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal
method.
10.5.13 [[CONSTRUCT]] ( ARGUMENTSLIST, NEWTARGET )
The [[Construct]] internal method of a Proxy exotic object O takes arguments
argumentsList (a List of ECMAScript language values) and newTarget (a
constructor) and returns either a normal completion containing an Object or a
throw completion. It performs the following steps when called:
1. 1. 1. Perform ? ValidateNonRevokedProxy(O).
2. 2. 2. Let target be O.[[ProxyTarget]].
3. 3. 3. Assert: IsConstructor(target) is true.
4. 4. 4. Let handler be O.[[ProxyHandler]].
5. 5. 5. Assert: handler is an Object.
6. 6. 6. Let trap be ? GetMethod(handler, "construct").
7. 7. 7. If trap is undefined, then
1. a. a. Return ? Construct(target, argumentsList, newTarget).
8. 8. 8. Let argArray be CreateArrayFromList(argumentsList).
9. 9. 9. Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
10. 10. 10. If newObj is not an Object, throw a TypeError exception.
11. 11. 11. Return newObj.
Note 1
A Proxy exotic object only has a [[Construct]] internal method if the initial
value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]]
internal method.
Note 2
[[Construct]] for Proxy objects enforces the following invariants:
* The result of [[Construct]] must be an Object.
10.5.14 VALIDATENONREVOKEDPROXY ( PROXY )
The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy
exotic object) and returns either a normal completion containing unused or a
throw completion. It throws a TypeError exception if proxy has been revoked. It
performs the following steps when called:
1. 1. 1. If proxy.[[ProxyTarget]] is null, throw a TypeError exception.
2. 2. 2. Assert: proxy.[[ProxyHandler]] is not null.
3. 3. 3. Return unused.
10.5.15 PROXYCREATE ( TARGET, HANDLER )
The abstract operation ProxyCreate takes arguments target (an ECMAScript
language value) and handler (an ECMAScript language value) and returns either a
normal completion containing a Proxy exotic object or a throw completion. It is
used to specify the creation of new Proxy objects. It performs the following
steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. If handler is not an Object, throw a TypeError exception.
3. 3. 3. Let P be MakeBasicObject(« [[ProxyHandler]], [[ProxyTarget]] »).
4. 4. 4. Set P's essential internal methods, except for [[Call]] and
[[Construct]], to the definitions specified in 10.5.
5. 5. 5. If IsCallable(target) is true, then
1. a. a. Set P.[[Call]] as specified in 10.5.12.
2. b. b. If IsConstructor(target) is true, then
1. i. i. Set P.[[Construct]] as specified in 10.5.13.
6. 6. 6. Set P.[[ProxyTarget]] to target.
7. 7. 7. Set P.[[ProxyHandler]] to handler.
8. 8. 8. Return P.
11 ECMASCRIPT LANGUAGE: SOURCE TEXT
11.1 SOURCE TEXT
SYNTAX
SourceCharacter :: any Unicode code point
ECMAScript source text is a sequence of Unicode code points. All Unicode code
point values from U+0000 to U+10FFFF, including surrogate code points, may occur
in ECMAScript source text where permitted by the ECMAScript grammars. The actual
encodings used to store and interchange ECMAScript source text is not relevant
to this specification. Regardless of the external source text encoding, a
conforming ECMAScript implementation processes the source text as if it was an
equivalent sequence of SourceCharacter values, each SourceCharacter being a
Unicode code point. Conforming ECMAScript implementations are not required to
perform any normalization of source text, or behave as though they were
performing normalization of source text.
The components of a combining character sequence are treated as individual
Unicode code points even though a user might think of the whole sequence as a
single character.
Note
In string literals, regular expression literals, template literals and
identifiers, any Unicode code point may also be expressed using Unicode escape
sequences that explicitly express a code point's numeric value. Within a
comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of
Unicode escape sequences. In a Java program, if the Unicode escape sequence
\u000A, for example, occurs within a single-line comment, it is interpreted as a
line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the
next code point is not part of the comment. Similarly, if the Unicode escape
sequence \u000A occurs within a string literal in a Java program, it is likewise
interpreted as a line terminator, which is not allowed within a string
literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part
of the String value of a string literal. In an ECMAScript program, a Unicode
escape sequence occurring within a comment is never interpreted and therefore
cannot contribute to termination of the comment. Similarly, a Unicode escape
sequence occurring within a string literal in an ECMAScript program always
contributes to the literal and is never interpreted as a line terminator or as a
code point that might terminate the string literal.
11.1.1 STATIC SEMANTICS: UTF16ENCODECODEPOINT ( CP )
The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code
point) and returns a String. It performs the following steps when called:
1. 1. 1. Assert: 0 ≤ cp ≤ 0x10FFFF.
2. 2. 2. If cp ≤ 0xFFFF, return the String value consisting of the code unit
whose numeric value is cp.
3. 3. 3. Let cu1 be the code unit whose numeric value is floor((cp - 0x10000) /
0x400) + 0xD800.
4. 4. 4. Let cu2 be the code unit whose numeric value is ((cp - 0x10000) modulo
0x400) + 0xDC00.
5. 5. 5. Return the string-concatenation of cu1 and cu2.
11.1.2 STATIC SEMANTICS: CODEPOINTSTOSTRING ( TEXT )
The abstract operation CodePointsToString takes argument text (a sequence of
Unicode code points) and returns a String. It converts text into a String value,
as described in 6.1.4. It performs the following steps when called:
1. 1. 1. Let result be the empty String.
2. 2. 2. For each code point cp of text, do
1. a. a. Set result to the string-concatenation of result and
UTF16EncodeCodePoint(cp).
3. 3. 3. Return result.
11.1.3 STATIC SEMANTICS: UTF16SURROGATEPAIRTOCODEPOINT ( LEAD, TRAIL )
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a
code unit) and trail (a code unit) and returns a code point. Two code units that
form a UTF-16 surrogate pair are converted to a code point. It performs the
following steps when called:
1. 1. 1. Assert: lead is a leading surrogate and trail is a trailing surrogate.
2. 2. 2. Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
3. 3. 3. Return the code point cp.
11.1.4 STATIC SEMANTICS: CODEPOINTAT ( STRING, POSITION )
The abstract operation CodePointAt takes arguments string (a String) and
position (a non-negative integer) and returns a Record with fields [[CodePoint]]
(a code point), [[CodeUnitCount]] (a positive integer), and
[[IsUnpairedSurrogate]] (a Boolean). It interprets string as a sequence of
UTF-16 encoded code points, as described in 6.1.4, and reads from it a single
code point starting with the code unit at index position. It performs the
following steps when called:
1. 1. 1. Let size be the length of string.
2. 2. 2. Assert: position ≥ 0 and position < size.
3. 3. 3. Let first be the code unit at index position within string.
4. 4. 4. Let cp be the code point whose numeric value is the numeric value of
first.
5. 5. 5. If first is neither a leading surrogate nor a trailing surrogate,
then
1. a. a. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1,
[[IsUnpairedSurrogate]]: false }.
6. 6. 6. If first is a trailing surrogate or position + 1 = size, then
1. a. a. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1,
[[IsUnpairedSurrogate]]: true }.
7. 7. 7. Let second be the code unit at index position + 1 within string.
8. 8. 8. If second is not a trailing surrogate, then
1. a. a. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1,
[[IsUnpairedSurrogate]]: true }.
9. 9. 9. Set cp to UTF16SurrogatePairToCodePoint(first, second).
10. 10. 10. Return the Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 2,
[[IsUnpairedSurrogate]]: false }.
11.1.5 STATIC SEMANTICS: STRINGTOCODEPOINTS ( STRING )
The abstract operation StringToCodePoints takes argument string (a String) and
returns a List of code points. It returns the sequence of Unicode code points
that results from interpreting string as UTF-16 encoded Unicode text as
described in 6.1.4. It performs the following steps when called:
1. 1. 1. Let codePoints be a new empty List.
2. 2. 2. Let size be the length of string.
3. 3. 3. Let position be 0.
4. 4. 4. Repeat, while position < size,
1. a. a. Let cp be CodePointAt(string, position).
2. b. b. Append cp.[[CodePoint]] to codePoints.
3. c. c. Set position to position + cp.[[CodeUnitCount]].
5. 5. 5. Return codePoints.
11.1.6 STATIC SEMANTICS: PARSETEXT ( SOURCETEXT, GOALSYMBOL )
The abstract operation ParseText takes arguments sourceText (a sequence of
Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript
grammars) and returns a Parse Node or a non-empty List of SyntaxError objects.
It performs the following steps when called:
1. 1. 1. Attempt to parse sourceText using goalSymbol as the goal symbol, and
analyse the parse result for any early error conditions. Parsing and early
error detection may be interleaved in an implementation-defined manner.
2. 2. 2. If the parse succeeded and no early errors were found, return the
Parse Node (an instance of goalSymbol) at the root of the parse tree
resulting from the parse.
3. 3. 3. Otherwise, return a List of one or more SyntaxError objects
representing the parsing errors and/or early errors. If more than one
parsing error or early error is present, the number and ordering of error
objects in the list is implementation-defined, but at least one must be
present.
Note 1
Consider a text that has an early error at a particular point, and also a syntax
error at a later point. An implementation that does a parse pass followed by an
early errors pass might report the syntax error and not proceed to the early
errors pass. An implementation that interleaves the two activities might report
the early error and not proceed to find the syntax error. A third implementation
might report both errors. All of these behaviours are conformant.
Note 2
See also clause 17.
11.2 TYPES OF SOURCE CODE
There are four types of ECMAScript code:
* Global code is source text that is treated as an ECMAScript Script. The
global code of a particular Script does not include any source text that is
parsed as part of a FunctionDeclaration, FunctionExpression,
GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration,
AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression,
MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or
ClassExpression.
* Eval code is the source text supplied to the built-in eval function. More
precisely, if the parameter to the built-in eval function is a String, it is
treated as an ECMAScript Script. The eval code for a particular invocation of
eval is the global code portion of that Script.
* Function code is source text that is parsed to supply the value of the
[[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 10.2) of an
ECMAScript function object. The function code of a particular ECMAScript
function does not include any source text that is parsed as the function code
of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration,
GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression,
AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition,
ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.
In addition, if the source text referred to above is parsed as:
* the FormalParameters and FunctionBody of a FunctionDeclaration or
FunctionExpression,
* the FormalParameters and GeneratorBody of a GeneratorDeclaration or
GeneratorExpression,
* the FormalParameters and AsyncFunctionBody of an AsyncFunctionDeclaration
or AsyncFunctionExpression, or
* the FormalParameters and AsyncGeneratorBody of an AsyncGeneratorDeclaration
or AsyncGeneratorExpression,
then the source text matched by the BindingIdentifier (if any) of that
declaration or expression is also included in the function code of the
corresponding function.
* Module code is source text that is code that is provided as a ModuleBody. It
is the code that is directly evaluated when a module is initialized. The
module code of a particular module does not include any source text that is
parsed as part of a nested FunctionDeclaration, FunctionExpression,
GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration,
AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression,
MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or
ClassExpression.
Note 1
Function code is generally provided as the bodies of Function Definitions
(15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator
Function Definitions (15.5), Async Function Definitions (15.8), Async Generator
Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is
also derived from the arguments to the Function constructor (20.2.1.1), the
GeneratorFunction constructor (27.3.1.1), and the AsyncFunction constructor
(27.7.1.1).
Note 2
The practical effect of including the BindingIdentifier in function code is that
the Early Errors for strict mode code are applied to a BindingIdentifier that is
the name of a function whose body contains a "use strict" directive, even if the
surrounding code is not strict mode code.
11.2.1 DIRECTIVE PROLOGUES AND THE USE STRICT DIRECTIVE
A Directive Prologue is the longest sequence of ExpressionStatements occurring
as the initial StatementListItems or ModuleItems of a FunctionBody, a
ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence
consists entirely of a StringLiteral token followed by a semicolon. The
semicolon may appear explicitly or may be inserted by automatic semicolon
insertion (12.10). A Directive Prologue may be an empty sequence.
A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose
StringLiteral is either of the exact code point sequences "use strict" or 'use
strict'. A Use Strict Directive may not contain an EscapeSequence or
LineContinuation.
A Directive Prologue may contain more than one Use Strict Directive. However, an
implementation may issue a warning if this occurs.
Note
The ExpressionStatements of a Directive Prologue are evaluated normally during
evaluation of the containing production. Implementations may define
implementation specific meanings for ExpressionStatements which are not a Use
Strict Directive and which occur in a Directive Prologue. If an appropriate
notification mechanism exists, an implementation should issue a warning if it
encounters in a Directive Prologue an ExpressionStatement that is not a Use
Strict Directive and which does not have a meaning defined by the
implementation.
11.2.2 STRICT MODE CODE
An ECMAScript syntactic unit may be processed using either unrestricted or
strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode
code in the following situations:
* Global code is strict mode code if it begins with a Directive Prologue that
contains a Use Strict Directive.
* Module code is always strict mode code.
* All parts of a ClassDeclaration or a ClassExpression are strict mode code.
* Eval code is strict mode code if it begins with a Directive Prologue that
contains a Use Strict Directive or if the call to eval is a direct eval that
is contained in strict mode code.
* Function code is strict mode code if the associated FunctionDeclaration,
FunctionExpression, GeneratorDeclaration, GeneratorExpression,
AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration,
AsyncGeneratorExpression, MethodDefinition, ArrowFunction, or
AsyncArrowFunction is contained in strict mode code or if the code that
produces the value of the function's [[ECMAScriptCode]] internal slot begins
with a Directive Prologue that contains a Use Strict Directive.
* Function code that is supplied as the arguments to the built-in Function,
Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code
if the last argument is a String that when processed is a FunctionBody that
begins with a Directive Prologue that contains a Use Strict Directive.
ECMAScript code that is not strict mode code is called non-strict code.
11.2.3 NON-ECMASCRIPT FUNCTIONS
An ECMAScript implementation may support the evaluation of function exotic
objects whose evaluative behaviour is expressed in some host-defined form of
executable code other than ECMAScript source text. Whether a function object is
defined within ECMAScript code or is a built-in function is not observable from
the perspective of ECMAScript code that calls or is called by such a function
object.
12 ECMASCRIPT LANGUAGE: LEXICAL GRAMMAR
The source text of an ECMAScript Script or Module is first converted into a
sequence of input elements, which are tokens, line terminators, comments, or
white space. The source text is scanned from left to right, repeatedly taking
the longest possible sequence of code points as the next input element.
There are several situations where the identification of lexical input elements
is sensitive to the syntactic grammar context that is consuming the input
elements. This requires multiple goal symbols for the lexical grammar. The
InputElementHashbangOrRegExp goal is used at the start of a Script or Module.
The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts
where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is
permitted. The InputElementRegExp goal symbol is used in all syntactic grammar
contexts where a RegularExpressionLiteral is permitted but neither a
TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail
goal is used in all syntactic grammar contexts where a TemplateMiddle or a
TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In
all other contexts, InputElementDiv is used as the lexical goal symbol.
Note
The use of multiple lexical goals ensures that there are no lexical ambiguities
that would affect automatic semicolon insertion. For example, there are no
syntactic grammar contexts where both a leading division or division-assignment,
and a leading RegularExpressionLiteral are permitted. This is not affected by
semicolon insertion (see 12.10); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is
U+002F (SOLIDUS) and the syntactic context allows division or
division-assignment, no semicolon is inserted at the LineTerminator. That is,
the above example is interpreted in the same way as:
a = b / hi / g.exec(c).map(d);
SYNTAX
InputElementDiv :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator
RightBracePunctuator InputElementRegExp :: WhiteSpace LineTerminator Comment
CommonToken RightBracePunctuator RegularExpressionLiteral
InputElementRegExpOrTemplateTail :: WhiteSpace LineTerminator Comment
CommonToken RegularExpressionLiteral TemplateSubstitutionTail
InputElementTemplateTail :: WhiteSpace LineTerminator Comment CommonToken
DivPunctuator TemplateSubstitutionTail InputElementHashbangOrRegExp ::
WhiteSpace LineTerminator Comment CommonToken HashbangComment
RegularExpressionLiteral
12.1 UNICODE FORMAT-CONTROL CHARACTERS
The Unicode format-control characters (i.e., the characters in category “Cf” in
the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK)
are control codes used to control the formatting of a range of text in the
absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate
editing and display. All format control characters may be used within comments,
and within string literals, template literals, and regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control
characters that are used to make necessary distinctions when forming words or
phrases in certain languages. In ECMAScript source text these code points may
also be used in an IdentifierName after the first character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily
at the start of a text to mark it as Unicode and to allow detection of the
text's encoding and byte order. <ZWNBSP> characters intended for this purpose
can sometimes also appear after the start of a text, for example as a result of
concatenating files. In ECMAScript source text <ZWNBSP> code points are treated
as white space characters (see 12.2).
The special treatment of certain format-control characters outside of comments,
string literals, and regular expression literals is summarized in Table 34.
Table 34: Format-Control Code Point Usage
Code Point Name Abbreviation Usage U+200C ZERO WIDTH NON-JOINER <ZWNJ>
IdentifierPart U+200D ZERO WIDTH JOINER <ZWJ> IdentifierPart U+FEFF ZERO WIDTH
NO-BREAK SPACE <ZWNBSP> WhiteSpace
12.2 WHITE SPACE
White space code points are used to improve source text readability and to
separate tokens (indivisible lexical units) from each other, but are otherwise
insignificant. White space code points may occur between any two tokens and at
the start or end of input. White space code points may occur within a
StringLiteral, a RegularExpressionLiteral, a Template, or a
TemplateSubstitutionTail where they are considered significant code points
forming part of a literal value. They may also occur within a Comment, but
cannot appear within any other kind of token.
The ECMAScript white space code points are listed in Table 35.
Table 35: White Space Code Points
Code Points Name Abbreviation U+0009 CHARACTER TABULATION <TAB> U+000B LINE
TABULATION <VT> U+000C FORM FEED (FF) <FF> U+FEFF ZERO WIDTH NO-BREAK SPACE
<ZWNBSP> any code point in general category “Space_Separator” <USP>
Note 1
U+0020 (SPACE) and U+00A0 (NO-BREAK SPACE) code points are part of <USP>.
Note 2
Other than for the code points listed in Table 35, ECMAScript WhiteSpace
intentionally excludes all code points that have the Unicode “White_Space”
property but which are not classified in general category “Space_Separator”
(“Zs”).
SYNTAX
WhiteSpace :: <TAB> <VT> <FF> <ZWNBSP> <USP>
12.3 LINE TERMINATORS
Like white space code points, line terminator code points are used to improve
source text readability and to separate tokens (indivisible lexical units) from
each other. However, unlike white space code points, line terminators have some
influence over the behaviour of the syntactic grammar. In general, line
terminators may occur between any two tokens, but there are a few places where
they are forbidden by the syntactic grammar. Line terminators also affect the
process of automatic semicolon insertion (12.10). A line terminator cannot occur
within any token except a StringLiteral, Template, or TemplateSubstitutionTail.
<LF> and <CR> line terminators cannot occur within a StringLiteral token except
as part of a LineContinuation.
A line terminator can occur within a MultiLineComment but cannot occur within a
SingleLineComment.
Line terminators are included in the set of white space code points that are
matched by the \s class in regular expressions.
The ECMAScript line terminator code points are listed in Table 36.
Table 36: Line Terminator Code Points
Code Point Unicode Name Abbreviation U+000A LINE FEED (LF) <LF> U+000D CARRIAGE
RETURN (CR) <CR> U+2028 LINE SEPARATOR <LS> U+2029 PARAGRAPH SEPARATOR <PS>
Only the Unicode code points in Table 36 are treated as line terminators. Other
new line or line breaking Unicode code points are not treated as line
terminators but are treated as white space if they meet the requirements listed
in Table 35. The sequence <CR><LF> is commonly used as a line terminator. It
should be considered a single SourceCharacter for the purpose of reporting line
numbers.
SYNTAX
LineTerminator :: <LF> <CR> <LS> <PS> LineTerminatorSequence :: <LF> <CR>
[lookahead ≠ <LF>] <LS> <PS> <CR> <LF>
12.4 COMMENTS
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a
LineTerminator code point, and because of the general rule that a token is
always as long as possible, a single-line comment always consists of all code
points from the // marker to the end of the line. However, the LineTerminator at
the end of the line is not considered to be part of the single-line comment; it
is recognized separately by the lexical grammar and becomes part of the stream
of input elements for the syntactic grammar. This point is very important,
because it implies that the presence or absence of single-line comments does not
affect the process of automatic semicolon insertion (see 12.10).
Comments behave like white space and are discarded except that, if a
MultiLineComment contains a line terminator code point, then the entire comment
is considered to be a LineTerminator for purposes of parsing by the syntactic
grammar.
SYNTAX
Comment :: MultiLineComment SingleLineComment MultiLineComment :: /*
MultiLineCommentCharsopt */ MultiLineCommentChars :: MultiLineNotAsteriskChar
MultiLineCommentCharsopt * PostAsteriskCommentCharsopt PostAsteriskCommentChars
:: MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt *
PostAsteriskCommentCharsopt MultiLineNotAsteriskChar :: SourceCharacter but not
* MultiLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or
* SingleLineComment :: // SingleLineCommentCharsopt SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt SingleLineCommentChar ::
SourceCharacter but not LineTerminator
A number of productions in this section are given alternative definitions in
section B.1.1
12.5 HASHBANG COMMENTS
Hashbang Comments are location-sensitive and like other types of comments are
discarded from the stream of input elements for the syntactic grammar.
SYNTAX
HashbangComment :: #! SingleLineCommentCharsopt
12.6 TOKENS
SYNTAX
CommonToken :: IdentifierName PrivateIdentifier Punctuator NumericLiteral
StringLiteral Template Note
The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and
TemplateSubstitutionTail productions derive additional tokens that are not
included in the CommonToken production.
12.7 NAMES AND KEYWORDS
IdentifierName and ReservedWord are tokens that are interpreted according to the
Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and
Pattern Syntax, with some small modifications. ReservedWord is an enumerated
subset of IdentifierName. The syntactic grammar defines Identifier as an
IdentifierName that is not a ReservedWord. The Unicode identifier grammar is
based on character properties specified by the Unicode Standard. The Unicode
code points in the specified categories in the latest version of the Unicode
Standard must be treated as in those categories by all conforming ECMAScript
implementations. ECMAScript implementations may recognize identifier code points
defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and
U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code
points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are
permitted anywhere after the first code point of an IdentifierName.
SYNTAX
PrivateIdentifier :: # IdentifierName IdentifierName :: IdentifierStart
IdentifierName IdentifierPart IdentifierStart :: IdentifierStartChar \
UnicodeEscapeSequence IdentifierPart :: IdentifierPartChar \
UnicodeEscapeSequence IdentifierStartChar :: UnicodeIDStart $ _
IdentifierPartChar :: UnicodeIDContinue $ <ZWNJ> <ZWJ> AsciiLetter :: one of a b
c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P
Q R S T U V W X Y Z UnicodeIDStart :: any Unicode code point with the Unicode
property “ID_Start” UnicodeIDContinue :: any Unicode code point with the Unicode
property “ID_Continue”
The definitions of the nonterminal UnicodeEscapeSequence is given in 12.9.4.
Note 2
The nonterminal IdentifierPart derives _ via UnicodeIDContinue.
Note 3
The sets of code points with Unicode properties “ID_Start” and “ID_Continue”
include, respectively, the code points with Unicode properties “Other_ID_Start”
and “Other_ID_Continue”.
12.7.1 IDENTIFIER NAMES
Unicode escape sequences are permitted in an IdentifierName, where they
contribute a single Unicode code point equal to the IdentifierCodePoint of the
UnicodeEscapeSequence. The \ preceding the UnicodeEscapeSequence does not
contribute any code points. A UnicodeEscapeSequence cannot be used to contribute
a code point to an IdentifierName that would otherwise be invalid. In other
words, if a \ UnicodeEscapeSequence sequence were replaced by the
SourceCharacter it contributes, the result must still be a valid IdentifierName
that has the exact same sequence of SourceCharacter elements as the original
IdentifierName. All interpretations of IdentifierName within this specification
are based upon their actual code points regardless of whether or not an escape
sequence was used to contribute any particular code point.
Two IdentifierNames that are canonically equivalent according to the Unicode
Standard are not equal unless, after replacement of each UnicodeEscapeSequence,
they are represented by the exact same sequence of code points.
12.7.1.1 STATIC SEMANTICS: EARLY ERRORS
IdentifierStart :: \ UnicodeEscapeSequence
* It is a Syntax Error if IdentifierCodePoint of UnicodeEscapeSequence is not
some Unicode code point matched by the IdentifierStartChar lexical grammar
production.
IdentifierPart :: \ UnicodeEscapeSequence
* It is a Syntax Error if IdentifierCodePoint of UnicodeEscapeSequence is not
some Unicode code point matched by the IdentifierPartChar lexical grammar
production.
12.7.1.2 STATIC SEMANTICS: IDENTIFIERCODEPOINTS
The syntax-directed operation IdentifierCodePoints takes no arguments and
returns a List of code points. It is defined piecewise over the following
productions:
IdentifierName :: IdentifierStart
1. 1. 1. Let cp be IdentifierCodePoint of IdentifierStart.
2. 2. 2. Return « cp ».
IdentifierName :: IdentifierName IdentifierPart
1. 1. 1. Let cps be IdentifierCodePoints of the derived IdentifierName.
2. 2. 2. Let cp be IdentifierCodePoint of IdentifierPart.
3. 3. 3. Return the list-concatenation of cps and « cp ».
12.7.1.3 STATIC SEMANTICS: IDENTIFIERCODEPOINT
The syntax-directed operation IdentifierCodePoint takes no arguments and returns
a code point. It is defined piecewise over the following productions:
IdentifierStart :: IdentifierStartChar
1. 1. 1. Return the code point matched by IdentifierStartChar.
IdentifierPart :: IdentifierPartChar
1. 1. 1. Return the code point matched by IdentifierPartChar.
UnicodeEscapeSequence :: u Hex4Digits
1. 1. 1. Return the code point whose numeric value is the MV of Hex4Digits.
UnicodeEscapeSequence :: u{ CodePoint }
1. 1. 1. Return the code point whose numeric value is the MV of CodePoint.
12.7.2 KEYWORDS AND RESERVED WORDS
A keyword is a token that matches IdentifierName, but also has a syntactic use;
that is, it appears literally, in a fixed width font, in some syntactic
production. The keywords of ECMAScript include if, while, async, await, and many
others.
A reserved word is an IdentifierName that cannot be used as an identifier. Many
keywords are reserved words, but some are not, and some are reserved only in
certain contexts. if and while are reserved words. await is reserved only inside
async functions and modules. async is not reserved; it can be used as a variable
name or statement label without restriction.
This specification uses a combination of grammatical productions and early error
rules to specify which names are valid identifiers and which are reserved words.
All tokens in the ReservedWord list below, except for await and yield, are
unconditionally reserved. Exceptions for await and yield are specified in 13.1,
using parameterized syntactic productions. Lastly, several early error rules
restrict the set of valid identifiers. See 13.1.1, 14.3.1.1, 14.7.5.1, and
15.7.1. In summary, there are five categories of identifier names:
* Those that are always allowed as identifiers, and are not keywords, such as
Math, window, toString, and _;
* Those that are never allowed as identifiers, namely the ReservedWords listed
below except await and yield;
* Those that are contextually allowed as identifiers, namely await and yield;
* Those that are contextually disallowed as identifiers, in strict mode code:
let, static, implements, interface, package, private, protected, and public;
* Those that are always allowed as identifiers, but also appear as keywords
within certain syntactic productions, at places where Identifier is not
allowed: as, async, from, get, meta, of, set, and target.
The term conditional keyword, or contextual keyword, is sometimes used to refer
to the keywords that fall in the last three categories, and thus can be used as
identifiers in some contexts and as keywords in others.
SYNTAX
ReservedWord :: one of await break case catch class const continue debugger
default delete do else enum export extends false finally for function if import
in instanceof new null return super switch this throw true try typeof var void
while with yield Note 1
Per 5.1.5, keywords in the grammar match literal sequences of specific
SourceCharacter elements. A code point in a keyword cannot be expressed by a \
UnicodeEscapeSequence.
An IdentifierName can contain \ UnicodeEscapeSequences, but it is not possible
to declare a variable named "else" by spelling it els\u{65}. The early error
rules in 13.1.1 rule out identifiers with the same StringValue as a reserved
word.
Note 2
enum is not currently used as a keyword in this specification. It is a future
reserved word, set aside for use as a keyword in future language extensions.
Similarly, implements, interface, package, private, protected, and public are
future reserved words in strict mode code.
Note 3
The names arguments and eval are not keywords, but they are subject to some
restrictions in strict mode code. See 13.1.1, 8.6.4, 15.2.1, 15.5.1, 15.6.1, and
15.8.1.
12.8 PUNCTUATORS
SYNTAX
Punctuator :: OptionalChainingPunctuator OtherPunctuator
OptionalChainingPunctuator :: ?. [lookahead ∉ DecimalDigit] OtherPunctuator ::
one of { ( ) [ ] . ... ; , < > <= >= == != === !== + - * % ** ++ -- << >> >>> &
| ^ ! ~ && || ?? ? : = += -= *= %= **= <<= >>= >>>= &= |= ^= &&= ||= ??= =>
DivPunctuator :: / /= RightBracePunctuator :: }
12.9 LITERALS
12.9.1 NULL LITERALS
SYNTAX
NullLiteral :: null
12.9.2 BOOLEAN LITERALS
SYNTAX
BooleanLiteral :: true false
12.9.3 NUMERIC LITERALS
SYNTAX
NumericLiteralSeparator :: _ NumericLiteral :: DecimalLiteral
DecimalBigIntegerLiteral NonDecimalIntegerLiteral[+Sep]
NonDecimalIntegerLiteral[+Sep] BigIntLiteralSuffix LegacyOctalIntegerLiteral
DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix NonZeroDigit
DecimalDigits[+Sep]opt BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator
DecimalDigits[+Sep] BigIntLiteralSuffix NonDecimalIntegerLiteral[Sep] ::
BinaryIntegerLiteral[?Sep] OctalIntegerLiteral[?Sep] HexIntegerLiteral[?Sep]
BigIntLiteralSuffix :: n DecimalLiteral :: DecimalIntegerLiteral .
DecimalDigits[+Sep]opt ExponentPart[+Sep]opt . DecimalDigits[+Sep]
ExponentPart[+Sep]opt DecimalIntegerLiteral ExponentPart[+Sep]opt
DecimalIntegerLiteral :: 0 NonZeroDigit NonZeroDigit NumericLiteralSeparatoropt
DecimalDigits[+Sep] NonOctalDecimalIntegerLiteral DecimalDigits[Sep] ::
DecimalDigit DecimalDigits[?Sep] DecimalDigit [+Sep] DecimalDigits[+Sep]
NumericLiteralSeparator DecimalDigit DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9
NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9 ExponentPart[Sep] :: ExponentIndicator
SignedInteger[?Sep] ExponentIndicator :: one of e E SignedInteger[Sep] ::
DecimalDigits[?Sep] + DecimalDigits[?Sep] - DecimalDigits[?Sep]
BinaryIntegerLiteral[Sep] :: 0b BinaryDigits[?Sep] 0B BinaryDigits[?Sep]
BinaryDigits[Sep] :: BinaryDigit BinaryDigits[?Sep] BinaryDigit [+Sep]
BinaryDigits[+Sep] NumericLiteralSeparator BinaryDigit BinaryDigit :: one of 0 1
OctalIntegerLiteral[Sep] :: 0o OctalDigits[?Sep] 0O OctalDigits[?Sep]
OctalDigits[Sep] :: OctalDigit OctalDigits[?Sep] OctalDigit [+Sep]
OctalDigits[+Sep] NumericLiteralSeparator OctalDigit LegacyOctalIntegerLiteral
:: 0 OctalDigit LegacyOctalIntegerLiteral OctalDigit
NonOctalDecimalIntegerLiteral :: 0 NonOctalDigit
LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit NonOctalDecimalIntegerLiteral
DecimalDigit LegacyOctalLikeDecimalIntegerLiteral :: 0 OctalDigit
LegacyOctalLikeDecimalIntegerLiteral OctalDigit OctalDigit :: one of 0 1 2 3 4 5
6 7 NonOctalDigit :: one of 8 9 HexIntegerLiteral[Sep] :: 0x HexDigits[?Sep] 0X
HexDigits[?Sep] HexDigits[Sep] :: HexDigit HexDigits[?Sep] HexDigit [+Sep]
HexDigits[+Sep] NumericLiteralSeparator HexDigit HexDigit :: one of 0 1 2 3 4 5
6 7 8 9 a b c d e f A B C D E F
The SourceCharacter immediately following a NumericLiteral must not be an
IdentifierStart or DecimalDigit.
Note
For example: 3in is an error and not the two input elements 3 and in.
12.9.3.1 STATIC SEMANTICS: EARLY ERRORS
NumericLiteral :: LegacyOctalIntegerLiteral DecimalIntegerLiteral ::
NonOctalDecimalIntegerLiteral
* It is a Syntax Error if the source text matched by this production is strict
mode code.
Note
In non-strict code, this syntax is Legacy.
12.9.3.2 STATIC SEMANTICS: MV
A numeric literal stands for a value of the Number type or the BigInt type.
* The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV
of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10-n), where n is
the number of code points in DecimalDigits, excluding all occurrences of
NumericLiteralSeparator.
* The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of
DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.
* The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits
ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of
DecimalDigits × 10-n)) × 10e, where n is the number of code points in
DecimalDigits, excluding all occurrences of NumericLiteralSeparator and e is
the MV of ExponentPart.
* The MV of DecimalLiteral :: . DecimalDigits is the MV of DecimalDigits ×
10-n, where n is the number of code points in DecimalDigits, excluding all
occurrences of NumericLiteralSeparator.
* The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of
DecimalDigits × 10e - n, where n is the number of code points in
DecimalDigits, excluding all occurrences of NumericLiteralSeparator, and e is
the MV of ExponentPart.
* The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of
DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.
* The MV of DecimalIntegerLiteral :: 0 is 0.
* The MV of DecimalIntegerLiteral :: NonZeroDigit NumericLiteralSeparatoropt
DecimalDigits is (the MV of NonZeroDigit × 10n) plus the MV of DecimalDigits,
where n is the number of code points in DecimalDigits, excluding all
occurrences of NumericLiteralSeparator.
* The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of
DecimalDigits × 10) plus the MV of DecimalDigit.
* The MV of DecimalDigits :: DecimalDigits NumericLiteralSeparator DecimalDigit
is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.
* The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of
SignedInteger.
* The MV of SignedInteger :: - DecimalDigits is the negative of the MV of
DecimalDigits.
* The MV of DecimalDigit :: 0 or of HexDigit :: 0 or of OctalDigit :: 0 or of
LegacyOctalEscapeSequence :: 0 or of BinaryDigit :: 0 is 0.
* The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 or of
OctalDigit :: 1 or of BinaryDigit :: 1 is 1.
* The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 or of
OctalDigit :: 2 is 2.
* The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 or of
OctalDigit :: 3 is 3.
* The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 or of
OctalDigit :: 4 is 4.
* The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 or of
OctalDigit :: 5 is 5.
* The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 or of
OctalDigit :: 6 is 6.
* The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 or of
OctalDigit :: 7 is 7.
* The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of NonOctalDigit :: 8
or of HexDigit :: 8 is 8.
* The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of NonOctalDigit :: 9
or of HexDigit :: 9 is 9.
* The MV of HexDigit :: a or of HexDigit :: A is 10.
* The MV of HexDigit :: b or of HexDigit :: B is 11.
* The MV of HexDigit :: c or of HexDigit :: C is 12.
* The MV of HexDigit :: d or of HexDigit :: D is 13.
* The MV of HexDigit :: e or of HexDigit :: E is 14.
* The MV of HexDigit :: f or of HexDigit :: F is 15.
* The MV of BinaryDigits :: BinaryDigits BinaryDigit is (the MV of BinaryDigits
× 2) plus the MV of BinaryDigit.
* The MV of BinaryDigits :: BinaryDigits NumericLiteralSeparator BinaryDigit is
(the MV of BinaryDigits × 2) plus the MV of BinaryDigit.
* The MV of OctalDigits :: OctalDigits OctalDigit is (the MV of OctalDigits ×
8) plus the MV of OctalDigit.
* The MV of OctalDigits :: OctalDigits NumericLiteralSeparator OctalDigit is
(the MV of OctalDigits × 8) plus the MV of OctalDigit.
* The MV of LegacyOctalIntegerLiteral :: LegacyOctalIntegerLiteral OctalDigit
is (the MV of LegacyOctalIntegerLiteral times 8) plus the MV of OctalDigit.
* The MV of NonOctalDecimalIntegerLiteral ::
LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit is (the MV of
LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV of NonOctalDigit.
* The MV of NonOctalDecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral
DecimalDigit is (the MV of NonOctalDecimalIntegerLiteral times 10) plus the
MV of DecimalDigit.
* The MV of LegacyOctalLikeDecimalIntegerLiteral ::
LegacyOctalLikeDecimalIntegerLiteral OctalDigit is (the MV of
LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV of OctalDigit.
* The MV of HexDigits :: HexDigits HexDigit is (the MV of HexDigits × 16) plus
the MV of HexDigit.
* The MV of HexDigits :: HexDigits NumericLiteralSeparator HexDigit is (the MV
of HexDigits × 16) plus the MV of HexDigit.
12.9.3.3 STATIC SEMANTICS: NUMERICVALUE
The syntax-directed operation NumericValue takes no arguments and returns a
Number or a BigInt. It is defined piecewise over the following productions:
NumericLiteral :: DecimalLiteral
1. 1. 1. Return RoundMVResult(MV of DecimalLiteral).
NumericLiteral :: NonDecimalIntegerLiteral
1. 1. 1. Return 𝔽(MV of NonDecimalIntegerLiteral).
NumericLiteral :: LegacyOctalIntegerLiteral
1. 1. 1. Return 𝔽(MV of LegacyOctalIntegerLiteral).
NumericLiteral :: NonDecimalIntegerLiteral BigIntLiteralSuffix
1. 1. 1. Return the BigInt value that represents the MV of
NonDecimalIntegerLiteral.
DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix
1. 1. 1. Return 0ℤ.
DecimalBigIntegerLiteral :: NonZeroDigit BigIntLiteralSuffix
1. 1. 1. Return the BigInt value that represents the MV of NonZeroDigit.
DecimalBigIntegerLiteral :: NonZeroDigit DecimalDigits BigIntLiteralSuffix
NonZeroDigit NumericLiteralSeparator DecimalDigits BigIntLiteralSuffix
1. 1. 1. Let n be the number of code points in DecimalDigits, excluding all
occurrences of NumericLiteralSeparator.
2. 2. 2. Let mv be (the MV of NonZeroDigit × 10n) plus the MV of DecimalDigits.
3. 3. 3. Return ℤ(mv).
12.9.4 STRING LITERALS
Note 1
A string literal is 0 or more Unicode code points enclosed in single or double
quotes. Unicode code points may also be represented by an escape sequence. All
code points may appear literally in a string literal except for the closing
quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and
U+000A (LINE FEED). Any code points may appear in the form of an escape
sequence. String literals evaluate to ECMAScript String values. When generating
these String values Unicode code points are UTF-16 encoded as defined in 11.1.1.
Code points belonging to the Basic Multilingual Plane are encoded as a single
code unit element of the string. All other code points are encoded as two code
unit elements of the string.
SYNTAX
StringLiteral :: " DoubleStringCharactersopt " ' SingleStringCharactersopt '
DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters :: SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator
<LS> <PS> \ EscapeSequence LineContinuation SingleStringCharacter ::
SourceCharacter but not one of ' or \ or LineTerminator <LS> <PS> \
EscapeSequence LineContinuation LineContinuation :: \ LineTerminatorSequence
EscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit]
LegacyOctalEscapeSequence NonOctalDecimalEscapeSequence HexEscapeSequence
UnicodeEscapeSequence CharacterEscapeSequence :: SingleEscapeCharacter
NonEscapeCharacter SingleEscapeCharacter :: one of ' " \ b f n r t v
NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or
LineTerminator EscapeCharacter :: SingleEscapeCharacter DecimalDigit x u
LegacyOctalEscapeSequence :: 0 [lookahead ∈ { 8, 9 }] NonZeroOctalDigit
[lookahead ∉ OctalDigit] ZeroToThree OctalDigit [lookahead ∉ OctalDigit]
FourToSeven OctalDigit ZeroToThree OctalDigit OctalDigit NonZeroOctalDigit ::
OctalDigit but not 0 ZeroToThree :: one of 0 1 2 3 FourToSeven :: one of 4 5 6 7
NonOctalDecimalEscapeSequence :: one of 8 9 HexEscapeSequence :: x HexDigit
HexDigit UnicodeEscapeSequence :: u Hex4Digits u{ CodePoint } Hex4Digits ::
HexDigit HexDigit HexDigit HexDigit
The definition of the nonterminal HexDigit is given in 12.9.3. SourceCharacter
is defined in 11.1.
Note 2
<LF> and <CR> cannot appear in a string literal, except as part of a
LineContinuation to produce the empty code points sequence. The proper way to
include either in the String value of a string literal is to use an escape
sequence such as \n or \u000A.
12.9.4.1 STATIC SEMANTICS: EARLY ERRORS
EscapeSequence :: LegacyOctalEscapeSequence NonOctalDecimalEscapeSequence
* It is a Syntax Error if the source text matched by this production is strict
mode code.
Note 1
In non-strict code, this syntax is Legacy.
Note 2
It is possible for string literals to precede a Use Strict Directive that places
the enclosing code in strict mode, and implementations must take care to enforce
the above rules for such literals. For example, the following source text
contains a Syntax Error:
function invalid() { "\7"; "use strict"; }
12.9.4.2 STATIC SEMANTICS: SV
The syntax-directed operation SV takes no arguments and returns a String.
A string literal stands for a value of the String type. SV produces String
values for string literals through recursive application on the various parts of
the string literal. As part of this process, some Unicode code points within the
string literal are interpreted as having a mathematical value, as described
below or in 12.9.3.
* The SV of StringLiteral :: " " is the empty String.
* The SV of StringLiteral :: ' ' is the empty String.
* The SV of DoubleStringCharacters :: DoubleStringCharacter
DoubleStringCharacters is the string-concatenation of the SV of
DoubleStringCharacter and the SV of DoubleStringCharacters.
* The SV of SingleStringCharacters :: SingleStringCharacter
SingleStringCharacters is the string-concatenation of the SV of
SingleStringCharacter and the SV of SingleStringCharacters.
* The SV of DoubleStringCharacter :: SourceCharacter but not one of " or \ or
LineTerminator is the result of performing UTF16EncodeCodePoint on the code
point matched by SourceCharacter.
* The SV of DoubleStringCharacter :: <LS> is the String value consisting of the
code unit 0x2028 (LINE SEPARATOR).
* The SV of DoubleStringCharacter :: <PS> is the String value consisting of the
code unit 0x2029 (PARAGRAPH SEPARATOR).
* The SV of DoubleStringCharacter :: LineContinuation is the empty String.
* The SV of SingleStringCharacter :: SourceCharacter but not one of ' or \ or
LineTerminator is the result of performing UTF16EncodeCodePoint on the code
point matched by SourceCharacter.
* The SV of SingleStringCharacter :: <LS> is the String value consisting of the
code unit 0x2028 (LINE SEPARATOR).
* The SV of SingleStringCharacter :: <PS> is the String value consisting of the
code unit 0x2029 (PARAGRAPH SEPARATOR).
* The SV of SingleStringCharacter :: LineContinuation is the empty String.
* The SV of EscapeSequence :: 0 is the String value consisting of the code unit
0x0000 (NULL).
* The SV of CharacterEscapeSequence :: SingleEscapeCharacter is the String
value consisting of the code unit whose numeric value is determined by the
SingleEscapeCharacter according to Table 37.
Table 37: String Single Character Escape Sequences
Escape Sequence Code Unit Value Unicode Character Name Symbol \b 0x0008
BACKSPACE <BS> \t 0x0009 CHARACTER TABULATION <HT> \n 0x000A LINE FEED (LF) <LF>
\v 0x000B LINE TABULATION <VT> \f 0x000C FORM FEED (FF) <FF> \r 0x000D CARRIAGE
RETURN (CR) <CR> \" 0x0022 QUOTATION MARK " \' 0x0027 APOSTROPHE ' \\ 0x005C
REVERSE SOLIDUS \
* The SV of NonEscapeCharacter :: SourceCharacter but not one of
EscapeCharacter or LineTerminator is the result of performing
UTF16EncodeCodePoint on the code point matched by SourceCharacter.
* The SV of EscapeSequence :: LegacyOctalEscapeSequence is the String value
consisting of the code unit whose numeric value is the MV of
LegacyOctalEscapeSequence.
* The SV of NonOctalDecimalEscapeSequence :: 8 is the String value consisting
of the code unit 0x0038 (DIGIT EIGHT).
* The SV of NonOctalDecimalEscapeSequence :: 9 is the String value consisting
of the code unit 0x0039 (DIGIT NINE).
* The SV of HexEscapeSequence :: x HexDigit HexDigit is the String value
consisting of the code unit whose numeric value is the MV of
HexEscapeSequence.
* The SV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the String
value consisting of the code unit whose numeric value is the MV of
Hex4Digits.
* The SV of UnicodeEscapeSequence :: u{ CodePoint } is the result of performing
UTF16EncodeCodePoint on the MV of CodePoint.
* The SV of TemplateEscapeSequence :: 0 is the String value consisting of the
code unit 0x0000 (NULL).
12.9.4.3 STATIC SEMANTICS: MV
* The MV of LegacyOctalEscapeSequence :: ZeroToThree OctalDigit is (8 times the
MV of ZeroToThree) plus the MV of OctalDigit.
* The MV of LegacyOctalEscapeSequence :: FourToSeven OctalDigit is (8 times the
MV of FourToSeven) plus the MV of OctalDigit.
* The MV of LegacyOctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit is
(64 (that is, 82) times the MV of ZeroToThree) plus (8 times the MV of the
first OctalDigit) plus the MV of the second OctalDigit.
* The MV of ZeroToThree :: 0 is 0.
* The MV of ZeroToThree :: 1 is 1.
* The MV of ZeroToThree :: 2 is 2.
* The MV of ZeroToThree :: 3 is 3.
* The MV of FourToSeven :: 4 is 4.
* The MV of FourToSeven :: 5 is 5.
* The MV of FourToSeven :: 6 is 6.
* The MV of FourToSeven :: 7 is 7.
* The MV of HexEscapeSequence :: x HexDigit HexDigit is (16 times the MV of the
first HexDigit) plus the MV of the second HexDigit.
* The MV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is (0x1000 × the
MV of the first HexDigit) plus (0x100 × the MV of the second HexDigit) plus
(0x10 × the MV of the third HexDigit) plus the MV of the fourth HexDigit.
12.9.5 REGULAR EXPRESSION LITERALS
Note 1
A regular expression literal is an input element that is converted to a RegExp
object (see 22.2) each time the literal is evaluated. Two regular expression
literals in a program evaluate to regular expression objects that never compare
as === to each other even if the two literals' contents are identical. A RegExp
object may also be created at runtime by new RegExp or calling the RegExp
constructor as a function (see 22.2.4).
The productions below describe the syntax for a regular expression literal and
are used by the input element scanner to find the end of the regular expression
literal. The source text comprising the RegularExpressionBody and the
RegularExpressionFlags are subsequently parsed again using the more stringent
ECMAScript Regular Expression grammar (22.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined
in 22.2.1, but it must not extend the RegularExpressionBody and
RegularExpressionFlags productions defined below or the productions used by
these productions.
SYNTAX
RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
RegularExpressionBody :: RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars :: [empty] RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar :: RegularExpressionNonTerminator but not one of * or
\ or / or [ RegularExpressionBackslashSequence RegularExpressionClass
RegularExpressionChar :: RegularExpressionNonTerminator but not one of \ or / or
[ RegularExpressionBackslashSequence RegularExpressionClass
RegularExpressionBackslashSequence :: \ RegularExpressionNonTerminator
RegularExpressionNonTerminator :: SourceCharacter but not LineTerminator
RegularExpressionClass :: [ RegularExpressionClassChars ]
RegularExpressionClassChars :: [empty] RegularExpressionClassChars
RegularExpressionClassChar RegularExpressionClassChar ::
RegularExpressionNonTerminator but not one of ] or \
RegularExpressionBackslashSequence RegularExpressionFlags :: [empty]
RegularExpressionFlags IdentifierPartChar Note 2
Regular expression literals may not be empty; instead of representing an empty
regular expression literal, the code unit sequence // starts a single-line
comment. To specify an empty regular expression, use: /(?:)/.
12.9.5.1 STATIC SEMANTICS: BODYTEXT
The syntax-directed operation BodyText takes no arguments and returns source
text. It is defined piecewise over the following productions:
RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
1. 1. 1. Return the source text that was recognized as RegularExpressionBody.
12.9.5.2 STATIC SEMANTICS: FLAGTEXT
The syntax-directed operation FlagText takes no arguments and returns source
text. It is defined piecewise over the following productions:
RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
1. 1. 1. Return the source text that was recognized as RegularExpressionFlags.
12.9.6 TEMPLATE LITERAL LEXICAL COMPONENTS
SYNTAX
Template :: NoSubstitutionTemplate TemplateHead NoSubstitutionTemplate :: `
TemplateCharactersopt ` TemplateHead :: ` TemplateCharactersopt ${
TemplateSubstitutionTail :: TemplateMiddle TemplateTail TemplateMiddle :: }
TemplateCharactersopt ${ TemplateTail :: } TemplateCharactersopt `
TemplateCharacters :: TemplateCharacter TemplateCharactersopt TemplateCharacter
:: $ [lookahead ≠ {] \ TemplateEscapeSequence \ NotEscapeSequence
LineContinuation LineTerminatorSequence SourceCharacter but not one of ` or \ or
$ or LineTerminator TemplateEscapeSequence :: CharacterEscapeSequence 0
[lookahead ∉ DecimalDigit] HexEscapeSequence UnicodeEscapeSequence
NotEscapeSequence :: 0 DecimalDigit DecimalDigit but not 0 x [lookahead ∉
HexDigit] x HexDigit [lookahead ∉ HexDigit] u [lookahead ∉ HexDigit] [lookahead
≠ {] u HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit [lookahead ∉
HexDigit] u HexDigit HexDigit HexDigit [lookahead ∉ HexDigit] u { [lookahead ∉
HexDigit] u { NotCodePoint [lookahead ∉ HexDigit] u { CodePoint [lookahead ∉
HexDigit] [lookahead ≠ }] NotCodePoint :: HexDigits[~Sep] but only if MV of
HexDigits > 0x10FFFF CodePoint :: HexDigits[~Sep] but only if MV of HexDigits ≤
0x10FFFF Note
TemplateSubstitutionTail is used by the InputElementTemplateTail alternative
lexical goal.
12.9.6.1 STATIC SEMANTICS: TV
The syntax-directed operation TV takes no arguments and returns a String or
undefined. A template literal component is interpreted by TV as a value of the
String type. TV is used to construct the indexed components of a template object
(colloquially, the template values). In TV, escape sequences are replaced by the
UTF-16 code unit(s) of the Unicode code point represented by the escape
sequence.
* The TV of NoSubstitutionTemplate :: ` ` is the empty String.
* The TV of TemplateHead :: ` ${ is the empty String.
* The TV of TemplateMiddle :: } ${ is the empty String.
* The TV of TemplateTail :: } ` is the empty String.
* The TV of TemplateCharacters :: TemplateCharacter TemplateCharacters is
undefined if the TV of TemplateCharacter is undefined or the TV of
TemplateCharacters is undefined. Otherwise, it is the string-concatenation of
the TV of TemplateCharacter and the TV of TemplateCharacters.
* The TV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ or
LineTerminator is the result of performing UTF16EncodeCodePoint on the code
point matched by SourceCharacter.
* The TV of TemplateCharacter :: $ is the String value consisting of the code
unit 0x0024 (DOLLAR SIGN).
* The TV of TemplateCharacter :: \ TemplateEscapeSequence is the SV of
TemplateEscapeSequence.
* The TV of TemplateCharacter :: \ NotEscapeSequence is undefined.
* The TV of TemplateCharacter :: LineTerminatorSequence is the TRV of
LineTerminatorSequence.
* The TV of LineContinuation :: \ LineTerminatorSequence is the empty String.
12.9.6.2 STATIC SEMANTICS: TRV
The syntax-directed operation TRV takes no arguments and returns a String. A
template literal component is interpreted by TRV as a value of the String type.
TRV is used to construct the raw components of a template object (colloquially,
the template raw values). TRV is similar to TV with the difference being that in
TRV, escape sequences are interpreted as they appear in the literal.
* The TRV of NoSubstitutionTemplate :: ` ` is the empty String.
* The TRV of TemplateHead :: ` ${ is the empty String.
* The TRV of TemplateMiddle :: } ${ is the empty String.
* The TRV of TemplateTail :: } ` is the empty String.
* The TRV of TemplateCharacters :: TemplateCharacter TemplateCharacters is the
string-concatenation of the TRV of TemplateCharacter and the TRV of
TemplateCharacters.
* The TRV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ or
LineTerminator is the result of performing UTF16EncodeCodePoint on the code
point matched by SourceCharacter.
* The TRV of TemplateCharacter :: $ is the String value consisting of the code
unit 0x0024 (DOLLAR SIGN).
* The TRV of TemplateCharacter :: \ TemplateEscapeSequence is the
string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV of
TemplateEscapeSequence.
* The TRV of TemplateCharacter :: \ NotEscapeSequence is the
string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV of
NotEscapeSequence.
* The TRV of TemplateEscapeSequence :: 0 is the String value consisting of the
code unit 0x0030 (DIGIT ZERO).
* The TRV of NotEscapeSequence :: 0 DecimalDigit is the string-concatenation of
the code unit 0x0030 (DIGIT ZERO) and the TRV of DecimalDigit.
* The TRV of NotEscapeSequence :: x [lookahead ∉ HexDigit] is the String value
consisting of the code unit 0x0078 (LATIN SMALL LETTER X).
* The TRV of NotEscapeSequence :: x HexDigit [lookahead ∉ HexDigit] is the
string-concatenation of the code unit 0x0078 (LATIN SMALL LETTER X) and the
TRV of HexDigit.
* The TRV of NotEscapeSequence :: u [lookahead ∉ HexDigit] [lookahead ≠ {] is
the String value consisting of the code unit 0x0075 (LATIN SMALL LETTER U).
* The TRV of NotEscapeSequence :: u HexDigit [lookahead ∉ HexDigit] is the
string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the
TRV of HexDigit.
* The TRV of NotEscapeSequence :: u HexDigit HexDigit [lookahead ∉ HexDigit] is
the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the
TRV of the first HexDigit, and the TRV of the second HexDigit.
* The TRV of NotEscapeSequence :: u HexDigit HexDigit HexDigit [lookahead ∉
HexDigit] is the string-concatenation of the code unit 0x0075 (LATIN SMALL
LETTER U), the TRV of the first HexDigit, the TRV of the second HexDigit, and
the TRV of the third HexDigit.
* The TRV of NotEscapeSequence :: u { [lookahead ∉ HexDigit] is the
string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the
code unit 0x007B (LEFT CURLY BRACKET).
* The TRV of NotEscapeSequence :: u { NotCodePoint [lookahead ∉ HexDigit] is
the string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the
code unit 0x007B (LEFT CURLY BRACKET), and the TRV of NotCodePoint.
* The TRV of NotEscapeSequence :: u { CodePoint [lookahead ∉ HexDigit]
[lookahead ≠ }] is the string-concatenation of the code unit 0x0075 (LATIN
SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), and the TRV of
CodePoint.
* The TRV of DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 is the result of
performing UTF16EncodeCodePoint on the single code point matched by this
production.
* The TRV of CharacterEscapeSequence :: NonEscapeCharacter is the SV of
NonEscapeCharacter.
* The TRV of SingleEscapeCharacter :: one of ' " \ b f n r t v is the result of
performing UTF16EncodeCodePoint on the single code point matched by this
production.
* The TRV of HexEscapeSequence :: x HexDigit HexDigit is the
string-concatenation of the code unit 0x0078 (LATIN SMALL LETTER X), the TRV
of the first HexDigit, and the TRV of the second HexDigit.
* The TRV of UnicodeEscapeSequence :: u Hex4Digits is the string-concatenation
of the code unit 0x0075 (LATIN SMALL LETTER U) and the TRV of Hex4Digits.
* The TRV of UnicodeEscapeSequence :: u{ CodePoint } is the
string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code
unit 0x007B (LEFT CURLY BRACKET), the TRV of CodePoint, and the code unit
0x007D (RIGHT CURLY BRACKET).
* The TRV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the
string-concatenation of the TRV of the first HexDigit, the TRV of the second
HexDigit, the TRV of the third HexDigit, and the TRV of the fourth HexDigit.
* The TRV of HexDigits :: HexDigits HexDigit is the string-concatenation of the
TRV of HexDigits and the TRV of HexDigit.
* The TRV of HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F is
the result of performing UTF16EncodeCodePoint on the single code point
matched by this production.
* The TRV of LineContinuation :: \ LineTerminatorSequence is the
string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV of
LineTerminatorSequence.
* The TRV of LineTerminatorSequence :: <LF> is the String value consisting of
the code unit 0x000A (LINE FEED).
* The TRV of LineTerminatorSequence :: <CR> is the String value consisting of
the code unit 0x000A (LINE FEED).
* The TRV of LineTerminatorSequence :: <LS> is the String value consisting of
the code unit 0x2028 (LINE SEPARATOR).
* The TRV of LineTerminatorSequence :: <PS> is the String value consisting of
the code unit 0x2029 (PARAGRAPH SEPARATOR).
* The TRV of LineTerminatorSequence :: <CR> <LF> is the String value consisting
of the code unit 0x000A (LINE FEED).
Note
TV excludes the code units of LineContinuation while TRV includes them. <CR><LF>
and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An
explicit TemplateEscapeSequence is needed to include a <CR> or <CR><LF>
sequence.
12.10 AUTOMATIC SEMICOLON INSERTION
Most ECMAScript statements and declarations must be terminated with a semicolon.
Such semicolons may always appear explicitly in the source text. For
convenience, however, such semicolons may be omitted from the source text in
certain situations. These situations are described by saying that semicolons are
automatically inserted into the source code token stream in those situations.
12.10.1 RULES OF AUTOMATIC SEMICOLON INSERTION
In the following rules, “token” means the actual recognized lexical token
determined using the current lexical goal symbol as described in clause 12.
There are three basic rules of semicolon insertion:
1. When, as the source text is parsed from left to right, a token (called the
offending token) is encountered that is not allowed by any production of the
grammar, then a semicolon is automatically inserted before the offending
token if one or more of the following conditions is true:
* The offending token is separated from the previous token by at least one
LineTerminator.
* The offending token is }.
* The previous token is ) and the inserted semicolon would then be parsed as
the terminating semicolon of a do-while statement (14.7.2).
2. When, as the source text is parsed from left to right, the end of the input
stream of tokens is encountered and the parser is unable to parse the input
token stream as a single instance of the goal nonterminal, then a semicolon
is automatically inserted at the end of the input stream.
3. When, as the source text is parsed from left to right, a token is
encountered that is allowed by some production of the grammar, but the
production is a restricted production and the token would be the first token
for a terminal or nonterminal immediately following the annotation “[no
LineTerminator here]” within the restricted production (and therefore such a
token is called a restricted token), and the restricted token is separated
from the previous token by at least one LineTerminator, then a semicolon is
automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a
semicolon is never inserted automatically if the semicolon would then be parsed
as an empty statement or if that semicolon would become one of the two
semicolons in the header of a for statement (see 14.7.4).
Note
The following are the only restricted productions in the grammar:
UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await] [no
LineTerminator here] ++ LeftHandSideExpression[?Yield, ?Await] [no
LineTerminator here] -- ContinueStatement[Yield, Await] : continue ; continue
[no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; BreakStatement[Yield,
Await] : break ; break [no LineTerminator here] LabelIdentifier[?Yield, ?Await]
; ReturnStatement[Yield, Await] : return ; return [no LineTerminator here]
Expression[+In, ?Yield, ?Await] ; ThrowStatement[Yield, Await] : throw [no
LineTerminator here] Expression[+In, ?Yield, ?Await] ; YieldExpression[In,
Await] : yield yield [no LineTerminator here] AssignmentExpression[?In, +Yield,
?Await] yield [no LineTerminator here] * AssignmentExpression[?In, +Yield,
?Await] ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no
LineTerminator here] => ConciseBody[?In] AsyncFunctionDeclaration[Yield, Await,
Default] : async [no LineTerminator here] function BindingIdentifier[?Yield,
?Await] ( FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } [+Default]
async [no LineTerminator here] function ( FormalParameters[~Yield, +Await] ) {
AsyncFunctionBody } AsyncFunctionExpression : async [no LineTerminator here]
function BindingIdentifier[~Yield, +Await]opt ( FormalParameters[~Yield, +Await]
) { AsyncFunctionBody } AsyncMethod[Yield, Await] : async [no LineTerminator
here] ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, +Await]
) { AsyncFunctionBody } AsyncGeneratorDeclaration[Yield, Await, Default] : async
[no LineTerminator here] function * BindingIdentifier[?Yield, ?Await] (
FormalParameters[+Yield, +Await] ) { AsyncGeneratorBody } [+Default] async [no
LineTerminator here] function * ( FormalParameters[+Yield, +Await] ) {
AsyncGeneratorBody } AsyncGeneratorExpression : async [no LineTerminator here]
function * BindingIdentifier[+Yield, +Await]opt ( FormalParameters[+Yield,
+Await] ) { AsyncGeneratorBody } AsyncGeneratorMethod[Yield, Await] : async [no
LineTerminator here] * ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[+Yield, +Await] ) { AsyncGeneratorBody }
AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here]
AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] =>
AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no
LineTerminator here] => AsyncConciseBody[?In] AsyncArrowHead : async [no
LineTerminator here] ArrowFormalParameters[~Yield, +Await]
The practical effect of these restricted productions is as follows:
* When a ++ or -- token is encountered where the parser would treat it as a
postfix operator, and at least one LineTerminator occurred between the
preceding token and the ++ or -- token, then a semicolon is automatically
inserted before the ++ or -- token.
* When a continue, break, return, throw, or yield token is encountered and a
LineTerminator is encountered before the next token, a semicolon is
automatically inserted after the continue, break, return, throw, or yield
token.
* When arrow function parameter(s) are followed by a LineTerminator before a =>
token, a semicolon is automatically inserted and the punctuator causes a
syntax error.
* When an async token is followed by a LineTerminator before a function or
IdentifierName or ( token, a semicolon is automatically inserted and the
async token is not treated as part of the same expression or class element as
the following tokens.
* When an async token is followed by a LineTerminator before a * token, a
semicolon is automatically inserted and the punctuator causes a syntax error.
The resulting practical advice to ECMAScript programmers is:
* A postfix ++ or -- operator should be on the same line as its operand.
* An Expression in a return or throw statement or an AssignmentExpression in a
yield expression should start on the same line as the return, throw, or yield
token.
* A LabelIdentifier in a break or continue statement should be on the same line
as the break or continue token.
* The end of an arrow function's parameter(s) and its => should be on the same
line.
* The async token preceding an asynchronous function or method should be on the
same line as the immediately following token.
12.10.2 EXAMPLES OF AUTOMATIC SEMICOLON INSERTION
This section is non-normative.
The source
{ 1 2 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic
semicolon insertion rules. In contrast, the source
{ 1
2 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic
semicolon insertion into the following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon
insertion because the semicolon is needed for the header of a for statement.
Automatic semicolon insertion never inserts one of the two semicolons in the
header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the return
statement, because a LineTerminator separates it from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable b,
because a LineTerminator occurs between b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon
insertion before the else token, even though no production of the grammar
applies at that point, because an automatically inserted semicolon would then be
parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized
expression that begins the second line can be interpreted as an argument list
for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left
parenthesis, it is a good idea for the programmer to provide an explicit
semicolon at the end of the preceding statement rather than to rely on automatic
semicolon insertion.
12.10.3 INTERESTING CASES OF AUTOMATIC SEMICOLON INSERTION
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons by
relying on automatic semicolon insertion. As described above, semicolons are not
inserted at every newline, and automatic semicolon insertion can depend on
multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar
productions could be added that cause lines relying on automatic semicolon
insertion preceding them to change grammar productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is
considered interesting if it is a place where a semicolon may or may not be
inserted, depending on the source text which precedes it. The rest of this
section describes a number of interesting cases of automatic semicolon insertion
in this version of ECMAScript.
12.10.3.1 INTERESTING CASES OF AUTOMATIC SEMICOLON INSERTION IN STATEMENT LISTS
In a StatementList, many StatementListItems end in semicolons, which may be
omitted using automatic semicolon insertion. As a consequence of the rules
above, at the end of a line ending an expression, a semicolon is required if the
following line begins with any of the following:
* An opening parenthesis ((). Without a semicolon, the two lines together are
treated as a CallExpression.
* An opening square bracket ([). Without a semicolon, the two lines together
are treated as property access, rather than an ArrayLiteral or
ArrayAssignmentPattern.
* A template literal (`). Without a semicolon, the two lines together are
interpreted as a tagged Template (13.3.11), with the previous expression as
the MemberExpression.
* Unary + or -. Without a semicolon, the two lines together are interpreted as
a usage of the corresponding binary operator.
* A RegExp literal. Without a semicolon, the two lines together may be parsed
instead as the / MultiplicativeOperator, for example if the RegExp has flags.
12.10.3.2 CASES OF AUTOMATIC SEMICOLON INSERTION AND “[NO LINETERMINATOR HERE]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no LineTerminator
here]”. These productions are sometimes a means to have optional operands in the
grammar. Introducing a LineTerminator in these locations would change the
grammar production of a source text by using the grammar production without the
optional operand.
The rest of this section describes a number of productions using “[no
LineTerminator here]” in this version of ECMAScript.
12.10.3.2.1 LIST OF GRAMMAR PRODUCTIONS WITH OPTIONAL OPERANDS AND “[NO
LINETERMINATOR HERE]”
* UpdateExpression.
* ContinueStatement.
* BreakStatement.
* ReturnStatement.
* YieldExpression.
* Async Function Definitions (15.8) with relation to Function Definitions
(15.2)
13 ECMASCRIPT LANGUAGE: EXPRESSIONS
13.1 IDENTIFIERS
SYNTAX
IdentifierReference[Yield, Await] : Identifier [~Yield] yield [~Await] await
BindingIdentifier[Yield, Await] : Identifier yield await LabelIdentifier[Yield,
Await] : Identifier [~Yield] yield [~Await] await Identifier : IdentifierName
but not ReservedWord Note
yield and await are permitted as BindingIdentifier in the grammar, and
prohibited with static semantics below, to prohibit automatic semicolon
insertion in cases such as
let
await 0;
13.1.1 STATIC SEMANTICS: EARLY ERRORS
BindingIdentifier : Identifier
* It is a Syntax Error if the source text matched by this production is
contained in strict mode code and the StringValue of Identifier is either
"arguments" or "eval".
IdentifierReference : yield BindingIdentifier : yield LabelIdentifier : yield
* It is a Syntax Error if the source text matched by this production is
contained in strict mode code.
IdentifierReference : await BindingIdentifier : await LabelIdentifier : await
* It is a Syntax Error if the goal symbol of the syntactic grammar is Module.
BindingIdentifier[Yield, Await] : yield
* It is a Syntax Error if this production has a [Yield] parameter.
BindingIdentifier[Yield, Await] : await
* It is a Syntax Error if this production has an [Await] parameter.
IdentifierReference[Yield, Await] : Identifier BindingIdentifier[Yield, Await] :
Identifier LabelIdentifier[Yield, Await] : Identifier
* It is a Syntax Error if this production has a [Yield] parameter and
StringValue of Identifier is "yield".
* It is a Syntax Error if this production has an [Await] parameter and
StringValue of Identifier is "await".
Identifier : IdentifierName but not ReservedWord
* It is a Syntax Error if this phrase is contained in strict mode code and the
StringValue of IdentifierName is one of "implements", "interface", "let",
"package", "private", "protected", "public", "static", or "yield".
* It is a Syntax Error if the goal symbol of the syntactic grammar is Module
and the StringValue of IdentifierName is "await".
* It is a Syntax Error if the StringValue of IdentifierName is the StringValue
of any ReservedWord except for yield or await.
Note
StringValue of IdentifierName normalizes any Unicode escape sequences in
IdentifierName hence such escapes cannot be used to write an Identifier whose
code point sequence is the same as a ReservedWord.
13.1.2 STATIC SEMANTICS: STRINGVALUE
The syntax-directed operation StringValue takes no arguments and returns a
String. It is defined piecewise over the following productions:
IdentifierName :: IdentifierStart IdentifierName IdentifierPart
1. 1. 1. Let idTextUnescaped be IdentifierCodePoints of IdentifierName.
2. 2. 2. Return CodePointsToString(idTextUnescaped).
IdentifierReference : yield BindingIdentifier : yield LabelIdentifier : yield
1. 1. 1. Return "yield".
IdentifierReference : await BindingIdentifier : await LabelIdentifier : await
1. 1. 1. Return "await".
Identifier : IdentifierName but not ReservedWord
1. 1. 1. Return the StringValue of IdentifierName.
PrivateIdentifier :: # IdentifierName
1. 1. 1. Return the string-concatenation of 0x0023 (NUMBER SIGN) and the
StringValue of IdentifierName.
ModuleExportName : StringLiteral
1. 1. 1. Return the SV of StringLiteral.
13.1.3 RUNTIME SEMANTICS: EVALUATION
IdentifierReference : Identifier
1. 1. 1. Return ? ResolveBinding(StringValue of Identifier).
IdentifierReference : yield
1. 1. 1. Return ? ResolveBinding("yield").
IdentifierReference : await
1. 1. 1. Return ? ResolveBinding("await").
Note 1
The result of evaluating an IdentifierReference is always a value of type
Reference.
Note 2
In non-strict code, the keyword yield may be used as an identifier. Evaluating
the IdentifierReference resolves the binding of yield as if it was an
Identifier. Early Error restriction ensures that such an evaluation only can
occur for non-strict code.
13.2 PRIMARY EXPRESSION
SYNTAX
PrimaryExpression[Yield, Await] : this IdentifierReference[?Yield, ?Await]
Literal ArrayLiteral[?Yield, ?Await] ObjectLiteral[?Yield, ?Await]
FunctionExpression ClassExpression[?Yield, ?Await] GeneratorExpression
AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral
TemplateLiteral[?Yield, ?Await, ~Tagged]
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList[Yield, Await] : (
Expression[+In, ?Yield, ?Await] ) ( Expression[+In, ?Yield, ?Await] , ) ( ) (
... BindingIdentifier[?Yield, ?Await] ) ( ... BindingPattern[?Yield, ?Await] ) (
Expression[+In, ?Yield, ?Await] , ... BindingIdentifier[?Yield, ?Await] ) (
Expression[+In, ?Yield, ?Await] , ... BindingPattern[?Yield, ?Await] )
SUPPLEMENTAL SYNTAX
When processing an instance of the production
PrimaryExpression[Yield, Await] :
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is
refined using the following grammar:
ParenthesizedExpression[Yield, Await] : ( Expression[+In, ?Yield, ?Await] )
13.2.1 THE THIS KEYWORD
13.2.1.1 RUNTIME SEMANTICS: EVALUATION
PrimaryExpression : this
1. 1. 1. Return ? ResolveThisBinding().
13.2.2 IDENTIFIER REFERENCE
See 13.1 for IdentifierReference.
13.2.3 LITERALS
SYNTAX
Literal : NullLiteral BooleanLiteral NumericLiteral StringLiteral
13.2.3.1 RUNTIME SEMANTICS: EVALUATION
Literal : NullLiteral
1. 1. 1. Return null.
Literal : BooleanLiteral
1. 1. 1. If BooleanLiteral is the token false, return false.
2. 2. 2. If BooleanLiteral is the token true, return true.
Literal : NumericLiteral
1. 1. 1. Return the NumericValue of NumericLiteral as defined in 12.9.3.
Literal : StringLiteral
1. 1. 1. Return the SV of StringLiteral as defined in 12.9.4.2.
13.2.4 ARRAY INITIALIZER
Note
An ArrayLiteral is an expression describing the initialization of an Array,
using a list, of zero or more expressions each of which represents an array
element, enclosed in square brackets. The elements need not be literals; they
are evaluated each time the array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the element
list. Whenever a comma in the element list is not preceded by an
AssignmentExpression (i.e., a comma at the beginning or after another comma),
the missing array element contributes to the length of the Array and increases
the index of subsequent elements. Elided array elements are not defined. If an
element is elided at the end of an array, that element does not contribute to
the length of the Array.
SYNTAX
ArrayLiteral[Yield, Await] : [ Elisionopt ] [ ElementList[?Yield, ?Await] ] [
ElementList[?Yield, ?Await] , Elisionopt ] ElementList[Yield, Await] :
Elisionopt AssignmentExpression[+In, ?Yield, ?Await] Elisionopt
SpreadElement[?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt
AssignmentExpression[+In, ?Yield, ?Await] ElementList[?Yield, ?Await] ,
Elisionopt SpreadElement[?Yield, ?Await] Elision : , Elision ,
SpreadElement[Yield, Await] : ... AssignmentExpression[+In, ?Yield, ?Await]
13.2.4.1 RUNTIME SEMANTICS: ARRAYACCUMULATION
The syntax-directed operation ArrayAccumulation takes arguments array (an Array)
and nextIndex (an integer) and returns either a normal completion containing an
integer or an abrupt completion. It is defined piecewise over the following
productions:
Elision : ,
1. 1. 1. Let len be nextIndex + 1.
2. 2. 2. Perform ? Set(array, "length", 𝔽(len), true).
3. 3. 3. NOTE: The above step throws if len exceeds 232-1.
4. 4. 4. Return len.
Elision : Elision ,
1. 1. 1. Return ? ArrayAccumulation of Elision with arguments array and
(nextIndex + 1).
ElementList : Elisionopt AssignmentExpression
1. 1. 1. If Elision is present, then
1. a. a. Set nextIndex to ? ArrayAccumulation of Elision with arguments
array and nextIndex.
2. 2. 2. Let initResult be ? Evaluation of AssignmentExpression.
3. 3. 3. Let initValue be ? GetValue(initResult).
4. 4. 4. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)),
initValue).
5. 5. 5. Return nextIndex + 1.
ElementList : Elisionopt SpreadElement
1. 1. 1. If Elision is present, then
1. a. a. Set nextIndex to ? ArrayAccumulation of Elision with arguments
array and nextIndex.
2. 2. 2. Return ? ArrayAccumulation of SpreadElement with arguments array and
nextIndex.
ElementList : ElementList , Elisionopt AssignmentExpression
1. 1. 1. Set nextIndex to ? ArrayAccumulation of ElementList with arguments
array and nextIndex.
2. 2. 2. If Elision is present, then
1. a. a. Set nextIndex to ? ArrayAccumulation of Elision with arguments
array and nextIndex.
3. 3. 3. Let initResult be ? Evaluation of AssignmentExpression.
4. 4. 4. Let initValue be ? GetValue(initResult).
5. 5. 5. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)),
initValue).
6. 6. 6. Return nextIndex + 1.
ElementList : ElementList , Elisionopt SpreadElement
1. 1. 1. Set nextIndex to ? ArrayAccumulation of ElementList with arguments
array and nextIndex.
2. 2. 2. If Elision is present, then
1. a. a. Set nextIndex to ? ArrayAccumulation of Elision with arguments
array and nextIndex.
3. 3. 3. Return ? ArrayAccumulation of SpreadElement with arguments array and
nextIndex.
SpreadElement : ... AssignmentExpression
1. 1. 1. Let spreadRef be ? Evaluation of AssignmentExpression.
2. 2. 2. Let spreadObj be ? GetValue(spreadRef).
3. 3. 3. Let iteratorRecord be ? GetIterator(spreadObj, sync).
4. 4. 4. Repeat,
1. a. a. Let next be ? IteratorStep(iteratorRecord).
2. b. b. If next is false, return nextIndex.
3. c. c. Let nextValue be ? IteratorValue(next).
4. d. d. Perform ! CreateDataPropertyOrThrow(array,
! ToString(𝔽(nextIndex)), nextValue).
5. e. e. Set nextIndex to nextIndex + 1.
Note
CreateDataPropertyOrThrow is used to ensure that own properties are defined for
the array even if the standard built-in Array prototype object has been modified
in a manner that would preclude the creation of new own properties using
[[Set]].
13.2.4.2 RUNTIME SEMANTICS: EVALUATION
ArrayLiteral : [ Elisionopt ]
1. 1. 1. Let array be ! ArrayCreate(0).
2. 2. 2. If Elision is present, then
1. a. a. Perform ? ArrayAccumulation of Elision with arguments array and 0.
3. 3. 3. Return array.
ArrayLiteral : [ ElementList ]
1. 1. 1. Let array be ! ArrayCreate(0).
2. 2. 2. Perform ? ArrayAccumulation of ElementList with arguments array and 0.
3. 3. 3. Return array.
ArrayLiteral : [ ElementList , Elisionopt ]
1. 1. 1. Let array be ! ArrayCreate(0).
2. 2. 2. Let nextIndex be ? ArrayAccumulation of ElementList with arguments
array and 0.
3. 3. 3. If Elision is present, then
1. a. a. Perform ? ArrayAccumulation of Elision with arguments array and
nextIndex.
4. 4. 4. Return array.
13.2.5 OBJECT INITIALIZER
Note 1
An object initializer is an expression describing the initialization of an
Object, written in a form resembling a literal. It is a list of zero or more
pairs of property keys and associated values, enclosed in curly brackets. The
values need not be literals; they are evaluated each time the object initializer
is evaluated.
SYNTAX
ObjectLiteral[Yield, Await] : { } { PropertyDefinitionList[?Yield, ?Await] } {
PropertyDefinitionList[?Yield, ?Await] , } PropertyDefinitionList[Yield, Await]
: PropertyDefinition[?Yield, ?Await] PropertyDefinitionList[?Yield, ?Await] ,
PropertyDefinition[?Yield, ?Await] PropertyDefinition[Yield, Await] :
IdentifierReference[?Yield, ?Await] CoverInitializedName[?Yield, ?Await]
PropertyName[?Yield, ?Await] : AssignmentExpression[+In, ?Yield, ?Await]
MethodDefinition[?Yield, ?Await] ... AssignmentExpression[+In, ?Yield, ?Await]
PropertyName[Yield, Await] : LiteralPropertyName ComputedPropertyName[?Yield,
?Await] LiteralPropertyName : IdentifierName StringLiteral NumericLiteral
ComputedPropertyName[Yield, Await] : [ AssignmentExpression[+In, ?Yield, ?Await]
] CoverInitializedName[Yield, Await] : IdentifierReference[?Yield, ?Await]
Initializer[+In, ?Yield, ?Await] Initializer[In, Yield, Await] : =
AssignmentExpression[?In, ?Yield, ?Await] Note 2
MethodDefinition is defined in 15.4.
Note 3
In certain contexts, ObjectLiteral is used as a cover grammar for a more
restricted secondary grammar. The CoverInitializedName production is necessary
to fully cover these secondary grammars. However, use of this production results
in an early Syntax Error in normal contexts where an actual ObjectLiteral is
expected.
13.2.5.1 STATIC SEMANTICS: EARLY ERRORS
PropertyDefinition : MethodDefinition
* It is a Syntax Error if HasDirectSuper of MethodDefinition is true.
* It is a Syntax Error if PrivateBoundIdentifiers of MethodDefinition is not
empty.
In addition to describing an actual object initializer the ObjectLiteral
productions are also used as a cover grammar for ObjectAssignmentPattern and may
be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList.
When ObjectLiteral appears in a context where ObjectAssignmentPattern is
required the following Early Error rules are not applied. In addition, they are
not applied when initially parsing a
CoverParenthesizedExpressionAndArrowParameterList or
CoverCallExpressionAndAsyncArrowHead.
PropertyDefinition : CoverInitializedName
* It is a Syntax Error if any source text is matched by this production.
Note 1
This production exists so that ObjectLiteral can serve as a cover grammar for
ObjectAssignmentPattern. It cannot occur in an actual object initializer.
ObjectLiteral : { PropertyDefinitionList } { PropertyDefinitionList , }
* It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains
any duplicate entries for "__proto__" and at least two of those entries were
obtained from productions of the form PropertyDefinition : PropertyName :
AssignmentExpression . This rule is not applied if this ObjectLiteral is
contained within a Script that is being parsed for JSON.parse (see step 4 of
JSON.parse).
Note 2
The List returned by PropertyNameList does not include property names defined
using a ComputedPropertyName.
13.2.5.2 STATIC SEMANTICS: ISCOMPUTEDPROPERTYKEY
The syntax-directed operation IsComputedPropertyKey takes no arguments and
returns a Boolean. It is defined piecewise over the following productions:
PropertyName : LiteralPropertyName
1. 1. 1. Return false.
PropertyName : ComputedPropertyName
1. 1. 1. Return true.
13.2.5.3 STATIC SEMANTICS: PROPERTYNAMELIST
The syntax-directed operation PropertyNameList takes no arguments and returns a
List of Strings. It is defined piecewise over the following productions:
PropertyDefinitionList : PropertyDefinition
1. 1. 1. Let propName be PropName of PropertyDefinition.
2. 2. 2. If propName is empty, return a new empty List.
3. 3. 3. Return « propName ».
PropertyDefinitionList : PropertyDefinitionList , PropertyDefinition
1. 1. 1. Let list be PropertyNameList of PropertyDefinitionList.
2. 2. 2. Let propName be PropName of PropertyDefinition.
3. 3. 3. If propName is empty, return list.
4. 4. 4. Return the list-concatenation of list and « propName ».
13.2.5.4 RUNTIME SEMANTICS: EVALUATION
ObjectLiteral : { }
1. 1. 1. Return OrdinaryObjectCreate(%Object.prototype%).
ObjectLiteral : { PropertyDefinitionList } { PropertyDefinitionList , }
1. 1. 1. Let obj be OrdinaryObjectCreate(%Object.prototype%).
2. 2. 2. Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with
argument obj.
3. 3. 3. Return obj.
LiteralPropertyName : IdentifierName
1. 1. 1. Return StringValue of IdentifierName.
LiteralPropertyName : StringLiteral
1. 1. 1. Return the SV of StringLiteral.
LiteralPropertyName : NumericLiteral
1. 1. 1. Let nbr be the NumericValue of NumericLiteral.
2. 2. 2. Return ! ToString(nbr).
ComputedPropertyName : [ AssignmentExpression ]
1. 1. 1. Let exprValue be ? Evaluation of AssignmentExpression.
2. 2. 2. Let propName be ? GetValue(exprValue).
3. 3. 3. Return ? ToPropertyKey(propName).
13.2.5.5 RUNTIME SEMANTICS: PROPERTYDEFINITIONEVALUATION
The syntax-directed operation PropertyDefinitionEvaluation takes argument object
(an Object) and returns either a normal completion containing unused or an
abrupt completion. It is defined piecewise over the following productions:
PropertyDefinitionList : PropertyDefinitionList , PropertyDefinition
1. 1. 1. Perform ? PropertyDefinitionEvaluation of PropertyDefinitionList with
argument object.
2. 2. 2. Perform ? PropertyDefinitionEvaluation of PropertyDefinition with
argument object.
3. 3. 3. Return unused.
PropertyDefinition : ... AssignmentExpression
1. 1. 1. Let exprValue be ? Evaluation of AssignmentExpression.
2. 2. 2. Let fromValue be ? GetValue(exprValue).
3. 3. 3. Let excludedNames be a new empty List.
4. 4. 4. Perform ? CopyDataProperties(object, fromValue, excludedNames).
5. 5. 5. Return unused.
PropertyDefinition : IdentifierReference
1. 1. 1. Let propName be StringValue of IdentifierReference.
2. 2. 2. Let exprValue be ? Evaluation of IdentifierReference.
3. 3. 3. Let propValue be ? GetValue(exprValue).
4. 4. 4. Assert: object is an ordinary, extensible object with no
non-configurable properties.
5. 5. 5. Perform ! CreateDataPropertyOrThrow(object, propName, propValue).
6. 6. 6. Return unused.
PropertyDefinition : PropertyName : AssignmentExpression
1. 1. 1. Let propKey be ? Evaluation of PropertyName.
2. 2. 2. If this PropertyDefinition is contained within a Script that is being
evaluated for JSON.parse (see step 7 of JSON.parse), then
1. a. a. Let isProtoSetter be false.
3. 3. 3. Else if propKey is "__proto__" and IsComputedPropertyKey of
PropertyName is false, then
1. a. a. Let isProtoSetter be true.
4. 4. 4. Else,
1. a. a. Let isProtoSetter be false.
5. 5. 5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and
isProtoSetter is false, then
1. a. a. Let propValue be ? NamedEvaluation of AssignmentExpression with
argument propKey.
6. 6. 6. Else,
1. a. a. Let exprValueRef be ? Evaluation of AssignmentExpression.
2. b. b. Let propValue be ? GetValue(exprValueRef).
7. 7. 7. If isProtoSetter is true, then
1. a. a. If propValue is an Object or propValue is null, then
1. i. i. Perform ! object.[[SetPrototypeOf]](propValue).
2. b. b. Return unused.
8. 8. 8. Assert: object is an ordinary, extensible object with no
non-configurable properties.
9. 9. 9. Perform ! CreateDataPropertyOrThrow(object, propKey, propValue).
10. 10. 10. Return unused.
PropertyDefinition : MethodDefinition
1. 1. 1. Perform ? MethodDefinitionEvaluation of MethodDefinition with
arguments object and true.
2. 2. 2. Return unused.
13.2.6 FUNCTION DEFINING EXPRESSIONS
See 15.2 for PrimaryExpression : FunctionExpression .
See 15.5 for PrimaryExpression : GeneratorExpression .
See 15.7 for PrimaryExpression : ClassExpression .
See 15.8 for PrimaryExpression : AsyncFunctionExpression .
See 15.6 for PrimaryExpression : AsyncGeneratorExpression .
13.2.7 REGULAR EXPRESSION LITERALS
SYNTAX
See 12.9.5.
13.2.7.1 STATIC SEMANTICS: EARLY ERRORS
PrimaryExpression : RegularExpressionLiteral
* It is a Syntax Error if
IsValidRegularExpressionLiteral(RegularExpressionLiteral) is false.
13.2.7.2 STATIC SEMANTICS: ISVALIDREGULAREXPRESSIONLITERAL ( LITERAL )
The abstract operation IsValidRegularExpressionLiteral takes argument literal (a
RegularExpressionLiteral Parse Node) and returns a Boolean. It determines if its
argument is a valid regular expression literal. It performs the following steps
when called:
1. 1. 1. Let flags be FlagText of literal.
2. 2. 2. If flags contains any code points other than d, g, i, m, s, u, or y,
or if flags contains any code point more than once, return false.
3. 3. 3. If flags contains u, let u be true; else let u be false.
4. 4. 4. Let patternText be BodyText of literal.
5. 5. 5. If u is false, then
1. a. a. Let stringValue be CodePointsToString(patternText).
2. b. b. Set patternText to the sequence of code points resulting from
interpreting each of the 16-bit elements of stringValue as a Unicode BMP
code point. UTF-16 decoding is not applied to the elements.
6. 6. 6. Let parseResult be ParsePattern(patternText, u).
7. 7. 7. If parseResult is a Parse Node, return true; else return false.
13.2.7.3 RUNTIME SEMANTICS: EVALUATION
PrimaryExpression : RegularExpressionLiteral
1. 1. 1. Let pattern be CodePointsToString(BodyText of
RegularExpressionLiteral).
2. 2. 2. Let flags be CodePointsToString(FlagText of RegularExpressionLiteral).
3. 3. 3. Return ! RegExpCreate(pattern, flags).
13.2.8 TEMPLATE LITERALS
SYNTAX
TemplateLiteral[Yield, Await, Tagged] : NoSubstitutionTemplate
SubstitutionTemplate[?Yield, ?Await, ?Tagged] SubstitutionTemplate[Yield, Await,
Tagged] : TemplateHead Expression[+In, ?Yield, ?Await] TemplateSpans[?Yield,
?Await, ?Tagged] TemplateSpans[Yield, Await, Tagged] : TemplateTail
TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateTail
TemplateMiddleList[Yield, Await, Tagged] : TemplateMiddle Expression[+In,
?Yield, ?Await] TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateMiddle
Expression[+In, ?Yield, ?Await]
13.2.8.1 STATIC SEMANTICS: EARLY ERRORS
TemplateLiteral[Yield, Await, Tagged] : NoSubstitutionTemplate
* It is a Syntax Error if the [Tagged] parameter was not set and
NoSubstitutionTemplate Contains NotEscapeSequence.
TemplateLiteral[Yield, Await, Tagged] : SubstitutionTemplate[?Yield, ?Await,
?Tagged]
* It is a Syntax Error if the number of elements in the result of
TemplateStrings of TemplateLiteral with argument false is greater than or
equal to 232.
SubstitutionTemplate[Yield, Await, Tagged] : TemplateHead Expression[+In,
?Yield, ?Await] TemplateSpans[?Yield, ?Await, ?Tagged]
* It is a Syntax Error if the [Tagged] parameter was not set and TemplateHead
Contains NotEscapeSequence.
TemplateSpans[Yield, Await, Tagged] : TemplateTail
* It is a Syntax Error if the [Tagged] parameter was not set and TemplateTail
Contains NotEscapeSequence.
TemplateMiddleList[Yield, Await, Tagged] : TemplateMiddle Expression[+In,
?Yield, ?Await] TemplateMiddleList[?Yield, ?Await, ?Tagged] TemplateMiddle
Expression[+In, ?Yield, ?Await]
* It is a Syntax Error if the [Tagged] parameter was not set and TemplateMiddle
Contains NotEscapeSequence.
13.2.8.2 STATIC SEMANTICS: TEMPLATESTRINGS
The syntax-directed operation TemplateStrings takes argument raw (a Boolean) and
returns a List of Strings. It is defined piecewise over the following
productions:
TemplateLiteral : NoSubstitutionTemplate
1. 1. 1. Return « TemplateString(NoSubstitutionTemplate, raw) ».
SubstitutionTemplate : TemplateHead Expression TemplateSpans
1. 1. 1. Let head be « TemplateString(TemplateHead, raw) ».
2. 2. 2. Let tail be TemplateStrings of TemplateSpans with argument raw.
3. 3. 3. Return the list-concatenation of head and tail.
TemplateSpans : TemplateTail
1. 1. 1. Return « TemplateString(TemplateTail, raw) ».
TemplateSpans : TemplateMiddleList TemplateTail
1. 1. 1. Let middle be TemplateStrings of TemplateMiddleList with argument raw.
2. 2. 2. Let tail be « TemplateString(TemplateTail, raw) ».
3. 3. 3. Return the list-concatenation of middle and tail.
TemplateMiddleList : TemplateMiddle Expression
1. 1. 1. Return « TemplateString(TemplateMiddle, raw) ».
TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression
1. 1. 1. Let front be TemplateStrings of TemplateMiddleList with argument raw.
2. 2. 2. Let last be « TemplateString(TemplateMiddle, raw) ».
3. 3. 3. Return the list-concatenation of front and last.
13.2.8.3 STATIC SEMANTICS: TEMPLATESTRING ( TEMPLATETOKEN, RAW )
The abstract operation TemplateString takes arguments templateToken (a
NoSubstitutionTemplate Parse Node, a TemplateHead Parse Node, a TemplateMiddle
Parse Node, or a TemplateTail Parse Node) and raw (a Boolean) and returns a
String. It performs the following steps when called:
1. 1. 1. If raw is true, then
1. a. a. Let string be the TRV of templateToken.
2. 2. 2. Else,
1. a. a. Let string be the TV of templateToken.
3. 3. 3. Return string.
13.2.8.4 GETTEMPLATEOBJECT ( TEMPLATELITERAL )
The abstract operation GetTemplateObject takes argument templateLiteral (a Parse
Node) and returns an Array. It performs the following steps when called:
1. 1. 1. Let realm be the current Realm Record.
2. 2. 2. Let templateRegistry be realm.[[TemplateMap]].
3. 3. 3. For each element e of templateRegistry, do
1. a. a. If e.[[Site]] is the same Parse Node as templateLiteral, then
1. i. i. Return e.[[Array]].
4. 4. 4. Let rawStrings be TemplateStrings of templateLiteral with argument
true.
5. 5. 5. Let cookedStrings be TemplateStrings of templateLiteral with argument
false.
6. 6. 6. Let count be the number of elements in the List cookedStrings.
7. 7. 7. Assert: count ≤ 232 - 1.
8. 8. 8. Let template be ! ArrayCreate(count).
9. 9. 9. Let rawObj be ! ArrayCreate(count).
10. 10. 10. Let index be 0.
11. 11. 11. Repeat, while index < count,
1. a. a. Let prop be ! ToString(𝔽(index)).
2. b. b. Let cookedValue be cookedStrings[index].
3. c. c. Perform ! DefinePropertyOrThrow(template, prop, PropertyDescriptor
{ [[Value]]: cookedValue, [[Writable]]: false, [[Enumerable]]: true,
[[Configurable]]: false }).
4. d. d. Let rawValue be the String value rawStrings[index].
5. e. e. Perform ! DefinePropertyOrThrow(rawObj, prop, PropertyDescriptor {
[[Value]]: rawValue, [[Writable]]: false, [[Enumerable]]: true,
[[Configurable]]: false }).
6. f. f. Set index to index + 1.
12. 12. 12. Perform ! SetIntegrityLevel(rawObj, frozen).
13. 13. 13. Perform ! DefinePropertyOrThrow(template, "raw", PropertyDescriptor
{ [[Value]]: rawObj, [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }).
14. 14. 14. Perform ! SetIntegrityLevel(template, frozen).
15. 15. 15. Append the Record { [[Site]]: templateLiteral, [[Array]]: template
} to realm.[[TemplateMap]].
16. 16. 16. Return template.
Note 1
The creation of a template object cannot result in an abrupt completion.
Note 2
Each TemplateLiteral in the program code of a realm is associated with a unique
template object that is used in the evaluation of tagged Templates (13.2.8.6).
The template objects are frozen and the same template object is used each time a
specific tagged Template is evaluated. Whether template objects are created
lazily upon first evaluation of the TemplateLiteral or eagerly prior to first
evaluation is an implementation choice that is not observable to ECMAScript
code.
Note 3
Future editions of this specification may define additional non-enumerable
properties of template objects.
13.2.8.5 RUNTIME SEMANTICS: SUBSTITUTIONEVALUATION
The syntax-directed operation SubstitutionEvaluation takes no arguments and
returns either a normal completion containing a List of ECMAScript language
values or an abrupt completion. It is defined piecewise over the following
productions:
TemplateSpans : TemplateTail
1. 1. 1. Return a new empty List.
TemplateSpans : TemplateMiddleList TemplateTail
1. 1. 1. Return ? SubstitutionEvaluation of TemplateMiddleList.
TemplateMiddleList : TemplateMiddle Expression
1. 1. 1. Let subRef be ? Evaluation of Expression.
2. 2. 2. Let sub be ? GetValue(subRef).
3. 3. 3. Return « sub ».
TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression
1. 1. 1. Let preceding be ? SubstitutionEvaluation of TemplateMiddleList.
2. 2. 2. Let nextRef be ? Evaluation of Expression.
3. 3. 3. Let next be ? GetValue(nextRef).
4. 4. 4. Return the list-concatenation of preceding and « next ».
13.2.8.6 RUNTIME SEMANTICS: EVALUATION
TemplateLiteral : NoSubstitutionTemplate
1. 1. 1. Return the TV of NoSubstitutionTemplate as defined in 12.9.6.
SubstitutionTemplate : TemplateHead Expression TemplateSpans
1. 1. 1. Let head be the TV of TemplateHead as defined in 12.9.6.
2. 2. 2. Let subRef be ? Evaluation of Expression.
3. 3. 3. Let sub be ? GetValue(subRef).
4. 4. 4. Let middle be ? ToString(sub).
5. 5. 5. Let tail be ? Evaluation of TemplateSpans.
6. 6. 6. Return the string-concatenation of head, middle, and tail.
Note 1
The string conversion semantics applied to the Expression value are like
String.prototype.concat rather than the + operator.
TemplateSpans : TemplateTail
1. 1. 1. Return the TV of TemplateTail as defined in 12.9.6.
TemplateSpans : TemplateMiddleList TemplateTail
1. 1. 1. Let head be ? Evaluation of TemplateMiddleList.
2. 2. 2. Let tail be the TV of TemplateTail as defined in 12.9.6.
3. 3. 3. Return the string-concatenation of head and tail.
TemplateMiddleList : TemplateMiddle Expression
1. 1. 1. Let head be the TV of TemplateMiddle as defined in 12.9.6.
2. 2. 2. Let subRef be ? Evaluation of Expression.
3. 3. 3. Let sub be ? GetValue(subRef).
4. 4. 4. Let middle be ? ToString(sub).
5. 5. 5. Return the string-concatenation of head and middle.
Note 2
The string conversion semantics applied to the Expression value are like
String.prototype.concat rather than the + operator.
TemplateMiddleList : TemplateMiddleList TemplateMiddle Expression
1. 1. 1. Let rest be ? Evaluation of TemplateMiddleList.
2. 2. 2. Let middle be the TV of TemplateMiddle as defined in 12.9.6.
3. 3. 3. Let subRef be ? Evaluation of Expression.
4. 4. 4. Let sub be ? GetValue(subRef).
5. 5. 5. Let last be ? ToString(sub).
6. 6. 6. Return the string-concatenation of rest, middle, and last.
Note 3
The string conversion semantics applied to the Expression value are like
String.prototype.concat rather than the + operator.
13.2.9 THE GROUPING OPERATOR
13.2.9.1 STATIC SEMANTICS: EARLY ERRORS
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
* CoverParenthesizedExpressionAndArrowParameterList must cover a
ParenthesizedExpression.
13.2.9.2 RUNTIME SEMANTICS: EVALUATION
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let expr be the ParenthesizedExpression that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return ? Evaluation of expr.
ParenthesizedExpression : ( Expression )
1. 1. 1. Return ? Evaluation of Expression. This may be of type Reference.
Note
This algorithm does not apply GetValue to Evaluation of Expression. The
principal motivation for this is so that operators such as delete and typeof may
be applied to parenthesized expressions.
13.3 LEFT-HAND-SIDE EXPRESSIONS
SYNTAX
MemberExpression[Yield, Await] : PrimaryExpression[?Yield, ?Await]
MemberExpression[?Yield, ?Await] [ Expression[+In, ?Yield, ?Await] ]
MemberExpression[?Yield, ?Await] . IdentifierName MemberExpression[?Yield,
?Await] TemplateLiteral[?Yield, ?Await, +Tagged] SuperProperty[?Yield, ?Await]
MetaProperty new MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]
MemberExpression[?Yield, ?Await] . PrivateIdentifier SuperProperty[Yield, Await]
: super [ Expression[+In, ?Yield, ?Await] ] super . IdentifierName MetaProperty
: NewTarget ImportMeta NewTarget : new . target ImportMeta : import . meta
NewExpression[Yield, Await] : MemberExpression[?Yield, ?Await] new
NewExpression[?Yield, ?Await] CallExpression[Yield, Await] :
CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] SuperCall[?Yield, ?Await]
ImportCall[?Yield, ?Await] CallExpression[?Yield, ?Await] Arguments[?Yield,
?Await] CallExpression[?Yield, ?Await] [ Expression[+In, ?Yield, ?Await] ]
CallExpression[?Yield, ?Await] . IdentifierName CallExpression[?Yield, ?Await]
TemplateLiteral[?Yield, ?Await, +Tagged] CallExpression[?Yield, ?Await] .
PrivateIdentifier SuperCall[Yield, Await] : super Arguments[?Yield, ?Await]
ImportCall[Yield, Await] : import ( AssignmentExpression[+In, ?Yield, ?Await] )
Arguments[Yield, Await] : ( ) ( ArgumentList[?Yield, ?Await] ) (
ArgumentList[?Yield, ?Await] , ) ArgumentList[Yield, Await] :
AssignmentExpression[+In, ?Yield, ?Await] ... AssignmentExpression[+In, ?Yield,
?Await] ArgumentList[?Yield, ?Await] , AssignmentExpression[+In, ?Yield, ?Await]
ArgumentList[?Yield, ?Await] , ... AssignmentExpression[+In, ?Yield, ?Await]
OptionalExpression[Yield, Await] : MemberExpression[?Yield, ?Await]
OptionalChain[?Yield, ?Await] CallExpression[?Yield, ?Await]
OptionalChain[?Yield, ?Await] OptionalExpression[?Yield, ?Await]
OptionalChain[?Yield, ?Await] OptionalChain[Yield, Await] : ?. Arguments[?Yield,
?Await] ?. [ Expression[+In, ?Yield, ?Await] ] ?. IdentifierName ?.
TemplateLiteral[?Yield, ?Await, +Tagged] ?. PrivateIdentifier
OptionalChain[?Yield, ?Await] Arguments[?Yield, ?Await] OptionalChain[?Yield,
?Await] [ Expression[+In, ?Yield, ?Await] ] OptionalChain[?Yield, ?Await] .
IdentifierName OptionalChain[?Yield, ?Await] TemplateLiteral[?Yield, ?Await,
+Tagged] OptionalChain[?Yield, ?Await] . PrivateIdentifier
LeftHandSideExpression[Yield, Await] : NewExpression[?Yield, ?Await]
CallExpression[?Yield, ?Await] OptionalExpression[?Yield, ?Await]
SUPPLEMENTAL SYNTAX
When processing an instance of the production
CallExpression : CoverCallExpressionAndAsyncArrowHead
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the
following grammar:
CallMemberExpression[Yield, Await] : MemberExpression[?Yield, ?Await]
Arguments[?Yield, ?Await]
13.3.1 STATIC SEMANTICS
13.3.1.1 STATIC SEMANTICS: EARLY ERRORS
OptionalChain : ?. TemplateLiteral OptionalChain TemplateLiteral
* It is a Syntax Error if any source text is matched by this production.
Note
This production exists in order to prevent automatic semicolon insertion rules
(12.10) from being applied to the following code:
a?.b
`c`
so that it would be interpreted as two valid statements. The purpose is to
maintain consistency with similar code without optional chaining:
a.b
`c`
which is a valid statement and where automatic semicolon insertion does not
apply.
ImportMeta : import . meta
* It is a Syntax Error if the syntactic goal symbol is not Module.
13.3.2 PROPERTY ACCESSORS
Note
Properties are accessed by name, using either the dot notation:
MemberExpression . IdentifierName
CallExpression . IdentifierName
or the bracket notation:
MemberExpression [ Expression ]
CallExpression [ Expression ]
The dot notation is explained by the following syntactic conversion:
MemberExpression . IdentifierName
is identical in its behaviour to
MemberExpression [ <identifier-name-string> ]
and similarly
CallExpression . IdentifierName
is identical in its behaviour to
CallExpression [ <identifier-name-string> ]
where <identifier-name-string> is the result of evaluating StringValue of
IdentifierName.
13.3.2.1 RUNTIME SEMANTICS: EVALUATION
MemberExpression : MemberExpression [ Expression ]
1. 1. 1. Let baseReference be ? Evaluation of MemberExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If the source text matched by this MemberExpression is strict mode
code, let strict be true; else let strict be false.
4. 4. 4. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue,
Expression, strict).
MemberExpression : MemberExpression . IdentifierName
1. 1. 1. Let baseReference be ? Evaluation of MemberExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If the source text matched by this MemberExpression is strict mode
code, let strict be true; else let strict be false.
4. 4. 4. Return EvaluatePropertyAccessWithIdentifierKey(baseValue,
IdentifierName, strict).
MemberExpression : MemberExpression . PrivateIdentifier
1. 1. 1. Let baseReference be ? Evaluation of MemberExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. Let fieldNameString be the StringValue of PrivateIdentifier.
4. 4. 4. Return MakePrivateReference(baseValue, fieldNameString).
CallExpression : CallExpression [ Expression ]
1. 1. 1. Let baseReference be ? Evaluation of CallExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If the source text matched by this CallExpression is strict mode code,
let strict be true; else let strict be false.
4. 4. 4. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue,
Expression, strict).
CallExpression : CallExpression . IdentifierName
1. 1. 1. Let baseReference be ? Evaluation of CallExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If the source text matched by this CallExpression is strict mode code,
let strict be true; else let strict be false.
4. 4. 4. Return EvaluatePropertyAccessWithIdentifierKey(baseValue,
IdentifierName, strict).
CallExpression : CallExpression . PrivateIdentifier
1. 1. 1. Let baseReference be ? Evaluation of CallExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. Let fieldNameString be the StringValue of PrivateIdentifier.
4. 4. 4. Return MakePrivateReference(baseValue, fieldNameString).
13.3.3 EVALUATEPROPERTYACCESSWITHEXPRESSIONKEY ( BASEVALUE, EXPRESSION, STRICT )
The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments
baseValue (an ECMAScript language value), expression (a Parse Node), and strict
(a Boolean) and returns either a normal completion containing a Reference Record
or an abrupt completion. It performs the following steps when called:
1. 1. 1. Let propertyNameReference be ? Evaluation of expression.
2. 2. 2. Let propertyNameValue be ? GetValue(propertyNameReference).
3. 3. 3. Let propertyKey be ? ToPropertyKey(propertyNameValue).
4. 4. 4. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
propertyKey, [[Strict]]: strict, [[ThisValue]]: empty }.
13.3.4 EVALUATEPROPERTYACCESSWITHIDENTIFIERKEY ( BASEVALUE, IDENTIFIERNAME,
STRICT )
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments
baseValue (an ECMAScript language value), identifierName (an IdentifierName
Parse Node), and strict (a Boolean) and returns a Reference Record. It performs
the following steps when called:
1. 1. 1. Let propertyNameString be StringValue of identifierName.
2. 2. 2. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
propertyNameString, [[Strict]]: strict, [[ThisValue]]: empty }.
13.3.5 THE NEW OPERATOR
13.3.5.1 RUNTIME SEMANTICS: EVALUATION
NewExpression : new NewExpression
1. 1. 1. Return ? EvaluateNew(NewExpression, empty).
MemberExpression : new MemberExpression Arguments
1. 1. 1. Return ? EvaluateNew(MemberExpression, Arguments).
13.3.5.1.1 EVALUATENEW ( CONSTRUCTEXPR, ARGUMENTS )
The abstract operation EvaluateNew takes arguments constructExpr (a
NewExpression Parse Node or a MemberExpression Parse Node) and arguments (empty
or an Arguments Parse Node) and returns either a normal completion containing an
ECMAScript language value or an abrupt completion. It performs the following
steps when called:
1. 1. 1. Let ref be ? Evaluation of constructExpr.
2. 2. 2. Let constructor be ? GetValue(ref).
3. 3. 3. If arguments is empty, let argList be a new empty List.
4. 4. 4. Else,
1. a. a. Let argList be ? ArgumentListEvaluation of arguments.
5. 5. 5. If IsConstructor(constructor) is false, throw a TypeError exception.
6. 6. 6. Return ? Construct(constructor, argList).
13.3.6 FUNCTION CALLS
13.3.6.1 RUNTIME SEMANTICS: EVALUATION
CallExpression : CoverCallExpressionAndAsyncArrowHead
1. 1. 1. Let expr be the CallMemberExpression that is covered by
CoverCallExpressionAndAsyncArrowHead.
2. 2. 2. Let memberExpr be the MemberExpression of expr.
3. 3. 3. Let arguments be the Arguments of expr.
4. 4. 4. Let ref be ? Evaluation of memberExpr.
5. 5. 5. Let func be ? GetValue(ref).
6. 6. 6. If ref is a Reference Record, IsPropertyReference(ref) is false, and
ref.[[ReferencedName]] is "eval", then
1. a. a. If SameValue(func, %eval%) is true, then
1. i. i. Let argList be ? ArgumentListEvaluation of arguments.
2. ii. ii. If argList has no elements, return undefined.
3. iii. iii. Let evalArg be the first element of argList.
4. iv. iv. If the source text matched by this CallExpression is strict
mode code, let strictCaller be true. Otherwise let strictCaller be
false.
5. v. v. Return ? PerformEval(evalArg, strictCaller, true).
7. 7. 7. Let thisCall be this CallExpression.
8. 8. 8. Let tailCall be IsInTailPosition(thisCall).
9. 9. 9. Return ? EvaluateCall(func, ref, arguments, tailCall).
A CallExpression evaluation that executes step 6.a.v is a direct eval.
CallExpression : CallExpression Arguments
1. 1. 1. Let ref be ? Evaluation of CallExpression.
2. 2. 2. Let func be ? GetValue(ref).
3. 3. 3. Let thisCall be this CallExpression.
4. 4. 4. Let tailCall be IsInTailPosition(thisCall).
5. 5. 5. Return ? EvaluateCall(func, ref, Arguments, tailCall).
13.3.6.2 EVALUATECALL ( FUNC, REF, ARGUMENTS, TAILPOSITION )
The abstract operation EvaluateCall takes arguments func (an ECMAScript language
value), ref (an ECMAScript language value or a Reference Record), arguments (a
Parse Node), and tailPosition (a Boolean) and returns either a normal completion
containing an ECMAScript language value or an abrupt completion. It performs the
following steps when called:
1. 1. 1. If ref is a Reference Record, then
1. a. a. If IsPropertyReference(ref) is true, then
1. i. i. Let thisValue be GetThisValue(ref).
2. b. b. Else,
1. i. i. Let refEnv be ref.[[Base]].
2. ii. ii. Assert: refEnv is an Environment Record.
3. iii. iii. Let thisValue be refEnv.WithBaseObject().
2. 2. 2. Else,
1. a. a. Let thisValue be undefined.
3. 3. 3. Let argList be ? ArgumentListEvaluation of arguments.
4. 4. 4. If func is not an Object, throw a TypeError exception.
5. 5. 5. If IsCallable(func) is false, throw a TypeError exception.
6. 6. 6. If tailPosition is true, perform PrepareForTailCall().
7. 7. 7. Return ? Call(func, thisValue, argList).
13.3.7 THE SUPER KEYWORD
13.3.7.1 RUNTIME SEMANTICS: EVALUATION
SuperProperty : super [ Expression ]
1. 1. 1. Let env be GetThisEnvironment().
2. 2. 2. Let actualThis be ? env.GetThisBinding().
3. 3. 3. Let propertyNameReference be ? Evaluation of Expression.
4. 4. 4. Let propertyNameValue be ? GetValue(propertyNameReference).
5. 5. 5. Let propertyKey be ? ToPropertyKey(propertyNameValue).
6. 6. 6. If the source text matched by this SuperProperty is strict mode code,
let strict be true; else let strict be false.
7. 7. 7. Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).
SuperProperty : super . IdentifierName
1. 1. 1. Let env be GetThisEnvironment().
2. 2. 2. Let actualThis be ? env.GetThisBinding().
3. 3. 3. Let propertyKey be StringValue of IdentifierName.
4. 4. 4. If the source text matched by this SuperProperty is strict mode code,
let strict be true; else let strict be false.
5. 5. 5. Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).
SuperCall : super Arguments
1. 1. 1. Let newTarget be GetNewTarget().
2. 2. 2. Assert: newTarget is an Object.
3. 3. 3. Let func be GetSuperConstructor().
4. 4. 4. Let argList be ? ArgumentListEvaluation of Arguments.
5. 5. 5. If IsConstructor(func) is false, throw a TypeError exception.
6. 6. 6. Let result be ? Construct(func, argList, newTarget).
7. 7. 7. Let thisER be GetThisEnvironment().
8. 8. 8. Perform ? thisER.BindThisValue(result).
9. 9. 9. Let F be thisER.[[FunctionObject]].
10. 10. 10. Assert: F is an ECMAScript function object.
11. 11. 11. Perform ? InitializeInstanceElements(result, F).
12. 12. 12. Return result.
13.3.7.2 GETSUPERCONSTRUCTOR ( )
The abstract operation GetSuperConstructor takes no arguments and returns an
ECMAScript language value. It performs the following steps when called:
1. 1. 1. Let envRec be GetThisEnvironment().
2. 2. 2. Assert: envRec is a Function Environment Record.
3. 3. 3. Let activeFunction be envRec.[[FunctionObject]].
4. 4. 4. Assert: activeFunction is an ECMAScript function object.
5. 5. 5. Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
6. 6. 6. Return superConstructor.
13.3.7.3 MAKESUPERPROPERTYREFERENCE ( ACTUALTHIS, PROPERTYKEY, STRICT )
The abstract operation MakeSuperPropertyReference takes arguments actualThis (an
ECMAScript language value), propertyKey (a property key), and strict (a Boolean)
and returns either a normal completion containing a Super Reference Record or a
throw completion. It performs the following steps when called:
1. 1. 1. Let env be GetThisEnvironment().
2. 2. 2. Assert: env.HasSuperBinding() is true.
3. 3. 3. Let baseValue be ? env.GetSuperBase().
4. 4. 4. Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
propertyKey, [[Strict]]: strict, [[ThisValue]]: actualThis }.
13.3.8 ARGUMENT LISTS
Note
The evaluation of an argument list produces a List of values.
13.3.8.1 RUNTIME SEMANTICS: ARGUMENTLISTEVALUATION
The syntax-directed operation ArgumentListEvaluation takes no arguments and
returns either a normal completion containing a List of ECMAScript language
values or an abrupt completion. It is defined piecewise over the following
productions:
Arguments : ( )
1. 1. 1. Return a new empty List.
ArgumentList : AssignmentExpression
1. 1. 1. Let ref be ? Evaluation of AssignmentExpression.
2. 2. 2. Let arg be ? GetValue(ref).
3. 3. 3. Return « arg ».
ArgumentList : ... AssignmentExpression
1. 1. 1. Let list be a new empty List.
2. 2. 2. Let spreadRef be ? Evaluation of AssignmentExpression.
3. 3. 3. Let spreadObj be ? GetValue(spreadRef).
4. 4. 4. Let iteratorRecord be ? GetIterator(spreadObj, sync).
5. 5. 5. Repeat,
1. a. a. Let next be ? IteratorStep(iteratorRecord).
2. b. b. If next is false, return list.
3. c. c. Let nextArg be ? IteratorValue(next).
4. d. d. Append nextArg to list.
ArgumentList : ArgumentList , AssignmentExpression
1. 1. 1. Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
2. 2. 2. Let ref be ? Evaluation of AssignmentExpression.
3. 3. 3. Let arg be ? GetValue(ref).
4. 4. 4. Return the list-concatenation of precedingArgs and « arg ».
ArgumentList : ArgumentList , ... AssignmentExpression
1. 1. 1. Let precedingArgs be ? ArgumentListEvaluation of ArgumentList.
2. 2. 2. Let spreadRef be ? Evaluation of AssignmentExpression.
3. 3. 3. Let iteratorRecord be ? GetIterator(? GetValue(spreadRef), sync).
4. 4. 4. Repeat,
1. a. a. Let next be ? IteratorStep(iteratorRecord).
2. b. b. If next is false, return precedingArgs.
3. c. c. Let nextArg be ? IteratorValue(next).
4. d. d. Append nextArg to precedingArgs.
TemplateLiteral : NoSubstitutionTemplate
1. 1. 1. Let templateLiteral be this TemplateLiteral.
2. 2. 2. Let siteObj be GetTemplateObject(templateLiteral).
3. 3. 3. Return « siteObj ».
TemplateLiteral : SubstitutionTemplate
1. 1. 1. Let templateLiteral be this TemplateLiteral.
2. 2. 2. Let siteObj be GetTemplateObject(templateLiteral).
3. 3. 3. Let remaining be ? ArgumentListEvaluation of SubstitutionTemplate.
4. 4. 4. Return the list-concatenation of « siteObj » and remaining.
SubstitutionTemplate : TemplateHead Expression TemplateSpans
1. 1. 1. Let firstSubRef be ? Evaluation of Expression.
2. 2. 2. Let firstSub be ? GetValue(firstSubRef).
3. 3. 3. Let restSub be ? SubstitutionEvaluation of TemplateSpans.
4. 4. 4. Assert: restSub is a possibly empty List.
5. 5. 5. Return the list-concatenation of « firstSub » and restSub.
13.3.9 OPTIONAL CHAINS
Note
An optional chain is a chain of one or more property accesses and function
calls, the first of which begins with the token ?..
13.3.9.1 RUNTIME SEMANTICS: EVALUATION
OptionalExpression : MemberExpression OptionalChain
1. 1. 1. Let baseReference be ? Evaluation of MemberExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If baseValue is either undefined or null, then
1. a. a. Return undefined.
4. 4. 4. Return ? ChainEvaluation of OptionalChain with arguments baseValue and
baseReference.
OptionalExpression : CallExpression OptionalChain
1. 1. 1. Let baseReference be ? Evaluation of CallExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If baseValue is either undefined or null, then
1. a. a. Return undefined.
4. 4. 4. Return ? ChainEvaluation of OptionalChain with arguments baseValue and
baseReference.
OptionalExpression : OptionalExpression OptionalChain
1. 1. 1. Let baseReference be ? Evaluation of OptionalExpression.
2. 2. 2. Let baseValue be ? GetValue(baseReference).
3. 3. 3. If baseValue is either undefined or null, then
1. a. a. Return undefined.
4. 4. 4. Return ? ChainEvaluation of OptionalChain with arguments baseValue and
baseReference.
13.3.9.2 RUNTIME SEMANTICS: CHAINEVALUATION
The syntax-directed operation ChainEvaluation takes arguments baseValue (an
ECMAScript language value) and baseReference (an ECMAScript language value or a
Reference Record) and returns either a normal completion containing either an
ECMAScript language value or a Reference Record, or an abrupt completion. It is
defined piecewise over the following productions:
OptionalChain : ?. Arguments
1. 1. 1. Let thisChain be this OptionalChain.
2. 2. 2. Let tailCall be IsInTailPosition(thisChain).
3. 3. 3. Return ? EvaluateCall(baseValue, baseReference, Arguments, tailCall).
OptionalChain : ?. [ Expression ]
1. 1. 1. If the source text matched by this OptionalChain is strict mode code,
let strict be true; else let strict be false.
2. 2. 2. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue,
Expression, strict).
OptionalChain : ?. IdentifierName
1. 1. 1. If the source text matched by this OptionalChain is strict mode code,
let strict be true; else let strict be false.
2. 2. 2. Return EvaluatePropertyAccessWithIdentifierKey(baseValue,
IdentifierName, strict).
OptionalChain : ?. PrivateIdentifier
1. 1. 1. Let fieldNameString be the StringValue of PrivateIdentifier.
2. 2. 2. Return MakePrivateReference(baseValue, fieldNameString).
OptionalChain : OptionalChain Arguments
1. 1. 1. Let optionalChain be OptionalChain.
2. 2. 2. Let newReference be ? ChainEvaluation of optionalChain with arguments
baseValue and baseReference.
3. 3. 3. Let newValue be ? GetValue(newReference).
4. 4. 4. Let thisChain be this OptionalChain.
5. 5. 5. Let tailCall be IsInTailPosition(thisChain).
6. 6. 6. Return ? EvaluateCall(newValue, newReference, Arguments, tailCall).
OptionalChain : OptionalChain [ Expression ]
1. 1. 1. Let optionalChain be OptionalChain.
2. 2. 2. Let newReference be ? ChainEvaluation of optionalChain with arguments
baseValue and baseReference.
3. 3. 3. Let newValue be ? GetValue(newReference).
4. 4. 4. If the source text matched by this OptionalChain is strict mode code,
let strict be true; else let strict be false.
5. 5. 5. Return ? EvaluatePropertyAccessWithExpressionKey(newValue, Expression,
strict).
OptionalChain : OptionalChain . IdentifierName
1. 1. 1. Let optionalChain be OptionalChain.
2. 2. 2. Let newReference be ? ChainEvaluation of optionalChain with arguments
baseValue and baseReference.
3. 3. 3. Let newValue be ? GetValue(newReference).
4. 4. 4. If the source text matched by this OptionalChain is strict mode code,
let strict be true; else let strict be false.
5. 5. 5. Return EvaluatePropertyAccessWithIdentifierKey(newValue,
IdentifierName, strict).
OptionalChain : OptionalChain . PrivateIdentifier
1. 1. 1. Let optionalChain be OptionalChain.
2. 2. 2. Let newReference be ? ChainEvaluation of optionalChain with arguments
baseValue and baseReference.
3. 3. 3. Let newValue be ? GetValue(newReference).
4. 4. 4. Let fieldNameString be the StringValue of PrivateIdentifier.
5. 5. 5. Return MakePrivateReference(newValue, fieldNameString).
13.3.10 IMPORT CALLS
13.3.10.1 RUNTIME SEMANTICS: EVALUATION
ImportCall : import ( AssignmentExpression )
1. 1. 1. Let referrer be GetActiveScriptOrModule().
2. 2. 2. If referrer is null, set referrer to the current Realm Record.
3. 3. 3. Let argRef be ? Evaluation of AssignmentExpression.
4. 4. 4. Let specifier be ? GetValue(argRef).
5. 5. 5. Let promiseCapability be ! NewPromiseCapability(%Promise%).
6. 6. 6. Let specifierString be Completion(ToString(specifier)).
7. 7. 7. IfAbruptRejectPromise(specifierString, promiseCapability).
8. 8. 8. Perform HostLoadImportedModule(referrer, specifierString, empty,
promiseCapability).
9. 9. 9. Return promiseCapability.[[Promise]].
13.3.10.1.1 CONTINUEDYNAMICIMPORT ( PROMISECAPABILITY, MODULECOMPLETION )
The abstract operation ContinueDynamicImport takes arguments promiseCapability
(a PromiseCapability Record) and moduleCompletion (either a normal completion
containing a Module Record or a throw completion) and returns unused. It
completes the process of a dynamic import originally started by an import()
call, resolving or rejecting the promise returned by that call as appropriate.
It performs the following steps when called:
1. 1. 1. If moduleCompletion is an abrupt completion, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, «
moduleCompletion.[[Value]] »).
2. b. b. Return unused.
2. 2. 2. Let module be moduleCompletion.[[Value]].
3. 3. 3. Let loadPromise be module.LoadRequestedModules().
4. 4. 4. Let rejectedClosure be a new Abstract Closure with parameters (reason)
that captures promiseCapability and performs the following steps when
called:
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, « reason
»).
2. b. b. Return unused.
5. 5. 5. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
6. 6. 6. Let linkAndEvaluateClosure be a new Abstract Closure with no
parameters that captures module, promiseCapability, and onRejected and
performs the following steps when called:
1. a. a. Let link be Completion(module.Link()).
2. b. b. If link is an abrupt completion, then
1. i. i. Perform ! Call(promiseCapability.[[Reject]], undefined, «
link.[[Value]] »).
2. ii. ii. Return unused.
3. c. c. Let evaluatePromise be module.Evaluate().
4. d. d. Let fulfilledClosure be a new Abstract Closure with no parameters
that captures module and promiseCapability and performs the following
steps when called:
1. i. i. Let namespace be GetModuleNamespace(module).
2. ii. ii. Perform ! Call(promiseCapability.[[Resolve]], undefined, «
namespace »).
3. iii. iii. Return unused.
5. e. e. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 0, "", «
»).
6. f. f. Perform PerformPromiseThen(evaluatePromise, onFulfilled,
onRejected).
7. g. g. Return unused.
7. 7. 7. Let linkAndEvaluate be CreateBuiltinFunction(linkAndEvaluateClosure,
0, "", « »).
8. 8. 8. Perform PerformPromiseThen(loadPromise, linkAndEvaluate, onRejected).
9. 9. 9. Return unused.
13.3.11 TAGGED TEMPLATES
Note
A tagged template is a function call where the arguments of the call are derived
from a TemplateLiteral (13.2.8). The actual arguments include a template object
(13.2.8.4) and the values produced by evaluating the expressions embedded within
the TemplateLiteral.
13.3.11.1 RUNTIME SEMANTICS: EVALUATION
MemberExpression : MemberExpression TemplateLiteral
1. 1. 1. Let tagRef be ? Evaluation of MemberExpression.
2. 2. 2. Let tagFunc be ? GetValue(tagRef).
3. 3. 3. Let thisCall be this MemberExpression.
4. 4. 4. Let tailCall be IsInTailPosition(thisCall).
5. 5. 5. Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).
CallExpression : CallExpression TemplateLiteral
1. 1. 1. Let tagRef be ? Evaluation of CallExpression.
2. 2. 2. Let tagFunc be ? GetValue(tagRef).
3. 3. 3. Let thisCall be this CallExpression.
4. 4. 4. Let tailCall be IsInTailPosition(thisCall).
5. 5. 5. Return ? EvaluateCall(tagFunc, tagRef, TemplateLiteral, tailCall).
13.3.12 META PROPERTIES
13.3.12.1 RUNTIME SEMANTICS: EVALUATION
NewTarget : new . target
1. 1. 1. Return GetNewTarget().
ImportMeta : import . meta
1. 1. 1. Let module be GetActiveScriptOrModule().
2. 2. 2. Assert: module is a Source Text Module Record.
3. 3. 3. Let importMeta be module.[[ImportMeta]].
4. 4. 4. If importMeta is empty, then
1. a. a. Set importMeta to OrdinaryObjectCreate(null).
2. b. b. Let importMetaValues be HostGetImportMetaProperties(module).
3. c. c. For each Record { [[Key]], [[Value]] } p of importMetaValues, do
1. i. i. Perform ! CreateDataPropertyOrThrow(importMeta, p.[[Key]],
p.[[Value]]).
4. d. d. Perform HostFinalizeImportMeta(importMeta, module).
5. e. e. Set module.[[ImportMeta]] to importMeta.
6. f. f. Return importMeta.
5. 5. 5. Else,
1. a. a. Assert: importMeta is an Object.
2. b. b. Return importMeta.
13.3.12.1.1 HOSTGETIMPORTMETAPROPERTIES ( MODULERECORD )
The host-defined abstract operation HostGetImportMetaProperties takes argument
moduleRecord (a Module Record) and returns a List of Records with fields [[Key]]
(a property key) and [[Value]] (an ECMAScript language value). It allows hosts
to provide property keys and values for the object returned from import.meta.
An implementation of HostGetImportMetaProperties must conform to the following
requirements:
* It must return a List whose values are all Records with two fields, [[Key]]
and [[Value]].
* Each such Record's [[Key]] field must be a property key, i.e., IsPropertyKey
must return true when applied to it.
* Each such Record's [[Value]] field must be an ECMAScript language value.
The default implementation of HostGetImportMetaProperties is to return a new
empty List.
13.3.12.1.2 HOSTFINALIZEIMPORTMETA ( IMPORTMETA, MODULERECORD )
The host-defined abstract operation HostFinalizeImportMeta takes arguments
importMeta (an Object) and moduleRecord (a Module Record) and returns unused. It
allows hosts to perform any extraordinary operations to prepare the object
returned from import.meta.
Most hosts will be able to simply define HostGetImportMetaProperties, and leave
HostFinalizeImportMeta with its default behaviour. However,
HostFinalizeImportMeta provides an "escape hatch" for hosts which need to
directly manipulate the object before it is exposed to ECMAScript code.
An implementation of HostFinalizeImportMeta must conform to the following
requirements:
* It must return unused.
The default implementation of HostFinalizeImportMeta is to return unused.
13.4 UPDATE EXPRESSIONS
SYNTAX
UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await]
LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] ++
LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] -- ++
UnaryExpression[?Yield, ?Await] -- UnaryExpression[?Yield, ?Await]
13.4.1 STATIC SEMANTICS: EARLY ERRORS
UpdateExpression : LeftHandSideExpression ++ LeftHandSideExpression --
* It is an early Syntax Error if AssignmentTargetType of LeftHandSideExpression
is not simple.
UpdateExpression : ++ UnaryExpression -- UnaryExpression
* It is an early Syntax Error if AssignmentTargetType of UnaryExpression is not
simple.
13.4.2 POSTFIX INCREMENT OPERATOR
13.4.2.1 RUNTIME SEMANTICS: EVALUATION
UpdateExpression : LeftHandSideExpression ++
1. 1. 1. Let lhs be ? Evaluation of LeftHandSideExpression.
2. 2. 2. Let oldValue be ? ToNumeric(? GetValue(lhs)).
3. 3. 3. If oldValue is a Number, then
1. a. a. Let newValue be Number::add(oldValue, 1𝔽).
4. 4. 4. Else,
1. a. a. Assert: oldValue is a BigInt.
2. b. b. Let newValue be BigInt::add(oldValue, 1ℤ).
5. 5. 5. Perform ? PutValue(lhs, newValue).
6. 6. 6. Return oldValue.
13.4.3 POSTFIX DECREMENT OPERATOR
13.4.3.1 RUNTIME SEMANTICS: EVALUATION
UpdateExpression : LeftHandSideExpression --
1. 1. 1. Let lhs be ? Evaluation of LeftHandSideExpression.
2. 2. 2. Let oldValue be ? ToNumeric(? GetValue(lhs)).
3. 3. 3. If oldValue is a Number, then
1. a. a. Let newValue be Number::subtract(oldValue, 1𝔽).
4. 4. 4. Else,
1. a. a. Assert: oldValue is a BigInt.
2. b. b. Let newValue be BigInt::subtract(oldValue, 1ℤ).
5. 5. 5. Perform ? PutValue(lhs, newValue).
6. 6. 6. Return oldValue.
13.4.4 PREFIX INCREMENT OPERATOR
13.4.4.1 RUNTIME SEMANTICS: EVALUATION
UpdateExpression : ++ UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Let oldValue be ? ToNumeric(? GetValue(expr)).
3. 3. 3. If oldValue is a Number, then
1. a. a. Let newValue be Number::add(oldValue, 1𝔽).
4. 4. 4. Else,
1. a. a. Assert: oldValue is a BigInt.
2. b. b. Let newValue be BigInt::add(oldValue, 1ℤ).
5. 5. 5. Perform ? PutValue(expr, newValue).
6. 6. 6. Return newValue.
13.4.5 PREFIX DECREMENT OPERATOR
13.4.5.1 RUNTIME SEMANTICS: EVALUATION
UpdateExpression : -- UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Let oldValue be ? ToNumeric(? GetValue(expr)).
3. 3. 3. If oldValue is a Number, then
1. a. a. Let newValue be Number::subtract(oldValue, 1𝔽).
4. 4. 4. Else,
1. a. a. Assert: oldValue is a BigInt.
2. b. b. Let newValue be BigInt::subtract(oldValue, 1ℤ).
5. 5. 5. Perform ? PutValue(expr, newValue).
6. 6. 6. Return newValue.
13.5 UNARY OPERATORS
SYNTAX
UnaryExpression[Yield, Await] : UpdateExpression[?Yield, ?Await] delete
UnaryExpression[?Yield, ?Await] void UnaryExpression[?Yield, ?Await] typeof
UnaryExpression[?Yield, ?Await] + UnaryExpression[?Yield, ?Await] -
UnaryExpression[?Yield, ?Await] ~ UnaryExpression[?Yield, ?Await] !
UnaryExpression[?Yield, ?Await] [+Await] AwaitExpression[?Yield]
13.5.1 THE DELETE OPERATOR
13.5.1.1 STATIC SEMANTICS: EARLY ERRORS
UnaryExpression : delete UnaryExpression
* It is a Syntax Error if the UnaryExpression is contained in strict mode code
and the derived UnaryExpression is PrimaryExpression : IdentifierReference ,
MemberExpression : MemberExpression . PrivateIdentifier , CallExpression :
CallExpression . PrivateIdentifier , OptionalChain : ?. PrivateIdentifier ,
or OptionalChain : OptionalChain . PrivateIdentifier .
* It is a Syntax Error if the derived UnaryExpression is
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
and CoverParenthesizedExpressionAndArrowParameterList ultimately derives a
phrase that, if used in place of UnaryExpression, would produce a Syntax
Error according to these rules. This rule is recursively applied.
Note
The last rule means that expressions such as delete (((foo))) produce early
errors because of recursive application of the first rule.
13.5.1.2 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : delete UnaryExpression
1. 1. 1. Let ref be ? Evaluation of UnaryExpression.
2. 2. 2. If ref is not a Reference Record, return true.
3. 3. 3. If IsUnresolvableReference(ref) is true, then
1. a. a. Assert: ref.[[Strict]] is false.
2. b. b. Return true.
4. 4. 4. If IsPropertyReference(ref) is true, then
1. a. a. Assert: IsPrivateReference(ref) is false.
2. b. b. If IsSuperReference(ref) is true, throw a ReferenceError exception.
3. c. c. Let baseObj be ? ToObject(ref.[[Base]]).
4. d. d. Let deleteStatus be ? baseObj.[[Delete]](ref.[[ReferencedName]]).
5. e. e. If deleteStatus is false and ref.[[Strict]] is true, throw a
TypeError exception.
6. f. f. Return deleteStatus.
5. 5. 5. Else,
1. a. a. Let base be ref.[[Base]].
2. b. b. Assert: base is an Environment Record.
3. c. c. Return ? base.DeleteBinding(ref.[[ReferencedName]]).
Note 1
When a delete operator occurs within strict mode code, a SyntaxError exception
is thrown if its UnaryExpression is a direct reference to a variable, function
argument, or function name. In addition, if a delete operator occurs within
strict mode code and the property to be deleted has the attribute {
[[Configurable]]: false } (or otherwise cannot be deleted), a TypeError
exception is thrown.
Note 2
The object that may be created in step 4.c is not accessible outside of the
above abstract operation and the ordinary object [[Delete]] internal method. An
implementation might choose to avoid the actual creation of that object.
13.5.2 THE VOID OPERATOR
13.5.2.1 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : void UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Perform ? GetValue(expr).
3. 3. 3. Return undefined.
Note
GetValue must be called even though its value is not used because it may have
observable side-effects.
13.5.3 THE TYPEOF OPERATOR
13.5.3.1 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : typeof UnaryExpression
1. 1. 1. Let val be ? Evaluation of UnaryExpression.
2. 2. 2. If val is a Reference Record, then
1. a. a. If IsUnresolvableReference(val) is true, return "undefined".
3. 3. 3. Set val to ? GetValue(val).
4. 4. 4. If val is undefined, return "undefined".
5. 5. 5. If val is null, return "object".
6. 6. 6. If val is a String, return "string".
7. 7. 7. If val is a Symbol, return "symbol".
8. 8. 8. If val is a Boolean, return "boolean".
9. 9. 9. If val is a Number, return "number".
10. 10. 10. If val is a BigInt, return "bigint".
11. 11. 11. Assert: val is an Object.
12. 12. 12. NOTE: This step is replaced in section B.3.6.3.
13. 13. 13. If val has a [[Call]] internal slot, return "function".
14. 14. 14. Return "object".
13.5.4 UNARY + OPERATOR
Note
The unary + operator converts its operand to Number type.
13.5.4.1 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : + UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Return ? ToNumber(? GetValue(expr)).
13.5.5 UNARY - OPERATOR
Note
The unary - operator converts its operand to a numeric value and then negates
it. Negating +0𝔽 produces -0𝔽, and negating -0𝔽 produces +0𝔽.
13.5.5.1 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : - UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Let oldValue be ? ToNumeric(? GetValue(expr)).
3. 3. 3. If oldValue is a Number, then
1. a. a. Return Number::unaryMinus(oldValue).
4. 4. 4. Else,
1. a. a. Assert: oldValue is a BigInt.
2. b. b. Return BigInt::unaryMinus(oldValue).
13.5.6 BITWISE NOT OPERATOR ( ~ )
13.5.6.1 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : ~ UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Let oldValue be ? ToNumeric(? GetValue(expr)).
3. 3. 3. If oldValue is a Number, then
1. a. a. Return Number::bitwiseNOT(oldValue).
4. 4. 4. Else,
1. a. a. Assert: oldValue is a BigInt.
2. b. b. Return BigInt::bitwiseNOT(oldValue).
13.5.7 LOGICAL NOT OPERATOR ( ! )
13.5.7.1 RUNTIME SEMANTICS: EVALUATION
UnaryExpression : ! UnaryExpression
1. 1. 1. Let expr be ? Evaluation of UnaryExpression.
2. 2. 2. Let oldValue be ToBoolean(? GetValue(expr)).
3. 3. 3. If oldValue is true, return false.
4. 4. 4. Return true.
13.6 EXPONENTIATION OPERATOR
SYNTAX
ExponentiationExpression[Yield, Await] : UnaryExpression[?Yield, ?Await]
UpdateExpression[?Yield, ?Await] ** ExponentiationExpression[?Yield, ?Await]
13.6.1 RUNTIME SEMANTICS: EVALUATION
ExponentiationExpression : UpdateExpression ** ExponentiationExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(UpdateExpression, **,
ExponentiationExpression).
13.7 MULTIPLICATIVE OPERATORS
SYNTAX
MultiplicativeExpression[Yield, Await] : ExponentiationExpression[?Yield,
?Await] MultiplicativeExpression[?Yield, ?Await] MultiplicativeOperator
ExponentiationExpression[?Yield, ?Await] MultiplicativeOperator : one of * / %
Note
* The * operator performs multiplication, producing the product of its
operands.
* The / operator performs division, producing the quotient of its operands.
* The % operator yields the remainder of its operands from an implied division.
13.7.1 RUNTIME SEMANTICS: EVALUATION
MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator
ExponentiationExpression
1. 1. 1. Let opText be the source text matched by MultiplicativeOperator.
2. 2. 2. Return
? EvaluateStringOrNumericBinaryExpression(MultiplicativeExpression, opText,
ExponentiationExpression).
13.8 ADDITIVE OPERATORS
SYNTAX
AdditiveExpression[Yield, Await] : MultiplicativeExpression[?Yield, ?Await]
AdditiveExpression[?Yield, ?Await] + MultiplicativeExpression[?Yield, ?Await]
AdditiveExpression[?Yield, ?Await] - MultiplicativeExpression[?Yield, ?Await]
13.8.1 THE ADDITION OPERATOR ( + )
Note
The addition operator either performs string concatenation or numeric addition.
13.8.1.1 RUNTIME SEMANTICS: EVALUATION
AdditiveExpression : AdditiveExpression + MultiplicativeExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression,
+, MultiplicativeExpression).
13.8.2 THE SUBTRACTION OPERATOR ( - )
Note
The - operator performs subtraction, producing the difference of its operands.
13.8.2.1 RUNTIME SEMANTICS: EVALUATION
AdditiveExpression : AdditiveExpression - MultiplicativeExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression,
-, MultiplicativeExpression).
13.9 BITWISE SHIFT OPERATORS
SYNTAX
ShiftExpression[Yield, Await] : AdditiveExpression[?Yield, ?Await]
ShiftExpression[?Yield, ?Await] << AdditiveExpression[?Yield, ?Await]
ShiftExpression[?Yield, ?Await] >> AdditiveExpression[?Yield, ?Await]
ShiftExpression[?Yield, ?Await] >>> AdditiveExpression[?Yield, ?Await]
13.9.1 THE LEFT SHIFT OPERATOR ( << )
Note
Performs a bitwise left shift operation on the left operand by the amount
specified by the right operand.
13.9.1.1 RUNTIME SEMANTICS: EVALUATION
ShiftExpression : ShiftExpression << AdditiveExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, <<,
AdditiveExpression).
13.9.2 THE SIGNED RIGHT SHIFT OPERATOR ( >> )
Note
Performs a sign-filling bitwise right shift operation on the left operand by the
amount specified by the right operand.
13.9.2.1 RUNTIME SEMANTICS: EVALUATION
ShiftExpression : ShiftExpression >> AdditiveExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>,
AdditiveExpression).
13.9.3 THE UNSIGNED RIGHT SHIFT OPERATOR ( >>> )
Note
Performs a zero-filling bitwise right shift operation on the left operand by the
amount specified by the right operand.
13.9.3.1 RUNTIME SEMANTICS: EVALUATION
ShiftExpression : ShiftExpression >>> AdditiveExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>>,
AdditiveExpression).
13.10 RELATIONAL OPERATORS
Note 1
The result of evaluating a relational operator is always of type Boolean,
reflecting whether the relationship named by the operator holds between its two
operands.
SYNTAX
RelationalExpression[In, Yield, Await] : ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] < ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] > ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] <= ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] >= ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] instanceof ShiftExpression[?Yield,
?Await] [+In] RelationalExpression[+In, ?Yield, ?Await] in
ShiftExpression[?Yield, ?Await] [+In] PrivateIdentifier in
ShiftExpression[?Yield, ?Await] Note 2
The [In] grammar parameter is needed to avoid confusing the in operator in a
relational expression with the in operator in a for statement.
13.10.1 RUNTIME SEMANTICS: EVALUATION
RelationalExpression : RelationalExpression < ShiftExpression
1. 1. 1. Let lref be ? Evaluation of RelationalExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of ShiftExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let r be ? IsLessThan(lval, rval, true).
6. 6. 6. If r is undefined, return false. Otherwise, return r.
RelationalExpression : RelationalExpression > ShiftExpression
1. 1. 1. Let lref be ? Evaluation of RelationalExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of ShiftExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let r be ? IsLessThan(rval, lval, false).
6. 6. 6. If r is undefined, return false. Otherwise, return r.
RelationalExpression : RelationalExpression <= ShiftExpression
1. 1. 1. Let lref be ? Evaluation of RelationalExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of ShiftExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let r be ? IsLessThan(rval, lval, false).
6. 6. 6. If r is either true or undefined, return false. Otherwise, return
true.
RelationalExpression : RelationalExpression >= ShiftExpression
1. 1. 1. Let lref be ? Evaluation of RelationalExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of ShiftExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let r be ? IsLessThan(lval, rval, true).
6. 6. 6. If r is either true or undefined, return false. Otherwise, return
true.
RelationalExpression : RelationalExpression instanceof ShiftExpression
1. 1. 1. Let lref be ? Evaluation of RelationalExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of ShiftExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Return ? InstanceofOperator(lval, rval).
RelationalExpression : RelationalExpression in ShiftExpression
1. 1. 1. Let lref be ? Evaluation of RelationalExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of ShiftExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. If rval is not an Object, throw a TypeError exception.
6. 6. 6. Return ? HasProperty(rval, ? ToPropertyKey(lval)).
RelationalExpression : PrivateIdentifier in ShiftExpression
1. 1. 1. Let privateIdentifier be the StringValue of PrivateIdentifier.
2. 2. 2. Let rref be ? Evaluation of ShiftExpression.
3. 3. 3. Let rval be ? GetValue(rref).
4. 4. 4. If rval is not an Object, throw a TypeError exception.
5. 5. 5. Let privateEnv be the running execution context's PrivateEnvironment.
6. 6. 6. Let privateName be ResolvePrivateIdentifier(privateEnv,
privateIdentifier).
7. 7. 7. If PrivateElementFind(rval, privateName) is not empty, return true.
8. 8. 8. Return false.
13.10.2 INSTANCEOFOPERATOR ( V, TARGET )
The abstract operation InstanceofOperator takes arguments V (an ECMAScript
language value) and target (an ECMAScript language value) and returns either a
normal completion containing a Boolean or a throw completion. It implements the
generic algorithm for determining if V is an instance of target either by
consulting target's @@hasInstance method or, if absent, determining whether the
value of target's "prototype" property is present in V's prototype chain. It
performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let instOfHandler be ? GetMethod(target, @@hasInstance).
3. 3. 3. If instOfHandler is not undefined, then
1. a. a. Return ToBoolean(? Call(instOfHandler, target, « V »)).
4. 4. 4. If IsCallable(target) is false, throw a TypeError exception.
5. 5. 5. Return ? OrdinaryHasInstance(target, V).
Note
Steps 4 and 5 provide compatibility with previous editions of ECMAScript that
did not use a @@hasInstance method to define the instanceof operator semantics.
If an object does not define or inherit @@hasInstance it uses the default
instanceof semantics.
13.11 EQUALITY OPERATORS
Note
The result of evaluating an equality operator is always of type Boolean,
reflecting whether the relationship named by the operator holds between its two
operands.
SYNTAX
EqualityExpression[In, Yield, Await] : RelationalExpression[?In, ?Yield, ?Await]
EqualityExpression[?In, ?Yield, ?Await] == RelationalExpression[?In, ?Yield,
?Await] EqualityExpression[?In, ?Yield, ?Await] != RelationalExpression[?In,
?Yield, ?Await] EqualityExpression[?In, ?Yield, ?Await] ===
RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield,
?Await] !== RelationalExpression[?In, ?Yield, ?Await]
13.11.1 RUNTIME SEMANTICS: EVALUATION
EqualityExpression : EqualityExpression == RelationalExpression
1. 1. 1. Let lref be ? Evaluation of EqualityExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of RelationalExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Return ? IsLooselyEqual(rval, lval).
EqualityExpression : EqualityExpression != RelationalExpression
1. 1. 1. Let lref be ? Evaluation of EqualityExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of RelationalExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let r be ? IsLooselyEqual(rval, lval).
6. 6. 6. If r is true, return false. Otherwise, return true.
EqualityExpression : EqualityExpression === RelationalExpression
1. 1. 1. Let lref be ? Evaluation of EqualityExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of RelationalExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Return IsStrictlyEqual(rval, lval).
EqualityExpression : EqualityExpression !== RelationalExpression
1. 1. 1. Let lref be ? Evaluation of EqualityExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of RelationalExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let r be IsStrictlyEqual(rval, lval).
6. 6. 6. If r is true, return false. Otherwise, return true.
Note 1
Given the above definition of equality:
* String comparison can be forced by: `${a}` == `${b}`.
* Numeric comparison can be forced by: +a == +b.
* Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
* A != B is equivalent to !(A == B).
* A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3
The equality operator is not always transitive. For example, there might be two
distinct String objects, each representing the same String value; each String
object would be considered equal to the String value by the == operator, but the
two String objects would not be equal to each other. For example:
* new String("a") == "a" and "a" == new String("a") are both true.
* new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit
values. There is no attempt to use the more complex, semantically oriented
definitions of character or string equality and collating order defined in the
Unicode specification. Therefore Strings values that are canonically equal
according to the Unicode Standard could test as unequal. In effect this
algorithm assumes that both Strings are already in normalized form.
13.12 BINARY BITWISE OPERATORS
SYNTAX
BitwiseANDExpression[In, Yield, Await] : EqualityExpression[?In, ?Yield, ?Await]
BitwiseANDExpression[?In, ?Yield, ?Await] & EqualityExpression[?In, ?Yield,
?Await] BitwiseXORExpression[In, Yield, Await] : BitwiseANDExpression[?In,
?Yield, ?Await] BitwiseXORExpression[?In, ?Yield, ?Await] ^
BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseORExpression[In, Yield, Await]
: BitwiseXORExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In, ?Yield,
?Await] | BitwiseXORExpression[?In, ?Yield, ?Await]
13.12.1 RUNTIME SEMANTICS: EVALUATION
BitwiseANDExpression : BitwiseANDExpression & EqualityExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseANDExpression,
&, EqualityExpression).
BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseXORExpression,
^, BitwiseANDExpression).
BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression
1. 1. 1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseORExpression,
|, BitwiseXORExpression).
13.13 BINARY LOGICAL OPERATORS
SYNTAX
LogicalANDExpression[In, Yield, Await] : BitwiseORExpression[?In, ?Yield,
?Await] LogicalANDExpression[?In, ?Yield, ?Await] && BitwiseORExpression[?In,
?Yield, ?Await] LogicalORExpression[In, Yield, Await] :
LogicalANDExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield,
?Await] || LogicalANDExpression[?In, ?Yield, ?Await] CoalesceExpression[In,
Yield, Await] : CoalesceExpressionHead[?In, ?Yield, ?Await] ??
BitwiseORExpression[?In, ?Yield, ?Await] CoalesceExpressionHead[In, Yield,
Await] : CoalesceExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In,
?Yield, ?Await] ShortCircuitExpression[In, Yield, Await] :
LogicalORExpression[?In, ?Yield, ?Await] CoalesceExpression[?In, ?Yield, ?Await]
Note
The value produced by a && or || operator is not necessarily of type Boolean.
The value produced will always be the value of one of the two operand
expressions.
13.13.1 RUNTIME SEMANTICS: EVALUATION
LogicalANDExpression : LogicalANDExpression && BitwiseORExpression
1. 1. 1. Let lref be ? Evaluation of LogicalANDExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let lbool be ToBoolean(lval).
4. 4. 4. If lbool is false, return lval.
5. 5. 5. Let rref be ? Evaluation of BitwiseORExpression.
6. 6. 6. Return ? GetValue(rref).
LogicalORExpression : LogicalORExpression || LogicalANDExpression
1. 1. 1. Let lref be ? Evaluation of LogicalORExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let lbool be ToBoolean(lval).
4. 4. 4. If lbool is true, return lval.
5. 5. 5. Let rref be ? Evaluation of LogicalANDExpression.
6. 6. 6. Return ? GetValue(rref).
CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression
1. 1. 1. Let lref be ? Evaluation of CoalesceExpressionHead.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. If lval is either undefined or null, then
1. a. a. Let rref be ? Evaluation of BitwiseORExpression.
2. b. b. Return ? GetValue(rref).
4. 4. 4. Otherwise, return lval.
13.14 CONDITIONAL OPERATOR ( ? : )
SYNTAX
ConditionalExpression[In, Yield, Await] : ShortCircuitExpression[?In, ?Yield,
?Await] ShortCircuitExpression[?In, ?Yield, ?Await] ? AssignmentExpression[+In,
?Yield, ?Await] : AssignmentExpression[?In, ?Yield, ?Await] Note
The grammar for a ConditionalExpression in ECMAScript is slightly different from
that in C and Java, which each allow the second subexpression to be an
Expression but restrict the third expression to be a ConditionalExpression. The
motivation for this difference in ECMAScript is to allow an assignment
expression to be governed by either arm of a conditional and to eliminate the
confusing and fairly useless case of a comma expression as the centre
expression.
13.14.1 RUNTIME SEMANTICS: EVALUATION
ConditionalExpression : ShortCircuitExpression ? AssignmentExpression :
AssignmentExpression
1. 1. 1. Let lref be ? Evaluation of ShortCircuitExpression.
2. 2. 2. Let lval be ToBoolean(? GetValue(lref)).
3. 3. 3. If lval is true, then
1. a. a. Let trueRef be ? Evaluation of the first AssignmentExpression.
2. b. b. Return ? GetValue(trueRef).
4. 4. 4. Else,
1. a. a. Let falseRef be ? Evaluation of the second AssignmentExpression.
2. b. b. Return ? GetValue(falseRef).
13.15 ASSIGNMENT OPERATORS
SYNTAX
AssignmentExpression[In, Yield, Await] : ConditionalExpression[?In, ?Yield,
?Await] [+Yield] YieldExpression[?In, ?Await] ArrowFunction[?In, ?Yield, ?Await]
AsyncArrowFunction[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] =
AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]
AssignmentOperator AssignmentExpression[?In, ?Yield, ?Await]
LeftHandSideExpression[?Yield, ?Await] &&= AssignmentExpression[?In, ?Yield,
?Await] LeftHandSideExpression[?Yield, ?Await] ||= AssignmentExpression[?In,
?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] ??=
AssignmentExpression[?In, ?Yield, ?Await] AssignmentOperator : one of *= /= %=
+= -= <<= >>= >>>= &= ^= |= **=
13.15.1 STATIC SEMANTICS: EARLY ERRORS
AssignmentExpression : LeftHandSideExpression = AssignmentExpression
If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the
following Early Error rules are applied:
* LeftHandSideExpression must cover an AssignmentPattern.
If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the
following Early Error rule is applied:
* It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not
simple.
AssignmentExpression : LeftHandSideExpression AssignmentOperator
AssignmentExpression LeftHandSideExpression &&= AssignmentExpression
LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??=
AssignmentExpression
* It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not
simple.
13.15.2 RUNTIME SEMANTICS: EVALUATION
AssignmentExpression : LeftHandSideExpression = AssignmentExpression
1. 1. 1. If LeftHandSideExpression is neither an ObjectLiteral nor an
ArrayLiteral, then
1. a. a. Let lref be ? Evaluation of LeftHandSideExpression.
2. b. b. If IsAnonymousFunctionDefinition(AssignmentExpression) and
IsIdentifierRef of LeftHandSideExpression are both true, then
1. i. i. Let rval be ? NamedEvaluation of AssignmentExpression with
argument lref.[[ReferencedName]].
3. c. c. Else,
1. i. i. Let rref be ? Evaluation of AssignmentExpression.
2. ii. ii. Let rval be ? GetValue(rref).
4. d. d. Perform ? PutValue(lref, rval).
5. e. e. Return rval.
2. 2. 2. Let assignmentPattern be the AssignmentPattern that is covered by
LeftHandSideExpression.
3. 3. 3. Let rref be ? Evaluation of AssignmentExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Perform ? DestructuringAssignmentEvaluation of assignmentPattern with
argument rval.
6. 6. 6. Return rval.
AssignmentExpression : LeftHandSideExpression AssignmentOperator
AssignmentExpression
1. 1. 1. Let lref be ? Evaluation of LeftHandSideExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of AssignmentExpression.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Let assignmentOpText be the source text matched by AssignmentOperator.
6. 6. 6. Let opText be the sequence of Unicode code points associated with
assignmentOpText in the following table:
assignmentOpText opText **= ** *= * /= / %= % += + -= - <<= << >>= >> >>>=
>>> &= & ^= ^ |= |
7. 7. 7. Let r be ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).
8. 8. 8. Perform ? PutValue(lref, r).
9. 9. 9. Return r.
AssignmentExpression : LeftHandSideExpression &&= AssignmentExpression
1. 1. 1. Let lref be ? Evaluation of LeftHandSideExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let lbool be ToBoolean(lval).
4. 4. 4. If lbool is false, return lval.
5. 5. 5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and
IsIdentifierRef of LeftHandSideExpression is true, then
1. a. a. Let rval be ? NamedEvaluation of AssignmentExpression with argument
lref.[[ReferencedName]].
6. 6. 6. Else,
1. a. a. Let rref be ? Evaluation of AssignmentExpression.
2. b. b. Let rval be ? GetValue(rref).
7. 7. 7. Perform ? PutValue(lref, rval).
8. 8. 8. Return rval.
AssignmentExpression : LeftHandSideExpression ||= AssignmentExpression
1. 1. 1. Let lref be ? Evaluation of LeftHandSideExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let lbool be ToBoolean(lval).
4. 4. 4. If lbool is true, return lval.
5. 5. 5. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and
IsIdentifierRef of LeftHandSideExpression is true, then
1. a. a. Let rval be ? NamedEvaluation of AssignmentExpression with argument
lref.[[ReferencedName]].
6. 6. 6. Else,
1. a. a. Let rref be ? Evaluation of AssignmentExpression.
2. b. b. Let rval be ? GetValue(rref).
7. 7. 7. Perform ? PutValue(lref, rval).
8. 8. 8. Return rval.
AssignmentExpression : LeftHandSideExpression ??= AssignmentExpression
1. 1. 1. Let lref be ? Evaluation of LeftHandSideExpression.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. If lval is neither undefined nor null, return lval.
4. 4. 4. If IsAnonymousFunctionDefinition(AssignmentExpression) is true and
IsIdentifierRef of LeftHandSideExpression is true, then
1. a. a. Let rval be ? NamedEvaluation of AssignmentExpression with argument
lref.[[ReferencedName]].
5. 5. 5. Else,
1. a. a. Let rref be ? Evaluation of AssignmentExpression.
2. b. b. Let rval be ? GetValue(rref).
6. 6. 6. Perform ? PutValue(lref, rval).
7. 7. 7. Return rval.
Note
When this expression occurs within strict mode code, it is a runtime error if
lref in step 1.d, 2, 2, 2, 2 is an unresolvable reference. If it is, a
ReferenceError exception is thrown. Additionally, it is a runtime error if the
lref in step 8, 7, 7, 6 is a reference to a data property with the attribute
value { [[Writable]]: false }, to an accessor property with the attribute value
{ [[Set]]: undefined }, or to a non-existent property of an object for which the
IsExtensible predicate returns the value false. In these cases a TypeError
exception is thrown.
13.15.3 APPLYSTRINGORNUMERICBINARYOPERATOR ( LVAL, OPTEXT, RVAL )
The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval
(an ECMAScript language value), opText (**, *, /, %, +, -, <<, >>, >>>, &, ^, or
|), and rval (an ECMAScript language value) and returns either a normal
completion containing either a String, a BigInt, or a Number, or a throw
completion. It performs the following steps when called:
1. 1. 1. If opText is +, then
1. a. a. Let lprim be ? ToPrimitive(lval).
2. b. b. Let rprim be ? ToPrimitive(rval).
3. c. c. If lprim is a String or rprim is a String, then
1. i. i. Let lstr be ? ToString(lprim).
2. ii. ii. Let rstr be ? ToString(rprim).
3. iii. iii. Return the string-concatenation of lstr and rstr.
4. d. d. Set lval to lprim.
5. e. e. Set rval to rprim.
2. 2. 2. NOTE: At this point, it must be a numeric operation.
3. 3. 3. Let lnum be ? ToNumeric(lval).
4. 4. 4. Let rnum be ? ToNumeric(rval).
5. 5. 5. If Type(lnum) is not Type(rnum), throw a TypeError exception.
6. 6. 6. If lnum is a BigInt, then
1. a. a. If opText is **, return ? BigInt::exponentiate(lnum, rnum).
2. b. b. If opText is /, return ? BigInt::divide(lnum, rnum).
3. c. c. If opText is %, return ? BigInt::remainder(lnum, rnum).
4. d. d. If opText is >>>, return ? BigInt::unsignedRightShift(lnum, rnum).
7. 7. 7. Let operation be the abstract operation associated with opText and
Type(lnum) in the following table:
opText Type(lnum) operation ** Number Number::exponentiate * Number
Number::multiply * BigInt BigInt::multiply / Number Number::divide % Number
Number::remainder + Number Number::add + BigInt BigInt::add - Number
Number::subtract - BigInt BigInt::subtract << Number Number::leftShift <<
BigInt BigInt::leftShift >> Number Number::signedRightShift >> BigInt
BigInt::signedRightShift >>> Number Number::unsignedRightShift & Number
Number::bitwiseAND & BigInt BigInt::bitwiseAND ^ Number Number::bitwiseXOR ^
BigInt BigInt::bitwiseXOR | Number Number::bitwiseOR | BigInt
BigInt::bitwiseOR
8. 8. 8. Return operation(lnum, rnum).
Note 1
No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All
standard objects except Dates handle the absence of a hint as if number were
given; Dates handle the absence of a hint as if string were given. Exotic
objects may handle the absence of a hint in some other manner.
Note 2
Step 1.c differs from step 3 of the IsLessThan algorithm, by using the
logical-or operation instead of the logical-and operation.
13.15.4 EVALUATESTRINGORNUMERICBINARYEXPRESSION ( LEFTOPERAND, OPTEXT,
RIGHTOPERAND )
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments
leftOperand (a Parse Node), opText (a sequence of Unicode code points), and
rightOperand (a Parse Node) and returns either a normal completion containing
either a String, a BigInt, or a Number, or an abrupt completion. It performs the
following steps when called:
1. 1. 1. Let lref be ? Evaluation of leftOperand.
2. 2. 2. Let lval be ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of rightOperand.
4. 4. 4. Let rval be ? GetValue(rref).
5. 5. 5. Return ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).
13.15.5 DESTRUCTURING ASSIGNMENT
SUPPLEMENTAL SYNTAX
In certain circumstances when processing an instance of the production
AssignmentExpression : LeftHandSideExpression = AssignmentExpression
the interpretation of LeftHandSideExpression is refined using the following
grammar:
AssignmentPattern[Yield, Await] : ObjectAssignmentPattern[?Yield, ?Await]
ArrayAssignmentPattern[?Yield, ?Await] ObjectAssignmentPattern[Yield, Await] : {
} { AssignmentRestProperty[?Yield, ?Await] } { AssignmentPropertyList[?Yield,
?Await] } { AssignmentPropertyList[?Yield, ?Await] ,
AssignmentRestProperty[?Yield, ?Await]opt } ArrayAssignmentPattern[Yield, Await]
: [ Elisionopt AssignmentRestElement[?Yield, ?Await]opt ] [
AssignmentElementList[?Yield, ?Await] ] [ AssignmentElementList[?Yield, ?Await]
, Elisionopt AssignmentRestElement[?Yield, ?Await]opt ]
AssignmentRestProperty[Yield, Await] : ... DestructuringAssignmentTarget[?Yield,
?Await] AssignmentPropertyList[Yield, Await] : AssignmentProperty[?Yield,
?Await] AssignmentPropertyList[?Yield, ?Await] , AssignmentProperty[?Yield,
?Await] AssignmentElementList[Yield, Await] : AssignmentElisionElement[?Yield,
?Await] AssignmentElementList[?Yield, ?Await] , AssignmentElisionElement[?Yield,
?Await] AssignmentElisionElement[Yield, Await] : Elisionopt
AssignmentElement[?Yield, ?Await] AssignmentProperty[Yield, Await] :
IdentifierReference[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt
PropertyName[?Yield, ?Await] : AssignmentElement[?Yield, ?Await]
AssignmentElement[Yield, Await] : DestructuringAssignmentTarget[?Yield, ?Await]
Initializer[+In, ?Yield, ?Await]opt AssignmentRestElement[Yield, Await] : ...
DestructuringAssignmentTarget[?Yield, ?Await]
DestructuringAssignmentTarget[Yield, Await] : LeftHandSideExpression[?Yield,
?Await]
13.15.5.1 STATIC SEMANTICS: EARLY ERRORS
AssignmentProperty : IdentifierReference Initializeropt
* It is a Syntax Error if AssignmentTargetType of IdentifierReference is not
simple.
AssignmentRestProperty : ... DestructuringAssignmentTarget
* It is a Syntax Error if DestructuringAssignmentTarget is either an
ArrayLiteral or an ObjectLiteral.
DestructuringAssignmentTarget : LeftHandSideExpression
If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the
following Early Error rules are applied:
* LeftHandSideExpression must cover an AssignmentPattern.
If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the
following Early Error rule is applied:
* It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not
simple.
13.15.5.2 RUNTIME SEMANTICS: DESTRUCTURINGASSIGNMENTEVALUATION
The syntax-directed operation DestructuringAssignmentEvaluation takes argument
value (an ECMAScript language value) and returns either a normal completion
containing unused or an abrupt completion. It is defined piecewise over the
following productions:
ObjectAssignmentPattern : { }
1. 1. 1. Perform ? RequireObjectCoercible(value).
2. 2. 2. Return unused.
ObjectAssignmentPattern : { AssignmentPropertyList } { AssignmentPropertyList ,
}
1. 1. 1. Perform ? RequireObjectCoercible(value).
2. 2. 2. Perform ? PropertyDestructuringAssignmentEvaluation of
AssignmentPropertyList with argument value.
3. 3. 3. Return unused.
ObjectAssignmentPattern : { AssignmentRestProperty }
1. 1. 1. Perform ? RequireObjectCoercible(value).
2. 2. 2. Let excludedNames be a new empty List.
3. 3. 3. Return ? RestDestructuringAssignmentEvaluation of
AssignmentRestProperty with arguments value and excludedNames.
ObjectAssignmentPattern : { AssignmentPropertyList , AssignmentRestProperty }
1. 1. 1. Perform ? RequireObjectCoercible(value).
2. 2. 2. Let excludedNames be ? PropertyDestructuringAssignmentEvaluation of
AssignmentPropertyList with argument value.
3. 3. 3. Return ? RestDestructuringAssignmentEvaluation of
AssignmentRestProperty with arguments value and excludedNames.
ArrayAssignmentPattern : [ ]
1. 1. 1. Let iteratorRecord be ? GetIterator(value, sync).
2. 2. 2. Return ? IteratorClose(iteratorRecord, NormalCompletion(unused)).
ArrayAssignmentPattern : [ Elision ]
1. 1. 1. Let iteratorRecord be ? GetIterator(value, sync).
2. 2. 2. Let result be Completion(IteratorDestructuringAssignmentEvaluation of
Elision with argument iteratorRecord).
3. 3. 3. If iteratorRecord.[[Done]] is false, return
? IteratorClose(iteratorRecord, result).
4. 4. 4. Return result.
ArrayAssignmentPattern : [ Elisionopt AssignmentRestElement ]
1. 1. 1. Let iteratorRecord be ? GetIterator(value, sync).
2. 2. 2. If Elision is present, then
1. a. a. Let status be Completion(IteratorDestructuringAssignmentEvaluation
of Elision with argument iteratorRecord).
2. b. b. If status is an abrupt completion, then
1. i. i. Assert: iteratorRecord.[[Done]] is true.
2. ii. ii. Return ? status.
3. 3. 3. Let result be Completion(IteratorDestructuringAssignmentEvaluation of
AssignmentRestElement with argument iteratorRecord).
4. 4. 4. If iteratorRecord.[[Done]] is false, return
? IteratorClose(iteratorRecord, result).
5. 5. 5. Return result.
ArrayAssignmentPattern : [ AssignmentElementList ]
1. 1. 1. Let iteratorRecord be ? GetIterator(value, sync).
2. 2. 2. Let result be Completion(IteratorDestructuringAssignmentEvaluation of
AssignmentElementList with argument iteratorRecord).
3. 3. 3. If iteratorRecord.[[Done]] is false, return
? IteratorClose(iteratorRecord, result).
4. 4. 4. Return result.
ArrayAssignmentPattern : [ AssignmentElementList , Elisionopt
AssignmentRestElementopt ]
1. 1. 1. Let iteratorRecord be ? GetIterator(value, sync).
2. 2. 2. Let status be Completion(IteratorDestructuringAssignmentEvaluation of
AssignmentElementList with argument iteratorRecord).
3. 3. 3. If status is an abrupt completion, then
1. a. a. If iteratorRecord.[[Done]] is false, return
? IteratorClose(iteratorRecord, status).
2. b. b. Return ? status.
4. 4. 4. If Elision is present, then
1. a. a. Set status to Completion(IteratorDestructuringAssignmentEvaluation
of Elision with argument iteratorRecord).
2. b. b. If status is an abrupt completion, then
1. i. i. Assert: iteratorRecord.[[Done]] is true.
2. ii. ii. Return ? status.
5. 5. 5. If AssignmentRestElement is present, then
1. a. a. Set status to Completion(IteratorDestructuringAssignmentEvaluation
of AssignmentRestElement with argument iteratorRecord).
6. 6. 6. If iteratorRecord.[[Done]] is false, return
? IteratorClose(iteratorRecord, status).
7. 7. 7. Return ? status.
13.15.5.3 RUNTIME SEMANTICS: PROPERTYDESTRUCTURINGASSIGNMENTEVALUATION
The syntax-directed operation PropertyDestructuringAssignmentEvaluation takes
argument value (an ECMAScript language value) and returns either a normal
completion containing a List of property keys or an abrupt completion. It
collects a list of all destructured property keys. It is defined piecewise over
the following productions:
AssignmentPropertyList : AssignmentPropertyList , AssignmentProperty
1. 1. 1. Let propertyNames be ? PropertyDestructuringAssignmentEvaluation of
AssignmentPropertyList with argument value.
2. 2. 2. Let nextNames be ? PropertyDestructuringAssignmentEvaluation of
AssignmentProperty with argument value.
3. 3. 3. Return the list-concatenation of propertyNames and nextNames.
AssignmentProperty : IdentifierReference Initializeropt
1. 1. 1. Let P be StringValue of IdentifierReference.
2. 2. 2. Let lref be ? ResolveBinding(P).
3. 3. 3. Let v be ? GetV(value, P).
4. 4. 4. If Initializeropt is present and v is undefined, then
1. a. a. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. i. i. Set v to ? NamedEvaluation of Initializer with argument P.
2. b. b. Else,
1. i. i. Let defaultValue be ? Evaluation of Initializer.
2. ii. ii. Set v to ? GetValue(defaultValue).
5. 5. 5. Perform ? PutValue(lref, v).
6. 6. 6. Return « P ».
AssignmentProperty : PropertyName : AssignmentElement
1. 1. 1. Let name be ? Evaluation of PropertyName.
2. 2. 2. Perform ? KeyedDestructuringAssignmentEvaluation of AssignmentElement
with arguments value and name.
3. 3. 3. Return « name ».
13.15.5.4 RUNTIME SEMANTICS: RESTDESTRUCTURINGASSIGNMENTEVALUATION
The syntax-directed operation RestDestructuringAssignmentEvaluation takes
arguments value (an ECMAScript language value) and excludedNames (a List of
property keys) and returns either a normal completion containing unused or an
abrupt completion. It is defined piecewise over the following productions:
AssignmentRestProperty : ... DestructuringAssignmentTarget
1. 1. 1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
2. 2. 2. Let restObj be OrdinaryObjectCreate(%Object.prototype%).
3. 3. 3. Perform ? CopyDataProperties(restObj, value, excludedNames).
4. 4. 4. Return ? PutValue(lref, restObj).
13.15.5.5 RUNTIME SEMANTICS: ITERATORDESTRUCTURINGASSIGNMENTEVALUATION
The syntax-directed operation IteratorDestructuringAssignmentEvaluation takes
argument iteratorRecord (an Iterator Record) and returns either a normal
completion containing unused or an abrupt completion. It is defined piecewise
over the following productions:
AssignmentElementList : AssignmentElisionElement
1. 1. 1. Return ? IteratorDestructuringAssignmentEvaluation of
AssignmentElisionElement with argument iteratorRecord.
AssignmentElementList : AssignmentElementList , AssignmentElisionElement
1. 1. 1. Perform ? IteratorDestructuringAssignmentEvaluation of
AssignmentElementList with argument iteratorRecord.
2. 2. 2. Return ? IteratorDestructuringAssignmentEvaluation of
AssignmentElisionElement with argument iteratorRecord.
AssignmentElisionElement : AssignmentElement
1. 1. 1. Return ? IteratorDestructuringAssignmentEvaluation of
AssignmentElement with argument iteratorRecord.
AssignmentElisionElement : Elision AssignmentElement
1. 1. 1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
2. 2. 2. Return ? IteratorDestructuringAssignmentEvaluation of
AssignmentElement with argument iteratorRecord.
Elision : ,
1. 1. 1. If iteratorRecord.[[Done]] is false, then
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, set iteratorRecord.[[Done]] to true.
2. 2. 2. Return unused.
Elision : Elision ,
1. 1. 1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with
argument iteratorRecord.
2. 2. 2. If iteratorRecord.[[Done]] is false, then
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, set iteratorRecord.[[Done]] to true.
3. 3. 3. Return unused.
AssignmentElement : DestructuringAssignmentTarget Initializeropt
1. 1. 1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an
ArrayLiteral, then
1. a. a. Let lref be ? Evaluation of DestructuringAssignmentTarget.
2. 2. 2. If iteratorRecord.[[Done]] is false, then
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, set iteratorRecord.[[Done]] to true.
5. e. e. Else,
1. i. i. Let value be Completion(IteratorValue(next)).
2. ii. ii. If value is an abrupt completion, set iteratorRecord.[[Done]]
to true.
3. iii. iii. ReturnIfAbrupt(value).
3. 3. 3. If iteratorRecord.[[Done]] is true, let value be undefined.
4. 4. 4. If Initializer is present and value is undefined, then
1. a. a. If IsAnonymousFunctionDefinition(Initializer) is true and
IsIdentifierRef of DestructuringAssignmentTarget is true, then
1. i. i. Let v be ? NamedEvaluation of Initializer with argument
lref.[[ReferencedName]].
2. b. b. Else,
1. i. i. Let defaultValue be ? Evaluation of Initializer.
2. ii. ii. Let v be ? GetValue(defaultValue).
5. 5. 5. Else, let v be value.
6. 6. 6. If DestructuringAssignmentTarget is either an ObjectLiteral or an
ArrayLiteral, then
1. a. a. Let nestedAssignmentPattern be the AssignmentPattern that is
covered by DestructuringAssignmentTarget.
2. b. b. Return ? DestructuringAssignmentEvaluation of
nestedAssignmentPattern with argument v.
7. 7. 7. Return ? PutValue(lref, v).
Note
Left to right evaluation order is maintained by evaluating a
DestructuringAssignmentTarget that is not a destructuring pattern prior to
accessing the iterator or evaluating the Initializer.
AssignmentRestElement : ... DestructuringAssignmentTarget
1. 1. 1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an
ArrayLiteral, then
1. a. a. Let lref be ? Evaluation of DestructuringAssignmentTarget.
2. 2. 2. Let A be ! ArrayCreate(0).
3. 3. 3. Let n be 0.
4. 4. 4. Repeat, while iteratorRecord.[[Done]] is false,
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, set iteratorRecord.[[Done]] to true.
5. e. e. Else,
1. i. i. Let nextValue be Completion(IteratorValue(next)).
2. ii. ii. If nextValue is an abrupt completion, set
iteratorRecord.[[Done]] to true.
3. iii. iii. ReturnIfAbrupt(nextValue).
4. iv. iv. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
nextValue).
5. v. v. Set n to n + 1.
5. 5. 5. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an
ArrayLiteral, then
1. a. a. Return ? PutValue(lref, A).
6. 6. 6. Let nestedAssignmentPattern be the AssignmentPattern that is covered
by DestructuringAssignmentTarget.
7. 7. 7. Return ? DestructuringAssignmentEvaluation of nestedAssignmentPattern
with argument A.
13.15.5.6 RUNTIME SEMANTICS: KEYEDDESTRUCTURINGASSIGNMENTEVALUATION
The syntax-directed operation KeyedDestructuringAssignmentEvaluation takes
arguments value (an ECMAScript language value) and propertyName (a property key)
and returns either a normal completion containing unused or an abrupt
completion. It is defined piecewise over the following productions:
AssignmentElement : DestructuringAssignmentTarget Initializeropt
1. 1. 1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an
ArrayLiteral, then
1. a. a. Let lref be ? Evaluation of DestructuringAssignmentTarget.
2. 2. 2. Let v be ? GetV(value, propertyName).
3. 3. 3. If Initializer is present and v is undefined, then
1. a. a. If IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef
of DestructuringAssignmentTarget are both true, then
1. i. i. Let rhsValue be ? NamedEvaluation of Initializer with argument
lref.[[ReferencedName]].
2. b. b. Else,
1. i. i. Let defaultValue be ? Evaluation of Initializer.
2. ii. ii. Let rhsValue be ? GetValue(defaultValue).
4. 4. 4. Else, let rhsValue be v.
5. 5. 5. If DestructuringAssignmentTarget is either an ObjectLiteral or an
ArrayLiteral, then
1. a. a. Let assignmentPattern be the AssignmentPattern that is covered by
DestructuringAssignmentTarget.
2. b. b. Return ? DestructuringAssignmentEvaluation of assignmentPattern
with argument rhsValue.
6. 6. 6. Return ? PutValue(lref, rhsValue).
13.16 COMMA OPERATOR ( , )
SYNTAX
Expression[In, Yield, Await] : AssignmentExpression[?In, ?Yield, ?Await]
Expression[?In, ?Yield, ?Await] , AssignmentExpression[?In, ?Yield, ?Await]
13.16.1 RUNTIME SEMANTICS: EVALUATION
Expression : Expression , AssignmentExpression
1. 1. 1. Let lref be ? Evaluation of Expression.
2. 2. 2. Perform ? GetValue(lref).
3. 3. 3. Let rref be ? Evaluation of AssignmentExpression.
4. 4. 4. Return ? GetValue(rref).
Note
GetValue must be called even though its value is not used because it may have
observable side-effects.
14 ECMASCRIPT LANGUAGE: STATEMENTS AND DECLARATIONS
SYNTAX
Statement[Yield, Await, Return] : BlockStatement[?Yield, ?Await, ?Return]
VariableStatement[?Yield, ?Await] EmptyStatement ExpressionStatement[?Yield,
?Await] IfStatement[?Yield, ?Await, ?Return] BreakableStatement[?Yield, ?Await,
?Return] ContinueStatement[?Yield, ?Await] BreakStatement[?Yield, ?Await]
[+Return] ReturnStatement[?Yield, ?Await] WithStatement[?Yield, ?Await, ?Return]
LabelledStatement[?Yield, ?Await, ?Return] ThrowStatement[?Yield, ?Await]
TryStatement[?Yield, ?Await, ?Return] DebuggerStatement Declaration[Yield,
Await] : HoistableDeclaration[?Yield, ?Await, ~Default] ClassDeclaration[?Yield,
?Await, ~Default] LexicalDeclaration[+In, ?Yield, ?Await]
HoistableDeclaration[Yield, Await, Default] : FunctionDeclaration[?Yield,
?Await, ?Default] GeneratorDeclaration[?Yield, ?Await, ?Default]
AsyncFunctionDeclaration[?Yield, ?Await, ?Default]
AsyncGeneratorDeclaration[?Yield, ?Await, ?Default] BreakableStatement[Yield,
Await, Return] : IterationStatement[?Yield, ?Await, ?Return]
SwitchStatement[?Yield, ?Await, ?Return]
14.1 STATEMENT SEMANTICS
14.1.1 RUNTIME SEMANTICS: EVALUATION
HoistableDeclaration : GeneratorDeclaration AsyncFunctionDeclaration
AsyncGeneratorDeclaration
1. 1. 1. Return empty.
HoistableDeclaration : FunctionDeclaration
1. 1. 1. Return ? Evaluation of FunctionDeclaration.
BreakableStatement : IterationStatement SwitchStatement
1. 1. 1. Let newLabelSet be a new empty List.
2. 2. 2. Return ? LabelledEvaluation of this BreakableStatement with argument
newLabelSet.
14.2 BLOCK
SYNTAX
BlockStatement[Yield, Await, Return] : Block[?Yield, ?Await, ?Return]
Block[Yield, Await, Return] : { StatementList[?Yield, ?Await, ?Return]opt }
StatementList[Yield, Await, Return] : StatementListItem[?Yield, ?Await, ?Return]
StatementList[?Yield, ?Await, ?Return] StatementListItem[?Yield, ?Await,
?Return] StatementListItem[Yield, Await, Return] : Statement[?Yield, ?Await,
?Return] Declaration[?Yield, ?Await]
14.2.1 STATIC SEMANTICS: EARLY ERRORS
Block : { StatementList }
* It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains
any duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
StatementList also occurs in the VarDeclaredNames of StatementList.
14.2.2 RUNTIME SEMANTICS: EVALUATION
Block : { }
1. 1. 1. Return empty.
Block : { StatementList }
1. 1. 1. Let oldEnv be the running execution context's LexicalEnvironment.
2. 2. 2. Let blockEnv be NewDeclarativeEnvironment(oldEnv).
3. 3. 3. Perform BlockDeclarationInstantiation(StatementList, blockEnv).
4. 4. 4. Set the running execution context's LexicalEnvironment to blockEnv.
5. 5. 5. Let blockValue be Completion(Evaluation of StatementList).
6. 6. 6. Set the running execution context's LexicalEnvironment to oldEnv.
7. 7. 7. Return ? blockValue.
Note 1
No matter how control leaves the Block the LexicalEnvironment is always restored
to its former state.
StatementList : StatementList StatementListItem
1. 1. 1. Let sl be ? Evaluation of StatementList.
2. 2. 2. Let s be Completion(Evaluation of StatementListItem).
3. 3. 3. Return ? UpdateEmpty(s, sl).
Note 2
The value of a StatementList is the value of the last value-producing item in
the StatementList. For example, the following calls to the eval function all
return the value 1:
eval("1;;;;;")
eval("1;{}")
eval("1;var a;")
14.2.3 BLOCKDECLARATIONINSTANTIATION ( CODE, ENV )
The abstract operation BlockDeclarationInstantiation takes arguments code (a
Parse Node) and env (a Declarative Environment Record) and returns unused. code
is the Parse Node corresponding to the body of the block. env is the Environment
Record in which bindings are to be created.
Note
When a Block or CaseBlock is evaluated a new Declarative Environment Record is
created and bindings for each block scoped variable, constant, function, or
class declared in the block are instantiated in the Environment Record.
It performs the following steps when called:
1. 1. 1. Let declarations be the LexicallyScopedDeclarations of code.
2. 2. 2. Let privateEnv be the running execution context's PrivateEnvironment.
3. 3. 3. For each element d of declarations, do
1. a. a. For each element dn of the BoundNames of d, do
1. i. i. If IsConstantDeclaration of d is true, then
1. 1. 1. Perform ! env.CreateImmutableBinding(dn, true).
2. ii. ii. Else,
1. 1. 1. Perform ! env.CreateMutableBinding(dn, false). NOTE: This
step is replaced in section B.3.2.6.
2. b. b. If d is either a FunctionDeclaration, a GeneratorDeclaration, an
AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
1. i. i. Let fn be the sole element of the BoundNames of d.
2. ii. ii. Let fo be InstantiateFunctionObject of d with arguments env
and privateEnv.
3. iii. iii. Perform ! env.InitializeBinding(fn, fo). NOTE: This step is
replaced in section B.3.2.6.
4. 4. 4. Return unused.
14.3 DECLARATIONS AND THE VARIABLE STATEMENT
14.3.1 LET AND CONST DECLARATIONS
Note
let and const declarations define variables that are scoped to the running
execution context's LexicalEnvironment. The variables are created when their
containing Environment Record is instantiated but may not be accessed in any way
until the variable's LexicalBinding is evaluated. A variable defined by a
LexicalBinding with an Initializer is assigned the value of its Initializer's
AssignmentExpression when the LexicalBinding is evaluated, not when the variable
is created. If a LexicalBinding in a let declaration does not have an
Initializer the variable is assigned the value undefined when the LexicalBinding
is evaluated.
SYNTAX
LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield,
?Await] ; LetOrConst : let const BindingList[In, Yield, Await] :
LexicalBinding[?In, ?Yield, ?Await] BindingList[?In, ?Yield, ?Await] ,
LexicalBinding[?In, ?Yield, ?Await] LexicalBinding[In, Yield, Await] :
BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt
BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]
14.3.1.1 STATIC SEMANTICS: EARLY ERRORS
LexicalDeclaration : LetOrConst BindingList ;
* It is a Syntax Error if the BoundNames of BindingList contains "let".
* It is a Syntax Error if the BoundNames of BindingList contains any duplicate
entries.
LexicalBinding : BindingIdentifier Initializeropt
* It is a Syntax Error if Initializer is not present and IsConstantDeclaration
of the LexicalDeclaration containing this LexicalBinding is true.
14.3.1.2 RUNTIME SEMANTICS: EVALUATION
LexicalDeclaration : LetOrConst BindingList ;
1. 1. 1. Perform ? Evaluation of BindingList.
2. 2. 2. Return empty.
BindingList : BindingList , LexicalBinding
1. 1. 1. Perform ? Evaluation of BindingList.
2. 2. 2. Return ? Evaluation of LexicalBinding.
LexicalBinding : BindingIdentifier
1. 1. 1. Let lhs be ! ResolveBinding(StringValue of BindingIdentifier).
2. 2. 2. Perform ! InitializeReferencedBinding(lhs, undefined).
3. 3. 3. Return empty.
Note
A static semantics rule ensures that this form of LexicalBinding never occurs in
a const declaration.
LexicalBinding : BindingIdentifier Initializer
1. 1. 1. Let bindingId be StringValue of BindingIdentifier.
2. 2. 2. Let lhs be ! ResolveBinding(bindingId).
3. 3. 3. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. a. a. Let value be ? NamedEvaluation of Initializer with argument
bindingId.
4. 4. 4. Else,
1. a. a. Let rhs be ? Evaluation of Initializer.
2. b. b. Let value be ? GetValue(rhs).
5. 5. 5. Perform ! InitializeReferencedBinding(lhs, value).
6. 6. 6. Return empty.
LexicalBinding : BindingPattern Initializer
1. 1. 1. Let rhs be ? Evaluation of Initializer.
2. 2. 2. Let value be ? GetValue(rhs).
3. 3. 3. Let env be the running execution context's LexicalEnvironment.
4. 4. 4. Return ? BindingInitialization of BindingPattern with arguments value
and env.
14.3.2 VARIABLE STATEMENT
Note
A var statement declares variables that are scoped to the running execution
context's VariableEnvironment. Var variables are created when their containing
Environment Record is instantiated and are initialized to undefined when
created. Within the scope of any VariableEnvironment a common BindingIdentifier
may appear in more than one VariableDeclaration but those declarations
collectively define only one variable. A variable defined by a
VariableDeclaration with an Initializer is assigned the value of its
Initializer's AssignmentExpression when the VariableDeclaration is executed, not
when the variable is created.
SYNTAX
VariableStatement[Yield, Await] : var VariableDeclarationList[+In, ?Yield,
?Await] ; VariableDeclarationList[In, Yield, Await] : VariableDeclaration[?In,
?Yield, ?Await] VariableDeclarationList[?In, ?Yield, ?Await] ,
VariableDeclaration[?In, ?Yield, ?Await] VariableDeclaration[In, Yield, Await] :
BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt
BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]
14.3.2.1 RUNTIME SEMANTICS: EVALUATION
VariableStatement : var VariableDeclarationList ;
1. 1. 1. Perform ? Evaluation of VariableDeclarationList.
2. 2. 2. Return empty.
VariableDeclarationList : VariableDeclarationList , VariableDeclaration
1. 1. 1. Perform ? Evaluation of VariableDeclarationList.
2. 2. 2. Return ? Evaluation of VariableDeclaration.
VariableDeclaration : BindingIdentifier
1. 1. 1. Return empty.
VariableDeclaration : BindingIdentifier Initializer
1. 1. 1. Let bindingId be StringValue of BindingIdentifier.
2. 2. 2. Let lhs be ? ResolveBinding(bindingId).
3. 3. 3. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. a. a. Let value be ? NamedEvaluation of Initializer with argument
bindingId.
4. 4. 4. Else,
1. a. a. Let rhs be ? Evaluation of Initializer.
2. b. b. Let value be ? GetValue(rhs).
5. 5. 5. Perform ? PutValue(lhs, value).
6. 6. 6. Return empty.
Note
If a VariableDeclaration is nested within a with statement and the
BindingIdentifier in the VariableDeclaration is the same as a property name of
the binding object of the with statement's Object Environment Record, then step
5 will assign value to the property instead of assigning to the
VariableEnvironment binding of the Identifier.
VariableDeclaration : BindingPattern Initializer
1. 1. 1. Let rhs be ? Evaluation of Initializer.
2. 2. 2. Let rval be ? GetValue(rhs).
3. 3. 3. Return ? BindingInitialization of BindingPattern with arguments rval
and undefined.
14.3.3 DESTRUCTURING BINDING PATTERNS
SYNTAX
BindingPattern[Yield, Await] : ObjectBindingPattern[?Yield, ?Await]
ArrayBindingPattern[?Yield, ?Await] ObjectBindingPattern[Yield, Await] : { } {
BindingRestProperty[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] } {
BindingPropertyList[?Yield, ?Await] , BindingRestProperty[?Yield, ?Await]opt }
ArrayBindingPattern[Yield, Await] : [ Elisionopt BindingRestElement[?Yield,
?Await]opt ] [ BindingElementList[?Yield, ?Await] ] [ BindingElementList[?Yield,
?Await] , Elisionopt BindingRestElement[?Yield, ?Await]opt ]
BindingRestProperty[Yield, Await] : ... BindingIdentifier[?Yield, ?Await]
BindingPropertyList[Yield, Await] : BindingProperty[?Yield, ?Await]
BindingPropertyList[?Yield, ?Await] , BindingProperty[?Yield, ?Await]
BindingElementList[Yield, Await] : BindingElisionElement[?Yield, ?Await]
BindingElementList[?Yield, ?Await] , BindingElisionElement[?Yield, ?Await]
BindingElisionElement[Yield, Await] : Elisionopt BindingElement[?Yield, ?Await]
BindingProperty[Yield, Await] : SingleNameBinding[?Yield, ?Await]
PropertyName[?Yield, ?Await] : BindingElement[?Yield, ?Await]
BindingElement[Yield, Await] : SingleNameBinding[?Yield, ?Await]
BindingPattern[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt
SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await]
Initializer[+In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ...
BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await]
14.3.3.1 RUNTIME SEMANTICS: PROPERTYBINDINGINITIALIZATION
The syntax-directed operation PropertyBindingInitialization takes arguments
value (an ECMAScript language value) and environment (an Environment Record or
undefined) and returns either a normal completion containing a List of property
keys or an abrupt completion. It collects a list of all bound property names. It
is defined piecewise over the following productions:
BindingPropertyList : BindingPropertyList , BindingProperty
1. 1. 1. Let boundNames be ? PropertyBindingInitialization of
BindingPropertyList with arguments value and environment.
2. 2. 2. Let nextNames be ? PropertyBindingInitialization of BindingProperty
with arguments value and environment.
3. 3. 3. Return the list-concatenation of boundNames and nextNames.
BindingProperty : SingleNameBinding
1. 1. 1. Let name be the sole element of the BoundNames of SingleNameBinding.
2. 2. 2. Perform ? KeyedBindingInitialization of SingleNameBinding with
arguments value, environment, and name.
3. 3. 3. Return « name ».
BindingProperty : PropertyName : BindingElement
1. 1. 1. Let P be ? Evaluation of PropertyName.
2. 2. 2. Perform ? KeyedBindingInitialization of BindingElement with arguments
value, environment, and P.
3. 3. 3. Return « P ».
14.3.3.2 RUNTIME SEMANTICS: RESTBINDINGINITIALIZATION
The syntax-directed operation RestBindingInitialization takes arguments value
(an ECMAScript language value), environment (an Environment Record or
undefined), and excludedNames (a List of property keys) and returns either a
normal completion containing unused or an abrupt completion. It is defined
piecewise over the following productions:
BindingRestProperty : ... BindingIdentifier
1. 1. 1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier,
environment).
2. 2. 2. Let restObj be OrdinaryObjectCreate(%Object.prototype%).
3. 3. 3. Perform ? CopyDataProperties(restObj, value, excludedNames).
4. 4. 4. If environment is undefined, return ? PutValue(lhs, restObj).
5. 5. 5. Return ? InitializeReferencedBinding(lhs, restObj).
14.3.3.3 RUNTIME SEMANTICS: KEYEDBINDINGINITIALIZATION
The syntax-directed operation KeyedBindingInitialization takes arguments value
(an ECMAScript language value), environment (an Environment Record or
undefined), and propertyName (a property key) and returns either a normal
completion containing unused or an abrupt completion.
Note
When undefined is passed for environment it indicates that a PutValue operation
should be used to assign the initialization value. This is the case for formal
parameter lists of non-strict functions. In that case the formal parameter
bindings are preinitialized in order to deal with the possibility of multiple
parameters with the same name.
It is defined piecewise over the following productions:
BindingElement : BindingPattern Initializeropt
1. 1. 1. Let v be ? GetV(value, propertyName).
2. 2. 2. If Initializer is present and v is undefined, then
1. a. a. Let defaultValue be ? Evaluation of Initializer.
2. b. b. Set v to ? GetValue(defaultValue).
3. 3. 3. Return ? BindingInitialization of BindingPattern with arguments v and
environment.
SingleNameBinding : BindingIdentifier Initializeropt
1. 1. 1. Let bindingId be StringValue of BindingIdentifier.
2. 2. 2. Let lhs be ? ResolveBinding(bindingId, environment).
3. 3. 3. Let v be ? GetV(value, propertyName).
4. 4. 4. If Initializer is present and v is undefined, then
1. a. a. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. i. i. Set v to ? NamedEvaluation of Initializer with argument
bindingId.
2. b. b. Else,
1. i. i. Let defaultValue be ? Evaluation of Initializer.
2. ii. ii. Set v to ? GetValue(defaultValue).
5. 5. 5. If environment is undefined, return ? PutValue(lhs, v).
6. 6. 6. Return ? InitializeReferencedBinding(lhs, v).
14.4 EMPTY STATEMENT
SYNTAX
EmptyStatement : ;
14.4.1 RUNTIME SEMANTICS: EVALUATION
EmptyStatement : ;
1. 1. 1. Return empty.
14.5 EXPRESSION STATEMENT
SYNTAX
ExpressionStatement[Yield, Await] : [lookahead ∉ { {, function, async [no
LineTerminator here] function, class, let [ }] Expression[+In, ?Yield, ?Await] ;
Note
An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because
that might make it ambiguous with a Block. An ExpressionStatement cannot start
with the function or class keywords because that would make it ambiguous with a
FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An
ExpressionStatement cannot start with async function because that would make it
ambiguous with an AsyncFunctionDeclaration or a AsyncGeneratorDeclaration. An
ExpressionStatement cannot start with the two token sequence let [ because that
would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding
was an ArrayBindingPattern.
14.5.1 RUNTIME SEMANTICS: EVALUATION
ExpressionStatement : Expression ;
1. 1. 1. Let exprRef be ? Evaluation of Expression.
2. 2. 2. Return ? GetValue(exprRef).
14.6 THE IF STATEMENT
SYNTAX
IfStatement[Yield, Await, Return] : if ( Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] else Statement[?Yield, ?Await, ?Return] if (
Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [lookahead
≠ else] Note
The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling
else" problem in the usual way. That is, when the choice of associated if is
otherwise ambiguous, the else is associated with the nearest (innermost) of the
candidate ifs
14.6.1 STATIC SEMANTICS: EARLY ERRORS
IfStatement : if ( Expression ) Statement else Statement
* It is a Syntax Error if IsLabelledFunction(the first Statement) is true.
* It is a Syntax Error if IsLabelledFunction(the second Statement) is true.
IfStatement : if ( Expression ) Statement
* It is a Syntax Error if IsLabelledFunction(Statement) is true.
Note
It is only necessary to apply this rule if the extension specified in B.3.1 is
implemented.
14.6.2 RUNTIME SEMANTICS: EVALUATION
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let exprRef be ? Evaluation of Expression.
2. 2. 2. Let exprValue be ToBoolean(? GetValue(exprRef)).
3. 3. 3. If exprValue is true, then
1. a. a. Let stmtCompletion be Completion(Evaluation of the first
Statement).
4. 4. 4. Else,
1. a. a. Let stmtCompletion be Completion(Evaluation of the second
Statement).
5. 5. 5. Return ? UpdateEmpty(stmtCompletion, undefined).
IfStatement : if ( Expression ) Statement
1. 1. 1. Let exprRef be ? Evaluation of Expression.
2. 2. 2. Let exprValue be ToBoolean(? GetValue(exprRef)).
3. 3. 3. If exprValue is false, then
1. a. a. Return undefined.
4. 4. 4. Else,
1. a. a. Let stmtCompletion be Completion(Evaluation of Statement).
2. b. b. Return ? UpdateEmpty(stmtCompletion, undefined).
14.7 ITERATION STATEMENTS
SYNTAX
IterationStatement[Yield, Await, Return] : DoWhileStatement[?Yield, ?Await,
?Return] WhileStatement[?Yield, ?Await, ?Return] ForStatement[?Yield, ?Await,
?Return] ForInOfStatement[?Yield, ?Await, ?Return]
14.7.1 SEMANTICS
14.7.1.1 LOOPCONTINUES ( COMPLETION, LABELSET )
The abstract operation LoopContinues takes arguments completion (a Completion
Record) and labelSet (a List of Strings) and returns a Boolean. It performs the
following steps when called:
1. 1. 1. If completion.[[Type]] is normal, return true.
2. 2. 2. If completion.[[Type]] is not continue, return false.
3. 3. 3. If completion.[[Target]] is empty, return true.
4. 4. 4. If labelSet contains completion.[[Target]], return true.
5. 5. 5. Return false.
Note
Within the Statement part of an IterationStatement a ContinueStatement may be
used to begin a new iteration.
14.7.1.2 RUNTIME SEMANTICS: LOOPEVALUATION
The syntax-directed operation LoopEvaluation takes argument labelSet (a List of
Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
IterationStatement : DoWhileStatement
1. 1. 1. Return ? DoWhileLoopEvaluation of DoWhileStatement with argument
labelSet.
IterationStatement : WhileStatement
1. 1. 1. Return ? WhileLoopEvaluation of WhileStatement with argument labelSet.
IterationStatement : ForStatement
1. 1. 1. Return ? ForLoopEvaluation of ForStatement with argument labelSet.
IterationStatement : ForInOfStatement
1. 1. 1. Return ? ForInOfLoopEvaluation of ForInOfStatement with argument
labelSet.
14.7.2 THE DO-WHILE STATEMENT
SYNTAX
DoWhileStatement[Yield, Await, Return] : do Statement[?Yield, ?Await, ?Return]
while ( Expression[+In, ?Yield, ?Await] ) ;
14.7.2.1 STATIC SEMANTICS: EARLY ERRORS
DoWhileStatement : do Statement while ( Expression ) ;
* It is a Syntax Error if IsLabelledFunction(Statement) is true.
Note
It is only necessary to apply this rule if the extension specified in B.3.1 is
implemented.
14.7.2.2 RUNTIME SEMANTICS: DOWHILELOOPEVALUATION
The syntax-directed operation DoWhileLoopEvaluation takes argument labelSet (a
List of Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
DoWhileStatement : do Statement while ( Expression ) ;
1. 1. 1. Let V be undefined.
2. 2. 2. Repeat,
1. a. a. Let stmtResult be Completion(Evaluation of Statement).
2. b. b. If LoopContinues(stmtResult, labelSet) is false, return
? UpdateEmpty(stmtResult, V).
3. c. c. If stmtResult.[[Value]] is not empty, set V to
stmtResult.[[Value]].
4. d. d. Let exprRef be ? Evaluation of Expression.
5. e. e. Let exprValue be ? GetValue(exprRef).
6. f. f. If ToBoolean(exprValue) is false, return V.
14.7.3 THE WHILE STATEMENT
SYNTAX
WhileStatement[Yield, Await, Return] : while ( Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return]
14.7.3.1 STATIC SEMANTICS: EARLY ERRORS
WhileStatement : while ( Expression ) Statement
* It is a Syntax Error if IsLabelledFunction(Statement) is true.
Note
It is only necessary to apply this rule if the extension specified in B.3.1 is
implemented.
14.7.3.2 RUNTIME SEMANTICS: WHILELOOPEVALUATION
The syntax-directed operation WhileLoopEvaluation takes argument labelSet (a
List of Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
WhileStatement : while ( Expression ) Statement
1. 1. 1. Let V be undefined.
2. 2. 2. Repeat,
1. a. a. Let exprRef be ? Evaluation of Expression.
2. b. b. Let exprValue be ? GetValue(exprRef).
3. c. c. If ToBoolean(exprValue) is false, return V.
4. d. d. Let stmtResult be Completion(Evaluation of Statement).
5. e. e. If LoopContinues(stmtResult, labelSet) is false, return
? UpdateEmpty(stmtResult, V).
6. f. f. If stmtResult.[[Value]] is not empty, set V to
stmtResult.[[Value]].
14.7.4 THE FOR STATEMENT
SYNTAX
ForStatement[Yield, Await, Return] : for ( [lookahead ≠ let [] Expression[~In,
?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ; Expression[+In,
?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( var
VariableDeclarationList[~In, ?Yield, ?Await] ; Expression[+In, ?Yield,
?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await,
?Return] for ( LexicalDeclaration[~In, ?Yield, ?Await] Expression[+In, ?Yield,
?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await,
?Return]
14.7.4.1 STATIC SEMANTICS: EARLY ERRORS
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
* It is a Syntax Error if IsLabelledFunction(Statement) is true.
Note
It is only necessary to apply this rule if the extension specified in B.3.1 is
implemented.
ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt )
Statement
* It is a Syntax Error if any element of the BoundNames of LexicalDeclaration
also occurs in the VarDeclaredNames of Statement.
14.7.4.2 RUNTIME SEMANTICS: FORLOOPEVALUATION
The syntax-directed operation ForLoopEvaluation takes argument labelSet (a List
of Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
ForStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
1. 1. 1. If the first Expression is present, then
1. a. a. Let exprRef be ? Evaluation of the first Expression.
2. b. b. Perform ? GetValue(exprRef).
2. 2. 2. If the second Expression is present, let test be the second
Expression; otherwise, let test be empty.
3. 3. 3. If the third Expression is present, let increment be the third
Expression; otherwise, let increment be empty.
4. 4. 4. Return ? ForBodyEvaluation(test, increment, Statement, « », labelSet).
ForStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt
) Statement
1. 1. 1. Perform ? Evaluation of VariableDeclarationList.
2. 2. 2. If the first Expression is present, let test be the first Expression;
otherwise, let test be empty.
3. 3. 3. If the second Expression is present, let increment be the second
Expression; otherwise, let increment be empty.
4. 4. 4. Return ? ForBodyEvaluation(test, increment, Statement, « », labelSet).
ForStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt )
Statement
1. 1. 1. Let oldEnv be the running execution context's LexicalEnvironment.
2. 2. 2. Let loopEnv be NewDeclarativeEnvironment(oldEnv).
3. 3. 3. Let isConst be IsConstantDeclaration of LexicalDeclaration.
4. 4. 4. Let boundNames be the BoundNames of LexicalDeclaration.
5. 5. 5. For each element dn of boundNames, do
1. a. a. If isConst is true, then
1. i. i. Perform ! loopEnv.CreateImmutableBinding(dn, true).
2. b. b. Else,
1. i. i. Perform ! loopEnv.CreateMutableBinding(dn, false).
6. 6. 6. Set the running execution context's LexicalEnvironment to loopEnv.
7. 7. 7. Let forDcl be Completion(Evaluation of LexicalDeclaration).
8. 8. 8. If forDcl is an abrupt completion, then
1. a. a. Set the running execution context's LexicalEnvironment to oldEnv.
2. b. b. Return ? forDcl.
9. 9. 9. If isConst is false, let perIterationLets be boundNames; otherwise
let perIterationLets be a new empty List.
10. 10. 10. If the first Expression is present, let test be the first
Expression; otherwise, let test be empty.
11. 11. 11. If the second Expression is present, let increment be the second
Expression; otherwise, let increment be empty.
12. 12. 12. Let bodyResult be Completion(ForBodyEvaluation(test, increment,
Statement, perIterationLets, labelSet)).
13. 13. 13. Set the running execution context's LexicalEnvironment to oldEnv.
14. 14. 14. Return ? bodyResult.
14.7.4.3 FORBODYEVALUATION ( TEST, INCREMENT, STMT, PERITERATIONBINDINGS,
LABELSET )
The abstract operation ForBodyEvaluation takes arguments test (an Expression
Parse Node or empty), increment (an Expression Parse Node or empty), stmt (a
Statement Parse Node), perIterationBindings (a List of Strings), and labelSet (a
List of Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It performs the following steps when
called:
1. 1. 1. Let V be undefined.
2. 2. 2. Perform ? CreatePerIterationEnvironment(perIterationBindings).
3. 3. 3. Repeat,
1. a. a. If test is not empty, then
1. i. i. Let testRef be ? Evaluation of test.
2. ii. ii. Let testValue be ? GetValue(testRef).
3. iii. iii. If ToBoolean(testValue) is false, return V.
2. b. b. Let result be Completion(Evaluation of stmt).
3. c. c. If LoopContinues(result, labelSet) is false, return
? UpdateEmpty(result, V).
4. d. d. If result.[[Value]] is not empty, set V to result.[[Value]].
5. e. e. Perform ? CreatePerIterationEnvironment(perIterationBindings).
6. f. f. If increment is not empty, then
1. i. i. Let incRef be ? Evaluation of increment.
2. ii. ii. Perform ? GetValue(incRef).
14.7.4.4 CREATEPERITERATIONENVIRONMENT ( PERITERATIONBINDINGS )
The abstract operation CreatePerIterationEnvironment takes argument
perIterationBindings (a List of Strings) and returns either a normal completion
containing unused or a throw completion. It performs the following steps when
called:
1. 1. 1. If perIterationBindings has any elements, then
1. a. a. Let lastIterationEnv be the running execution context's
LexicalEnvironment.
2. b. b. Let outer be lastIterationEnv.[[OuterEnv]].
3. c. c. Assert: outer is not null.
4. d. d. Let thisIterationEnv be NewDeclarativeEnvironment(outer).
5. e. e. For each element bn of perIterationBindings, do
1. i. i. Perform ! thisIterationEnv.CreateMutableBinding(bn, false).
2. ii. ii. Let lastValue be ? lastIterationEnv.GetBindingValue(bn, true).
3. iii. iii. Perform ! thisIterationEnv.InitializeBinding(bn, lastValue).
6. f. f. Set the running execution context's LexicalEnvironment to
thisIterationEnv.
2. 2. 2. Return unused.
14.7.5 THE FOR-IN, FOR-OF, AND FOR-AWAIT-OF STATEMENTS
SYNTAX
ForInOfStatement[Yield, Await, Return] : for ( [lookahead ≠ let []
LeftHandSideExpression[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] in
Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for (
ForDeclaration[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] for ( [lookahead ∉ { let, async of }]
LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield,
?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield,
?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await,
?Return] for ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[+In,
?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [+Await] for await (
[lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of
AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
[+Await] for await ( var ForBinding[?Yield, ?Await] of AssignmentExpression[+In,
?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [+Await] for await (
ForDeclaration[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] ForDeclaration[Yield, Await] : LetOrConst
ForBinding[?Yield, ?Await] ForBinding[Yield, Await] : BindingIdentifier[?Yield,
?Await] BindingPattern[?Yield, ?Await] Note
This section is extended by Annex B.3.5.
14.7.5.1 STATIC SEMANTICS: EARLY ERRORS
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
var ForBinding in Expression ) Statement for ( ForDeclaration in Expression )
Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for (
var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of
AssignmentExpression ) Statement for await ( LeftHandSideExpression of
AssignmentExpression ) Statement for await ( var ForBinding of
AssignmentExpression ) Statement for await ( ForDeclaration of
AssignmentExpression ) Statement
* It is a Syntax Error if IsLabelledFunction(Statement) is true.
Note
It is only necessary to apply this rule if the extension specified in B.3.1 is
implemented.
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement for (
LeftHandSideExpression of AssignmentExpression ) Statement for await (
LeftHandSideExpression of AssignmentExpression ) Statement
If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the
following Early Error rules are applied:
* LeftHandSideExpression must cover an AssignmentPattern.
If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the
following Early Error rule is applied:
* It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not
simple.
ForInOfStatement : for ( ForDeclaration in Expression ) Statement for (
ForDeclaration of AssignmentExpression ) Statement for await ( ForDeclaration of
AssignmentExpression ) Statement
* It is a Syntax Error if the BoundNames of ForDeclaration contains "let".
* It is a Syntax Error if any element of the BoundNames of ForDeclaration also
occurs in the VarDeclaredNames of Statement.
* It is a Syntax Error if the BoundNames of ForDeclaration contains any
duplicate entries.
14.7.5.2 STATIC SEMANTICS: ISDESTRUCTURING
The syntax-directed operation IsDestructuring takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
MemberExpression : PrimaryExpression
1. 1. 1. If PrimaryExpression is either an ObjectLiteral or an ArrayLiteral,
return true.
2. 2. 2. Return false.
MemberExpression : MemberExpression [ Expression ] MemberExpression .
IdentifierName MemberExpression TemplateLiteral SuperProperty MetaProperty new
MemberExpression Arguments MemberExpression . PrivateIdentifier NewExpression :
new NewExpression LeftHandSideExpression : CallExpression OptionalExpression
1. 1. 1. Return false.
ForDeclaration : LetOrConst ForBinding
1. 1. 1. Return IsDestructuring of ForBinding.
ForBinding : BindingIdentifier
1. 1. 1. Return false.
ForBinding : BindingPattern
1. 1. 1. Return true.
Note
This section is extended by Annex B.3.5.
14.7.5.3 RUNTIME SEMANTICS: FORDECLARATIONBINDINGINITIALIZATION
The syntax-directed operation ForDeclarationBindingInitialization takes
arguments value (an ECMAScript language value) and environment (an Environment
Record or undefined) and returns either a normal completion containing unused or
an abrupt completion.
Note
undefined is passed for environment to indicate that a PutValue operation should
be used to assign the initialization value. This is the case for var statements
and the formal parameter lists of some non-strict functions (see 10.2.11). In
those cases a lexical binding is hoisted and preinitialized prior to evaluation
of its initializer.
It is defined piecewise over the following productions:
ForDeclaration : LetOrConst ForBinding
1. 1. 1. Return ? BindingInitialization of ForBinding with arguments value and
environment.
14.7.5.4 RUNTIME SEMANTICS: FORDECLARATIONBINDINGINSTANTIATION
The syntax-directed operation ForDeclarationBindingInstantiation takes argument
environment (a Declarative Environment Record) and returns unused. It is defined
piecewise over the following productions:
ForDeclaration : LetOrConst ForBinding
1. 1. 1. For each element name of the BoundNames of ForBinding, do
1. a. a. If IsConstantDeclaration of LetOrConst is true, then
1. i. i. Perform ! environment.CreateImmutableBinding(name, true).
2. b. b. Else,
1. i. i. Perform ! environment.CreateMutableBinding(name, false).
2. 2. 2. Return unused.
14.7.5.5 RUNTIME SEMANTICS: FORINOFLOOPEVALUATION
The syntax-directed operation ForInOfLoopEvaluation takes argument labelSet (a
List of Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
ForInOfStatement : for ( LeftHandSideExpression in Expression ) Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement,
keyResult, enumerate, assignment, labelSet).
ForInOfStatement : for ( var ForBinding in Expression ) Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult,
enumerate, varBinding, labelSet).
ForInOfStatement : for ( ForDeclaration in Expression ) Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of
ForDeclaration, Expression, enumerate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult,
enumerate, lexicalBinding, labelSet).
ForInOfStatement : for ( LeftHandSideExpression of AssignmentExpression )
Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression,
iterate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement,
keyResult, iterate, assignment, labelSet).
ForInOfStatement : for ( var ForBinding of AssignmentExpression ) Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression,
iterate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult,
iterate, varBinding, labelSet).
ForInOfStatement : for ( ForDeclaration of AssignmentExpression ) Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of
ForDeclaration, AssignmentExpression, iterate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult,
iterate, lexicalBinding, labelSet).
ForInOfStatement : for await ( LeftHandSideExpression of AssignmentExpression )
Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression,
async-iterate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement,
keyResult, iterate, assignment, labelSet, async).
ForInOfStatement : for await ( var ForBinding of AssignmentExpression )
Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression,
async-iterate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult,
iterate, varBinding, labelSet, async).
ForInOfStatement : for await ( ForDeclaration of AssignmentExpression )
Statement
1. 1. 1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of
ForDeclaration, AssignmentExpression, async-iterate).
2. 2. 2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult,
iterate, lexicalBinding, labelSet, async).
Note
This section is extended by Annex B.3.5.
14.7.5.6 FORIN/OFHEADEVALUATION ( UNINITIALIZEDBOUNDNAMES, EXPR, ITERATIONKIND )
The abstract operation ForIn/OfHeadEvaluation takes arguments
uninitializedBoundNames (a List of Strings), expr (an Expression Parse Node or
an AssignmentExpression Parse Node), and iterationKind (enumerate, iterate, or
async-iterate) and returns either a normal completion containing an Iterator
Record or an abrupt completion. It performs the following steps when called:
1. 1. 1. Let oldEnv be the running execution context's LexicalEnvironment.
2. 2. 2. If uninitializedBoundNames is not empty, then
1. a. a. Assert: uninitializedBoundNames has no duplicate entries.
2. b. b. Let newEnv be NewDeclarativeEnvironment(oldEnv).
3. c. c. For each String name of uninitializedBoundNames, do
1. i. i. Perform ! newEnv.CreateMutableBinding(name, false).
4. d. d. Set the running execution context's LexicalEnvironment to newEnv.
3. 3. 3. Let exprRef be Completion(Evaluation of expr).
4. 4. 4. Set the running execution context's LexicalEnvironment to oldEnv.
5. 5. 5. Let exprValue be ? GetValue(? exprRef).
6. 6. 6. If iterationKind is enumerate, then
1. a. a. If exprValue is either undefined or null, then
1. i. i. Return Completion Record { [[Type]]: break, [[Value]]: empty,
[[Target]]: empty }.
2. b. b. Let obj be ! ToObject(exprValue).
3. c. c. Let iterator be EnumerateObjectProperties(obj).
4. d. d. Let nextMethod be ! GetV(iterator, "next").
5. e. e. Return the Iterator Record { [[Iterator]]: iterator,
[[NextMethod]]: nextMethod, [[Done]]: false }.
7. 7. 7. Else,
1. a. a. Assert: iterationKind is either iterate or async-iterate.
2. b. b. If iterationKind is async-iterate, let iteratorKind be async.
3. c. c. Else, let iteratorKind be sync.
4. d. d. Return ? GetIterator(exprValue, iteratorKind).
14.7.5.7 FORIN/OFBODYEVALUATION ( LHS, STMT, ITERATORRECORD, ITERATIONKIND,
LHSKIND, LABELSET [ , ITERATORKIND ] )
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse
Node), stmt (a Statement Parse Node), iteratorRecord (an Iterator Record),
iterationKind (enumerate or iterate), lhsKind (assignment, varBinding, or
lexicalBinding), and labelSet (a List of Strings) and optional argument
iteratorKind (sync or async) and returns either a normal completion containing
an ECMAScript language value or an abrupt completion. It performs the following
steps when called:
1. 1. 1. If iteratorKind is not present, set iteratorKind to sync.
2. 2. 2. Let oldEnv be the running execution context's LexicalEnvironment.
3. 3. 3. Let V be undefined.
4. 4. 4. Let destructuring be IsDestructuring of lhs.
5. 5. 5. If destructuring is true and lhsKind is assignment, then
1. a. a. Assert: lhs is a LeftHandSideExpression.
2. b. b. Let assignmentPattern be the AssignmentPattern that is covered by
lhs.
6. 6. 6. Repeat,
1. a. a. Let nextResult be ? Call(iteratorRecord.[[NextMethod]],
iteratorRecord.[[Iterator]]).
2. b. b. If iteratorKind is async, set nextResult to ? Await(nextResult).
3. c. c. If nextResult is not an Object, throw a TypeError exception.
4. d. d. Let done be ? IteratorComplete(nextResult).
5. e. e. If done is true, return V.
6. f. f. Let nextValue be ? IteratorValue(nextResult).
7. g. g. If lhsKind is either assignment or varBinding, then
1. i. i. If destructuring is true, then
1. 1. 1. If lhsKind is assignment, then
1. a. a. Let status be
Completion(DestructuringAssignmentEvaluation of
assignmentPattern with argument nextValue).
2. 2. 2. Else,
1. a. a. Assert: lhsKind is varBinding.
2. b. b. Assert: lhs is a ForBinding.
3. c. c. Let status be Completion(BindingInitialization of lhs
with arguments nextValue and undefined).
2. ii. ii. Else,
1. 1. 1. Let lhsRef be Completion(Evaluation of lhs). (It may be
evaluated repeatedly.)
2. 2. 2. If lhsRef is an abrupt completion, then
1. a. a. Let status be lhsRef.
3. 3. 3. Else,
1. a. a. Let status be Completion(PutValue(lhsRef.[[Value]],
nextValue)).
8. h. h. Else,
1. i. i. Assert: lhsKind is lexicalBinding.
2. ii. ii. Assert: lhs is a ForDeclaration.
3. iii. iii. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
4. iv. iv. Perform ForDeclarationBindingInstantiation of lhs with
argument iterationEnv.
5. v. v. Set the running execution context's LexicalEnvironment to
iterationEnv.
6. vi. vi. If destructuring is true, then
1. 1. 1. Let status be Completion(ForDeclarationBindingInitialization
of lhs with arguments nextValue and iterationEnv).
7. vii. vii. Else,
1. 1. 1. Assert: lhs binds a single name.
2. 2. 2. Let lhsName be the sole element of BoundNames of lhs.
3. 3. 3. Let lhsRef be ! ResolveBinding(lhsName).
4. 4. 4. Let status be Completion(InitializeReferencedBinding(lhsRef,
nextValue)).
9. i. i. If status is an abrupt completion, then
1. i. i. Set the running execution context's LexicalEnvironment to
oldEnv.
2. ii. ii. If iteratorKind is async, return
? AsyncIteratorClose(iteratorRecord, status).
3. iii. iii. If iterationKind is enumerate, then
1. 1. 1. Return ? status.
4. iv. iv. Else,
1. 1. 1. Assert: iterationKind is iterate.
2. 2. 2. Return ? IteratorClose(iteratorRecord, status).
10. j. j. Let result be Completion(Evaluation of stmt).
11. k. k. Set the running execution context's LexicalEnvironment to oldEnv.
12. l. l. If LoopContinues(result, labelSet) is false, then
1. i. i. If iterationKind is enumerate, then
1. 1. 1. Return ? UpdateEmpty(result, V).
2. ii. ii. Else,
1. 1. 1. Assert: iterationKind is iterate.
2. 2. 2. Set status to Completion(UpdateEmpty(result, V)).
3. 3. 3. If iteratorKind is async, return
? AsyncIteratorClose(iteratorRecord, status).
4. 4. 4. Return ? IteratorClose(iteratorRecord, status).
13. m. m. If result.[[Value]] is not empty, set V to result.[[Value]].
14.7.5.8 RUNTIME SEMANTICS: EVALUATION
BindingIdentifier : Identifier yield await
1. 1. 1. Let bindingId be StringValue of BindingIdentifier.
2. 2. 2. Return ? ResolveBinding(bindingId).
14.7.5.9 ENUMERATEOBJECTPROPERTIES ( O )
The abstract operation EnumerateObjectProperties takes argument O (an Object)
and returns an Iterator. It performs the following steps when called:
1. 1. 1. Return an Iterator object (27.1.1.2) whose next method iterates over
all the String-valued keys of enumerable properties of O. The iterator
object is never directly accessible to ECMAScript code. The mechanics and
order of enumerating the properties is not specified but must conform to the
rules specified below.
The iterator's throw and return methods are null and are never invoked. The
iterator's next method processes object properties to determine whether the
property key should be returned as an iterator value. Returned property keys do
not include keys that are Symbols. Properties of the target object may be
deleted during enumeration. A property that is deleted before it is processed by
the iterator's next method is ignored. If new properties are added to the target
object during enumeration, the newly added properties are not guaranteed to be
processed in the active enumeration. A property name will be returned by the
iterator's next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties
of its prototype, and the prototype of the prototype, and so on, recursively;
but a property of a prototype is not processed if it has the same name as a
property that has already been processed by the iterator's next method. The
values of [[Enumerable]] attributes are not considered when determining if a
property of a prototype object has already been processed. The enumerable
property names of prototype objects must be obtained by invoking
EnumerateObjectProperties passing the prototype object as the argument.
EnumerateObjectProperties must obtain the own property keys of the target object
by calling its [[OwnPropertyKeys]] internal method. Property attributes of the
target object must be obtained by calling its [[GetOwnProperty]] internal
method.
In addition, if neither O nor any object in its prototype chain is a Proxy
exotic object, Integer-Indexed exotic object, module namespace exotic object, or
implementation provided exotic object, then the iterator must behave as would
the iterator given by CreateForInIterator(O) until one of the following occurs:
* the value of the [[Prototype]] internal slot of O or an object in its
prototype chain changes,
* a property is removed from O or an object in its prototype chain,
* a property is added to an object in O's prototype chain, or
* the value of the [[Enumerable]] attribute of a property of O or an object in
its prototype chain changes.
Note 1
ECMAScript implementations are not required to implement the algorithm in
14.7.5.10.2.1 directly. They may choose any implementation whose behaviour will
not deviate from that algorithm unless one of the constraints in the previous
paragraph is violated.
The following is an informative definition of an ECMAScript generator function
that conforms to these rules:
function* EnumerateObjectProperties(obj) {
const visited = new Set();
for (const key of Reflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey of EnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic objects for which implementations are not required to match
CreateForInIterator was chosen because implementations historically differed in
behaviour for those cases, and agreed in all others.
14.7.5.10 FOR-IN ITERATOR OBJECTS
A For-In Iterator is an object that represents a specific iteration over some
specific object. For-In Iterator objects are never directly accessible to
ECMAScript code; they exist solely to illustrate the behaviour of
EnumerateObjectProperties.
14.7.5.10.1 CREATEFORINITERATOR ( OBJECT )
The abstract operation CreateForInIterator takes argument object (an Object) and
returns a For-In Iterator. It is used to create a For-In Iterator object which
iterates over the own and inherited enumerable string properties of object in a
specific order. It performs the following steps when called:
1. 1. 1. Let iterator be OrdinaryObjectCreate(%ForInIteratorPrototype%, «
[[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »).
2. 2. 2. Set iterator.[[Object]] to object.
3. 3. 3. Set iterator.[[ObjectWasVisited]] to false.
4. 4. 4. Set iterator.[[VisitedKeys]] to a new empty List.
5. 5. 5. Set iterator.[[RemainingKeys]] to a new empty List.
6. 6. 6. Return iterator.
14.7.5.10.2 THE %FORINITERATORPROTOTYPE% OBJECT
The %ForInIteratorPrototype% object:
* has properties that are inherited by all For-In Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* is never directly accessible to ECMAScript code.
* has the following properties:
14.7.5.10.2.1 %FORINITERATORPROTOTYPE%.NEXT ( )
1. 1. 1. Let O be the this value.
2. 2. 2. Assert: O is an Object.
3. 3. 3. Assert: O has all of the internal slots of a For-In Iterator Instance
(14.7.5.10.3).
4. 4. 4. Let object be O.[[Object]].
5. 5. 5. Repeat,
1. a. a. If O.[[ObjectWasVisited]] is false, then
1. i. i. Let keys be ? object.[[OwnPropertyKeys]]().
2. ii. ii. For each element key of keys, do
1. 1. 1. If key is a String, then
1. a. a. Append key to O.[[RemainingKeys]].
3. iii. iii. Set O.[[ObjectWasVisited]] to true.
2. b. b. Repeat, while O.[[RemainingKeys]] is not empty,
1. i. i. Let r be the first element of O.[[RemainingKeys]].
2. ii. ii. Remove the first element from O.[[RemainingKeys]].
3. iii. iii. If there does not exist an element v of O.[[VisitedKeys]]
such that SameValue(r, v) is true, then
1. 1. 1. Let desc be ? object.[[GetOwnProperty]](r).
2. 2. 2. If desc is not undefined, then
1. a. a. Append r to O.[[VisitedKeys]].
2. b. b. If desc.[[Enumerable]] is true, return
CreateIterResultObject(r, false).
3. c. c. Set object to ? object.[[GetPrototypeOf]]().
4. d. d. Set O.[[Object]] to object.
5. e. e. Set O.[[ObjectWasVisited]] to false.
6. f. f. If object is null, return CreateIterResultObject(undefined, true).
14.7.5.10.3 PROPERTIES OF FOR-IN ITERATOR INSTANCES
For-In Iterator instances are ordinary objects that inherit properties from the
%ForInIteratorPrototype% intrinsic object. For-In Iterator instances are
initially created with the internal slots listed in Table 38.
Table 38: Internal Slots of For-In Iterator Instances
Internal Slot Type Description [[Object]] an Object The Object value whose
properties are being iterated. [[ObjectWasVisited]] a Boolean true if the
iterator has invoked [[OwnPropertyKeys]] on [[Object]], false otherwise.
[[VisitedKeys]] a List of Strings The values that have been emitted by this
iterator thus far. [[RemainingKeys]] a List of Strings The values remaining to
be emitted for the current object, before iterating the properties of its
prototype (if its prototype is not null).
14.8 THE CONTINUE STATEMENT
SYNTAX
ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here]
LabelIdentifier[?Yield, ?Await] ;
14.8.1 STATIC SEMANTICS: EARLY ERRORS
ContinueStatement : continue ; continue LabelIdentifier ;
* It is a Syntax Error if this ContinueStatement is not nested, directly or
indirectly (but not crossing function or static initialization block
boundaries), within an IterationStatement.
14.8.2 RUNTIME SEMANTICS: EVALUATION
ContinueStatement : continue ;
1. 1. 1. Return Completion Record { [[Type]]: continue, [[Value]]: empty,
[[Target]]: empty }.
ContinueStatement : continue LabelIdentifier ;
1. 1. 1. Let label be the StringValue of LabelIdentifier.
2. 2. 2. Return Completion Record { [[Type]]: continue, [[Value]]: empty,
[[Target]]: label }.
14.9 THE BREAK STATEMENT
SYNTAX
BreakStatement[Yield, Await] : break ; break [no LineTerminator here]
LabelIdentifier[?Yield, ?Await] ;
14.9.1 STATIC SEMANTICS: EARLY ERRORS
BreakStatement : break ;
* It is a Syntax Error if this BreakStatement is not nested, directly or
indirectly (but not crossing function or static initialization block
boundaries), within an IterationStatement or a SwitchStatement.
14.9.2 RUNTIME SEMANTICS: EVALUATION
BreakStatement : break ;
1. 1. 1. Return Completion Record { [[Type]]: break, [[Value]]: empty,
[[Target]]: empty }.
BreakStatement : break LabelIdentifier ;
1. 1. 1. Let label be the StringValue of LabelIdentifier.
2. 2. 2. Return Completion Record { [[Type]]: break, [[Value]]: empty,
[[Target]]: label }.
14.10 THE RETURN STATEMENT
SYNTAX
ReturnStatement[Yield, Await] : return ; return [no LineTerminator here]
Expression[+In, ?Yield, ?Await] ; Note
A return statement causes a function to cease execution and, in most cases,
returns a value to the caller. If Expression is omitted, the return value is
undefined. Otherwise, the return value is the value of Expression. A return
statement may not actually return a value to the caller depending on surrounding
context. For example, in a try block, a return statement's Completion Record may
be replaced with another Completion Record during evaluation of the finally
block.
14.10.1 RUNTIME SEMANTICS: EVALUATION
ReturnStatement : return ;
1. 1. 1. Return Completion Record { [[Type]]: return, [[Value]]: undefined,
[[Target]]: empty }.
ReturnStatement : return Expression ;
1. 1. 1. Let exprRef be ? Evaluation of Expression.
2. 2. 2. Let exprValue be ? GetValue(exprRef).
3. 3. 3. If GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
4. 4. 4. Return Completion Record { [[Type]]: return, [[Value]]: exprValue,
[[Target]]: empty }.
Legacy
14.11 THE WITH STATEMENT
Note 1
Use of the Legacy with statement is discouraged in new ECMAScript code. Consider
alternatives that are permitted in both strict mode code and non-strict code,
such as destructuring assignment.
SYNTAX
WithStatement[Yield, Await, Return] : with ( Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] Note 2
The with statement adds an Object Environment Record for a computed object to
the lexical environment of the running execution context. It then executes a
statement using this augmented lexical environment. Finally, it restores the
original lexical environment.
14.11.1 STATIC SEMANTICS: EARLY ERRORS
WithStatement : with ( Expression ) Statement
* It is a Syntax Error if the source text matched by this production is
contained in strict mode code.
* It is a Syntax Error if IsLabelledFunction(Statement) is true.
Note
It is only necessary to apply the second rule if the extension specified in
B.3.1 is implemented.
14.11.2 RUNTIME SEMANTICS: EVALUATION
WithStatement : with ( Expression ) Statement
1. 1. 1. Let val be ? Evaluation of Expression.
2. 2. 2. Let obj be ? ToObject(? GetValue(val)).
3. 3. 3. Let oldEnv be the running execution context's LexicalEnvironment.
4. 4. 4. Let newEnv be NewObjectEnvironment(obj, true, oldEnv).
5. 5. 5. Set the running execution context's LexicalEnvironment to newEnv.
6. 6. 6. Let C be Completion(Evaluation of Statement).
7. 7. 7. Set the running execution context's LexicalEnvironment to oldEnv.
8. 8. 8. Return ? UpdateEmpty(C, undefined).
Note
No matter how control leaves the embedded Statement, whether normally or by some
form of abrupt completion or exception, the LexicalEnvironment is always
restored to its former state.
14.12 THE SWITCH STATEMENT
SYNTAX
SwitchStatement[Yield, Await, Return] : switch ( Expression[+In, ?Yield, ?Await]
) CaseBlock[?Yield, ?Await, ?Return] CaseBlock[Yield, Await, Return] : {
CaseClauses[?Yield, ?Await, ?Return]opt } { CaseClauses[?Yield, ?Await,
?Return]opt DefaultClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await,
?Return]opt } CaseClauses[Yield, Await, Return] : CaseClause[?Yield, ?Await,
?Return] CaseClauses[?Yield, ?Await, ?Return] CaseClause[?Yield, ?Await,
?Return] CaseClause[Yield, Await, Return] : case Expression[+In, ?Yield, ?Await]
: StatementList[?Yield, ?Await, ?Return]opt DefaultClause[Yield, Await, Return]
: default : StatementList[?Yield, ?Await, ?Return]opt
14.12.1 STATIC SEMANTICS: EARLY ERRORS
SwitchStatement : switch ( Expression ) CaseBlock
* It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any
duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
CaseBlock also occurs in the VarDeclaredNames of CaseBlock.
14.12.2 RUNTIME SEMANTICS: CASEBLOCKEVALUATION
The syntax-directed operation CaseBlockEvaluation takes argument input (an
ECMAScript language value) and returns either a normal completion containing an
ECMAScript language value or an abrupt completion. It is defined piecewise over
the following productions:
CaseBlock : { }
1. 1. 1. Return undefined.
CaseBlock : { CaseClauses }
1. 1. 1. Let V be undefined.
2. 2. 2. Let A be the List of CaseClause items in CaseClauses, in source text
order.
3. 3. 3. Let found be false.
4. 4. 4. For each CaseClause C of A, do
1. a. a. If found is false, then
1. i. i. Set found to ? CaseClauseIsSelected(C, input).
2. b. b. If found is true, then
1. i. i. Let R be Completion(Evaluation of C).
2. ii. ii. If R.[[Value]] is not empty, set V to R.[[Value]].
3. iii. iii. If R is an abrupt completion, return ? UpdateEmpty(R, V).
5. 5. 5. Return V.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. Let V be undefined.
2. 2. 2. If the first CaseClauses is present, then
1. a. a. Let A be the List of CaseClause items in the first CaseClauses, in
source text order.
3. 3. 3. Else,
1. a. a. Let A be a new empty List.
4. 4. 4. Let found be false.
5. 5. 5. For each CaseClause C of A, do
1. a. a. If found is false, then
1. i. i. Set found to ? CaseClauseIsSelected(C, input).
2. b. b. If found is true, then
1. i. i. Let R be Completion(Evaluation of C).
2. ii. ii. If R.[[Value]] is not empty, set V to R.[[Value]].
3. iii. iii. If R is an abrupt completion, return ? UpdateEmpty(R, V).
6. 6. 6. Let foundInB be false.
7. 7. 7. If the second CaseClauses is present, then
1. a. a. Let B be the List of CaseClause items in the second CaseClauses,
in source text order.
8. 8. 8. Else,
1. a. a. Let B be a new empty List.
9. 9. 9. If found is false, then
1. a. a. For each CaseClause C of B, do
1. i. i. If foundInB is false, then
1. 1. 1. Set foundInB to ? CaseClauseIsSelected(C, input).
2. ii. ii. If foundInB is true, then
1. 1. 1. Let R be Completion(Evaluation of CaseClause C).
2. 2. 2. If R.[[Value]] is not empty, set V to R.[[Value]].
3. 3. 3. If R is an abrupt completion, return ? UpdateEmpty(R, V).
10. 10. 10. If foundInB is true, return V.
11. 11. 11. Let R be Completion(Evaluation of DefaultClause).
12. 12. 12. If R.[[Value]] is not empty, set V to R.[[Value]].
13. 13. 13. If R is an abrupt completion, return ? UpdateEmpty(R, V).
14. 14. 14. NOTE: The following is another complete iteration of the second
CaseClauses.
15. 15. 15. For each CaseClause C of B, do
1. a. a. Let R be Completion(Evaluation of CaseClause C).
2. b. b. If R.[[Value]] is not empty, set V to R.[[Value]].
3. c. c. If R is an abrupt completion, return ? UpdateEmpty(R, V).
16. 16. 16. Return V.
14.12.3 CASECLAUSEISSELECTED ( C, INPUT )
The abstract operation CaseClauseIsSelected takes arguments C (a CaseClause
Parse Node) and input (an ECMAScript language value) and returns either a normal
completion containing a Boolean or an abrupt completion. It determines whether C
matches input. It performs the following steps when called:
1. 1. 1. Assert: C is an instance of the production CaseClause : case
Expression : StatementListopt .
2. 2. 2. Let exprRef be ? Evaluation of the Expression of C.
3. 3. 3. Let clauseSelector be ? GetValue(exprRef).
4. 4. 4. Return IsStrictlyEqual(input, clauseSelector).
Note
This operation does not execute C's StatementList (if any). The CaseBlock
algorithm uses its return value to determine which StatementList to start
executing.
14.12.4 RUNTIME SEMANTICS: EVALUATION
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Let exprRef be ? Evaluation of Expression.
2. 2. 2. Let switchValue be ? GetValue(exprRef).
3. 3. 3. Let oldEnv be the running execution context's LexicalEnvironment.
4. 4. 4. Let blockEnv be NewDeclarativeEnvironment(oldEnv).
5. 5. 5. Perform BlockDeclarationInstantiation(CaseBlock, blockEnv).
6. 6. 6. Set the running execution context's LexicalEnvironment to blockEnv.
7. 7. 7. Let R be Completion(CaseBlockEvaluation of CaseBlock with argument
switchValue).
8. 8. 8. Set the running execution context's LexicalEnvironment to oldEnv.
9. 9. 9. Return R.
Note
No matter how control leaves the SwitchStatement the LexicalEnvironment is
always restored to its former state.
CaseClause : case Expression :
1. 1. 1. Return empty.
CaseClause : case Expression : StatementList
1. 1. 1. Return ? Evaluation of StatementList.
DefaultClause : default :
1. 1. 1. Return empty.
DefaultClause : default : StatementList
1. 1. 1. Return ? Evaluation of StatementList.
14.13 LABELLED STATEMENTS
SYNTAX
LabelledStatement[Yield, Await, Return] : LabelIdentifier[?Yield, ?Await] :
LabelledItem[?Yield, ?Await, ?Return] LabelledItem[Yield, Await, Return] :
Statement[?Yield, ?Await, ?Return] FunctionDeclaration[?Yield, ?Await, ~Default]
Note
A Statement may be prefixed by a label. Labelled statements are only used in
conjunction with labelled break and continue statements. ECMAScript has no goto
statement. A Statement can be part of a LabelledStatement, which itself can be
part of a LabelledStatement, and so on. The labels introduced this way are
collectively referred to as the “current label set” when describing the
semantics of individual statements.
14.13.1 STATIC SEMANTICS: EARLY ERRORS
LabelledItem : FunctionDeclaration
* It is a Syntax Error if any source text is matched by this production.
Note
An alternative definition for this rule is provided in B.3.1.
14.13.2 STATIC SEMANTICS: ISLABELLEDFUNCTION ( STMT )
The abstract operation IsLabelledFunction takes argument stmt (a Statement Parse
Node) and returns a Boolean. It performs the following steps when called:
1. 1. 1. If stmt is not a LabelledStatement, return false.
2. 2. 2. Let item be the LabelledItem of stmt.
3. 3. 3. If item is LabelledItem : FunctionDeclaration , return true.
4. 4. 4. Let subStmt be the Statement of item.
5. 5. 5. Return IsLabelledFunction(subStmt).
14.13.3 RUNTIME SEMANTICS: EVALUATION
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return ? LabelledEvaluation of this LabelledStatement with argument «
».
14.13.4 RUNTIME SEMANTICS: LABELLEDEVALUATION
The syntax-directed operation LabelledEvaluation takes argument labelSet (a List
of Strings) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
BreakableStatement : IterationStatement
1. 1. 1. Let stmtResult be Completion(LoopEvaluation of IterationStatement with
argument labelSet).
2. 2. 2. If stmtResult.[[Type]] is break, then
1. a. a. If stmtResult.[[Target]] is empty, then
1. i. i. If stmtResult.[[Value]] is empty, set stmtResult to
NormalCompletion(undefined).
2. ii. ii. Else, set stmtResult to
NormalCompletion(stmtResult.[[Value]]).
3. 3. 3. Return ? stmtResult.
BreakableStatement : SwitchStatement
1. 1. 1. Let stmtResult be Completion(Evaluation of SwitchStatement).
2. 2. 2. If stmtResult.[[Type]] is break, then
1. a. a. If stmtResult.[[Target]] is empty, then
1. i. i. If stmtResult.[[Value]] is empty, set stmtResult to
NormalCompletion(undefined).
2. ii. ii. Else, set stmtResult to
NormalCompletion(stmtResult.[[Value]]).
3. 3. 3. Return ? stmtResult.
Note 1
A BreakableStatement is one that can be exited via an unlabelled BreakStatement.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Let label be the StringValue of LabelIdentifier.
2. 2. 2. Let newLabelSet be the list-concatenation of labelSet and « label ».
3. 3. 3. Let stmtResult be Completion(LabelledEvaluation of LabelledItem with
argument newLabelSet).
4. 4. 4. If stmtResult.[[Type]] is break and SameValue(stmtResult.[[Target]],
label) is true, then
1. a. a. Set stmtResult to NormalCompletion(stmtResult.[[Value]]).
5. 5. 5. Return ? stmtResult.
LabelledItem : FunctionDeclaration
1. 1. 1. Return ? Evaluation of FunctionDeclaration.
Statement : BlockStatement VariableStatement EmptyStatement ExpressionStatement
IfStatement ContinueStatement BreakStatement ReturnStatement WithStatement
ThrowStatement TryStatement DebuggerStatement
1. 1. 1. Return ? Evaluation of Statement.
Note 2
The only two productions of Statement which have special semantics for
LabelledEvaluation are BreakableStatement and LabelledStatement.
14.14 THE THROW STATEMENT
SYNTAX
ThrowStatement[Yield, Await] : throw [no LineTerminator here] Expression[+In,
?Yield, ?Await] ;
14.14.1 RUNTIME SEMANTICS: EVALUATION
ThrowStatement : throw Expression ;
1. 1. 1. Let exprRef be ? Evaluation of Expression.
2. 2. 2. Let exprValue be ? GetValue(exprRef).
3. 3. 3. Return ThrowCompletion(exprValue).
14.15 THE TRY STATEMENT
SYNTAX
TryStatement[Yield, Await, Return] : try Block[?Yield, ?Await, ?Return]
Catch[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return]
Finally[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return]
Catch[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] Catch[Yield,
Await, Return] : catch ( CatchParameter[?Yield, ?Await] ) Block[?Yield, ?Await,
?Return] catch Block[?Yield, ?Await, ?Return] Finally[Yield, Await, Return] :
finally Block[?Yield, ?Await, ?Return] CatchParameter[Yield, Await] :
BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] Note
The try statement encloses a block of code in which an exceptional condition can
occur, such as a runtime error or a throw statement. The catch clause provides
the exception-handling code. When a catch clause catches an exception, its
CatchParameter is bound to that exception.
14.15.1 STATIC SEMANTICS: EARLY ERRORS
Catch : catch ( CatchParameter ) Block
* It is a Syntax Error if BoundNames of CatchParameter contains any duplicate
elements.
* It is a Syntax Error if any element of the BoundNames of CatchParameter also
occurs in the LexicallyDeclaredNames of Block.
* It is a Syntax Error if any element of the BoundNames of CatchParameter also
occurs in the VarDeclaredNames of Block.
Note
An alternative static semantics for this production is given in B.3.4.
14.15.2 RUNTIME SEMANTICS: CATCHCLAUSEEVALUATION
The syntax-directed operation CatchClauseEvaluation takes argument thrownValue
(an ECMAScript language value) and returns either a normal completion containing
an ECMAScript language value or an abrupt completion. It is defined piecewise
over the following productions:
Catch : catch ( CatchParameter ) Block
1. 1. 1. Let oldEnv be the running execution context's LexicalEnvironment.
2. 2. 2. Let catchEnv be NewDeclarativeEnvironment(oldEnv).
3. 3. 3. For each element argName of the BoundNames of CatchParameter, do
1. a. a. Perform ! catchEnv.CreateMutableBinding(argName, false).
4. 4. 4. Set the running execution context's LexicalEnvironment to catchEnv.
5. 5. 5. Let status be Completion(BindingInitialization of CatchParameter with
arguments thrownValue and catchEnv).
6. 6. 6. If status is an abrupt completion, then
1. a. a. Set the running execution context's LexicalEnvironment to oldEnv.
2. b. b. Return ? status.
7. 7. 7. Let B be Completion(Evaluation of Block).
8. 8. 8. Set the running execution context's LexicalEnvironment to oldEnv.
9. 9. 9. Return ? B.
Catch : catch Block
1. 1. 1. Return ? Evaluation of Block.
Note
No matter how control leaves the Block the LexicalEnvironment is always restored
to its former state.
14.15.3 RUNTIME SEMANTICS: EVALUATION
TryStatement : try Block Catch
1. 1. 1. Let B be Completion(Evaluation of Block).
2. 2. 2. If B.[[Type]] is throw, let C be Completion(CatchClauseEvaluation of
Catch with argument B.[[Value]]).
3. 3. 3. Else, let C be B.
4. 4. 4. Return ? UpdateEmpty(C, undefined).
TryStatement : try Block Finally
1. 1. 1. Let B be Completion(Evaluation of Block).
2. 2. 2. Let F be Completion(Evaluation of Finally).
3. 3. 3. If F.[[Type]] is normal, set F to B.
4. 4. 4. Return ? UpdateEmpty(F, undefined).
TryStatement : try Block Catch Finally
1. 1. 1. Let B be Completion(Evaluation of Block).
2. 2. 2. If B.[[Type]] is throw, let C be Completion(CatchClauseEvaluation of
Catch with argument B.[[Value]]).
3. 3. 3. Else, let C be B.
4. 4. 4. Let F be Completion(Evaluation of Finally).
5. 5. 5. If F.[[Type]] is normal, set F to C.
6. 6. 6. Return ? UpdateEmpty(F, undefined).
14.16 THE DEBUGGER STATEMENT
SYNTAX
DebuggerStatement : debugger ;
14.16.1 RUNTIME SEMANTICS: EVALUATION
Note
Evaluating a DebuggerStatement may allow an implementation to cause a breakpoint
when run under a debugger. If a debugger is not present or active this statement
has no observable effect.
DebuggerStatement : debugger ;
1. 1. 1. If an implementation-defined debugging facility is available and
enabled, then
1. a. a. Perform an implementation-defined debugging action.
2. b. b. Return a new implementation-defined Completion Record.
2. 2. 2. Else,
1. a. a. Return empty.
15 ECMASCRIPT LANGUAGE: FUNCTIONS AND CLASSES
Note
Various ECMAScript language elements cause the creation of ECMAScript function
objects (10.2). Evaluation of such functions starts with the execution of their
[[Call]] internal method (10.2.1).
15.1 PARAMETER LISTS
SYNTAX
UniqueFormalParameters[Yield, Await] : FormalParameters[?Yield, ?Await]
FormalParameters[Yield, Await] : [empty] FunctionRestParameter[?Yield, ?Await]
FormalParameterList[?Yield, ?Await] FormalParameterList[?Yield, ?Await] ,
FormalParameterList[?Yield, ?Await] , FunctionRestParameter[?Yield, ?Await]
FormalParameterList[Yield, Await] : FormalParameter[?Yield, ?Await]
FormalParameterList[?Yield, ?Await] , FormalParameter[?Yield, ?Await]
FunctionRestParameter[Yield, Await] : BindingRestElement[?Yield, ?Await]
FormalParameter[Yield, Await] : BindingElement[?Yield, ?Await]
15.1.1 STATIC SEMANTICS: EARLY ERRORS
UniqueFormalParameters : FormalParameters
* It is a Syntax Error if BoundNames of FormalParameters contains any duplicate
elements.
FormalParameters : FormalParameterList
* It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false
and BoundNames of FormalParameterList contains any duplicate elements.
Note
Multiple occurrences of the same BindingIdentifier in a FormalParameterList is
only allowed for functions which have simple parameter lists and which are not
defined in strict mode code.
15.1.2 STATIC SEMANTICS: CONTAINSEXPRESSION
The syntax-directed operation ContainsExpression takes no arguments and returns
a Boolean. It is defined piecewise over the following productions:
ObjectBindingPattern : { } { BindingRestProperty }
1. 1. 1. Return false.
ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
1. 1. 1. Return ContainsExpression of BindingPropertyList.
ArrayBindingPattern : [ Elisionopt ]
1. 1. 1. Return false.
ArrayBindingPattern : [ Elisionopt BindingRestElement ]
1. 1. 1. Return ContainsExpression of BindingRestElement.
ArrayBindingPattern : [ BindingElementList , Elisionopt ]
1. 1. 1. Return ContainsExpression of BindingElementList.
ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
1. 1. 1. Let has be ContainsExpression of BindingElementList.
2. 2. 2. If has is true, return true.
3. 3. 3. Return ContainsExpression of BindingRestElement.
BindingPropertyList : BindingPropertyList , BindingProperty
1. 1. 1. Let has be ContainsExpression of BindingPropertyList.
2. 2. 2. If has is true, return true.
3. 3. 3. Return ContainsExpression of BindingProperty.
BindingElementList : BindingElementList , BindingElisionElement
1. 1. 1. Let has be ContainsExpression of BindingElementList.
2. 2. 2. If has is true, return true.
3. 3. 3. Return ContainsExpression of BindingElisionElement.
BindingElisionElement : Elisionopt BindingElement
1. 1. 1. Return ContainsExpression of BindingElement.
BindingProperty : PropertyName : BindingElement
1. 1. 1. Let has be IsComputedPropertyKey of PropertyName.
2. 2. 2. If has is true, return true.
3. 3. 3. Return ContainsExpression of BindingElement.
BindingElement : BindingPattern Initializer
1. 1. 1. Return true.
SingleNameBinding : BindingIdentifier
1. 1. 1. Return false.
SingleNameBinding : BindingIdentifier Initializer
1. 1. 1. Return true.
BindingRestElement : ... BindingIdentifier
1. 1. 1. Return false.
BindingRestElement : ... BindingPattern
1. 1. 1. Return ContainsExpression of BindingPattern.
FormalParameters : [empty]
1. 1. 1. Return false.
FormalParameters : FormalParameterList , FunctionRestParameter
1. 1. 1. If ContainsExpression of FormalParameterList is true, return true.
2. 2. 2. Return ContainsExpression of FunctionRestParameter.
FormalParameterList : FormalParameterList , FormalParameter
1. 1. 1. If ContainsExpression of FormalParameterList is true, return true.
2. 2. 2. Return ContainsExpression of FormalParameter.
ArrowParameters : BindingIdentifier
1. 1. 1. Return false.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let formals be the ArrowFormalParameters that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return ContainsExpression of formals.
AsyncArrowBindingIdentifier : BindingIdentifier
1. 1. 1. Return false.
15.1.3 STATIC SEMANTICS: ISSIMPLEPARAMETERLIST
The syntax-directed operation IsSimpleParameterList takes no arguments and
returns a Boolean. It is defined piecewise over the following productions:
BindingElement : BindingPattern
1. 1. 1. Return false.
BindingElement : BindingPattern Initializer
1. 1. 1. Return false.
SingleNameBinding : BindingIdentifier
1. 1. 1. Return true.
SingleNameBinding : BindingIdentifier Initializer
1. 1. 1. Return false.
FormalParameters : [empty]
1. 1. 1. Return true.
FormalParameters : FunctionRestParameter
1. 1. 1. Return false.
FormalParameters : FormalParameterList , FunctionRestParameter
1. 1. 1. Return false.
FormalParameterList : FormalParameterList , FormalParameter
1. 1. 1. If IsSimpleParameterList of FormalParameterList is false, return
false.
2. 2. 2. Return IsSimpleParameterList of FormalParameter.
FormalParameter : BindingElement
1. 1. 1. Return IsSimpleParameterList of BindingElement.
ArrowParameters : BindingIdentifier
1. 1. 1. Return true.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let formals be the ArrowFormalParameters that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return IsSimpleParameterList of formals.
AsyncArrowBindingIdentifier : BindingIdentifier
1. 1. 1. Return true.
CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments
1. 1. 1. Let head be the AsyncArrowHead that is covered by
CoverCallExpressionAndAsyncArrowHead.
2. 2. 2. Return IsSimpleParameterList of head.
15.1.4 STATIC SEMANTICS: HASINITIALIZER
The syntax-directed operation HasInitializer takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
BindingElement : BindingPattern
1. 1. 1. Return false.
BindingElement : BindingPattern Initializer
1. 1. 1. Return true.
SingleNameBinding : BindingIdentifier
1. 1. 1. Return false.
SingleNameBinding : BindingIdentifier Initializer
1. 1. 1. Return true.
FormalParameterList : FormalParameterList , FormalParameter
1. 1. 1. If HasInitializer of FormalParameterList is true, return true.
2. 2. 2. Return HasInitializer of FormalParameter.
15.1.5 STATIC SEMANTICS: EXPECTEDARGUMENTCOUNT
The syntax-directed operation ExpectedArgumentCount takes no arguments and
returns an integer. It is defined piecewise over the following productions:
FormalParameters : [empty] FunctionRestParameter
1. 1. 1. Return 0.
FormalParameters : FormalParameterList , FunctionRestParameter
1. 1. 1. Return ExpectedArgumentCount of FormalParameterList.
Note
The ExpectedArgumentCount of a FormalParameterList is the number of
FormalParameters to the left of either the rest parameter or the first
FormalParameter with an Initializer. A FormalParameter without an initializer is
allowed after the first parameter with an initializer but such parameters are
considered to be optional with undefined as their default value.
FormalParameterList : FormalParameter
1. 1. 1. If HasInitializer of FormalParameter is true, return 0.
2. 2. 2. Return 1.
FormalParameterList : FormalParameterList , FormalParameter
1. 1. 1. Let count be ExpectedArgumentCount of FormalParameterList.
2. 2. 2. If HasInitializer of FormalParameterList is true or HasInitializer of
FormalParameter is true, return count.
3. 3. 3. Return count + 1.
ArrowParameters : BindingIdentifier
1. 1. 1. Return 1.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let formals be the ArrowFormalParameters that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return ExpectedArgumentCount of formals.
PropertySetParameterList : FormalParameter
1. 1. 1. If HasInitializer of FormalParameter is true, return 0.
2. 2. 2. Return 1.
AsyncArrowBindingIdentifier : BindingIdentifier
1. 1. 1. Return 1.
15.2 FUNCTION DEFINITIONS
SYNTAX
FunctionDeclaration[Yield, Await, Default] : function BindingIdentifier[?Yield,
?Await] ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
[+Default] function ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield,
~Await] } FunctionExpression : function BindingIdentifier[~Yield, ~Await]opt (
FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
FunctionBody[Yield, Await] : FunctionStatementList[?Yield, ?Await]
FunctionStatementList[Yield, Await] : StatementList[?Yield, ?Await, +Return]opt
15.2.1 STATIC SEMANTICS: EARLY ERRORS
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression
: function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
* If the source text matched by FormalParameters is strict mode code, the Early
Error rules for UniqueFormalParameters : FormalParameters are applied.
* If BindingIdentifier is present and the source text matched by
BindingIdentifier is strict mode code, it is a Syntax Error if the
StringValue of BindingIdentifier is either "eval" or "arguments".
* It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
and IsSimpleParameterList of FormalParameters is false.
* It is a Syntax Error if any element of the BoundNames of FormalParameters
also occurs in the LexicallyDeclaredNames of FunctionBody.
* It is a Syntax Error if FormalParameters Contains SuperProperty is true.
* It is a Syntax Error if FunctionBody Contains SuperProperty is true.
* It is a Syntax Error if FormalParameters Contains SuperCall is true.
* It is a Syntax Error if FunctionBody Contains SuperCall is true.
Note
The LexicallyDeclaredNames of a FunctionBody does not include identifiers bound
using var or function declarations.
FunctionBody : FunctionStatementList
* It is a Syntax Error if the LexicallyDeclaredNames of FunctionStatementList
contains any duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
FunctionStatementList also occurs in the VarDeclaredNames of
FunctionStatementList.
* It is a Syntax Error if ContainsDuplicateLabels of FunctionStatementList with
argument « » is true.
* It is a Syntax Error if ContainsUndefinedBreakTarget of FunctionStatementList
with argument « » is true.
* It is a Syntax Error if ContainsUndefinedContinueTarget of
FunctionStatementList with arguments « » and « » is true.
15.2.2 STATIC SEMANTICS: FUNCTIONBODYCONTAINSUSESTRICT
The syntax-directed operation FunctionBodyContainsUseStrict takes no arguments
and returns a Boolean. It is defined piecewise over the following productions:
FunctionBody : FunctionStatementList
1. 1. 1. If the Directive Prologue of FunctionBody contains a Use Strict
Directive, return true; otherwise, return false.
15.2.3 RUNTIME SEMANTICS: EVALUATEFUNCTIONBODY
The syntax-directed operation EvaluateFunctionBody takes arguments
functionObject (a function object) and argumentsList (a List of ECMAScript
language values) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It is defined piecewise over the
following productions:
FunctionBody : FunctionStatementList
1. 1. 1. Perform ? FunctionDeclarationInstantiation(functionObject,
argumentsList).
2. 2. 2. Return ? Evaluation of FunctionStatementList.
15.2.4 RUNTIME SEMANTICS: INSTANTIATEORDINARYFUNCTIONOBJECT
The syntax-directed operation InstantiateOrdinaryFunctionObject takes arguments
env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null)
and returns a function object. It is defined piecewise over the following
productions:
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody }
1. 1. 1. Let name be StringValue of BindingIdentifier.
2. 2. 2. Let sourceText be the source text matched by FunctionDeclaration.
3. 3. 3. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText,
FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
4. 4. 4. Perform SetFunctionName(F, name).
5. 5. 5. Perform MakeConstructor(F).
6. 6. 6. Return F.
FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
1. 1. 1. Let sourceText be the source text matched by FunctionDeclaration.
2. 2. 2. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText,
FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
3. 3. 3. Perform SetFunctionName(F, "default").
4. 4. 4. Perform MakeConstructor(F).
5. 5. 5. Return F.
Note
An anonymous FunctionDeclaration can only occur as part of an export default
declaration, and its function code is therefore always strict mode code.
15.2.5 RUNTIME SEMANTICS: INSTANTIATEORDINARYFUNCTIONEXPRESSION
The syntax-directed operation InstantiateOrdinaryFunctionExpression takes
optional argument name (a property key or a Private Name) and returns a function
object. It is defined piecewise over the following productions:
FunctionExpression : function ( FormalParameters ) { FunctionBody }
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by FunctionExpression.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, FormalParameters, FunctionBody, non-lexical-this, env,
privateEnv).
6. 6. 6. Perform SetFunctionName(closure, name).
7. 7. 7. Perform MakeConstructor(closure).
8. 8. 8. Return closure.
FunctionExpression : function BindingIdentifier ( FormalParameters ) {
FunctionBody }
1. 1. 1. Assert: name is not present.
2. 2. 2. Set name to StringValue of BindingIdentifier.
3. 3. 3. Let outerEnv be the running execution context's LexicalEnvironment.
4. 4. 4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. 5. 5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. 6. 6. Let privateEnv be the running execution context's PrivateEnvironment.
7. 7. 7. Let sourceText be the source text matched by FunctionExpression.
8. 8. 8. Let closure be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, FormalParameters, FunctionBody, non-lexical-this, funcEnv,
privateEnv).
9. 9. 9. Perform SetFunctionName(closure, name).
10. 10. 10. Perform MakeConstructor(closure).
11. 11. 11. Perform ! funcEnv.InitializeBinding(name, closure).
12. 12. 12. Return closure.
Note
The BindingIdentifier in a FunctionExpression can be referenced from inside the
FunctionExpression's FunctionBody to allow the function to call itself
recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in
a FunctionExpression cannot be referenced from and does not affect the scope
enclosing the FunctionExpression.
15.2.6 RUNTIME SEMANTICS: EVALUATION
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody }
1. 1. 1. Return empty.
Note 1
An alternative semantics is provided in B.3.2.
FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
1. 1. 1. Return empty.
FunctionExpression : function BindingIdentifieropt ( FormalParameters ) {
FunctionBody }
1. 1. 1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression.
Note 2
A "prototype" property is automatically created for every function defined using
a FunctionDeclaration or FunctionExpression, to allow for the possibility that
the function will be used as a constructor.
FunctionStatementList : [empty]
1. 1. 1. Return undefined.
15.3 ARROW FUNCTION DEFINITIONS
SYNTAX
ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no
LineTerminator here] => ConciseBody[?In] ArrowParameters[Yield, Await] :
BindingIdentifier[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
ConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, ~Await] {
FunctionBody[~Yield, ~Await] } ExpressionBody[In, Await] :
AssignmentExpression[?In, ~Yield, ?Await]
SUPPLEMENTAL SYNTAX
When processing an instance of the production
ArrowParameters[Yield, Await] :
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is
refined using the following grammar:
ArrowFormalParameters[Yield, Await] : ( UniqueFormalParameters[?Yield, ?Await] )
15.3.1 STATIC SEMANTICS: EARLY ERRORS
ArrowFunction : ArrowParameters => ConciseBody
* It is a Syntax Error if ArrowParameters Contains YieldExpression is true.
* It is a Syntax Error if ArrowParameters Contains AwaitExpression is true.
* It is a Syntax Error if ConciseBodyContainsUseStrict of ConciseBody is true
and IsSimpleParameterList of ArrowParameters is false.
* It is a Syntax Error if any element of the BoundNames of ArrowParameters also
occurs in the LexicallyDeclaredNames of ConciseBody.
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
* CoverParenthesizedExpressionAndArrowParameterList must cover an
ArrowFormalParameters.
15.3.2 STATIC SEMANTICS: CONCISEBODYCONTAINSUSESTRICT
The syntax-directed operation ConciseBodyContainsUseStrict takes no arguments
and returns a Boolean. It is defined piecewise over the following productions:
ConciseBody : ExpressionBody
1. 1. 1. Return false.
ConciseBody : { FunctionBody }
1. 1. 1. Return FunctionBodyContainsUseStrict of FunctionBody.
15.3.3 RUNTIME SEMANTICS: EVALUATECONCISEBODY
The syntax-directed operation EvaluateConciseBody takes arguments functionObject
(a function object) and argumentsList (a List of ECMAScript language values) and
returns either a normal completion containing an ECMAScript language value or an
abrupt completion. It is defined piecewise over the following productions:
ConciseBody : ExpressionBody
1. 1. 1. Perform ? FunctionDeclarationInstantiation(functionObject,
argumentsList).
2. 2. 2. Return ? Evaluation of ExpressionBody.
15.3.4 RUNTIME SEMANTICS: INSTANTIATEARROWFUNCTIONEXPRESSION
The syntax-directed operation InstantiateArrowFunctionExpression takes optional
argument name (a property key or a Private Name) and returns a function object.
It is defined piecewise over the following productions:
ArrowFunction : ArrowParameters => ConciseBody
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by ArrowFunction.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, ArrowParameters, ConciseBody, lexical-this, env, privateEnv).
6. 6. 6. Perform SetFunctionName(closure, name).
7. 7. 7. Return closure.
Note
An ArrowFunction does not define local bindings for arguments, super, this, or
new.target. Any reference to arguments, super, this, or new.target within an
ArrowFunction must resolve to a binding in a lexically enclosing environment.
Typically this will be the Function Environment of an immediately enclosing
function. Even though an ArrowFunction may contain references to super, the
function object created in step 5 is not made into a method by performing
MakeMethod. An ArrowFunction that references super is always contained within a
non-ArrowFunction and the necessary state to implement super is accessible via
the env that is captured by the function object of the ArrowFunction.
15.3.5 RUNTIME SEMANTICS: EVALUATION
ArrowFunction : ArrowParameters => ConciseBody
1. 1. 1. Return InstantiateArrowFunctionExpression of ArrowFunction.
ExpressionBody : AssignmentExpression
1. 1. 1. Let exprRef be ? Evaluation of AssignmentExpression.
2. 2. 2. Let exprValue be ? GetValue(exprRef).
3. 3. 3. Return Completion Record { [[Type]]: return, [[Value]]: exprValue,
[[Target]]: empty }.
15.4 METHOD DEFINITIONS
SYNTAX
MethodDefinition[Yield, Await] : ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
GeneratorMethod[?Yield, ?Await] AsyncMethod[?Yield, ?Await]
AsyncGeneratorMethod[?Yield, ?Await] get ClassElementName[?Yield, ?Await] ( ) {
FunctionBody[~Yield, ~Await] } set ClassElementName[?Yield, ?Await] (
PropertySetParameterList ) { FunctionBody[~Yield, ~Await] }
PropertySetParameterList : FormalParameter[~Yield, ~Await]
15.4.1 STATIC SEMANTICS: EARLY ERRORS
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
* It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
and IsSimpleParameterList of UniqueFormalParameters is false.
* It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters also occurs in the LexicallyDeclaredNames of
FunctionBody.
MethodDefinition : set ClassElementName ( PropertySetParameterList ) {
FunctionBody }
* It is a Syntax Error if BoundNames of PropertySetParameterList contains any
duplicate elements.
* It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
and IsSimpleParameterList of PropertySetParameterList is false.
* It is a Syntax Error if any element of the BoundNames of
PropertySetParameterList also occurs in the LexicallyDeclaredNames of
FunctionBody.
15.4.2 STATIC SEMANTICS: HASDIRECTSUPER
The syntax-directed operation HasDirectSuper takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
1. 1. 1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. 2. 2. Return FunctionBody Contains SuperCall.
MethodDefinition : get ClassElementName ( ) { FunctionBody }
1. 1. 1. Return FunctionBody Contains SuperCall.
MethodDefinition : set ClassElementName ( PropertySetParameterList ) {
FunctionBody }
1. 1. 1. If PropertySetParameterList Contains SuperCall is true, return true.
2. 2. 2. Return FunctionBody Contains SuperCall.
GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody
}
1. 1. 1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. 2. 2. Return GeneratorBody Contains SuperCall.
AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. 2. 2. Return AsyncGeneratorBody Contains SuperCall.
AsyncMethod : async ClassElementName ( UniqueFormalParameters ) {
AsyncFunctionBody }
1. 1. 1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. 2. 2. Return AsyncFunctionBody Contains SuperCall.
15.4.3 STATIC SEMANTICS: SPECIALMETHOD
The syntax-directed operation SpecialMethod takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
1. 1. 1. Return false.
MethodDefinition : GeneratorMethod AsyncMethod AsyncGeneratorMethod get
ClassElementName ( ) { FunctionBody } set ClassElementName (
PropertySetParameterList ) { FunctionBody }
1. 1. 1. Return true.
15.4.4 RUNTIME SEMANTICS: DEFINEMETHOD
The syntax-directed operation DefineMethod takes argument object (an Object) and
optional argument functionPrototype (an Object) and returns either a normal
completion containing a Record with fields [[Key]] (a property key) and
[[Closure]] (a function object) or an abrupt completion. It is defined piecewise
over the following productions:
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
1. 1. 1. Let propKey be ? Evaluation of ClassElementName.
2. 2. 2. Let env be the running execution context's LexicalEnvironment.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. If functionPrototype is present, then
1. a. a. Let prototype be functionPrototype.
5. 5. 5. Else,
1. a. a. Let prototype be %Function.prototype%.
6. 6. 6. Let sourceText be the source text matched by MethodDefinition.
7. 7. 7. Let closure be OrdinaryFunctionCreate(prototype, sourceText,
UniqueFormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
8. 8. 8. Perform MakeMethod(closure, object).
9. 9. 9. Return the Record { [[Key]]: propKey, [[Closure]]: closure }.
15.4.5 RUNTIME SEMANTICS: METHODDEFINITIONEVALUATION
The syntax-directed operation MethodDefinitionEvaluation takes arguments object
(an Object) and enumerable (a Boolean) and returns either a normal completion
containing either a PrivateElement or unused, or an abrupt completion. It is
defined piecewise over the following productions:
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
1. 1. 1. Let methodDef be ? DefineMethod of MethodDefinition with argument
object.
2. 2. 2. Perform SetFunctionName(methodDef.[[Closure]], methodDef.[[Key]]).
3. 3. 3. Return DefineMethodProperty(object, methodDef.[[Key]],
methodDef.[[Closure]], enumerable).
MethodDefinition : get ClassElementName ( ) { FunctionBody }
1. 1. 1. Let propKey be ? Evaluation of ClassElementName.
2. 2. 2. Let env be the running execution context's LexicalEnvironment.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by MethodDefinition.
5. 5. 5. Let formalParameterList be an instance of the production
FormalParameters : [empty] .
6. 6. 6. Let closure be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, formalParameterList, FunctionBody, non-lexical-this, env,
privateEnv).
7. 7. 7. Perform MakeMethod(closure, object).
8. 8. 8. Perform SetFunctionName(closure, propKey, "get").
9. 9. 9. If propKey is a Private Name, then
1. a. a. Return PrivateElement { [[Key]]: propKey, [[Kind]]: accessor,
[[Get]]: closure, [[Set]]: undefined }.
10. 10. 10. Else,
1. a. a. Let desc be the PropertyDescriptor { [[Get]]: closure,
[[Enumerable]]: enumerable, [[Configurable]]: true }.
2. b. b. Perform ? DefinePropertyOrThrow(object, propKey, desc).
3. c. c. Return unused.
MethodDefinition : set ClassElementName ( PropertySetParameterList ) {
FunctionBody }
1. 1. 1. Let propKey be ? Evaluation of ClassElementName.
2. 2. 2. Let env be the running execution context's LexicalEnvironment.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by MethodDefinition.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, PropertySetParameterList, FunctionBody, non-lexical-this, env,
privateEnv).
6. 6. 6. Perform MakeMethod(closure, object).
7. 7. 7. Perform SetFunctionName(closure, propKey, "set").
8. 8. 8. If propKey is a Private Name, then
1. a. a. Return PrivateElement { [[Key]]: propKey, [[Kind]]: accessor,
[[Get]]: undefined, [[Set]]: closure }.
9. 9. 9. Else,
1. a. a. Let desc be the PropertyDescriptor { [[Set]]: closure,
[[Enumerable]]: enumerable, [[Configurable]]: true }.
2. b. b. Perform ? DefinePropertyOrThrow(object, propKey, desc).
3. c. c. Return unused.
GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody
}
1. 1. 1. Let propKey be ? Evaluation of ClassElementName.
2. 2. 2. Let env be the running execution context's LexicalEnvironment.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by GeneratorMethod.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%,
sourceText, UniqueFormalParameters, GeneratorBody, non-lexical-this, env,
privateEnv).
6. 6. 6. Perform MakeMethod(closure, object).
7. 7. 7. Perform SetFunctionName(closure, propKey).
8. 8. 8. Let prototype be
OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
9. 9. 9. Perform ! DefinePropertyOrThrow(closure, "prototype",
PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }).
10. 10. 10. Return DefineMethodProperty(object, propKey, closure, enumerable).
AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Let propKey be ? Evaluation of ClassElementName.
2. 2. 2. Let env be the running execution context's LexicalEnvironment.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by AsyncGeneratorMethod.
5. 5. 5. Let closure be
OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,
UniqueFormalParameters, AsyncGeneratorBody, non-lexical-this, env,
privateEnv).
6. 6. 6. Perform MakeMethod(closure, object).
7. 7. 7. Perform SetFunctionName(closure, propKey).
8. 8. 8. Let prototype be
OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
9. 9. 9. Perform ! DefinePropertyOrThrow(closure, "prototype",
PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }).
10. 10. 10. Return DefineMethodProperty(object, propKey, closure, enumerable).
AsyncMethod : async ClassElementName ( UniqueFormalParameters ) {
AsyncFunctionBody }
1. 1. 1. Let propKey be ? Evaluation of ClassElementName.
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by AsyncMethod.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%,
sourceText, UniqueFormalParameters, AsyncFunctionBody, non-lexical-this,
env, privateEnv).
6. 6. 6. Perform MakeMethod(closure, object).
7. 7. 7. Perform SetFunctionName(closure, propKey).
8. 8. 8. Return DefineMethodProperty(object, propKey, closure, enumerable).
15.5 GENERATOR FUNCTION DEFINITIONS
SYNTAX
GeneratorDeclaration[Yield, Await, Default] : function *
BindingIdentifier[?Yield, ?Await] ( FormalParameters[+Yield, ~Await] ) {
GeneratorBody } [+Default] function * ( FormalParameters[+Yield, ~Await] ) {
GeneratorBody } GeneratorExpression : function * BindingIdentifier[+Yield,
~Await]opt ( FormalParameters[+Yield, ~Await] ) { GeneratorBody }
GeneratorMethod[Yield, Await] : * ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[+Yield, ~Await] ) { GeneratorBody } GeneratorBody :
FunctionBody[+Yield, ~Await] YieldExpression[In, Await] : yield yield [no
LineTerminator here] AssignmentExpression[?In, +Yield, ?Await] yield [no
LineTerminator here] * AssignmentExpression[?In, +Yield, ?Await] Note 1
The syntactic context immediately following yield requires use of the
InputElementRegExpOrTemplateTail lexical goal.
Note 2
YieldExpression cannot be used within the FormalParameters of a generator
function because any expressions that are part of FormalParameters are evaluated
before the resulting Generator is in a resumable state.
Note 3
Abstract operations relating to Generators are defined in 27.5.3.
15.5.1 STATIC SEMANTICS: EARLY ERRORS
GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody
}
* It is a Syntax Error if HasDirectSuper of GeneratorMethod is true.
* It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is
true.
* It is a Syntax Error if FunctionBodyContainsUseStrict of GeneratorBody is
true and IsSimpleParameterList of UniqueFormalParameters is false.
* It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters also occurs in the LexicallyDeclaredNames of
GeneratorBody.
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) {
GeneratorBody }
* If the source text matched by FormalParameters is strict mode code, the Early
Error rules for UniqueFormalParameters : FormalParameters are applied.
* If BindingIdentifier is present and the source text matched by
BindingIdentifier is strict mode code, it is a Syntax Error if the
StringValue of BindingIdentifier is either "eval" or "arguments".
* It is a Syntax Error if FunctionBodyContainsUseStrict of GeneratorBody is
true and IsSimpleParameterList of FormalParameters is false.
* It is a Syntax Error if any element of the BoundNames of FormalParameters
also occurs in the LexicallyDeclaredNames of GeneratorBody.
* It is a Syntax Error if FormalParameters Contains YieldExpression is true.
* It is a Syntax Error if FormalParameters Contains SuperProperty is true.
* It is a Syntax Error if GeneratorBody Contains SuperProperty is true.
* It is a Syntax Error if FormalParameters Contains SuperCall is true.
* It is a Syntax Error if GeneratorBody Contains SuperCall is true.
15.5.2 RUNTIME SEMANTICS: EVALUATEGENERATORBODY
The syntax-directed operation EvaluateGeneratorBody takes arguments
functionObject (a function object) and argumentsList (a List of ECMAScript
language values) and returns a throw completion or a return completion. It is
defined piecewise over the following productions:
GeneratorBody : FunctionBody
1. 1. 1. Perform ? FunctionDeclarationInstantiation(functionObject,
argumentsList).
2. 2. 2. Let G be ? OrdinaryCreateFromConstructor(functionObject,
"%GeneratorFunction.prototype.prototype%", « [[GeneratorState]],
[[GeneratorContext]], [[GeneratorBrand]] »).
3. 3. 3. Set G.[[GeneratorBrand]] to empty.
4. 4. 4. Perform GeneratorStart(G, FunctionBody).
5. 5. 5. Return Completion Record { [[Type]]: return, [[Value]]: G, [[Target]]:
empty }.
15.5.3 RUNTIME SEMANTICS: INSTANTIATEGENERATORFUNCTIONOBJECT
The syntax-directed operation InstantiateGeneratorFunctionObject takes arguments
env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null)
and returns a function object. It is defined piecewise over the following
productions:
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody }
1. 1. 1. Let name be StringValue of BindingIdentifier.
2. 2. 2. Let sourceText be the source text matched by GeneratorDeclaration.
3. 3. 3. Let F be OrdinaryFunctionCreate(%GeneratorFunction.prototype%,
sourceText, FormalParameters, GeneratorBody, non-lexical-this, env,
privateEnv).
4. 4. 4. Perform SetFunctionName(F, name).
5. 5. 5. Let prototype be
OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
6. 6. 6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor {
[[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
7. 7. 7. Return F.
GeneratorDeclaration : function * ( FormalParameters ) { GeneratorBody }
1. 1. 1. Let sourceText be the source text matched by GeneratorDeclaration.
2. 2. 2. Let F be OrdinaryFunctionCreate(%GeneratorFunction.prototype%,
sourceText, FormalParameters, GeneratorBody, non-lexical-this, env,
privateEnv).
3. 3. 3. Perform SetFunctionName(F, "default").
4. 4. 4. Let prototype be
OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
5. 5. 5. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor {
[[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
6. 6. 6. Return F.
Note
An anonymous GeneratorDeclaration can only occur as part of an export default
declaration, and its function code is therefore always strict mode code.
15.5.4 RUNTIME SEMANTICS: INSTANTIATEGENERATORFUNCTIONEXPRESSION
The syntax-directed operation InstantiateGeneratorFunctionExpression takes
optional argument name (a property key or a Private Name) and returns a function
object. It is defined piecewise over the following productions:
GeneratorExpression : function * ( FormalParameters ) { GeneratorBody }
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by GeneratorExpression.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%,
sourceText, FormalParameters, GeneratorBody, non-lexical-this, env,
privateEnv).
6. 6. 6. Perform SetFunctionName(closure, name).
7. 7. 7. Let prototype be
OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
8. 8. 8. Perform ! DefinePropertyOrThrow(closure, "prototype",
PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }).
9. 9. 9. Return closure.
GeneratorExpression : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody }
1. 1. 1. Assert: name is not present.
2. 2. 2. Set name to StringValue of BindingIdentifier.
3. 3. 3. Let outerEnv be the running execution context's LexicalEnvironment.
4. 4. 4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. 5. 5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. 6. 6. Let privateEnv be the running execution context's PrivateEnvironment.
7. 7. 7. Let sourceText be the source text matched by GeneratorExpression.
8. 8. 8. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%,
sourceText, FormalParameters, GeneratorBody, non-lexical-this, funcEnv,
privateEnv).
9. 9. 9. Perform SetFunctionName(closure, name).
10. 10. 10. Let prototype be
OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
11. 11. 11. Perform ! DefinePropertyOrThrow(closure, "prototype",
PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }).
12. 12. 12. Perform ! funcEnv.InitializeBinding(name, closure).
13. 13. 13. Return closure.
Note
The BindingIdentifier in a GeneratorExpression can be referenced from inside the
GeneratorExpression's FunctionBody to allow the generator code to call itself
recursively. However, unlike in a GeneratorDeclaration, the BindingIdentifier in
a GeneratorExpression cannot be referenced from and does not affect the scope
enclosing the GeneratorExpression.
15.5.5 RUNTIME SEMANTICS: EVALUATION
GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) {
GeneratorBody }
1. 1. 1. Return InstantiateGeneratorFunctionExpression of GeneratorExpression.
YieldExpression : yield
1. 1. 1. Return ? Yield(undefined).
YieldExpression : yield AssignmentExpression
1. 1. 1. Let exprRef be ? Evaluation of AssignmentExpression.
2. 2. 2. Let value be ? GetValue(exprRef).
3. 3. 3. Return ? Yield(value).
YieldExpression : yield * AssignmentExpression
1. 1. 1. Let generatorKind be GetGeneratorKind().
2. 2. 2. Let exprRef be ? Evaluation of AssignmentExpression.
3. 3. 3. Let value be ? GetValue(exprRef).
4. 4. 4. Let iteratorRecord be ? GetIterator(value, generatorKind).
5. 5. 5. Let iterator be iteratorRecord.[[Iterator]].
6. 6. 6. Let received be NormalCompletion(undefined).
7. 7. 7. Repeat,
1. a. a. If received.[[Type]] is normal, then
1. i. i. Let innerResult be ? Call(iteratorRecord.[[NextMethod]],
iteratorRecord.[[Iterator]], « received.[[Value]] »).
2. ii. ii. If generatorKind is async, set innerResult to
? Await(innerResult).
3. iii. iii. If innerResult is not an Object, throw a TypeError
exception.
4. iv. iv. Let done be ? IteratorComplete(innerResult).
5. v. v. If done is true, then
1. 1. 1. Return ? IteratorValue(innerResult).
6. vi. vi. If generatorKind is async, set received to
Completion(AsyncGeneratorYield(? IteratorValue(innerResult))).
7. vii. vii. Else, set received to
Completion(GeneratorYield(innerResult)).
2. b. b. Else if received.[[Type]] is throw, then
1. i. i. Let throw be ? GetMethod(iterator, "throw").
2. ii. ii. If throw is not undefined, then
1. 1. 1. Let innerResult be ? Call(throw, iterator, «
received.[[Value]] »).
2. 2. 2. If generatorKind is async, set innerResult to
? Await(innerResult).
3. 3. 3. NOTE: Exceptions from the inner iterator throw method are
propagated. Normal completions from an inner throw method are
processed similarly to an inner next.
4. 4. 4. If innerResult is not an Object, throw a TypeError exception.
5. 5. 5. Let done be ? IteratorComplete(innerResult).
6. 6. 6. If done is true, then
1. a. a. Return ? IteratorValue(innerResult).
7. 7. 7. If generatorKind is async, set received to
Completion(AsyncGeneratorYield(? IteratorValue(innerResult))).
8. 8. 8. Else, set received to
Completion(GeneratorYield(innerResult)).
3. iii. iii. Else,
1. 1. 1. NOTE: If iterator does not have a throw method, this throw is
going to terminate the yield* loop. But first we need to give
iterator a chance to clean up.
2. 2. 2. Let closeCompletion be Completion Record { [[Type]]: normal,
[[Value]]: empty, [[Target]]: empty }.
3. 3. 3. If generatorKind is async, perform
? AsyncIteratorClose(iteratorRecord, closeCompletion).
4. 4. 4. Else, perform ? IteratorClose(iteratorRecord,
closeCompletion).
5. 5. 5. NOTE: The next step throws a TypeError to indicate that there
was a yield* protocol violation: iterator does not have a throw
method.
6. 6. 6. Throw a TypeError exception.
3. c. c. Else,
1. i. i. Assert: received.[[Type]] is return.
2. ii. ii. Let return be ? GetMethod(iterator, "return").
3. iii. iii. If return is undefined, then
1. 1. 1. Let value be received.[[Value]].
2. 2. 2. If generatorKind is async, then
1. a. a. Set value to ? Await(value).
3. 3. 3. Return Completion Record { [[Type]]: return, [[Value]]:
value, [[Target]]: empty }.
4. iv. iv. Let innerReturnResult be ? Call(return, iterator, «
received.[[Value]] »).
5. v. v. If generatorKind is async, set innerReturnResult to
? Await(innerReturnResult).
6. vi. vi. If innerReturnResult is not an Object, throw a TypeError
exception.
7. vii. vii. Let done be ? IteratorComplete(innerReturnResult).
8. viii. viii. If done is true, then
1. 1. 1. Let value be ? IteratorValue(innerReturnResult).
2. 2. 2. Return Completion Record { [[Type]]: return, [[Value]]:
value, [[Target]]: empty }.
9. ix. ix. If generatorKind is async, set received to
Completion(AsyncGeneratorYield(? IteratorValue(innerReturnResult))).
10. x. x. Else, set received to
Completion(GeneratorYield(innerReturnResult)).
15.6 ASYNC GENERATOR FUNCTION DEFINITIONS
SYNTAX
AsyncGeneratorDeclaration[Yield, Await, Default] : async [no LineTerminator
here] function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[+Yield,
+Await] ) { AsyncGeneratorBody } [+Default] async [no LineTerminator here]
function * ( FormalParameters[+Yield, +Await] ) { AsyncGeneratorBody }
AsyncGeneratorExpression : async [no LineTerminator here] function *
BindingIdentifier[+Yield, +Await]opt ( FormalParameters[+Yield, +Await] ) {
AsyncGeneratorBody } AsyncGeneratorMethod[Yield, Await] : async [no
LineTerminator here] * ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[+Yield, +Await] ) { AsyncGeneratorBody }
AsyncGeneratorBody : FunctionBody[+Yield, +Await] Note 1
YieldExpression and AwaitExpression cannot be used within the FormalParameters
of an async generator function because any expressions that are part of
FormalParameters are evaluated before the resulting AsyncGenerator is in a
resumable state.
Note 2
Abstract operations relating to AsyncGenerators are defined in 27.6.3.
15.6.1 STATIC SEMANTICS: EARLY ERRORS
AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) {
AsyncGeneratorBody }
* It is a Syntax Error if HasDirectSuper of AsyncGeneratorMethod is true.
* It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is
true.
* It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is
true.
* It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncGeneratorBody
is true and IsSimpleParameterList of UniqueFormalParameters is false.
* It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters also occurs in the LexicallyDeclaredNames of
AsyncGeneratorBody.
AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters )
{ AsyncGeneratorBody } AsyncGeneratorExpression : async function *
BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }
* If the source text matched by FormalParameters is strict mode code, the Early
Error rules for UniqueFormalParameters : FormalParameters are applied.
* If BindingIdentifier is present and the source text matched by
BindingIdentifier is strict mode code, it is a Syntax Error if the
StringValue of BindingIdentifier is either "eval" or "arguments".
* It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncGeneratorBody
is true and IsSimpleParameterList of FormalParameters is false.
* It is a Syntax Error if any element of the BoundNames of FormalParameters
also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.
* It is a Syntax Error if FormalParameters Contains YieldExpression is true.
* It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
* It is a Syntax Error if FormalParameters Contains SuperProperty is true.
* It is a Syntax Error if AsyncGeneratorBody Contains SuperProperty is true.
* It is a Syntax Error if FormalParameters Contains SuperCall is true.
* It is a Syntax Error if AsyncGeneratorBody Contains SuperCall is true.
15.6.2 RUNTIME SEMANTICS: EVALUATEASYNCGENERATORBODY
The syntax-directed operation EvaluateAsyncGeneratorBody takes arguments
functionObject (a function object) and argumentsList (a List of ECMAScript
language values) and returns a throw completion or a return completion. It is
defined piecewise over the following productions:
AsyncGeneratorBody : FunctionBody
1. 1. 1. Perform ? FunctionDeclarationInstantiation(functionObject,
argumentsList).
2. 2. 2. Let generator be ? OrdinaryCreateFromConstructor(functionObject,
"%AsyncGeneratorFunction.prototype.prototype%", « [[AsyncGeneratorState]],
[[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] »).
3. 3. 3. Set generator.[[GeneratorBrand]] to empty.
4. 4. 4. Perform AsyncGeneratorStart(generator, FunctionBody).
5. 5. 5. Return Completion Record { [[Type]]: return, [[Value]]: generator,
[[Target]]: empty }.
15.6.3 RUNTIME SEMANTICS: INSTANTIATEASYNCGENERATORFUNCTIONOBJECT
The syntax-directed operation InstantiateAsyncGeneratorFunctionObject takes
arguments env (an Environment Record) and privateEnv (a PrivateEnvironment
Record or null) and returns a function object. It is defined piecewise over the
following productions:
AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody }
1. 1. 1. Let name be StringValue of BindingIdentifier.
2. 2. 2. Let sourceText be the source text matched by
AsyncGeneratorDeclaration.
3. 3. 3. Let F be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%,
sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env,
privateEnv).
4. 4. 4. Perform SetFunctionName(F, name).
5. 5. 5. Let prototype be
OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
6. 6. 6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor {
[[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
7. 7. 7. Return F.
AsyncGeneratorDeclaration : async function * ( FormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Let sourceText be the source text matched by
AsyncGeneratorDeclaration.
2. 2. 2. Let F be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%,
sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env,
privateEnv).
3. 3. 3. Perform SetFunctionName(F, "default").
4. 4. 4. Let prototype be
OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
5. 5. 5. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor {
[[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
6. 6. 6. Return F.
Note
An anonymous AsyncGeneratorDeclaration can only occur as part of an export
default declaration.
15.6.4 RUNTIME SEMANTICS: INSTANTIATEASYNCGENERATORFUNCTIONEXPRESSION
The syntax-directed operation InstantiateAsyncGeneratorFunctionExpression takes
optional argument name (a property key or a Private Name) and returns a function
object. It is defined piecewise over the following productions:
AsyncGeneratorExpression : async function * ( FormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by AsyncGeneratorExpression.
5. 5. 5. Let closure be
OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,
FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
6. 6. 6. Perform SetFunctionName(closure, name).
7. 7. 7. Let prototype be
OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
8. 8. 8. Perform ! DefinePropertyOrThrow(closure, "prototype",
PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }).
9. 9. 9. Return closure.
AsyncGeneratorExpression : async function * BindingIdentifier ( FormalParameters
) { AsyncGeneratorBody }
1. 1. 1. Assert: name is not present.
2. 2. 2. Set name to StringValue of BindingIdentifier.
3. 3. 3. Let outerEnv be the running execution context's LexicalEnvironment.
4. 4. 4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. 5. 5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. 6. 6. Let privateEnv be the running execution context's PrivateEnvironment.
7. 7. 7. Let sourceText be the source text matched by
AsyncGeneratorExpression.
8. 8. 8. Let closure be
OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,
FormalParameters, AsyncGeneratorBody, non-lexical-this, funcEnv,
privateEnv).
9. 9. 9. Perform SetFunctionName(closure, name).
10. 10. 10. Let prototype be
OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
11. 11. 11. Perform ! DefinePropertyOrThrow(closure, "prototype",
PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }).
12. 12. 12. Perform ! funcEnv.InitializeBinding(name, closure).
13. 13. 13. Return closure.
Note
The BindingIdentifier in an AsyncGeneratorExpression can be referenced from
inside the AsyncGeneratorExpression's AsyncGeneratorBody to allow the generator
code to call itself recursively. However, unlike in an
AsyncGeneratorDeclaration, the BindingIdentifier in an AsyncGeneratorExpression
cannot be referenced from and does not affect the scope enclosing the
AsyncGeneratorExpression.
15.6.5 RUNTIME SEMANTICS: EVALUATION
AsyncGeneratorExpression : async function * BindingIdentifieropt (
FormalParameters ) { AsyncGeneratorBody }
1. 1. 1. Return InstantiateAsyncGeneratorFunctionExpression of
AsyncGeneratorExpression.
15.7 CLASS DEFINITIONS
SYNTAX
ClassDeclaration[Yield, Await, Default] : class BindingIdentifier[?Yield,
?Await] ClassTail[?Yield, ?Await] [+Default] class ClassTail[?Yield, ?Await]
ClassExpression[Yield, Await] : class BindingIdentifier[?Yield, ?Await]opt
ClassTail[?Yield, ?Await] ClassTail[Yield, Await] : ClassHeritage[?Yield,
?Await]opt { ClassBody[?Yield, ?Await]opt } ClassHeritage[Yield, Await] :
extends LeftHandSideExpression[?Yield, ?Await] ClassBody[Yield, Await] :
ClassElementList[?Yield, ?Await] ClassElementList[Yield, Await] :
ClassElement[?Yield, ?Await] ClassElementList[?Yield, ?Await]
ClassElement[?Yield, ?Await] ClassElement[Yield, Await] :
MethodDefinition[?Yield, ?Await] static MethodDefinition[?Yield, ?Await]
FieldDefinition[?Yield, ?Await] ; static FieldDefinition[?Yield, ?Await] ;
ClassStaticBlock ; FieldDefinition[Yield, Await] : ClassElementName[?Yield,
?Await] Initializer[+In, ?Yield, ?Await]opt ClassElementName[Yield, Await] :
PropertyName[?Yield, ?Await] PrivateIdentifier ClassStaticBlock : static {
ClassStaticBlockBody } ClassStaticBlockBody : ClassStaticBlockStatementList
ClassStaticBlockStatementList : StatementList[~Yield, +Await, ~Return]opt Note
A class definition is always strict mode code.
15.7.1 STATIC SEMANTICS: EARLY ERRORS
ClassTail : ClassHeritageopt { ClassBody }
* It is a Syntax Error if ClassHeritage is not present and the following
algorithm returns true:
1. 1. 1. Let constructor be ConstructorMethod of ClassBody.
2. 2. 2. If constructor is empty, return false.
3. 3. 3. Return HasDirectSuper of constructor.
ClassBody : ClassElementList
* It is a Syntax Error if PrototypePropertyNameList of ClassElementList
contains more than one occurrence of "constructor".
* It is a Syntax Error if PrivateBoundIdentifiers of ClassElementList contains
any duplicate entries, unless the name is used once for a getter and once for
a setter and in no other entries, and the getter and setter are either both
static or both non-static.
ClassElement : MethodDefinition
* It is a Syntax Error if PropName of MethodDefinition is not "constructor" and
HasDirectSuper of MethodDefinition is true.
* It is a Syntax Error if PropName of MethodDefinition is "constructor" and
SpecialMethod of MethodDefinition is true.
ClassElement : static MethodDefinition
* It is a Syntax Error if HasDirectSuper of MethodDefinition is true.
* It is a Syntax Error if PropName of MethodDefinition is "prototype".
ClassElement : FieldDefinition ;
* It is a Syntax Error if PropName of FieldDefinition is "constructor".
ClassElement : static FieldDefinition ;
* It is a Syntax Error if PropName of FieldDefinition is either "prototype" or
"constructor".
FieldDefinition : ClassElementName Initializeropt
* It is a Syntax Error if Initializer is present and ContainsArguments of
Initializer is true.
* It is a Syntax Error if Initializer is present and Initializer Contains
SuperCall is true.
ClassElementName : PrivateIdentifier
* It is a Syntax Error if StringValue of PrivateIdentifier is "#constructor".
ClassStaticBlockBody : ClassStaticBlockStatementList
* It is a Syntax Error if the LexicallyDeclaredNames of
ClassStaticBlockStatementList contains any duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
ClassStaticBlockStatementList also occurs in the VarDeclaredNames of
ClassStaticBlockStatementList.
* It is a Syntax Error if ContainsDuplicateLabels of
ClassStaticBlockStatementList with argument « » is true.
* It is a Syntax Error if ContainsUndefinedBreakTarget of
ClassStaticBlockStatementList with argument « » is true.
* It is a Syntax Error if ContainsUndefinedContinueTarget of
ClassStaticBlockStatementList with arguments « » and « » is true.
* It is a Syntax Error if ContainsArguments of ClassStaticBlockStatementList is
true.
* It is a Syntax Error if ClassStaticBlockStatementList Contains SuperCall is
true.
* It is a Syntax Error if ClassStaticBlockStatementList Contains await is true.
15.7.2 STATIC SEMANTICS: CLASSELEMENTKIND
The syntax-directed operation ClassElementKind takes no arguments and returns
ConstructorMethod, NonConstructorMethod, or empty. It is defined piecewise over
the following productions:
ClassElement : MethodDefinition
1. 1. 1. If PropName of MethodDefinition is "constructor", return
ConstructorMethod.
2. 2. 2. Return NonConstructorMethod.
ClassElement : static MethodDefinition FieldDefinition ; static FieldDefinition
;
1. 1. 1. Return NonConstructorMethod.
ClassElement : ClassStaticBlock
1. 1. 1. Return NonConstructorMethod.
ClassElement : ;
1. 1. 1. Return empty.
15.7.3 STATIC SEMANTICS: CONSTRUCTORMETHOD
The syntax-directed operation ConstructorMethod takes no arguments and returns a
ClassElement Parse Node or empty. It is defined piecewise over the following
productions:
ClassElementList : ClassElement
1. 1. 1. If ClassElementKind of ClassElement is ConstructorMethod, return
ClassElement.
2. 2. 2. Return empty.
ClassElementList : ClassElementList ClassElement
1. 1. 1. Let head be ConstructorMethod of ClassElementList.
2. 2. 2. If head is not empty, return head.
3. 3. 3. If ClassElementKind of ClassElement is ConstructorMethod, return
ClassElement.
4. 4. 4. Return empty.
Note
Early Error rules ensure that there is only one method definition named
"constructor" and that it is not an accessor property or generator definition.
15.7.4 STATIC SEMANTICS: ISSTATIC
The syntax-directed operation IsStatic takes no arguments and returns a Boolean.
It is defined piecewise over the following productions:
ClassElement : MethodDefinition
1. 1. 1. Return false.
ClassElement : static MethodDefinition
1. 1. 1. Return true.
ClassElement : FieldDefinition ;
1. 1. 1. Return false.
ClassElement : static FieldDefinition ;
1. 1. 1. Return true.
ClassElement : ClassStaticBlock
1. 1. 1. Return true.
ClassElement : ;
1. 1. 1. Return false.
15.7.5 STATIC SEMANTICS: NONCONSTRUCTORELEMENTS
The syntax-directed operation NonConstructorElements takes no arguments and
returns a List of ClassElement Parse Nodes. It is defined piecewise over the
following productions:
ClassElementList : ClassElement
1. 1. 1. If ClassElementKind of ClassElement is NonConstructorMethod, then
1. a. a. Return « ClassElement ».
2. 2. 2. Return a new empty List.
ClassElementList : ClassElementList ClassElement
1. 1. 1. Let list be NonConstructorElements of ClassElementList.
2. 2. 2. If ClassElementKind of ClassElement is NonConstructorMethod, then
1. a. a. Append ClassElement to the end of list.
3. 3. 3. Return list.
15.7.6 STATIC SEMANTICS: PROTOTYPEPROPERTYNAMELIST
The syntax-directed operation PrototypePropertyNameList takes no arguments and
returns a List of property keys. It is defined piecewise over the following
productions:
ClassElementList : ClassElement
1. 1. 1. Let propName be PropName of ClassElement.
2. 2. 2. If propName is empty, return a new empty List.
3. 3. 3. If IsStatic of ClassElement is true, return a new empty List.
4. 4. 4. Return « propName ».
ClassElementList : ClassElementList ClassElement
1. 1. 1. Let list be PrototypePropertyNameList of ClassElementList.
2. 2. 2. Let propName be PropName of ClassElement.
3. 3. 3. If propName is empty, return list.
4. 4. 4. If IsStatic of ClassElement is true, return list.
5. 5. 5. Return the list-concatenation of list and « propName ».
15.7.7 STATIC SEMANTICS: ALLPRIVATEIDENTIFIERSVALID
The syntax-directed operation AllPrivateIdentifiersValid takes argument names (a
List of Strings) and returns a Boolean.
Every grammar production alternative in this specification which is not listed
below implicitly has the following default definition of
AllPrivateIdentifiersValid:
1. 1. 1. For each child node child of this Parse Node, do
1. a. a. If child is an instance of a nonterminal, then
1. i. i. If AllPrivateIdentifiersValid of child with argument names is
false, return false.
2. 2. 2. Return true.
MemberExpression : MemberExpression . PrivateIdentifier
1. 1. 1. If names contains the StringValue of PrivateIdentifier, then
1. a. a. Return AllPrivateIdentifiersValid of MemberExpression with argument
names.
2. 2. 2. Return false.
CallExpression : CallExpression . PrivateIdentifier
1. 1. 1. If names contains the StringValue of PrivateIdentifier, then
1. a. a. Return AllPrivateIdentifiersValid of CallExpression with argument
names.
2. 2. 2. Return false.
OptionalChain : ?. PrivateIdentifier
1. 1. 1. If names contains the StringValue of PrivateIdentifier, return true.
2. 2. 2. Return false.
OptionalChain : OptionalChain . PrivateIdentifier
1. 1. 1. If names contains the StringValue of PrivateIdentifier, then
1. a. a. Return AllPrivateIdentifiersValid of OptionalChain with argument
names.
2. 2. 2. Return false.
ClassBody : ClassElementList
1. 1. 1. Let newNames be the list-concatenation of names and
PrivateBoundIdentifiers of ClassBody.
2. 2. 2. Return AllPrivateIdentifiersValid of ClassElementList with argument
newNames.
RelationalExpression : PrivateIdentifier in ShiftExpression
1. 1. 1. If names contains the StringValue of PrivateIdentifier, then
1. a. a. Return AllPrivateIdentifiersValid of ShiftExpression with argument
names.
2. 2. 2. Return false.
15.7.8 STATIC SEMANTICS: PRIVATEBOUNDIDENTIFIERS
The syntax-directed operation PrivateBoundIdentifiers takes no arguments and
returns a List of Strings. It is defined piecewise over the following
productions:
FieldDefinition : ClassElementName Initializeropt
1. 1. 1. Return PrivateBoundIdentifiers of ClassElementName.
ClassElementName : PrivateIdentifier
1. 1. 1. Return a List whose sole element is the StringValue of
PrivateIdentifier.
ClassElementName : PropertyName ClassElement : ClassStaticBlock ;
1. 1. 1. Return a new empty List.
ClassElementList : ClassElementList ClassElement
1. 1. 1. Let names1 be PrivateBoundIdentifiers of ClassElementList.
2. 2. 2. Let names2 be PrivateBoundIdentifiers of ClassElement.
3. 3. 3. Return the list-concatenation of names1 and names2.
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
get ClassElementName ( ) { FunctionBody } set ClassElementName (
PropertySetParameterList ) { FunctionBody } GeneratorMethod : * ClassElementName
( UniqueFormalParameters ) { GeneratorBody } AsyncMethod : async
ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) {
AsyncGeneratorBody }
1. 1. 1. Return PrivateBoundIdentifiers of ClassElementName.
15.7.9 STATIC SEMANTICS: CONTAINSARGUMENTS
The syntax-directed operation ContainsArguments takes no arguments and returns a
Boolean.
Every grammar production alternative in this specification which is not listed
below implicitly has the following default definition of ContainsArguments:
1. 1. 1. For each child node child of this Parse Node, do
1. a. a. If child is an instance of a nonterminal, then
1. i. i. If ContainsArguments of child is true, return true.
2. 2. 2. Return false.
IdentifierReference : Identifier
1. 1. 1. If the StringValue of Identifier is "arguments", return true.
2. 2. 2. Return false.
FunctionDeclaration : function BindingIdentifier ( FormalParameters ) {
FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression
: function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) {
GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) {
GeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier (
FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters )
{ AsyncGeneratorBody } AsyncGeneratorExpression : async function *
BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }
AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters
) { AsyncFunctionBody }
1. 1. 1. Return false.
MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }
get ClassElementName ( ) { FunctionBody } set ClassElementName (
PropertySetParameterList ) { FunctionBody } GeneratorMethod : * ClassElementName
( UniqueFormalParameters ) { GeneratorBody } AsyncGeneratorMethod : async *
ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody } AsyncMethod :
async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
1. 1. 1. Return ContainsArguments of ClassElementName.
15.7.10 RUNTIME SEMANTICS: CLASSFIELDDEFINITIONEVALUATION
The syntax-directed operation ClassFieldDefinitionEvaluation takes argument
homeObject (an Object) and returns either a normal completion containing a
ClassFieldDefinition Record or an abrupt completion. It is defined piecewise
over the following productions:
FieldDefinition : ClassElementName Initializeropt
1. 1. 1. Let name be ? Evaluation of ClassElementName.
2. 2. 2. If Initializeropt is present, then
1. a. a. Let formalParameterList be an instance of the production
FormalParameters : [empty] .
2. b. b. Let env be the LexicalEnvironment of the running execution context.
3. c. c. Let privateEnv be the running execution context's
PrivateEnvironment.
4. d. d. Let sourceText be the empty sequence of Unicode code points.
5. e. e. Let initializer be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, formalParameterList, Initializer, non-lexical-this, env,
privateEnv).
6. f. f. Perform MakeMethod(initializer, homeObject).
7. g. g. Set initializer.[[ClassFieldInitializerName]] to name.
3. 3. 3. Else,
1. a. a. Let initializer be empty.
4. 4. 4. Return the ClassFieldDefinition Record { [[Name]]: name,
[[Initializer]]: initializer }.
Note
The function created for initializer is never directly accessible to ECMAScript
code.
15.7.11 RUNTIME SEMANTICS: CLASSSTATICBLOCKDEFINITIONEVALUATION
The syntax-directed operation ClassStaticBlockDefinitionEvaluation takes
argument homeObject (an Object) and returns a ClassStaticBlockDefinition Record.
It is defined piecewise over the following productions:
ClassStaticBlock : static { ClassStaticBlockBody }
1. 1. 1. Let lex be the running execution context's LexicalEnvironment.
2. 2. 2. Let privateEnv be the running execution context's PrivateEnvironment.
3. 3. 3. Let sourceText be the empty sequence of Unicode code points.
4. 4. 4. Let formalParameters be an instance of the production FormalParameters
: [empty] .
5. 5. 5. Let bodyFunction be OrdinaryFunctionCreate(%Function.prototype%,
sourceText, formalParameters, ClassStaticBlockBody, non-lexical-this, lex,
privateEnv).
6. 6. 6. Perform MakeMethod(bodyFunction, homeObject).
7. 7. 7. Return the ClassStaticBlockDefinition Record { [[BodyFunction]]:
bodyFunction }.
Note
The function bodyFunction is never directly accessible to ECMAScript code.
15.7.12 RUNTIME SEMANTICS: EVALUATECLASSSTATICBLOCKBODY
The syntax-directed operation EvaluateClassStaticBlockBody takes argument
functionObject (a function object) and returns either a normal completion
containing an ECMAScript language value or an abrupt completion. It is defined
piecewise over the following productions:
ClassStaticBlockBody : ClassStaticBlockStatementList
1. 1. 1. Perform ? FunctionDeclarationInstantiation(functionObject, « »).
2. 2. 2. Return ? Evaluation of ClassStaticBlockStatementList.
15.7.13 RUNTIME SEMANTICS: CLASSELEMENTEVALUATION
The syntax-directed operation ClassElementEvaluation takes argument object (an
Object) and returns either a normal completion containing either a
ClassFieldDefinition Record, a ClassStaticBlockDefinition Record, a
PrivateElement, or unused, or an abrupt completion. It is defined piecewise over
the following productions:
ClassElement : FieldDefinition ; static FieldDefinition ;
1. 1. 1. Return ? ClassFieldDefinitionEvaluation of FieldDefinition with
argument object.
ClassElement : MethodDefinition static MethodDefinition
1. 1. 1. Return ? MethodDefinitionEvaluation of MethodDefinition with arguments
object and false.
ClassElement : ClassStaticBlock
1. 1. 1. Return ClassStaticBlockDefinitionEvaluation of ClassStaticBlock with
argument object.
ClassElement : ;
1. 1. 1. Return unused.
15.7.14 RUNTIME SEMANTICS: CLASSDEFINITIONEVALUATION
The syntax-directed operation ClassDefinitionEvaluation takes arguments
classBinding (a String or undefined) and className (a property key or a Private
Name) and returns either a normal completion containing a function object or an
abrupt completion.
Note
For ease of specification, private methods and accessors are included alongside
private fields in the [[PrivateElements]] slot of class instances. However, any
given object has either all or none of the private methods and accessors defined
by a given class. This feature has been designed so that implementations may
choose to implement private methods and accessors using a strategy which does
not require tracking each method or accessor individually.
For example, an implementation could directly associate instance private methods
with their corresponding Private Name and track, for each object, which class
constructors have run with that object as their this value. Looking up an
instance private method on an object then consists of checking that the class
constructor which defines the method has been used to initialize the object,
then returning the method associated with the Private Name.
This differs from private fields: because field initializers can throw during
class instantiation, an individual object may have some proper subset of the
private fields of a given class, and so private fields must in general be
tracked individually.
It is defined piecewise over the following productions:
ClassTail : ClassHeritageopt { ClassBodyopt }
1. 1. 1. Let env be the LexicalEnvironment of the running execution context.
2. 2. 2. Let classEnv be NewDeclarativeEnvironment(env).
3. 3. 3. If classBinding is not undefined, then
1. a. a. Perform ! classEnv.CreateImmutableBinding(classBinding, true).
4. 4. 4. Let outerPrivateEnvironment be the running execution context's
PrivateEnvironment.
5. 5. 5. Let classPrivateEnvironment be
NewPrivateEnvironment(outerPrivateEnvironment).
6. 6. 6. If ClassBodyopt is present, then
1. a. a. For each String dn of the PrivateBoundIdentifiers of ClassBodyopt,
do
1. i. i. If classPrivateEnvironment.[[Names]] contains a Private Name pn
such that pn.[[Description]] is dn, then
1. 1. 1. Assert: This is only possible for getter/setter pairs.
2. ii. ii. Else,
1. 1. 1. Let name be a new Private Name whose [[Description]] is dn.
2. 2. 2. Append name to classPrivateEnvironment.[[Names]].
7. 7. 7. If ClassHeritageopt is not present, then
1. a. a. Let protoParent be %Object.prototype%.
2. b. b. Let constructorParent be %Function.prototype%.
8. 8. 8. Else,
1. a. a. Set the running execution context's LexicalEnvironment to
classEnv.
2. b. b. NOTE: The running execution context's PrivateEnvironment is
outerPrivateEnvironment when evaluating ClassHeritage.
3. c. c. Let superclassRef be Completion(Evaluation of ClassHeritage).
4. d. d. Set the running execution context's LexicalEnvironment to env.
5. e. e. Let superclass be ? GetValue(? superclassRef).
6. f. f. If superclass is null, then
1. i. i. Let protoParent be null.
2. ii. ii. Let constructorParent be %Function.prototype%.
7. g. g. Else if IsConstructor(superclass) is false, throw a TypeError
exception.
8. h. h. Else,
1. i. i. Let protoParent be ? Get(superclass, "prototype").
2. ii. ii. If protoParent is not an Object and protoParent is not null,
throw a TypeError exception.
3. iii. iii. Let constructorParent be superclass.
9. 9. 9. Let proto be OrdinaryObjectCreate(protoParent).
10. 10. 10. If ClassBodyopt is not present, let constructor be empty.
11. 11. 11. Else, let constructor be ConstructorMethod of ClassBody.
12. 12. 12. Set the running execution context's LexicalEnvironment to classEnv.
13. 13. 13. Set the running execution context's PrivateEnvironment to
classPrivateEnvironment.
14. 14. 14. If constructor is empty, then
1. a. a. Let defaultConstructor be a new Abstract Closure with no
parameters that captures nothing and performs the following steps when
called:
1. i. i. Let args be the List of arguments that was passed to this
function by [[Call]] or [[Construct]].
2. ii. ii. If NewTarget is undefined, throw a TypeError exception.
3. iii. iii. Let F be the active function object.
4. iv. iv. If F.[[ConstructorKind]] is derived, then
1. 1. 1. NOTE: This branch behaves similarly to constructor(...args)
{ super(...args); }. The most notable distinction is that while
the aforementioned ECMAScript source text observably calls the
@@iterator method on %Array.prototype%, this function does not.
2. 2. 2. Let func be ! F.[[GetPrototypeOf]]().
3. 3. 3. If IsConstructor(func) is false, throw a TypeError
exception.
4. 4. 4. Let result be ? Construct(func, args, NewTarget).
5. v. v. Else,
1. 1. 1. NOTE: This branch behaves similarly to constructor() {}.
2. 2. 2. Let result be ? OrdinaryCreateFromConstructor(NewTarget,
"%Object.prototype%").
6. vi. vi. Perform ? InitializeInstanceElements(result, F).
7. vii. vii. Return result.
2. b. b. Let F be CreateBuiltinFunction(defaultConstructor, 0, className, «
[[ConstructorKind]], [[SourceText]] », the current Realm Record,
constructorParent).
15. 15. 15. Else,
1. a. a. Let constructorInfo be ! DefineMethod of constructor with
arguments proto and constructorParent.
2. b. b. Let F be constructorInfo.[[Closure]].
3. c. c. Perform MakeClassConstructor(F).
4. d. d. Perform SetFunctionName(F, className).
16. 16. 16. Perform MakeConstructor(F, false, proto).
17. 17. 17. If ClassHeritageopt is present, set F.[[ConstructorKind]] to
derived.
18. 18. 18. Perform CreateMethodProperty(proto, "constructor", F).
19. 19. 19. If ClassBodyopt is not present, let elements be a new empty List.
20. 20. 20. Else, let elements be NonConstructorElements of ClassBody.
21. 21. 21. Let instancePrivateMethods be a new empty List.
22. 22. 22. Let staticPrivateMethods be a new empty List.
23. 23. 23. Let instanceFields be a new empty List.
24. 24. 24. Let staticElements be a new empty List.
25. 25. 25. For each ClassElement e of elements, do
1. a. a. If IsStatic of e is false, then
1. i. i. Let element be Completion(ClassElementEvaluation of e with
argument proto).
2. b. b. Else,
1. i. i. Let element be Completion(ClassElementEvaluation of e with
argument F).
3. c. c. If element is an abrupt completion, then
1. i. i. Set the running execution context's LexicalEnvironment to env.
2. ii. ii. Set the running execution context's PrivateEnvironment to
outerPrivateEnvironment.
3. iii. iii. Return ? element.
4. d. d. Set element to element.[[Value]].
5. e. e. If element is a PrivateElement, then
1. i. i. Assert: element.[[Kind]] is either method or accessor.
2. ii. ii. If IsStatic of e is false, let container be
instancePrivateMethods.
3. iii. iii. Else, let container be staticPrivateMethods.
4. iv. iv. If container contains a PrivateElement pe such that
pe.[[Key]] is element.[[Key]], then
1. 1. 1. Assert: element.[[Kind]] and pe.[[Kind]] are both accessor.
2. 2. 2. If element.[[Get]] is undefined, then
1. a. a. Let combined be PrivateElement { [[Key]]:
element.[[Key]], [[Kind]]: accessor, [[Get]]: pe.[[Get]],
[[Set]]: element.[[Set]] }.
3. 3. 3. Else,
1. a. a. Let combined be PrivateElement { [[Key]]:
element.[[Key]], [[Kind]]: accessor, [[Get]]: element.[[Get]],
[[Set]]: pe.[[Set]] }.
4. 4. 4. Replace pe in container with combined.
5. v. v. Else,
1. 1. 1. Append element to container.
6. f. f. Else if element is a ClassFieldDefinition Record, then
1. i. i. If IsStatic of e is false, append element to instanceFields.
2. ii. ii. Else, append element to staticElements.
7. g. g. Else if element is a ClassStaticBlockDefinition Record, then
1. i. i. Append element to staticElements.
26. 26. 26. Set the running execution context's LexicalEnvironment to env.
27. 27. 27. If classBinding is not undefined, then
1. a. a. Perform ! classEnv.InitializeBinding(classBinding, F).
28. 28. 28. Set F.[[PrivateMethods]] to instancePrivateMethods.
29. 29. 29. Set F.[[Fields]] to instanceFields.
30. 30. 30. For each PrivateElement method of staticPrivateMethods, do
1. a. a. Perform ! PrivateMethodOrAccessorAdd(F, method).
31. 31. 31. For each element elementRecord of staticElements, do
1. a. a. If elementRecord is a ClassFieldDefinition Record, then
1. i. i. Let result be Completion(DefineField(F, elementRecord)).
2. b. b. Else,
1. i. i. Assert: elementRecord is a ClassStaticBlockDefinition Record.
2. ii. ii. Let result be Completion(Call(elementRecord.[[BodyFunction]],
F)).
3. c. c. If result is an abrupt completion, then
1. i. i. Set the running execution context's PrivateEnvironment to
outerPrivateEnvironment.
2. ii. ii. Return ? result.
32. 32. 32. Set the running execution context's PrivateEnvironment to
outerPrivateEnvironment.
33. 33. 33. Return F.
15.7.15 RUNTIME SEMANTICS: BINDINGCLASSDECLARATIONEVALUATION
The syntax-directed operation BindingClassDeclarationEvaluation takes no
arguments and returns either a normal completion containing a function object or
an abrupt completion. It is defined piecewise over the following productions:
ClassDeclaration : class BindingIdentifier ClassTail
1. 1. 1. Let className be StringValue of BindingIdentifier.
2. 2. 2. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments
className and className.
3. 3. 3. Set value.[[SourceText]] to the source text matched by
ClassDeclaration.
4. 4. 4. Let env be the running execution context's LexicalEnvironment.
5. 5. 5. Perform ? InitializeBoundName(className, value, env).
6. 6. 6. Return value.
ClassDeclaration : class ClassTail
1. 1. 1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments
undefined and "default".
2. 2. 2. Set value.[[SourceText]] to the source text matched by
ClassDeclaration.
3. 3. 3. Return value.
Note
ClassDeclaration : class ClassTail only occurs as part of an ExportDeclaration
and establishing its binding is handled as part of the evaluation action for
that production. See 16.2.3.7.
15.7.16 RUNTIME SEMANTICS: EVALUATION
ClassDeclaration : class BindingIdentifier ClassTail
1. 1. 1. Perform ? BindingClassDeclarationEvaluation of this ClassDeclaration.
2. 2. 2. Return empty.
Note
ClassDeclaration : class ClassTail only occurs as part of an ExportDeclaration
and is never directly evaluated.
ClassExpression : class ClassTail
1. 1. 1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments
undefined and "".
2. 2. 2. Set value.[[SourceText]] to the source text matched by
ClassExpression.
3. 3. 3. Return value.
ClassExpression : class BindingIdentifier ClassTail
1. 1. 1. Let className be StringValue of BindingIdentifier.
2. 2. 2. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments
className and className.
3. 3. 3. Set value.[[SourceText]] to the source text matched by
ClassExpression.
4. 4. 4. Return value.
ClassElementName : PrivateIdentifier
1. 1. 1. Let privateIdentifier be StringValue of PrivateIdentifier.
2. 2. 2. Let privateEnvRec be the running execution context's
PrivateEnvironment.
3. 3. 3. Let names be privateEnvRec.[[Names]].
4. 4. 4. Assert: Exactly one element of names is a Private Name whose
[[Description]] is privateIdentifier.
5. 5. 5. Let privateName be the Private Name in names whose [[Description]] is
privateIdentifier.
6. 6. 6. Return privateName.
ClassStaticBlockStatementList : [empty]
1. 1. 1. Return undefined.
15.8 ASYNC FUNCTION DEFINITIONS
SYNTAX
AsyncFunctionDeclaration[Yield, Await, Default] : async [no LineTerminator here]
function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, +Await] )
{ AsyncFunctionBody } [+Default] async [no LineTerminator here] function (
FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncFunctionExpression
: async [no LineTerminator here] function BindingIdentifier[~Yield, +Await]opt (
FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncMethod[Yield,
Await] : async [no LineTerminator here] ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncFunctionBody
: FunctionBody[~Yield, +Await] AwaitExpression[Yield] : await
UnaryExpression[?Yield, +Await] Note 1
await is parsed as a keyword of an AwaitExpression when the [Await] parameter is
present. The [Await] parameter is present in the top level of the following
contexts, although the parameter may be absent in some contexts depending on the
nonterminals, such as FunctionBody:
* In an AsyncFunctionBody.
* In the FormalParameters of an AsyncFunctionDeclaration,
AsyncFunctionExpression, AsyncGeneratorDeclaration, or
AsyncGeneratorExpression. AwaitExpression in this position is a Syntax error
via static semantics.
* In a Module.
When Script is the syntactic goal symbol, await may be parsed as an identifier
when the [Await] parameter is absent. This includes the following contexts:
* Anywhere outside of an AsyncFunctionBody or FormalParameters of an
AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration,
or AsyncGeneratorExpression.
* In the BindingIdentifier of a FunctionExpression, GeneratorExpression, or
AsyncGeneratorExpression.
Note 2
Unlike YieldExpression, it is a Syntax Error to omit the operand of an
AwaitExpression. You must await something.
15.8.1 STATIC SEMANTICS: EARLY ERRORS
AsyncMethod : async ClassElementName ( UniqueFormalParameters ) {
AsyncFunctionBody }
* It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncFunctionBody is
true and IsSimpleParameterList of UniqueFormalParameters is false.
* It is a Syntax Error if HasDirectSuper of AsyncMethod is true.
* It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is
true.
* It is a Syntax Error if any element of the BoundNames of
UniqueFormalParameters also occurs in the LexicallyDeclaredNames of
AsyncFunctionBody.
AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters
) { AsyncFunctionBody }
* It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncFunctionBody is
true and IsSimpleParameterList of FormalParameters is false.
* It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
* If the source text matched by FormalParameters is strict mode code, the Early
Error rules for UniqueFormalParameters : FormalParameters are applied.
* If BindingIdentifier is present and the source text matched by
BindingIdentifier is strict mode code, it is a Syntax Error if the
StringValue of BindingIdentifier is either "eval" or "arguments".
* It is a Syntax Error if any element of the BoundNames of FormalParameters
also occurs in the LexicallyDeclaredNames of AsyncFunctionBody.
* It is a Syntax Error if FormalParameters Contains SuperProperty is true.
* It is a Syntax Error if AsyncFunctionBody Contains SuperProperty is true.
* It is a Syntax Error if FormalParameters Contains SuperCall is true.
* It is a Syntax Error if AsyncFunctionBody Contains SuperCall is true.
15.8.2 RUNTIME SEMANTICS: INSTANTIATEASYNCFUNCTIONOBJECT
The syntax-directed operation InstantiateAsyncFunctionObject takes arguments env
(an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and
returns a function object. It is defined piecewise over the following
productions:
AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody }
1. 1. 1. Let name be StringValue of BindingIdentifier.
2. 2. 2. Let sourceText be the source text matched by AsyncFunctionDeclaration.
3. 3. 3. Let F be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,
FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
4. 4. 4. Perform SetFunctionName(F, name).
5. 5. 5. Return F.
AsyncFunctionDeclaration : async function ( FormalParameters ) {
AsyncFunctionBody }
1. 1. 1. Let sourceText be the source text matched by AsyncFunctionDeclaration.
2. 2. 2. Let F be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,
FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
3. 3. 3. Perform SetFunctionName(F, "default").
4. 4. 4. Return F.
15.8.3 RUNTIME SEMANTICS: INSTANTIATEASYNCFUNCTIONEXPRESSION
The syntax-directed operation InstantiateAsyncFunctionExpression takes optional
argument name (a property key or a Private Name) and returns a function object.
It is defined piecewise over the following productions:
AsyncFunctionExpression : async function ( FormalParameters ) {
AsyncFunctionBody }
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by AsyncFunctionExpression.
5. 5. 5. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%,
sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env,
privateEnv).
6. 6. 6. Perform SetFunctionName(closure, name).
7. 7. 7. Return closure.
AsyncFunctionExpression : async function BindingIdentifier ( FormalParameters )
{ AsyncFunctionBody }
1. 1. 1. Assert: name is not present.
2. 2. 2. Set name to StringValue of BindingIdentifier.
3. 3. 3. Let outerEnv be the LexicalEnvironment of the running execution
context.
4. 4. 4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. 5. 5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. 6. 6. Let privateEnv be the running execution context's PrivateEnvironment.
7. 7. 7. Let sourceText be the source text matched by AsyncFunctionExpression.
8. 8. 8. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%,
sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, funcEnv,
privateEnv).
9. 9. 9. Perform SetFunctionName(closure, name).
10. 10. 10. Perform ! funcEnv.InitializeBinding(name, closure).
11. 11. 11. Return closure.
Note
The BindingIdentifier in an AsyncFunctionExpression can be referenced from
inside the AsyncFunctionExpression's AsyncFunctionBody to allow the function to
call itself recursively. However, unlike in a FunctionDeclaration, the
BindingIdentifier in a AsyncFunctionExpression cannot be referenced from and
does not affect the scope enclosing the AsyncFunctionExpression.
15.8.4 RUNTIME SEMANTICS: EVALUATEASYNCFUNCTIONBODY
The syntax-directed operation EvaluateAsyncFunctionBody takes arguments
functionObject (a function object) and argumentsList (a List of ECMAScript
language values) and returns a return completion. It is defined piecewise over
the following productions:
AsyncFunctionBody : FunctionBody
1. 1. 1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
2. 2. 2. Let declResult be
Completion(FunctionDeclarationInstantiation(functionObject, argumentsList)).
3. 3. 3. If declResult is an abrupt completion, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, «
declResult.[[Value]] »).
4. 4. 4. Else,
1. a. a. Perform AsyncFunctionStart(promiseCapability, FunctionBody).
5. 5. 5. Return Completion Record { [[Type]]: return, [[Value]]:
promiseCapability.[[Promise]], [[Target]]: empty }.
15.8.5 RUNTIME SEMANTICS: EVALUATION
AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters
) { AsyncFunctionBody }
1. 1. 1. Return InstantiateAsyncFunctionExpression of AsyncFunctionExpression.
AwaitExpression : await UnaryExpression
1. 1. 1. Let exprRef be ? Evaluation of UnaryExpression.
2. 2. 2. Let value be ? GetValue(exprRef).
3. 3. 3. Return ? Await(value).
15.9 ASYNC ARROW FUNCTION DEFINITIONS
SYNTAX
AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here]
AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] =>
AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no
LineTerminator here] => AsyncConciseBody[?In] AsyncConciseBody[In] : [lookahead
≠ {] ExpressionBody[?In, +Await] { AsyncFunctionBody }
AsyncArrowBindingIdentifier[Yield] : BindingIdentifier[?Yield, +Await]
CoverCallExpressionAndAsyncArrowHead[Yield, Await] : MemberExpression[?Yield,
?Await] Arguments[?Yield, ?Await]
SUPPLEMENTAL SYNTAX
When processing an instance of the production
AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the
following grammar:
AsyncArrowHead : async [no LineTerminator here] ArrowFormalParameters[~Yield,
+Await]
15.9.1 STATIC SEMANTICS: EARLY ERRORS
AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
* It is a Syntax Error if any element of the BoundNames of
AsyncArrowBindingIdentifier also occurs in the LexicallyDeclaredNames of
AsyncConciseBody.
AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
* CoverCallExpressionAndAsyncArrowHead must cover an AsyncArrowHead.
* It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead Contains
YieldExpression is true.
* It is a Syntax Error if CoverCallExpressionAndAsyncArrowHead Contains
AwaitExpression is true.
* It is a Syntax Error if any element of the BoundNames of
CoverCallExpressionAndAsyncArrowHead also occurs in the
LexicallyDeclaredNames of AsyncConciseBody.
* It is a Syntax Error if AsyncConciseBodyContainsUseStrict of AsyncConciseBody
is true and IsSimpleParameterList of CoverCallExpressionAndAsyncArrowHead is
false.
15.9.2 STATIC SEMANTICS: ASYNCCONCISEBODYCONTAINSUSESTRICT
The syntax-directed operation AsyncConciseBodyContainsUseStrict takes no
arguments and returns a Boolean. It is defined piecewise over the following
productions:
AsyncConciseBody : ExpressionBody
1. 1. 1. Return false.
AsyncConciseBody : { AsyncFunctionBody }
1. 1. 1. Return FunctionBodyContainsUseStrict of AsyncFunctionBody.
15.9.3 RUNTIME SEMANTICS: EVALUATEASYNCCONCISEBODY
The syntax-directed operation EvaluateAsyncConciseBody takes arguments
functionObject (a function object) and argumentsList (a List of ECMAScript
language values) and returns a return completion. It is defined piecewise over
the following productions:
AsyncConciseBody : ExpressionBody
1. 1. 1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
2. 2. 2. Let declResult be
Completion(FunctionDeclarationInstantiation(functionObject, argumentsList)).
3. 3. 3. If declResult is an abrupt completion, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, «
declResult.[[Value]] »).
4. 4. 4. Else,
1. a. a. Perform AsyncFunctionStart(promiseCapability, ExpressionBody).
5. 5. 5. Return Completion Record { [[Type]]: return, [[Value]]:
promiseCapability.[[Promise]], [[Target]]: empty }.
15.9.4 RUNTIME SEMANTICS: INSTANTIATEASYNCARROWFUNCTIONEXPRESSION
The syntax-directed operation InstantiateAsyncArrowFunctionExpression takes
optional argument name (a property key or a Private Name) and returns a function
object. It is defined piecewise over the following productions:
AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by AsyncArrowFunction.
5. 5. 5. Let parameters be AsyncArrowBindingIdentifier.
6. 6. 6. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%,
sourceText, parameters, AsyncConciseBody, lexical-this, env, privateEnv).
7. 7. 7. Perform SetFunctionName(closure, name).
8. 8. 8. Return closure.
AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
1. 1. 1. If name is not present, set name to "".
2. 2. 2. Let env be the LexicalEnvironment of the running execution context.
3. 3. 3. Let privateEnv be the running execution context's PrivateEnvironment.
4. 4. 4. Let sourceText be the source text matched by AsyncArrowFunction.
5. 5. 5. Let head be the AsyncArrowHead that is covered by
CoverCallExpressionAndAsyncArrowHead.
6. 6. 6. Let parameters be the ArrowFormalParameters of head.
7. 7. 7. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%,
sourceText, parameters, AsyncConciseBody, lexical-this, env, privateEnv).
8. 8. 8. Perform SetFunctionName(closure, name).
9. 9. 9. Return closure.
15.9.5 RUNTIME SEMANTICS: EVALUATION
AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
1. 1. 1. Return InstantiateAsyncArrowFunctionExpression of AsyncArrowFunction.
15.10 TAIL POSITION CALLS
15.10.1 STATIC SEMANTICS: ISINTAILPOSITION ( CALL )
The abstract operation IsInTailPosition takes argument call (a CallExpression
Parse Node, a MemberExpression Parse Node, or an OptionalChain Parse Node) and
returns a Boolean. It performs the following steps when called:
1. 1. 1. If the source text matched by call is non-strict code, return false.
2. 2. 2. If call is not contained within a FunctionBody, a ConciseBody, or an
AsyncConciseBody, return false.
3. 3. 3. Let body be the FunctionBody, ConciseBody, or AsyncConciseBody that
most closely contains call.
4. 4. 4. If body is the FunctionBody of a GeneratorBody, return false.
5. 5. 5. If body is the FunctionBody of an AsyncFunctionBody, return false.
6. 6. 6. If body is the FunctionBody of an AsyncGeneratorBody, return false.
7. 7. 7. If body is an AsyncConciseBody, return false.
8. 8. 8. Return the result of HasCallInTailPosition of body with argument call.
Note
Tail Position calls are only defined in strict mode code because of a common
non-standard language extension (see 10.2.4) that enables observation of the
chain of caller contexts.
15.10.2 STATIC SEMANTICS: HASCALLINTAILPOSITION
The syntax-directed operation HasCallInTailPosition takes argument call (a
CallExpression Parse Node, a MemberExpression Parse Node, or an OptionalChain
Parse Node) and returns a Boolean.
Note 1
call is a Parse Node that represents a specific range of source text. When the
following algorithms compare call to another Parse Node, it is a test of whether
they represent the same source text.
Note 2
A potential tail position call that is immediately followed by return GetValue
of the call result is also a possible tail position call. A function call cannot
return a Reference Record, so such a GetValue operation will always return the
same value as the actual function call result.
It is defined piecewise over the following productions:
StatementList : StatementList StatementListItem
1. 1. 1. Let has be HasCallInTailPosition of StatementList with argument call.
2. 2. 2. If has is true, return true.
3. 3. 3. Return HasCallInTailPosition of StatementListItem with argument call.
FunctionStatementList : [empty] StatementListItem : Declaration Statement :
VariableStatement EmptyStatement ExpressionStatement ContinueStatement
BreakStatement ThrowStatement DebuggerStatement Block : { } ReturnStatement :
return ; LabelledItem : FunctionDeclaration ForInOfStatement : for (
LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding
of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression
) Statement CaseBlock : { }
1. 1. 1. Return false.
IfStatement : if ( Expression ) Statement else Statement
1. 1. 1. Let has be HasCallInTailPosition of the first Statement with argument
call.
2. 2. 2. If has is true, return true.
3. 3. 3. Return HasCallInTailPosition of the second Statement with argument
call.
IfStatement : if ( Expression ) Statement DoWhileStatement : do Statement while
( Expression ) ; WhileStatement : while ( Expression ) Statement ForStatement :
for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var
VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for (
LexicalDeclaration Expressionopt ; Expressionopt ) Statement ForInOfStatement :
for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in
Expression ) Statement for ( ForDeclaration in Expression ) Statement
WithStatement : with ( Expression ) Statement
1. 1. 1. Return HasCallInTailPosition of Statement with argument call.
LabelledStatement : LabelIdentifier : LabelledItem
1. 1. 1. Return HasCallInTailPosition of LabelledItem with argument call.
ReturnStatement : return Expression ;
1. 1. 1. Return HasCallInTailPosition of Expression with argument call.
SwitchStatement : switch ( Expression ) CaseBlock
1. 1. 1. Return HasCallInTailPosition of CaseBlock with argument call.
CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
1. 1. 1. Let has be false.
2. 2. 2. If the first CaseClauses is present, let has be HasCallInTailPosition
of the first CaseClauses with argument call.
3. 3. 3. If has is true, return true.
4. 4. 4. Let has be HasCallInTailPosition of DefaultClause with argument call.
5. 5. 5. If has is true, return true.
6. 6. 6. If the second CaseClauses is present, let has be HasCallInTailPosition
of the second CaseClauses with argument call.
7. 7. 7. Return has.
CaseClauses : CaseClauses CaseClause
1. 1. 1. Let has be HasCallInTailPosition of CaseClauses with argument call.
2. 2. 2. If has is true, return true.
3. 3. 3. Return HasCallInTailPosition of CaseClause with argument call.
CaseClause : case Expression : StatementListopt DefaultClause : default :
StatementListopt
1. 1. 1. If StatementList is present, return HasCallInTailPosition of
StatementList with argument call.
2. 2. 2. Return false.
TryStatement : try Block Catch
1. 1. 1. Return HasCallInTailPosition of Catch with argument call.
TryStatement : try Block Finally try Block Catch Finally
1. 1. 1. Return HasCallInTailPosition of Finally with argument call.
Catch : catch ( CatchParameter ) Block
1. 1. 1. Return HasCallInTailPosition of Block with argument call.
AssignmentExpression : YieldExpression ArrowFunction AsyncArrowFunction
LeftHandSideExpression = AssignmentExpression LeftHandSideExpression
AssignmentOperator AssignmentExpression LeftHandSideExpression &&=
AssignmentExpression LeftHandSideExpression ||= AssignmentExpression
LeftHandSideExpression ??= AssignmentExpression BitwiseANDExpression :
BitwiseANDExpression & EqualityExpression BitwiseXORExpression :
BitwiseXORExpression ^ BitwiseANDExpression BitwiseORExpression :
BitwiseORExpression | BitwiseXORExpression EqualityExpression :
EqualityExpression == RelationalExpression EqualityExpression !=
RelationalExpression EqualityExpression === RelationalExpression
EqualityExpression !== RelationalExpression RelationalExpression :
RelationalExpression < ShiftExpression RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression RelationalExpression >= ShiftExpression
RelationalExpression instanceof ShiftExpression RelationalExpression in
ShiftExpression PrivateIdentifier in ShiftExpression ShiftExpression :
ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression AdditiveExpression : AdditiveExpression +
MultiplicativeExpression AdditiveExpression - MultiplicativeExpression
MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator
ExponentiationExpression ExponentiationExpression : UpdateExpression **
ExponentiationExpression UpdateExpression : LeftHandSideExpression ++
LeftHandSideExpression -- ++ UnaryExpression -- UnaryExpression UnaryExpression
: delete UnaryExpression void UnaryExpression typeof UnaryExpression +
UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression
AwaitExpression CallExpression : SuperCall CallExpression [ Expression ]
CallExpression . IdentifierName CallExpression . PrivateIdentifier NewExpression
: new NewExpression MemberExpression : MemberExpression [ Expression ]
MemberExpression . IdentifierName SuperProperty MetaProperty new
MemberExpression Arguments MemberExpression . PrivateIdentifier
PrimaryExpression : this IdentifierReference Literal ArrayLiteral ObjectLiteral
FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression
AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral
1. 1. 1. Return false.
Expression : AssignmentExpression Expression , AssignmentExpression
1. 1. 1. Return HasCallInTailPosition of AssignmentExpression with argument
call.
ConditionalExpression : ShortCircuitExpression ? AssignmentExpression :
AssignmentExpression
1. 1. 1. Let has be HasCallInTailPosition of the first AssignmentExpression
with argument call.
2. 2. 2. If has is true, return true.
3. 3. 3. Return HasCallInTailPosition of the second AssignmentExpression with
argument call.
LogicalANDExpression : LogicalANDExpression && BitwiseORExpression
1. 1. 1. Return HasCallInTailPosition of BitwiseORExpression with argument
call.
LogicalORExpression : LogicalORExpression || LogicalANDExpression
1. 1. 1. Return HasCallInTailPosition of LogicalANDExpression with argument
call.
CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression
1. 1. 1. Return HasCallInTailPosition of BitwiseORExpression with argument
call.
CallExpression : CoverCallExpressionAndAsyncArrowHead CallExpression Arguments
CallExpression TemplateLiteral
1. 1. 1. If this CallExpression is call, return true.
2. 2. 2. Return false.
OptionalExpression : MemberExpression OptionalChain CallExpression OptionalChain
OptionalExpression OptionalChain
1. 1. 1. Return HasCallInTailPosition of OptionalChain with argument call.
OptionalChain : ?. [ Expression ] ?. IdentifierName ?. PrivateIdentifier
OptionalChain [ Expression ] OptionalChain . IdentifierName OptionalChain .
PrivateIdentifier
1. 1. 1. Return false.
OptionalChain : ?. Arguments OptionalChain Arguments
1. 1. 1. If this OptionalChain is call, return true.
2. 2. 2. Return false.
MemberExpression : MemberExpression TemplateLiteral
1. 1. 1. If this MemberExpression is call, return true.
2. 2. 2. Return false.
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
1. 1. 1. Let expr be the ParenthesizedExpression that is covered by
CoverParenthesizedExpressionAndArrowParameterList.
2. 2. 2. Return HasCallInTailPosition of expr with argument call.
ParenthesizedExpression : ( Expression )
1. 1. 1. Return HasCallInTailPosition of Expression with argument call.
15.10.3 PREPAREFORTAILCALL ( )
The abstract operation PrepareForTailCall takes no arguments and returns unused.
It performs the following steps when called:
1. 1. 1. Assert: The current execution context will not subsequently be used
for the evaluation of any ECMAScript code or built-in functions. The
invocation of Call subsequent to the invocation of this abstract operation
will create and push a new execution context before performing any such
evaluation.
2. 2. 2. Discard all resources associated with the current execution context.
3. 3. 3. Return unused.
A tail position call must either release any transient internal resources
associated with the currently executing function execution context before
invoking the target function or reuse those resources in support of the target
function.
Note
For example, a tail position call should only grow an implementation's
activation record stack by the amount that the size of the target function's
activation record exceeds the size of the calling function's activation record.
If the target function's activation record is smaller, then the total size of
the stack should decrease.
16 ECMASCRIPT LANGUAGE: SCRIPTS AND MODULES
16.1 SCRIPTS
SYNTAX
Script : ScriptBodyopt ScriptBody : StatementList[~Yield, ~Await, ~Return]
16.1.1 STATIC SEMANTICS: EARLY ERRORS
Script : ScriptBody
* It is a Syntax Error if the LexicallyDeclaredNames of ScriptBody contains any
duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
ScriptBody also occurs in the VarDeclaredNames of ScriptBody.
ScriptBody : StatementList
* It is a Syntax Error if StatementList Contains super unless the source text
containing super is eval code that is being processed by a direct eval.
Additional early error rules for super within direct eval are defined in
19.2.1.1.
* It is a Syntax Error if StatementList Contains NewTarget unless the source
text containing NewTarget is eval code that is being processed by a direct
eval. Additional early error rules for NewTarget in direct eval are defined
in 19.2.1.1.
* It is a Syntax Error if ContainsDuplicateLabels of StatementList with
argument « » is true.
* It is a Syntax Error if ContainsUndefinedBreakTarget of StatementList with
argument « » is true.
* It is a Syntax Error if ContainsUndefinedContinueTarget of StatementList with
arguments « » and « » is true.
* It is a Syntax Error if AllPrivateIdentifiersValid of StatementList with
argument « » is false unless the source text containing ScriptBody is eval
code that is being processed by a direct eval.
16.1.2 STATIC SEMANTICS: ISSTRICT
The syntax-directed operation IsStrict takes no arguments and returns a Boolean.
It is defined piecewise over the following productions:
Script : ScriptBodyopt
1. 1. 1. If ScriptBody is present and the Directive Prologue of ScriptBody
contains a Use Strict Directive, return true; otherwise, return false.
16.1.3 RUNTIME SEMANTICS: EVALUATION
Script : [empty]
1. 1. 1. Return undefined.
16.1.4 SCRIPT RECORDS
A Script Record encapsulates information about a script being evaluated. Each
script record contains the fields listed in Table 39.
Table 39: Script Record Fields
Field Name Value Type Meaning [[Realm]] a Realm Record or undefined The realm
within which this script was created. undefined if not yet assigned.
[[ECMAScriptCode]] a Script Parse Node The result of parsing the source text of
this script. [[LoadedModules]] a List of Records with fields [[Specifier]] (a
String) and [[Module]] (a Module Record) A map from the specifier strings
imported by this script to the resolved Module Record. The list does not contain
two different Records with the same [[Specifier]]. [[HostDefined]] anything
(default value is empty) Field reserved for use by host environments that need
to associate additional information with a script.
16.1.5 PARSESCRIPT ( SOURCETEXT, REALM, HOSTDEFINED )
The abstract operation ParseScript takes arguments sourceText (ECMAScript source
text), realm (a Realm Record or undefined), and hostDefined (anything) and
returns a Script Record or a non-empty List of SyntaxError objects. It creates a
Script Record based upon the result of parsing sourceText as a Script. It
performs the following steps when called:
1. 1. 1. Let script be ParseText(sourceText, Script).
2. 2. 2. If script is a List of errors, return script.
3. 3. 3. Return Script Record { [[Realm]]: realm, [[ECMAScriptCode]]: script,
[[LoadedModules]]: « », [[HostDefined]]: hostDefined }.
Note
An implementation may parse script source text and analyse it for Early Error
conditions prior to evaluation of ParseScript for that script source text.
However, the reporting of any errors must be deferred until the point where this
specification actually performs ParseScript upon that source text.
16.1.6 SCRIPTEVALUATION ( SCRIPTRECORD )
The abstract operation ScriptEvaluation takes argument scriptRecord (a Script
Record) and returns either a normal completion containing an ECMAScript language
value or an abrupt completion. It performs the following steps when called:
1. 1. 1. Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
2. 2. 2. Let scriptContext be a new ECMAScript code execution context.
3. 3. 3. Set the Function of scriptContext to null.
4. 4. 4. Set the Realm of scriptContext to scriptRecord.[[Realm]].
5. 5. 5. Set the ScriptOrModule of scriptContext to scriptRecord.
6. 6. 6. Set the VariableEnvironment of scriptContext to globalEnv.
7. 7. 7. Set the LexicalEnvironment of scriptContext to globalEnv.
8. 8. 8. Set the PrivateEnvironment of scriptContext to null.
9. 9. 9. Suspend the running execution context.
10. 10. 10. Push scriptContext onto the execution context stack; scriptContext
is now the running execution context.
11. 11. 11. Let script be scriptRecord.[[ECMAScriptCode]].
12. 12. 12. Let result be Completion(GlobalDeclarationInstantiation(script,
globalEnv)).
13. 13. 13. If result.[[Type]] is normal, then
1. a. a. Set result to Completion(Evaluation of script).
2. b. b. If result.[[Type]] is normal and result.[[Value]] is empty, then
1. i. i. Set result to NormalCompletion(undefined).
14. 14. 14. Suspend scriptContext and remove it from the execution context
stack.
15. 15. 15. Assert: The execution context stack is not empty.
16. 16. 16. Resume the context that is now on the top of the execution context
stack as the running execution context.
17. 17. 17. Return ? result.
16.1.7 GLOBALDECLARATIONINSTANTIATION ( SCRIPT, ENV )
The abstract operation GlobalDeclarationInstantiation takes arguments script (a
Script Parse Node) and env (a Global Environment Record) and returns either a
normal completion containing unused or a throw completion. script is the Script
for which the execution context is being established. env is the global
environment in which bindings are to be created.
Note 1
When an execution context is established for evaluating scripts, declarations
are instantiated in the current global environment. Each global binding declared
in the code is instantiated.
It performs the following steps when called:
1. 1. 1. Let lexNames be the LexicallyDeclaredNames of script.
2. 2. 2. Let varNames be the VarDeclaredNames of script.
3. 3. 3. For each element name of lexNames, do
1. a. a. If env.HasVarDeclaration(name) is true, throw a SyntaxError
exception.
2. b. b. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError
exception.
3. c. c. Let hasRestrictedGlobal be
? env.HasRestrictedGlobalProperty(name).
4. d. d. If hasRestrictedGlobal is true, throw a SyntaxError exception.
4. 4. 4. For each element name of varNames, do
1. a. a. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError
exception.
5. 5. 5. Let varDeclarations be the VarScopedDeclarations of script.
6. 6. 6. Let functionsToInitialize be a new empty List.
7. 7. 7. Let declaredFunctionNames be a new empty List.
8. 8. 8. For each element d of varDeclarations, in reverse List order, do
1. a. a. If d is not either a VariableDeclaration, a ForBinding, or a
BindingIdentifier, then
1. i. i. Assert: d is either a FunctionDeclaration, a
GeneratorDeclaration, an AsyncFunctionDeclaration, or an
AsyncGeneratorDeclaration.
2. ii. ii. NOTE: If there are multiple function declarations for the
same name, the last declaration is used.
3. iii. iii. Let fn be the sole element of the BoundNames of d.
4. iv. iv. If declaredFunctionNames does not contain fn, then
1. 1. 1. Let fnDefinable be ? env.CanDeclareGlobalFunction(fn).
2. 2. 2. If fnDefinable is false, throw a TypeError exception.
3. 3. 3. Append fn to declaredFunctionNames.
4. 4. 4. Insert d as the first element of functionsToInitialize.
9. 9. 9. Let declaredVarNames be a new empty List.
10. 10. 10. For each element d of varDeclarations, do
1. a. a. If d is either a VariableDeclaration, a ForBinding, or a
BindingIdentifier, then
1. i. i. For each String vn of the BoundNames of d, do
1. 1. 1. If declaredFunctionNames does not contain vn, then
1. a. a. Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
2. b. b. If vnDefinable is false, throw a TypeError exception.
3. c. c. If declaredVarNames does not contain vn, then
1. i. i. Append vn to declaredVarNames.
11. 11. 11. NOTE: No abnormal terminations occur after this algorithm step if
the global object is an ordinary object. However, if the global object is a
Proxy exotic object it may exhibit behaviours that cause abnormal
terminations in some of the following steps.
12. 12. 12. NOTE: Annex B.3.2.2 adds additional steps at this point.
13. 13. 13. Let lexDeclarations be the LexicallyScopedDeclarations of script.
14. 14. 14. Let privateEnv be null.
15. 15. 15. For each element d of lexDeclarations, do
1. a. a. NOTE: Lexically declared names are only instantiated here but not
initialized.
2. b. b. For each element dn of the BoundNames of d, do
1. i. i. If IsConstantDeclaration of d is true, then
1. 1. 1. Perform ? env.CreateImmutableBinding(dn, true).
2. ii. ii. Else,
1. 1. 1. Perform ? env.CreateMutableBinding(dn, false).
16. 16. 16. For each Parse Node f of functionsToInitialize, do
1. a. a. Let fn be the sole element of the BoundNames of f.
2. b. b. Let fo be InstantiateFunctionObject of f with arguments env and
privateEnv.
3. c. c. Perform ? env.CreateGlobalFunctionBinding(fn, fo, false).
17. 17. 17. For each String vn of declaredVarNames, do
1. a. a. Perform ? env.CreateGlobalVarBinding(vn, false).
18. 18. 18. Return unused.
Note 2
Early errors specified in 16.1.1 prevent name conflicts between function/var
declarations and let/const/class declarations as well as redeclaration of
let/const/class bindings for declaration contained within a single Script.
However, such conflicts and redeclarations that span more than one Script are
detected as runtime errors during GlobalDeclarationInstantiation. If any such
errors are detected, no bindings are instantiated for the script. However, if
the global object is defined using Proxy exotic objects then the runtime tests
for conflicting declarations may be unreliable resulting in an abrupt completion
and some global declarations not being instantiated. If this occurs, the code
for the Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly
created on the global object result in global bindings that may be shadowed by
let/const/class declarations.
16.2 MODULES
SYNTAX
Module : ModuleBodyopt ModuleBody : ModuleItemList ModuleItemList : ModuleItem
ModuleItemList ModuleItem ModuleItem : ImportDeclaration ExportDeclaration
StatementListItem[~Yield, +Await, ~Return] ModuleExportName : IdentifierName
StringLiteral
16.2.1 MODULE SEMANTICS
16.2.1.1 STATIC SEMANTICS: EARLY ERRORS
ModuleBody : ModuleItemList
* It is a Syntax Error if the LexicallyDeclaredNames of ModuleItemList contains
any duplicate entries.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
ModuleItemList also occurs in the VarDeclaredNames of ModuleItemList.
* It is a Syntax Error if the ExportedNames of ModuleItemList contains any
duplicate entries.
* It is a Syntax Error if any element of the ExportedBindings of ModuleItemList
does not also occur in either the VarDeclaredNames of ModuleItemList, or the
LexicallyDeclaredNames of ModuleItemList.
* It is a Syntax Error if ModuleItemList Contains super.
* It is a Syntax Error if ModuleItemList Contains NewTarget.
* It is a Syntax Error if ContainsDuplicateLabels of ModuleItemList with
argument « » is true.
* It is a Syntax Error if ContainsUndefinedBreakTarget of ModuleItemList with
argument « » is true.
* It is a Syntax Error if ContainsUndefinedContinueTarget of ModuleItemList
with arguments « » and « » is true.
* It is a Syntax Error if AllPrivateIdentifiersValid of ModuleItemList with
argument « » is false.
Note
The duplicate ExportedNames rule implies that multiple export default
ExportDeclaration items within a ModuleBody is a Syntax Error. Additional error
conditions relating to conflicting or duplicate declarations are checked during
module linking prior to evaluation of a Module. If any such errors are detected
the Module is not evaluated.
ModuleExportName : StringLiteral
* It is a Syntax Error if IsStringWellFormedUnicode(the SV of StringLiteral) is
false.
16.2.1.2 STATIC SEMANTICS: IMPORTEDLOCALNAMES ( IMPORTENTRIES )
The abstract operation ImportedLocalNames takes argument importEntries (a List
of ImportEntry Records) and returns a List of Strings. It creates a List of all
of the local name bindings defined by importEntries. It performs the following
steps when called:
1. 1. 1. Let localNames be a new empty List.
2. 2. 2. For each ImportEntry Record i of importEntries, do
1. a. a. Append i.[[LocalName]] to localNames.
3. 3. 3. Return localNames.
16.2.1.3 STATIC SEMANTICS: MODULEREQUESTS
The syntax-directed operation ModuleRequests takes no arguments and returns a
List of Strings. It is defined piecewise over the following productions:
Module : [empty]
1. 1. 1. Return a new empty List.
ModuleItemList : ModuleItem
1. 1. 1. Return ModuleRequests of ModuleItem.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let moduleNames be ModuleRequests of ModuleItemList.
2. 2. 2. Let additionalNames be ModuleRequests of ModuleItem.
3. 3. 3. For each String name of additionalNames, do
1. a. a. If moduleNames does not contain name, then
1. i. i. Append name to moduleNames.
4. 4. 4. Return moduleNames.
ModuleItem : StatementListItem
1. 1. 1. Return a new empty List.
ImportDeclaration : import ImportClause FromClause ;
1. 1. 1. Return ModuleRequests of FromClause.
ModuleSpecifier : StringLiteral
1. 1. 1. Return a List whose sole element is the SV of StringLiteral.
ExportDeclaration : export ExportFromClause FromClause ;
1. 1. 1. Return the ModuleRequests of FromClause.
ExportDeclaration : export NamedExports ; export VariableStatement export
Declaration export default HoistableDeclaration export default ClassDeclaration
export default AssignmentExpression ;
1. 1. 1. Return a new empty List.
16.2.1.4 ABSTRACT MODULE RECORDS
A Module Record encapsulates structural information about the imports and
exports of a single module. This information is used to link the imports and
exports of sets of connected modules. A Module Record includes four fields that
are only used when evaluating a module.
For specification purposes Module Record values are values of the Record
specification type and can be thought of as existing in a simple object-oriented
hierarchy where Module Record is an abstract class with both abstract and
concrete subclasses. This specification defines the abstract subclass named
Cyclic Module Record and its concrete subclass named Source Text Module Record.
Other specifications and implementations may define additional Module Record
subclasses corresponding to alternative module definition facilities that they
defined.
Module Record defines the fields listed in Table 40. All Module Definition
subclasses include at least those fields. Module Record also defines the
abstract method list in Table 41. All Module definition subclasses must provide
concrete implementations of these abstract methods.
Table 40: Module Record Fields
Field Name Value Type Meaning [[Realm]] a Realm Record The Realm within which
this module was created. [[Environment]] a Module Environment Record or empty
The Environment Record containing the top level bindings for this module. This
field is set when the module is linked. [[Namespace]] an Object or empty The
Module Namespace Object (28.3) if one has been created for this module.
[[HostDefined]] anything (default value is undefined) Field reserved for use by
host environments that need to associate additional information with a module.
Table 41: Abstract Methods of Module Records
Method Purpose LoadRequestedModules( [ hostDefined ] )
Prepares the module for linking by recursively loading all its dependencies, and
returns a promise.
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or indirectly exported from
this module.
LoadRequestedModules must have completed successfully prior to invoking this
method.
ResolveExport(exportName [, resolveSet])
Return the binding of a name exported by this module. Bindings are represented
by a ResolvedBinding Record, of the form { [[Module]]: Module Record,
[[BindingName]]: String | namespace }. If the export is a Module Namespace
Object without a direct binding in any module, [[BindingName]] will be set to
namespace. Return null if the name cannot be resolved, or ambiguous if multiple
bindings were found.
Each time this operation is called with a specific exportName, resolveSet pair
as arguments it must return the same result.
LoadRequestedModules must have completed successfully prior to invoking this
method.
Link()
Prepare the module for evaluation by transitively resolving all module
dependencies and creating a Module Environment Record.
LoadRequestedModules must have completed successfully prior to invoking this
method.
Evaluate()
Returns a promise for the evaluation of this module and its dependencies,
resolving on successful evaluation or if it has already been evaluated
successfully, and rejecting for an evaluation error or if it has already been
evaluated unsuccessfully. If the promise is rejected, hosts are expected to
handle the promise rejection and rethrow the evaluation error.
Link must have completed successfully prior to invoking this method.
16.2.1.5 CYCLIC MODULE RECORDS
A Cyclic Module Record is used to represent information about a module that can
participate in dependency cycles with other modules that are subclasses of the
Cyclic Module Record type. Module Records that are not subclasses of the Cyclic
Module Record type must not participate in dependency cycles with Source Text
Module Records.
In addition to the fields defined in Table 40 Cyclic Module Records have the
additional fields listed in Table 42
Table 42: Additional Fields of Cyclic Module Records
Field Name Value Type Meaning [[Status]] new, unlinked, linking, linked,
evaluating, evaluating-async, or evaluated Initially new. Transitions to
unlinked, linking, linked, evaluating, possibly evaluating-async, evaluated (in
that order) as the module progresses throughout its lifecycle. evaluating-async
indicates this module is queued to execute on completion of its asynchronous
dependencies or it is a module whose [[HasTLA]] field is true that has been
executed and is pending top-level completion. [[EvaluationError]] a throw
completion or empty A throw completion representing the exception that occurred
during evaluation. undefined if no exception occurred or if [[Status]] is not
evaluated. [[DFSIndex]] an integer or empty Auxiliary field used during Link and
Evaluate only. If [[Status]] is either linking or evaluating, this non-negative
number records the point at which the module was first visited during the
depth-first traversal of the dependency graph. [[DFSAncestorIndex]] an integer
or empty Auxiliary field used during Link and Evaluate only. If [[Status]] is
either linking or evaluating, this is either the module's own [[DFSIndex]] or
that of an "earlier" module in the same strongly connected component.
[[RequestedModules]] a List of Strings A List of all the ModuleSpecifier strings
used by the module represented by this record to request the importation of a
module. The List is in source text occurrence order. [[LoadedModules]] a List of
Records with fields [[Specifier]] (a String) and [[Module]] (a Module Record) A
map from the specifier strings used by the module represented by this record to
request the importation of a module to the resolved Module Record. The list does
not contain two different Records with the same [[Specifier]]. [[CycleRoot]] a
Cyclic Module Record or empty The first visited module of the cycle, the root
DFS ancestor of the strongly connected component. For a module not in a cycle,
this would be the module itself. Once Evaluate has completed, a module's
[[DFSAncestorIndex]] is the [[DFSIndex]] of its [[CycleRoot]]. [[HasTLA]] a
Boolean Whether this module is individually asynchronous (for example, if it's a
Source Text Module Record containing a top-level await). Having an asynchronous
dependency does not mean this field is true. This field must not change after
the module is parsed. [[AsyncEvaluation]] a Boolean Whether this module is
either itself asynchronous or has an asynchronous dependency. Note: The order in
which this field is set is used to order queued executions, see 16.2.1.5.3.4.
[[TopLevelCapability]] a PromiseCapability Record or empty If this module is the
[[CycleRoot]] of some cycle, and Evaluate() was called on some module in that
cycle, this field contains the PromiseCapability Record for that entire
evaluation. It is used to settle the Promise object that is returned from the
Evaluate() abstract method. This field will be empty for any dependencies of
that module, unless a top-level Evaluate() has been initiated for some of those
dependencies. [[AsyncParentModules]] a List of Cyclic Module Records If this
module or a dependency has [[HasTLA]] true, and execution is in progress, this
tracks the parent importers of this module for the top-level execution job.
These parent modules will not start executing before this module has
successfully completed execution. [[PendingAsyncDependencies]] an integer or
empty If this module has any asynchronous dependencies, this tracks the number
of asynchronous dependency modules remaining to execute for this module. A
module with asynchronous dependencies will be executed when this field reaches 0
and there are no execution errors.
In addition to the methods defined in Table 41 Cyclic Module Records have the
additional methods listed in Table 43
Table 43: Additional Abstract Methods of Cyclic Module Records
Method Purpose InitializeEnvironment() Initialize the Environment Record of the
module, including resolving all imported bindings, and create the module's
execution context. ExecuteModule( [ promiseCapability ] ) Evaluate the module's
code within its execution context. If this module has true in [[HasTLA]], then a
PromiseCapability Record is passed as an argument, and the method is expected to
resolve or reject the given capability. In this case, the method must not throw
an exception, but instead reject the PromiseCapability Record if necessary.
A GraphLoadingState Record is a Record that contains information about the
loading process of a module graph. It's used to continue loading after a call to
HostLoadImportedModule. Each GraphLoadingState Record has the fields defined in
Table 44:
Table 44: GraphLoadingState Record Fields
Field Name Value Type Meaning [[PromiseCapability]] a PromiseCapability Record
The promise to resolve when the loading process finishes. [[IsLoading]] a
Boolean It is true if the loading process has not finished yet, neither
successfully nor with an error. [[PendingModulesCount]] a non-negative integer
It tracks the number of pending HostLoadImportedModule calls. [[Visited]] a List
of Cyclic Module Records It is a list of the Cyclic Module Records that have
been already loaded by the current loading process, to avoid infinite loops with
circular dependencies. [[HostDefined]] anything (default value is empty) It
contains host-defined data to pass from the LoadRequestedModules caller to
HostLoadImportedModule.
16.2.1.5.1 LOADREQUESTEDMODULES ( [ HOSTDEFINED ] )
The LoadRequestedModules concrete method of a Cyclic Module Record module takes
optional argument hostDefined (anything) and returns a Promise. It populates the
[[LoadedModules]] of all the Module Records in the dependency graph of module
(most of the work is done by the auxiliary function InnerModuleLoading). It
takes an optional hostDefined parameter that is passed to the
HostLoadImportedModule hook. It performs the following steps when called:
1. 1. 1. If hostDefined is not present, let hostDefined be empty.
2. 2. 2. Let pc be ! NewPromiseCapability(%Promise%).
3. 3. 3. Let state be the GraphLoadingState Record { [[IsLoading]]: true,
[[PendingModulesCount]]: 1, [[Visited]]: « », [[PromiseCapability]]: pc,
[[HostDefined]]: hostDefined }.
4. 4. 4. Perform InnerModuleLoading(state, module).
5. 5. 5. Return pc.[[Promise]].
Note
The hostDefined parameter can be used to pass additional information necessary
to fetch the imported modules. It is used, for example, by HTML to set the
correct fetch destination for <link rel="preload" as="..."> tags. import()
expressions never set the hostDefined parameter.
16.2.1.5.1.1 INNERMODULELOADING ( STATE, MODULE )
The abstract operation InnerModuleLoading takes arguments state (a
GraphLoadingState Record) and module (a Module Record) and returns unused. It is
used by LoadRequestedModules to recursively perform the actual loading process
for module's dependency graph. It performs the following steps when called:
1. 1. 1. Assert: state.[[IsLoading]] is true.
2. 2. 2. If module is a Cyclic Module Record, module.[[Status]] is new, and
state.[[Visited]] does not contain module, then
1. a. a. Append module to state.[[Visited]].
2. b. b. Let requestedModulesCount be the number of elements in
module.[[RequestedModules]].
3. c. c. Set state.[[PendingModulesCount]] to state.[[PendingModulesCount]]
+ requestedModulesCount.
4. d. d. For each String required of module.[[RequestedModules]], do
1. i. i. If module.[[LoadedModules]] contains a Record whose
[[Specifier]] is required, then
1. 1. 1. Let record be that Record.
2. 2. 2. Perform InnerModuleLoading(state, record.[[Module]]).
2. ii. ii. Else,
1. 1. 1. Perform HostLoadImportedModule(module, required,
state.[[HostDefined]], state).
2. 2. 2. NOTE: HostLoadImportedModule will call
FinishLoadingImportedModule, which re-enters the graph loading
process through ContinueModuleLoading.
3. iii. iii. If state.[[IsLoading]] is false, return unused.
3. 3. 3. Assert: state.[[PendingModulesCount]] ≥ 1.
4. 4. 4. Set state.[[PendingModulesCount]] to state.[[PendingModulesCount]] -
1.
5. 5. 5. If state.[[PendingModulesCount]] = 0, then
1. a. a. Set state.[[IsLoading]] to false.
2. b. b. For each Cyclic Module Record loaded of state.[[Visited]], do
1. i. i. If loaded.[[Status]] is new, set loaded.[[Status]] to unlinked.
3. c. c. Perform ! Call(state.[[PromiseCapability]].[[Resolve]], undefined,
« undefined »).
6. 6. 6. Return unused.
16.2.1.5.1.2 CONTINUEMODULELOADING ( STATE, MODULECOMPLETION )
The abstract operation ContinueModuleLoading takes arguments state (a
GraphLoadingState Record) and moduleCompletion (either a normal completion
containing a Module Record or a throw completion) and returns unused. It is used
to re-enter the loading process after a call to HostLoadImportedModule. It
performs the following steps when called:
1. 1. 1. If state.[[IsLoading]] is false, return unused.
2. 2. 2. If moduleCompletion is a normal completion, then
1. a. a. Perform InnerModuleLoading(state, moduleCompletion.[[Value]]).
3. 3. 3. Else,
1. a. a. Set state.[[IsLoading]] to false.
2. b. b. Perform ! Call(state.[[PromiseCapability]].[[Reject]], undefined, «
moduleCompletion.[[Value]] »).
4. 4. 4. Return unused.
16.2.1.5.2 LINK ( )
The Link concrete method of a Cyclic Module Record module takes no arguments and
returns either a normal completion containing unused or a throw completion. On
success, Link transitions this module's [[Status]] from unlinked to linked. On
failure, an exception is thrown and this module's [[Status]] remains unlinked.
(Most of the work is done by the auxiliary function InnerModuleLinking.) It
performs the following steps when called:
1. 1. 1. Assert: module.[[Status]] is one of unlinked, linked,
evaluating-async, or evaluated.
2. 2. 2. Let stack be a new empty List.
3. 3. 3. Let result be Completion(InnerModuleLinking(module, stack, 0)).
4. 4. 4. If result is an abrupt completion, then
1. a. a. For each Cyclic Module Record m of stack, do
1. i. i. Assert: m.[[Status]] is linking.
2. ii. ii. Set m.[[Status]] to unlinked.
2. b. b. Assert: module.[[Status]] is unlinked.
3. c. c. Return ? result.
5. 5. 5. Assert: module.[[Status]] is one of linked, evaluating-async, or
evaluated.
6. 6. 6. Assert: stack is empty.
7. 7. 7. Return unused.
16.2.1.5.2.1 INNERMODULELINKING ( MODULE, STACK, INDEX )
The abstract operation InnerModuleLinking takes arguments module (a Module
Record), stack (a List of Cyclic Module Records), and index (a non-negative
integer) and returns either a normal completion containing a non-negative
integer or a throw completion. It is used by Link to perform the actual linking
process for module, as well as recursively on all other modules in the
dependency graph. The stack and index parameters, as well as a module's
[[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first
search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover
strongly connected components (SCCs), such that all modules in an SCC transition
to linked together. It performs the following steps when called:
1. 1. 1. If module is not a Cyclic Module Record, then
1. a. a. Perform ? module.Link().
2. b. b. Return index.
2. 2. 2. If module.[[Status]] is one of linking, linked, evaluating-async, or
evaluated, then
1. a. a. Return index.
3. 3. 3. Assert: module.[[Status]] is unlinked.
4. 4. 4. Set module.[[Status]] to linking.
5. 5. 5. Set module.[[DFSIndex]] to index.
6. 6. 6. Set module.[[DFSAncestorIndex]] to index.
7. 7. 7. Set index to index + 1.
8. 8. 8. Append module to stack.
9. 9. 9. For each String required of module.[[RequestedModules]], do
1. a. a. Let requiredModule be GetImportedModule(module, required).
2. b. b. Set index to ? InnerModuleLinking(requiredModule, stack, index).
3. c. c. If requiredModule is a Cyclic Module Record, then
1. i. i. Assert: requiredModule.[[Status]] is one of linking, linked,
evaluating-async, or evaluated.
2. ii. ii. Assert: requiredModule.[[Status]] is linking if and only if
stack contains requiredModule.
3. iii. iii. If requiredModule.[[Status]] is linking, then
1. 1. 1. Set module.[[DFSAncestorIndex]] to
min(module.[[DFSAncestorIndex]],
requiredModule.[[DFSAncestorIndex]]).
10. 10. 10. Perform ? module.InitializeEnvironment().
11. 11. 11. Assert: module occurs exactly once in stack.
12. 12. 12. Assert: module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].
13. 13. 13. If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
1. a. a. Let done be false.
2. b. b. Repeat, while done is false,
1. i. i. Let requiredModule be the last element of stack.
2. ii. ii. Remove the last element of stack.
3. iii. iii. Assert: requiredModule is a Cyclic Module Record.
4. iv. iv. Set requiredModule.[[Status]] to linked.
5. v. v. If requiredModule and module are the same Module Record, set
done to true.
14. 14. 14. Return index.
16.2.1.5.3 EVALUATE ( )
The Evaluate concrete method of a Cyclic Module Record module takes no arguments
and returns a Promise. Evaluate transitions this module's [[Status]] from linked
to either evaluating-async or evaluated. The first time it is called on a module
in a given strongly connected component, Evaluate creates and returns a Promise
which resolves when the module has finished evaluating. This Promise is stored
in the [[TopLevelCapability]] field of the [[CycleRoot]] for the component.
Future invocations of Evaluate on any module in the component return the same
Promise. (Most of the work is done by the auxiliary function
InnerModuleEvaluation.) It performs the following steps when called:
1. 1. 1. Assert: This call to Evaluate is not happening at the same time as
another call to Evaluate within the surrounding agent.
2. 2. 2. Assert: module.[[Status]] is one of linked, evaluating-async, or
evaluated.
3. 3. 3. If module.[[Status]] is either evaluating-async or evaluated, set
module to module.[[CycleRoot]].
4. 4. 4. If module.[[TopLevelCapability]] is not empty, then
1. a. a. Return module.[[TopLevelCapability]].[[Promise]].
5. 5. 5. Let stack be a new empty List.
6. 6. 6. Let capability be ! NewPromiseCapability(%Promise%).
7. 7. 7. Set module.[[TopLevelCapability]] to capability.
8. 8. 8. Let result be Completion(InnerModuleEvaluation(module, stack, 0)).
9. 9. 9. If result is an abrupt completion, then
1. a. a. For each Cyclic Module Record m of stack, do
1. i. i. Assert: m.[[Status]] is evaluating.
2. ii. ii. Set m.[[Status]] to evaluated.
3. iii. iii. Set m.[[EvaluationError]] to result.
2. b. b. Assert: module.[[Status]] is evaluated.
3. c. c. Assert: module.[[EvaluationError]] is result.
4. d. d. Perform ! Call(capability.[[Reject]], undefined, «
result.[[Value]] »).
10. 10. 10. Else,
1. a. a. Assert: module.[[Status]] is either evaluating-async or evaluated.
2. b. b. Assert: module.[[EvaluationError]] is empty.
3. c. c. If module.[[AsyncEvaluation]] is false, then
1. i. i. Assert: module.[[Status]] is evaluated.
2. ii. ii. Perform ! Call(capability.[[Resolve]], undefined, « undefined
»).
4. d. d. Assert: stack is empty.
11. 11. 11. Return capability.[[Promise]].
16.2.1.5.3.1 INNERMODULEEVALUATION ( MODULE, STACK, INDEX )
The abstract operation InnerModuleEvaluation takes arguments module (a Module
Record), stack (a List of Cyclic Module Records), and index (a non-negative
integer) and returns either a normal completion containing a non-negative
integer or a throw completion. It is used by Evaluate to perform the actual
evaluation process for module, as well as recursively on all other modules in
the dependency graph. The stack and index parameters, as well as module's
[[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the same way as in
InnerModuleLinking. It performs the following steps when called:
1. 1. 1. If module is not a Cyclic Module Record, then
1. a. a. Let promise be ! module.Evaluate().
2. b. b. Assert: promise.[[PromiseState]] is not pending.
3. c. c. If promise.[[PromiseState]] is rejected, then
1. i. i. Return ThrowCompletion(promise.[[PromiseResult]]).
4. d. d. Return index.
2. 2. 2. If module.[[Status]] is either evaluating-async or evaluated, then
1. a. a. If module.[[EvaluationError]] is empty, return index.
2. b. b. Otherwise, return ? module.[[EvaluationError]].
3. 3. 3. If module.[[Status]] is evaluating, return index.
4. 4. 4. Assert: module.[[Status]] is linked.
5. 5. 5. Set module.[[Status]] to evaluating.
6. 6. 6. Set module.[[DFSIndex]] to index.
7. 7. 7. Set module.[[DFSAncestorIndex]] to index.
8. 8. 8. Set module.[[PendingAsyncDependencies]] to 0.
9. 9. 9. Set index to index + 1.
10. 10. 10. Append module to stack.
11. 11. 11. For each String required of module.[[RequestedModules]], do
1. a. a. Let requiredModule be GetImportedModule(module, required).
2. b. b. Set index to ? InnerModuleEvaluation(requiredModule, stack,
index).
3. c. c. If requiredModule is a Cyclic Module Record, then
1. i. i. Assert: requiredModule.[[Status]] is one of evaluating,
evaluating-async, or evaluated.
2. ii. ii. Assert: requiredModule.[[Status]] is evaluating if and only
if stack contains requiredModule.
3. iii. iii. If requiredModule.[[Status]] is evaluating, then
1. 1. 1. Set module.[[DFSAncestorIndex]] to
min(module.[[DFSAncestorIndex]],
requiredModule.[[DFSAncestorIndex]]).
4. iv. iv. Else,
1. 1. 1. Set requiredModule to requiredModule.[[CycleRoot]].
2. 2. 2. Assert: requiredModule.[[Status]] is either evaluating-async
or evaluated.
3. 3. 3. If requiredModule.[[EvaluationError]] is not empty, return
? requiredModule.[[EvaluationError]].
5. v. v. If requiredModule.[[AsyncEvaluation]] is true, then
1. 1. 1. Set module.[[PendingAsyncDependencies]] to
module.[[PendingAsyncDependencies]] + 1.
2. 2. 2. Append module to requiredModule.[[AsyncParentModules]].
12. 12. 12. If module.[[PendingAsyncDependencies]] > 0 or module.[[HasTLA]] is
true, then
1. a. a. Assert: module.[[AsyncEvaluation]] is false and was never
previously set to true.
2. b. b. Set module.[[AsyncEvaluation]] to true.
3. c. c. NOTE: The order in which module records have their
[[AsyncEvaluation]] fields transition to true is significant. (See
16.2.1.5.3.4.)
4. d. d. If module.[[PendingAsyncDependencies]] = 0, perform
ExecuteAsyncModule(module).
13. 13. 13. Otherwise, perform ? module.ExecuteModule().
14. 14. 14. Assert: module occurs exactly once in stack.
15. 15. 15. Assert: module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].
16. 16. 16. If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
1. a. a. Let done be false.
2. b. b. Repeat, while done is false,
1. i. i. Let requiredModule be the last element of stack.
2. ii. ii. Remove the last element of stack.
3. iii. iii. Assert: requiredModule is a Cyclic Module Record.
4. iv. iv. If requiredModule.[[AsyncEvaluation]] is false, set
requiredModule.[[Status]] to evaluated.
5. v. v. Otherwise, set requiredModule.[[Status]] to evaluating-async.
6. vi. vi. If requiredModule and module are the same Module Record, set
done to true.
7. vii. vii. Set requiredModule.[[CycleRoot]] to module.
17. 17. 17. Return index.
Note 1
A module is evaluating while it is being traversed by InnerModuleEvaluation. A
module is evaluated on execution completion or evaluating-async during execution
if its [[HasTLA]] field is true or if it has asynchronous dependencies.
Note 2
Any modules depending on a module of an asynchronous cycle when that cycle is
not evaluating will instead depend on the execution of the root of the cycle via
[[CycleRoot]]. This ensures that the cycle state can be treated as a single
strongly connected component through its root module state.
16.2.1.5.3.2 EXECUTEASYNCMODULE ( MODULE )
The abstract operation ExecuteAsyncModule takes argument module (a Cyclic Module
Record) and returns unused. It performs the following steps when called:
1. 1. 1. Assert: module.[[Status]] is either evaluating or evaluating-async.
2. 2. 2. Assert: module.[[HasTLA]] is true.
3. 3. 3. Let capability be ! NewPromiseCapability(%Promise%).
4. 4. 4. Let fulfilledClosure be a new Abstract Closure with no parameters
that captures module and performs the following steps when called:
1. a. a. Perform AsyncModuleExecutionFulfilled(module).
2. b. b. Return undefined.
5. 5. 5. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 0, "", «
»).
6. 6. 6. Let rejectedClosure be a new Abstract Closure with parameters (error)
that captures module and performs the following steps when called:
1. a. a. Perform AsyncModuleExecutionRejected(module, error).
2. b. b. Return undefined.
7. 7. 7. Let onRejected be CreateBuiltinFunction(rejectedClosure, 0, "", « »).
8. 8. 8. Perform PerformPromiseThen(capability.[[Promise]], onFulfilled,
onRejected).
9. 9. 9. Perform ! module.ExecuteModule(capability).
10. 10. 10. Return unused.
16.2.1.5.3.3 GATHERAVAILABLEANCESTORS ( MODULE, EXECLIST )
The abstract operation GatherAvailableAncestors takes arguments module (a Cyclic
Module Record) and execList (a List of Cyclic Module Records) and returns
unused. It performs the following steps when called:
1. 1. 1. For each Cyclic Module Record m of module.[[AsyncParentModules]], do
1. a. a. If execList does not contain m and
m.[[CycleRoot]].[[EvaluationError]] is empty, then
1. i. i. Assert: m.[[Status]] is evaluating-async.
2. ii. ii. Assert: m.[[EvaluationError]] is empty.
3. iii. iii. Assert: m.[[AsyncEvaluation]] is true.
4. iv. iv. Assert: m.[[PendingAsyncDependencies]] > 0.
5. v. v. Set m.[[PendingAsyncDependencies]] to
m.[[PendingAsyncDependencies]] - 1.
6. vi. vi. If m.[[PendingAsyncDependencies]] = 0, then
1. 1. 1. Append m to execList.
2. 2. 2. If m.[[HasTLA]] is false, perform GatherAvailableAncestors(m,
execList).
2. 2. 2. Return unused.
Note
When an asynchronous execution for a root module is fulfilled, this function
determines the list of modules which are able to synchronously execute together
on this completion, populating them in execList.
16.2.1.5.3.4 ASYNCMODULEEXECUTIONFULFILLED ( MODULE )
The abstract operation AsyncModuleExecutionFulfilled takes argument module (a
Cyclic Module Record) and returns unused. It performs the following steps when
called:
1. 1. 1. If module.[[Status]] is evaluated, then
1. a. a. Assert: module.[[EvaluationError]] is not empty.
2. b. b. Return unused.
2. 2. 2. Assert: module.[[Status]] is evaluating-async.
3. 3. 3. Assert: module.[[AsyncEvaluation]] is true.
4. 4. 4. Assert: module.[[EvaluationError]] is empty.
5. 5. 5. Set module.[[AsyncEvaluation]] to false.
6. 6. 6. Set module.[[Status]] to evaluated.
7. 7. 7. If module.[[TopLevelCapability]] is not empty, then
1. a. a. Assert: module.[[CycleRoot]] is module.
2. b. b. Perform ! Call(module.[[TopLevelCapability]].[[Resolve]],
undefined, « undefined »).
8. 8. 8. Let execList be a new empty List.
9. 9. 9. Perform GatherAvailableAncestors(module, execList).
10. 10. 10. Let sortedExecList be a List whose elements are the elements of
execList, in the order in which they had their [[AsyncEvaluation]] fields
set to true in InnerModuleEvaluation.
11. 11. 11. Assert: All elements of sortedExecList have their
[[AsyncEvaluation]] field set to true, [[PendingAsyncDependencies]] field
set to 0, and [[EvaluationError]] field set to empty.
12. 12. 12. For each Cyclic Module Record m of sortedExecList, do
1. a. a. If m.[[Status]] is evaluated, then
1. i. i. Assert: m.[[EvaluationError]] is not empty.
2. b. b. Else if m.[[HasTLA]] is true, then
1. i. i. Perform ExecuteAsyncModule(m).
3. c. c. Else,
1. i. i. Let result be m.ExecuteModule().
2. ii. ii. If result is an abrupt completion, then
1. 1. 1. Perform AsyncModuleExecutionRejected(m, result.[[Value]]).
3. iii. iii. Else,
1. 1. 1. Set m.[[Status]] to evaluated.
2. 2. 2. If m.[[TopLevelCapability]] is not empty, then
1. a. a. Assert: m.[[CycleRoot]] is m.
2. b. b. Perform ! Call(m.[[TopLevelCapability]].[[Resolve]],
undefined, « undefined »).
13. 13. 13. Return unused.
16.2.1.5.3.5 ASYNCMODULEEXECUTIONREJECTED ( MODULE, ERROR )
The abstract operation AsyncModuleExecutionRejected takes arguments module (a
Cyclic Module Record) and error (an ECMAScript language value) and returns
unused. It performs the following steps when called:
1. 1. 1. If module.[[Status]] is evaluated, then
1. a. a. Assert: module.[[EvaluationError]] is not empty.
2. b. b. Return unused.
2. 2. 2. Assert: module.[[Status]] is evaluating-async.
3. 3. 3. Assert: module.[[AsyncEvaluation]] is true.
4. 4. 4. Assert: module.[[EvaluationError]] is empty.
5. 5. 5. Set module.[[EvaluationError]] to ThrowCompletion(error).
6. 6. 6. Set module.[[Status]] to evaluated.
7. 7. 7. For each Cyclic Module Record m of module.[[AsyncParentModules]], do
1. a. a. Perform AsyncModuleExecutionRejected(m, error).
8. 8. 8. If module.[[TopLevelCapability]] is not empty, then
1. a. a. Assert: module.[[CycleRoot]] is module.
2. b. b. Perform ! Call(module.[[TopLevelCapability]].[[Reject]], undefined,
« error »).
9. 9. 9. Return unused.
16.2.1.5.4 EXAMPLE CYCLIC MODULE RECORD GRAPHS
This non-normative section gives a series of examples of the linking and
evaluation of a few common module graphs, with a specific focus on how errors
can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When a host first calls
A.LoadRequestedModules(), this will complete successfully by assumption, and
recursively load the dependencies of B and C as well (respectively, C and none),
and then set A.[[Status]] = B.[[Status]] = C.[[Status]] = unlinked. Then, when
the host calls A.Link(), it will complete successfully (again by assumption)
such that A.[[Status]] = B.[[Status]] = C.[[Status]] = linked. These preparatory
steps can be performed at any time. Later, when the host is ready to incur any
possible side effects of the modules, it can call A.Evaluate(), which will
complete successfully, returning a Promise resolving to undefined (again by
assumption), recursively having evaluated first C and then B. Each module's
[[Status]] at this point will be evaluated.
Consider then cases involving linking errors, after a successful call to
A.LoadRequestedModules(). If InnerModuleLinking of C succeeds but, thereafter,
fails for B, for example because it imports something that C does not provide,
then the original A.Link() will fail, and both A and B's [[Status]] remain
unlinked. C's [[Status]] has become linked, though.
Finally, consider a case involving evaluation errors after a successful call to
Link(). If InnerModuleEvaluation of C succeeds but, thereafter, fails for B, for
example because B contains code that throws an exception, then the original
A.Evaluate() will fail, returning a rejected Promise. The resulting exception
will be recorded in both A and B's [[EvaluationError]] fields, and their
[[Status]] will become evaluated. C will also become evaluated but, in contrast
to A and B, will remain without an [[EvaluationError]], as it successfully
completed evaluation. Storing the exception ensures that any time a host tries
to reuse A or B by calling their Evaluate() method, it will encounter the same
exception. (Hosts are not required to reuse Cyclic Module Records; similarly,
hosts are not required to expose the exception objects thrown by these methods.
However, the specification enables such uses.)
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module A declares a dependency on some other module, but no
Module Record exists for that module, i.e. HostLoadImportedModule calls
FinishLoadingImportedModule with an exception when asked for it. This could
occur for a variety of reasons, such as the corresponding resource not existing,
or the resource existing but ParseModule returning some errors when trying to
parse the resulting source text. Hosts can choose to expose the cause of failure
via the completion they pass to FinishLoadingImportedModule. In any case, this
exception causes a loading failure, which results in A's [[Status]] remaining
new.
The difference here between loading, linking and evaluation errors is due to the
following characteristic:
* Evaluation must be only performed once, as it can cause side effects; it is
thus important to remember whether evaluation has already been performed,
even if unsuccessfully. (In the error case, it makes sense to also remember
the exception because otherwise subsequent Evaluate() calls would have to
synthesize a new one.)
* Linking, on the other hand, is side-effect-free, and thus even if it fails,
it can be retried at a later time with no issues.
* Loading closely interacts with the host, and it may be desiderable for some
of them to allow users to retry failed loads (for example, if the failure is
caused by temporarily bad network conditions).
Now, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module A, so that the host proceeds by
calling A.LoadRequestedModules(), which performs InnerModuleLoading on A. This
in turn calls InnerModuleLoading on B and C. Because of the cycle, this again
triggers InnerModuleLoading on A, but at this point it is a no-op since A's
dependencies loading has already been triggered during this LoadRequestedModules
process. When all the modules in the graph have been successfully loaded, their
[[Status]] transitions from new to unlinked at the same time.
Then the host proceeds by calling A.Link(), which performs InnerModuleLinking on
A. This in turn calls InnerModuleLinking on B. Because of the cycle, this again
triggers InnerModuleLinking on A, but at this point it is a no-op since
A.[[Status]] is already linking. B.[[Status]] itself remains linking when
control gets back to A and InnerModuleLinking is triggered on C. After this
returns with C.[[Status]] being linked, both A and B transition from linking to
linked together; this is by design, since they form a strongly connected
component. It's possible to transition the status of modules in the same SCC at
the same time because during this phase the module graph is traversed with a
depth-first search.
An analogous story occurs for the evaluation phase of a cyclic module graph, in
the success case.
Now consider a case where A has a linking error; for example, it tries to import
a binding from C that does not exist. In that case, the above steps still occur,
including the early return from the second call to InnerModuleLinking on A.
However, once we unwind back to the original InnerModuleLinking on A, it fails
during InitializeEnvironment, namely right after C.ResolveExport(). The thrown
SyntaxError exception propagates up to A.Link, which resets all modules that are
currently on its stack (these are always exactly the modules that are still
linking). Hence both A and B become unlinked. Note that C is left as linked.
Alternatively, consider a case where A has an evaluation error; for example, its
source code throws an exception. In that case, the evaluation-time analog of the
above steps still occurs, including the early return from the second call to
InnerModuleEvaluation on A. However, once we unwind back to the original
InnerModuleEvaluation on A, it fails by assumption. The exception thrown
propagates up to A.Evaluate(), which records the error in all modules that are
currently on its stack (i.e., the modules that are still evaluating) as well as
via [[AsyncParentModules]], which form a chain for modules which contain or
depend on top-level await through the whole dependency graph through the
AsyncModuleExecutionRejected algorithm. Hence both A and B become evaluated and
the exception is recorded in both A and B's [[EvaluationError]] fields, while C
is left as evaluated with no [[EvaluationError]].
Lastly, consider a module graph with a cycle, where all modules complete
asynchronously:
Figure 5: An asynchronous cyclic module graph
Loading and linking happen as before, and all modules end up with [[Status]] set
to linked.
Calling A.Evaluate() calls InnerModuleEvaluation on A, B, and D, which all
transition to evaluating. Then InnerModuleEvaluation is called on A again, which
is a no-op because it is already evaluating. At this point,
D.[[PendingAsyncDependencies]] is 0, so ExecuteAsyncModule(D) is called and we
call D.ExecuteModule with a new PromiseCapability tracking the asynchronous
execution of D. We unwind back to the InnerModuleEvaluation on B, setting
B.[[PendingAsyncDependencies]] to 1 and B.[[AsyncEvaluation]] to true. We unwind
back to the original InnerModuleEvaluation on A, setting
A.[[PendingAsyncDependencies]] to 1. In the next iteration of the loop over A's
dependencies, we call InnerModuleEvaluation on C and thus on D (again a no-op)
and E. As E has no dependencies and is not part of a cycle, we call
ExecuteAsyncModule(E) in the same manner as D and E is immediately removed from
the stack. We unwind once more to the original InnerModuleEvaluation on A,
setting C.[[AsyncEvaluation]] to true. Now we finish the loop over A's
dependencies, set A.[[AsyncEvaluation]] to true, and remove the entire strongly
connected component from the stack, transitioning all of the modules to
evaluating-async at once. At this point, the fields of the modules are as given
in Table 45.
Table 45: Module fields after the initial Evaluate() call
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] A 0 0 evaluating-async true
« » 2 (B and C) B 1 0 evaluating-async true « A » 1 (D) C 2 0 evaluating-async
true « A » 2 (D and E) D 3 0 evaluating-async true « B, C » 0 E 4 4
evaluating-async true « C » 0
Let us assume that E finishes executing first. When that happens,
AsyncModuleExecutionFulfilled is called, E.[[Status]] is set to evaluated and
C.[[PendingAsyncDependencies]] is decremented to become 1. The fields of the
updated modules are as given in Table 46.
Table 46: Module fields after module E finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] C 2 0 evaluating-async true
« A » 1 (D) E 4 4 evaluated true « C » 0
D is next to finish (as it was the only module that was still executing). When
that happens, AsyncModuleExecutionFulfilled is called again and D.[[Status]] is
set to evaluated. Then B.[[PendingAsyncDependencies]] is decremented to become
0, ExecuteAsyncModule is called on B, and it starts executing.
C.[[PendingAsyncDependencies]] is also decremented to become 0, and C starts
executing (potentially in parallel to B if B contains an await). The fields of
the updated modules are as given in Table 47.
Table 47: Module fields after module D finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] B 1 0 evaluating-async true
« A » 0 C 2 0 evaluating-async true « A » 0 D 3 0 evaluated true « B, C » 0
Let us assume that C finishes executing next. When that happens,
AsyncModuleExecutionFulfilled is called again, C.[[Status]] is set to evaluated
and A.[[PendingAsyncDependencies]] is decremented to become 1. The fields of the
updated modules are as given in Table 48.
Table 48: Module fields after module C finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] A 0 0 evaluating-async true
« » 1 (B) C 2 0 evaluated true « A » 0
Then, B finishes executing. When that happens, AsyncModuleExecutionFulfilled is
called again and B.[[Status]] is set to evaluated.
A.[[PendingAsyncDependencies]] is decremented to become 0, so ExecuteAsyncModule
is called and it starts executing. The fields of the updated modules are as
given in Table 49.
Table 49: Module fields after module B finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] A 0 0 evaluating-async true
« » 0 B 1 0 evaluated true « A » 0
Finally, A finishes executing. When that happens, AsyncModuleExecutionFulfilled
is called again and A.[[Status]] is set to evaluated. At this point, the Promise
in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is resolved,
and this concludes the handling of this module graph. The fields of the updated
module are as given in Table 50.
Table 50: Module fields after module A finishes executing
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] A 0 0 evaluated true « » 0
Alternatively, consider a failure case where C fails execution and returns an
error before B has finished executing. When that happens,
AsyncModuleExecutionRejected is called, which sets C.[[Status]] to evaluated and
C.[[EvaluationError]] to the error. It then propagates this error to all of the
AsyncParentModules by performing AsyncModuleExecutionRejected on each of them.
The fields of the updated modules are as given in Table 51.
Table 51: Module fields after module C finishes with an error
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] [[EvaluationError]] A 0 0
evaluated true « » 1 (B) empty C 2 1 evaluated true « A » 0 C's evaluation error
A will be rejected with the same error as C since C will call
AsyncModuleExecutionRejected on A with C's error. A.[[Status]] is set to
evaluated. At this point the Promise in A.[[TopLevelCapability]] (which was
returned from A.Evaluate()) is rejected. The fields of the updated module are as
given in Table 52.
Table 52: Module fields after module A is rejected
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] [[EvaluationError]] A 0 0
evaluated true « » 0 C's Evaluation Error
Then, B finishes executing without an error. When that happens,
AsyncModuleExecutionFulfilled is called again and B.[[Status]] is set to
evaluated. GatherAvailableAncestors is called on B. However, A.[[CycleRoot]] is
A which has an evaluation error, so it will not be added to the returned
sortedExecList and AsyncModuleExecutionFulfilled will return without further
processing. Any future importer of B will resolve the rejection of
B.[[CycleRoot]].[[EvaluationError]] from the evaluation error from C that was
set on the cycle root A. The fields of the updated modules are as given in Table
53.
Table 53: Module fields after module B finishes executing in an erroring graph
Module [[DFSIndex]] [[DFSAncestorIndex]] [[Status]] [[AsyncEvaluation]]
[[AsyncParentModules]] [[PendingAsyncDependencies]] [[EvaluationError]] A 0 0
evaluated true « » 0 C's Evaluation Error B 1 0 evaluated true « A » 0 empty
16.2.1.6 SOURCE TEXT MODULE RECORDS
A Source Text Module Record is used to represent information about a module that
was defined from ECMAScript source text (11) that was parsed using the goal
symbol Module. Its fields contain digested information about the names that are
imported by the module and its concrete methods use this digest to link, link,
and evaluate the module.
A Source Text Module Record can exist in a module graph with other subclasses of
the abstract Module Record type, and can participate in cycles with other
subclasses of the Cyclic Module Record type.
In addition to the fields defined in Table 42, Source Text Module Records have
the additional fields listed in Table 54. Each of these fields is initially set
in ParseModule.
Table 54: Additional Fields of Source Text Module Records
Field Name Value Type Meaning [[ECMAScriptCode]] a Parse Node The result of
parsing the source text of this module using Module as the goal symbol.
[[Context]] an ECMAScript code execution context or empty The execution context
associated with this module. It is empty until the module's environment has been
initialized. [[ImportMeta]] an Object or empty An object exposed through the
import.meta meta property. It is empty until it is accessed by ECMAScript code.
[[ImportEntries]] a List of ImportEntry Records A List of ImportEntry records
derived from the code of this module. [[LocalExportEntries]] a List of
ExportEntry Records A List of ExportEntry records derived from the code of this
module that correspond to declarations that occur within the module.
[[IndirectExportEntries]] a List of ExportEntry Records A List of ExportEntry
records derived from the code of this module that correspond to reexported
imports that occur within the module or exports from export * as namespace
declarations. [[StarExportEntries]] a List of ExportEntry Records A List of
ExportEntry records derived from the code of this module that correspond to
export * declarations that occur within the module, not including export * as
namespace declarations.
An ImportEntry Record is a Record that digests information about a single
declarative import. Each ImportEntry Record has the fields defined in Table 55:
Table 55: ImportEntry Record Fields
Field Name Value Type Meaning [[ModuleRequest]] a String String value of the
ModuleSpecifier of the ImportDeclaration. [[ImportName]] a String or
namespace-object The name under which the desired binding is exported by the
module identified by [[ModuleRequest]]. The value namespace-object indicates
that the import request is for the target module's namespace object.
[[LocalName]] a String The name that is used to locally access the imported
value from within the importing module.
Note 1
Table 56 gives examples of ImportEntry records fields used to represent the
syntactic import forms:
Table 56 (Informative): Import Forms Mappings to ImportEntry Records
Import Statement Form [[ModuleRequest]] [[ImportName]] [[LocalName]] import v
from "mod"; "mod" "default" "v" import * as ns from "mod"; "mod"
namespace-object "ns" import {x} from "mod"; "mod" "x" "x" import {x as v} from
"mod"; "mod" "x" "v" import "mod"; An ImportEntry Record is not created.
An ExportEntry Record is a Record that digests information about a single
declarative export. Each ExportEntry Record has the fields defined in Table 57:
Table 57: ExportEntry Record Fields
Field Name Value Type Meaning [[ExportName]] a String or null The name used to
export this binding by this module. [[ModuleRequest]] a String or null The
String value of the ModuleSpecifier of the ExportDeclaration. null if the
ExportDeclaration does not have a ModuleSpecifier. [[ImportName]] a String,
null, all, or all-but-default The name under which the desired binding is
exported by the module identified by [[ModuleRequest]]. null if the
ExportDeclaration does not have a ModuleSpecifier. all is used for export * as
ns from "mod" declarations. all-but-default is used for export * from "mod"
declarations. [[LocalName]] a String or null The name that is used to locally
access the exported value from within the importing module. null if the exported
value is not locally accessible from within the module.
Note 2
Table 58 gives examples of the ExportEntry record fields used to represent the
syntactic export forms:
Table 58 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form [[ExportName]] [[ModuleRequest]] [[ImportName]]
[[LocalName]] export var v; "v" null null "v" export default function f() {}
"default" null null "f" export default function () {} "default" null null
"*default*" export default 42; "default" null null "*default*" export {x}; "x"
null null "x" export {v as x}; "x" null null "v" export {x} from "mod"; "x"
"mod" "x" null export {v as x} from "mod"; "x" "mod" "v" null export * from
"mod"; null "mod" all-but-default null export * as ns from "mod"; "ns" "mod" all
null
The following definitions specify the required concrete methods and other
abstract operations for Source Text Module Records
16.2.1.6.1 PARSEMODULE ( SOURCETEXT, REALM, HOSTDEFINED )
The abstract operation ParseModule takes arguments sourceText (ECMAScript source
text), realm (a Realm Record), and hostDefined (anything) and returns a Source
Text Module Record or a non-empty List of SyntaxError objects. It creates a
Source Text Module Record based upon the result of parsing sourceText as a
Module. It performs the following steps when called:
1. 1. 1. Let body be ParseText(sourceText, Module).
2. 2. 2. If body is a List of errors, return body.
3. 3. 3. Let requestedModules be the ModuleRequests of body.
4. 4. 4. Let importEntries be ImportEntries of body.
5. 5. 5. Let importedBoundNames be ImportedLocalNames(importEntries).
6. 6. 6. Let indirectExportEntries be a new empty List.
7. 7. 7. Let localExportEntries be a new empty List.
8. 8. 8. Let starExportEntries be a new empty List.
9. 9. 9. Let exportEntries be ExportEntries of body.
10. 10. 10. For each ExportEntry Record ee of exportEntries, do
1. a. a. If ee.[[ModuleRequest]] is null, then
1. i. i. If importedBoundNames does not contain ee.[[LocalName]], then
1. 1. 1. Append ee to localExportEntries.
2. ii. ii. Else,
1. 1. 1. Let ie be the element of importEntries whose [[LocalName]]
is ee.[[LocalName]].
2. 2. 2. If ie.[[ImportName]] is namespace-object, then
1. a. a. NOTE: This is a re-export of an imported module namespace
object.
2. b. b. Append ee to localExportEntries.
3. 3. 3. Else,
1. a. a. NOTE: This is a re-export of a single name.
2. b. b. Append the ExportEntry Record { [[ModuleRequest]]:
ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]],
[[LocalName]]: null, [[ExportName]]: ee.[[ExportName]] } to
indirectExportEntries.
2. b. b. Else if ee.[[ImportName]] is all-but-default, then
1. i. i. Assert: ee.[[ExportName]] is null.
2. ii. ii. Append ee to starExportEntries.
3. c. c. Else,
1. i. i. Append ee to indirectExportEntries.
11. 11. 11. Let async be body Contains await.
12. 12. 12. Return Source Text Module Record { [[Realm]]: realm,
[[Environment]]: empty, [[Namespace]]: empty, [[CycleRoot]]: empty,
[[HasTLA]]: async, [[AsyncEvaluation]]: false, [[TopLevelCapability]]:
empty, [[AsyncParentModules]]: « », [[PendingAsyncDependencies]]: empty,
[[Status]]: new, [[EvaluationError]]: empty, [[HostDefined]]: hostDefined,
[[ECMAScriptCode]]: body, [[Context]]: empty, [[ImportMeta]]: empty,
[[RequestedModules]]: requestedModules, [[LoadedModules]]: « »,
[[ImportEntries]]: importEntries, [[LocalExportEntries]]:
localExportEntries, [[IndirectExportEntries]]: indirectExportEntries,
[[StarExportEntries]]: starExportEntries, [[DFSIndex]]: empty,
[[DFSAncestorIndex]]: empty }.
Note
An implementation may parse module source text and analyse it for Early Error
conditions prior to the evaluation of ParseModule for that module source text.
However, the reporting of any errors must be deferred until the point where this
specification actually performs ParseModule upon that source text.
16.2.1.6.2 GETEXPORTEDNAMES ( [ EXPORTSTARSET ] )
The GetExportedNames concrete method of a Source Text Module Record module takes
optional argument exportStarSet (a List of Source Text Module Records) and
returns a List of either Strings or null. It performs the following steps when
called:
1. 1. 1. Assert: module.[[Status]] is not new.
2. 2. 2. If exportStarSet is not present, set exportStarSet to a new empty
List.
3. 3. 3. If exportStarSet contains module, then
1. a. a. Assert: We've reached the starting point of an export *
circularity.
2. b. b. Return a new empty List.
4. 4. 4. Append module to exportStarSet.
5. 5. 5. Let exportedNames be a new empty List.
6. 6. 6. For each ExportEntry Record e of module.[[LocalExportEntries]], do
1. a. a. Assert: module provides the direct binding for this export.
2. b. b. Append e.[[ExportName]] to exportedNames.
7. 7. 7. For each ExportEntry Record e of module.[[IndirectExportEntries]], do
1. a. a. Assert: module imports a specific binding for this export.
2. b. b. Append e.[[ExportName]] to exportedNames.
8. 8. 8. For each ExportEntry Record e of module.[[StarExportEntries]], do
1. a. a. Let requestedModule be GetImportedModule(module,
e.[[ModuleRequest]]).
2. b. b. Let starNames be requestedModule.GetExportedNames(exportStarSet).
3. c. c. For each element n of starNames, do
1. i. i. If SameValue(n, "default") is false, then
1. 1. 1. If exportedNames does not contain n, then
1. a. a. Append n to exportedNames.
9. 9. 9. Return exportedNames.
Note
GetExportedNames does not filter out or throw an exception for names that have
ambiguous star export bindings.
16.2.1.6.3 RESOLVEEXPORT ( EXPORTNAME [ , RESOLVESET ] )
The ResolveExport concrete method of a Source Text Module Record module takes
argument exportName (a String) and optional argument resolveSet (a List of
Records with fields [[Module]] (a Module Record) and [[ExportName]] (a String))
and returns a ResolvedBinding Record, null, or ambiguous.
ResolveExport attempts to resolve an imported binding to the actual defining
module and local binding name. The defining module may be the module represented
by the Module Record this method was invoked on or some other module that is
imported by that module. The parameter resolveSet is used to detect unresolved
circular import/export paths. If a pair consisting of specific Module Record and
exportName is reached that is already in resolveSet, an import circularity has
been encountered. Before recursively calling ResolveExport, a pair consisting of
module and exportName is added to resolveSet.
If a defining module is found, a ResolvedBinding Record { [[Module]],
[[BindingName]] } is returned. This record identifies the resolved binding of
the originally requested export, unless this is the export of a namespace with
no local binding. In this case, [[BindingName]] will be set to namespace. If no
definition was found or the request is found to be circular, null is returned.
If the request is found to be ambiguous, ambiguous is returned.
It performs the following steps when called:
1. 1. 1. Assert: module.[[Status]] is not new.
2. 2. 2. If resolveSet is not present, set resolveSet to a new empty List.
3. 3. 3. For each Record { [[Module]], [[ExportName]] } r of resolveSet, do
1. a. a. If module and r.[[Module]] are the same Module Record and
SameValue(exportName, r.[[ExportName]]) is true, then
1. i. i. Assert: This is a circular import request.
2. ii. ii. Return null.
4. 4. 4. Append the Record { [[Module]]: module, [[ExportName]]: exportName }
to resolveSet.
5. 5. 5. For each ExportEntry Record e of module.[[LocalExportEntries]], do
1. a. a. If SameValue(exportName, e.[[ExportName]]) is true, then
1. i. i. Assert: module provides the direct binding for this export.
2. ii. ii. Return ResolvedBinding Record { [[Module]]: module,
[[BindingName]]: e.[[LocalName]] }.
6. 6. 6. For each ExportEntry Record e of module.[[IndirectExportEntries]], do
1. a. a. If SameValue(exportName, e.[[ExportName]]) is true, then
1. i. i. Let importedModule be GetImportedModule(module,
e.[[ModuleRequest]]).
2. ii. ii. If e.[[ImportName]] is all, then
1. 1. 1. Assert: module does not provide the direct binding for this
export.
2. 2. 2. Return ResolvedBinding Record { [[Module]]: importedModule,
[[BindingName]]: namespace }.
3. iii. iii. Else,
1. 1. 1. Assert: module imports a specific binding for this export.
2. 2. 2. Return importedModule.ResolveExport(e.[[ImportName]],
resolveSet).
7. 7. 7. If SameValue(exportName, "default") is true, then
1. a. a. Assert: A default export was not explicitly defined by this
module.
2. b. b. Return null.
3. c. c. NOTE: A default export cannot be provided by an export * from
"mod" declaration.
8. 8. 8. Let starResolution be null.
9. 9. 9. For each ExportEntry Record e of module.[[StarExportEntries]], do
1. a. a. Let importedModule be GetImportedModule(module,
e.[[ModuleRequest]]).
2. b. b. Let resolution be importedModule.ResolveExport(exportName,
resolveSet).
3. c. c. If resolution is ambiguous, return ambiguous.
4. d. d. If resolution is not null, then
1. i. i. Assert: resolution is a ResolvedBinding Record.
2. ii. ii. If starResolution is null, set starResolution to resolution.
3. iii. iii. Else,
1. 1. 1. Assert: There is more than one * import that includes the
requested name.
2. 2. 2. If resolution.[[Module]] and starResolution.[[Module]] are
not the same Module Record, return ambiguous.
3. 3. 3. If resolution.[[BindingName]] is not
starResolution.[[BindingName]] and either
resolution.[[BindingName]] or starResolution.[[BindingName]] is
namespace, return ambiguous.
4. 4. 4. If resolution.[[BindingName]] is a String,
starResolution.[[BindingName]] is a String, and
SameValue(resolution.[[BindingName]],
starResolution.[[BindingName]]) is false, return ambiguous.
10. 10. 10. Return starResolution.
16.2.1.6.4 INITIALIZEENVIRONMENT ( )
The InitializeEnvironment concrete method of a Source Text Module Record module
takes no arguments and returns either a normal completion containing unused or a
throw completion. It performs the following steps when called:
1. 1. 1. For each ExportEntry Record e of module.[[IndirectExportEntries]], do
1. a. a. Let resolution be module.ResolveExport(e.[[ExportName]]).
2. b. b. If resolution is either null or ambiguous, throw a SyntaxError
exception.
3. c. c. Assert: resolution is a ResolvedBinding Record.
2. 2. 2. Assert: All named exports from module are resolvable.
3. 3. 3. Let realm be module.[[Realm]].
4. 4. 4. Assert: realm is not undefined.
5. 5. 5. Let env be NewModuleEnvironment(realm.[[GlobalEnv]]).
6. 6. 6. Set module.[[Environment]] to env.
7. 7. 7. For each ImportEntry Record in of module.[[ImportEntries]], do
1. a. a. Let importedModule be GetImportedModule(module,
in.[[ModuleRequest]]).
2. b. b. If in.[[ImportName]] is namespace-object, then
1. i. i. Let namespace be GetModuleNamespace(importedModule).
2. ii. ii. Perform ! env.CreateImmutableBinding(in.[[LocalName]], true).
3. iii. iii. Perform ! env.InitializeBinding(in.[[LocalName]],
namespace).
3. c. c. Else,
1. i. i. Let resolution be
importedModule.ResolveExport(in.[[ImportName]]).
2. ii. ii. If resolution is either null or ambiguous, throw a
SyntaxError exception.
3. iii. iii. If resolution.[[BindingName]] is namespace, then
1. 1. 1. Let namespace be GetModuleNamespace(resolution.[[Module]]).
2. 2. 2. Perform ! env.CreateImmutableBinding(in.[[LocalName]],
true).
3. 3. 3. Perform ! env.InitializeBinding(in.[[LocalName]],
namespace).
4. iv. iv. Else,
1. 1. 1. Perform env.CreateImportBinding(in.[[LocalName]],
resolution.[[Module]], resolution.[[BindingName]]).
8. 8. 8. Let moduleContext be a new ECMAScript code execution context.
9. 9. 9. Set the Function of moduleContext to null.
10. 10. 10. Assert: module.[[Realm]] is not undefined.
11. 11. 11. Set the Realm of moduleContext to module.[[Realm]].
12. 12. 12. Set the ScriptOrModule of moduleContext to module.
13. 13. 13. Set the VariableEnvironment of moduleContext to
module.[[Environment]].
14. 14. 14. Set the LexicalEnvironment of moduleContext to
module.[[Environment]].
15. 15. 15. Set the PrivateEnvironment of moduleContext to null.
16. 16. 16. Set module.[[Context]] to moduleContext.
17. 17. 17. Push moduleContext onto the execution context stack; moduleContext
is now the running execution context.
18. 18. 18. Let code be module.[[ECMAScriptCode]].
19. 19. 19. Let varDeclarations be the VarScopedDeclarations of code.
20. 20. 20. Let declaredVarNames be a new empty List.
21. 21. 21. For each element d of varDeclarations, do
1. a. a. For each element dn of the BoundNames of d, do
1. i. i. If declaredVarNames does not contain dn, then
1. 1. 1. Perform ! env.CreateMutableBinding(dn, false).
2. 2. 2. Perform ! env.InitializeBinding(dn, undefined).
3. 3. 3. Append dn to declaredVarNames.
22. 22. 22. Let lexDeclarations be the LexicallyScopedDeclarations of code.
23. 23. 23. Let privateEnv be null.
24. 24. 24. For each element d of lexDeclarations, do
1. a. a. For each element dn of the BoundNames of d, do
1. i. i. If IsConstantDeclaration of d is true, then
1. 1. 1. Perform ! env.CreateImmutableBinding(dn, true).
2. ii. ii. Else,
1. 1. 1. Perform ! env.CreateMutableBinding(dn, false).
3. iii. iii. If d is either a FunctionDeclaration, a
GeneratorDeclaration, an AsyncFunctionDeclaration, or an
AsyncGeneratorDeclaration, then
1. 1. 1. Let fo be InstantiateFunctionObject of d with arguments env
and privateEnv.
2. 2. 2. Perform ! env.InitializeBinding(dn, fo).
25. 25. 25. Remove moduleContext from the execution context stack.
26. 26. 26. Return unused.
16.2.1.6.5 EXECUTEMODULE ( [ CAPABILITY ] )
The ExecuteModule concrete method of a Source Text Module Record module takes
optional argument capability (a PromiseCapability Record) and returns either a
normal completion containing unused or a throw completion. It performs the
following steps when called:
1. 1. 1. Let moduleContext be a new ECMAScript code execution context.
2. 2. 2. Set the Function of moduleContext to null.
3. 3. 3. Set the Realm of moduleContext to module.[[Realm]].
4. 4. 4. Set the ScriptOrModule of moduleContext to module.
5. 5. 5. Assert: module has been linked and declarations in its module
environment have been instantiated.
6. 6. 6. Set the VariableEnvironment of moduleContext to
module.[[Environment]].
7. 7. 7. Set the LexicalEnvironment of moduleContext to
module.[[Environment]].
8. 8. 8. Suspend the running execution context.
9. 9. 9. If module.[[HasTLA]] is false, then
1. a. a. Assert: capability is not present.
2. b. b. Push moduleContext onto the execution context stack; moduleContext
is now the running execution context.
3. c. c. Let result be Completion(Evaluation of module.[[ECMAScriptCode]]).
4. d. d. Suspend moduleContext and remove it from the execution context
stack.
5. e. e. Resume the context that is now on the top of the execution context
stack as the running execution context.
6. f. f. If result is an abrupt completion, then
1. i. i. Return ? result.
10. 10. 10. Else,
1. a. a. Assert: capability is a PromiseCapability Record.
2. b. b. Perform AsyncBlockStart(capability, module.[[ECMAScriptCode]],
moduleContext).
11. 11. 11. Return unused.
16.2.1.7 GETIMPORTEDMODULE ( REFERRER, SPECIFIER )
The abstract operation GetImportedModule takes arguments referrer (a Cyclic
Module Record) and specifier (a String) and returns a Module Record. It performs
the following steps when called:
1. 1. 1. Assert: Exactly one element of referrer.[[LoadedModules]] is a Record
whose [[Specifier]] is specifier, since LoadRequestedModules has completed
successfully on referrer prior to invoking this abstract operation.
2. 2. 2. Let record be the Record in referrer.[[LoadedModules]] whose
[[Specifier]] is specifier.
3. 3. 3. Return record.[[Module]].
16.2.1.8 HOSTLOADIMPORTEDMODULE ( REFERRER, SPECIFIER, HOSTDEFINED, PAYLOAD )
The host-defined abstract operation HostLoadImportedModule takes arguments
referrer (a Script Record, a Cyclic Module Record, or a Realm Record), specifier
(a String), hostDefined (anything), and payload (a GraphLoadingState Record or a
PromiseCapability Record) and returns unused.
Note
An example of when referrer can be a Realm Record is in a web browser host.
There, if a user clicks on a control given by
<button type="button" onclick="import('./foo.mjs')">Click me</button>
there will be no active script or module at the time the import() expression
runs. More generally, this can happen in any situation where the host pushes
execution contexts with null ScriptOrModule components onto the execution
context stack.
An implementation of HostLoadImportedModule must conform to the following
requirements:
* The host environment must perform FinishLoadingImportedModule(referrer,
specifier, payload, result), where result is either a normal completion
containing the loaded Module Record or a throw completion, either
synchronously or asynchronously.
* If this operation is called multiple times with the same (referrer,
specifier) pair and it performs FinishLoadingImportedModule(referrer,
specifier, payload, result) where result is a normal completion, then it must
perform FinishLoadingImportedModule(referrer, specifier, payload, result)
with the same result each time.
* The operation must treat payload as an opaque value to be passed through to
FinishLoadingImportedModule.
The actual process performed is host-defined, but typically consists of
performing whatever I/O operations are necessary to load the appropriate Module
Record. Multiple different (referrer, specifier) pairs may map to the same
Module Record instance. The actual mapping semantics is host-defined but
typically a normalization process is applied to specifier as part of the mapping
process. A typical normalization process would include actions such as expansion
of relative and abbreviated path specifiers.
16.2.1.9 FINISHLOADINGIMPORTEDMODULE ( REFERRER, SPECIFIER, PAYLOAD, RESULT )
The abstract operation FinishLoadingImportedModule takes arguments referrer (a
Script Record, a Cyclic Module Record, or a Realm Record), specifier (a String),
payload (a GraphLoadingState Record or a PromiseCapability Record), and result
(either a normal completion containing a Module Record or a throw completion)
and returns unused. It performs the following steps when called:
1. 1. 1. If result is a normal completion, then
1. a. a. If referrer.[[LoadedModules]] contains a Record whose [[Specifier]]
is specifier, then
1. i. i. Assert: That Record's [[Module]] is result.[[Value]].
2. b. b. Else, append the Record { [[Specifier]]: specifier, [[Module]]:
result.[[Value]] } to referrer.[[LoadedModules]].
2. 2. 2. If payload is a GraphLoadingState Record, then
1. a. a. Perform ContinueModuleLoading(payload, result).
3. 3. 3. Else,
1. a. a. Perform ContinueDynamicImport(payload, result).
4. 4. 4. Return unused.
16.2.1.10 GETMODULENAMESPACE ( MODULE )
The abstract operation GetModuleNamespace takes argument module (an instance of
a concrete subclass of Module Record) and returns a Module Namespace Object or
empty. It retrieves the Module Namespace Object representing module's exports,
lazily creating it the first time it was requested, and storing it in
module.[[Namespace]] for future retrieval. It performs the following steps when
called:
1. 1. 1. Assert: If module is a Cyclic Module Record, then module.[[Status]] is
not new or unlinked.
2. 2. 2. Let namespace be module.[[Namespace]].
3. 3. 3. If namespace is empty, then
1. a. a. Let exportedNames be module.GetExportedNames().
2. b. b. Let unambiguousNames be a new empty List.
3. c. c. For each element name of exportedNames, do
1. i. i. Let resolution be module.ResolveExport(name).
2. ii. ii. If resolution is a ResolvedBinding Record, append name to
unambiguousNames.
4. d. d. Set namespace to ModuleNamespaceCreate(module, unambiguousNames).
4. 4. 4. Return namespace.
Note
GetModuleNamespace never throws. Instead, unresolvable names are simply excluded
from the namespace at this point. They will lead to a real linking error later
unless they are all ambiguous star exports that are not explicitly requested
anywhere.
16.2.1.11 RUNTIME SEMANTICS: EVALUATION
Module : [empty]
1. 1. 1. Return undefined.
ModuleBody : ModuleItemList
1. 1. 1. Let result be Completion(Evaluation of ModuleItemList).
2. 2. 2. If result.[[Type]] is normal and result.[[Value]] is empty, then
1. a. a. Return undefined.
3. 3. 3. Return ? result.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let sl be ? Evaluation of ModuleItemList.
2. 2. 2. Let s be Completion(Evaluation of ModuleItem).
3. 3. 3. Return ? UpdateEmpty(s, sl).
Note
The value of a ModuleItemList is the value of the last value-producing item in
the ModuleItemList.
ModuleItem : ImportDeclaration
1. 1. 1. Return empty.
16.2.2 IMPORTS
SYNTAX
ImportDeclaration : import ImportClause FromClause ; import ModuleSpecifier ;
ImportClause : ImportedDefaultBinding NameSpaceImport NamedImports
ImportedDefaultBinding , NameSpaceImport ImportedDefaultBinding , NamedImports
ImportedDefaultBinding : ImportedBinding NameSpaceImport : * as ImportedBinding
NamedImports : { } { ImportsList } { ImportsList , } FromClause : from
ModuleSpecifier ImportsList : ImportSpecifier ImportsList , ImportSpecifier
ImportSpecifier : ImportedBinding ModuleExportName as ImportedBinding
ModuleSpecifier : StringLiteral ImportedBinding : BindingIdentifier[~Yield,
+Await]
16.2.2.1 STATIC SEMANTICS: EARLY ERRORS
ModuleItem : ImportDeclaration
* It is a Syntax Error if the BoundNames of ImportDeclaration contains any
duplicate entries.
16.2.2.2 STATIC SEMANTICS: IMPORTENTRIES
The syntax-directed operation ImportEntries takes no arguments and returns a
List of ImportEntry Records. It is defined piecewise over the following
productions:
Module : [empty]
1. 1. 1. Return a new empty List.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let entries1 be ImportEntries of ModuleItemList.
2. 2. 2. Let entries2 be ImportEntries of ModuleItem.
3. 3. 3. Return the list-concatenation of entries1 and entries2.
ModuleItem : ExportDeclaration StatementListItem
1. 1. 1. Return a new empty List.
ImportDeclaration : import ImportClause FromClause ;
1. 1. 1. Let module be the sole element of ModuleRequests of FromClause.
2. 2. 2. Return ImportEntriesForModule of ImportClause with argument module.
ImportDeclaration : import ModuleSpecifier ;
1. 1. 1. Return a new empty List.
16.2.2.3 STATIC SEMANTICS: IMPORTENTRIESFORMODULE
The syntax-directed operation ImportEntriesForModule takes argument module (a
String) and returns a List of ImportEntry Records. It is defined piecewise over
the following productions:
ImportClause : ImportedDefaultBinding , NameSpaceImport
1. 1. 1. Let entries1 be ImportEntriesForModule of ImportedDefaultBinding with
argument module.
2. 2. 2. Let entries2 be ImportEntriesForModule of NameSpaceImport with
argument module.
3. 3. 3. Return the list-concatenation of entries1 and entries2.
ImportClause : ImportedDefaultBinding , NamedImports
1. 1. 1. Let entries1 be ImportEntriesForModule of ImportedDefaultBinding with
argument module.
2. 2. 2. Let entries2 be ImportEntriesForModule of NamedImports with argument
module.
3. 3. 3. Return the list-concatenation of entries1 and entries2.
ImportedDefaultBinding : ImportedBinding
1. 1. 1. Let localName be the sole element of BoundNames of ImportedBinding.
2. 2. 2. Let defaultEntry be the ImportEntry Record { [[ModuleRequest]]:
module, [[ImportName]]: "default", [[LocalName]]: localName }.
3. 3. 3. Return « defaultEntry ».
NameSpaceImport : * as ImportedBinding
1. 1. 1. Let localName be the StringValue of ImportedBinding.
2. 2. 2. Let entry be the ImportEntry Record { [[ModuleRequest]]: module,
[[ImportName]]: namespace-object, [[LocalName]]: localName }.
3. 3. 3. Return « entry ».
NamedImports : { }
1. 1. 1. Return a new empty List.
ImportsList : ImportsList , ImportSpecifier
1. 1. 1. Let specs1 be the ImportEntriesForModule of ImportsList with argument
module.
2. 2. 2. Let specs2 be the ImportEntriesForModule of ImportSpecifier with
argument module.
3. 3. 3. Return the list-concatenation of specs1 and specs2.
ImportSpecifier : ImportedBinding
1. 1. 1. Let localName be the sole element of BoundNames of ImportedBinding.
2. 2. 2. Let entry be the ImportEntry Record { [[ModuleRequest]]: module,
[[ImportName]]: localName, [[LocalName]]: localName }.
3. 3. 3. Return « entry ».
ImportSpecifier : ModuleExportName as ImportedBinding
1. 1. 1. Let importName be the StringValue of ModuleExportName.
2. 2. 2. Let localName be the StringValue of ImportedBinding.
3. 3. 3. Let entry be the ImportEntry Record { [[ModuleRequest]]: module,
[[ImportName]]: importName, [[LocalName]]: localName }.
4. 4. 4. Return « entry ».
16.2.3 EXPORTS
SYNTAX
ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
export VariableStatement[~Yield, +Await] export Declaration[~Yield, +Await]
export default HoistableDeclaration[~Yield, +Await, +Default] export default
ClassDeclaration[~Yield, +Await, +Default] export default [lookahead ∉ {
function, async [no LineTerminator here] function, class }]
AssignmentExpression[+In, ~Yield, +Await] ; ExportFromClause : * * as
ModuleExportName NamedExports NamedExports : { } { ExportsList } { ExportsList ,
} ExportsList : ExportSpecifier ExportsList , ExportSpecifier ExportSpecifier :
ModuleExportName ModuleExportName as ModuleExportName
16.2.3.1 STATIC SEMANTICS: EARLY ERRORS
ExportDeclaration : export NamedExports ;
* It is a Syntax Error if ReferencedBindings of NamedExports contains any
StringLiterals.
* For each IdentifierName n in ReferencedBindings of NamedExports: It is a
Syntax Error if StringValue of n is a ReservedWord or the StringValue of n is
one of "implements", "interface", "let", "package", "private", "protected",
"public", or "static".
Note
The above rule means that each ReferencedBindings of NamedExports is treated as
an IdentifierReference.
16.2.3.2 STATIC SEMANTICS: EXPORTEDBINDINGS
The syntax-directed operation ExportedBindings takes no arguments and returns a
List of Strings.
Note
ExportedBindings are the locally bound names that are explicitly associated with
a Module's ExportedNames.
It is defined piecewise over the following productions:
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let names1 be ExportedBindings of ModuleItemList.
2. 2. 2. Let names2 be ExportedBindings of ModuleItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
ModuleItem : ImportDeclaration StatementListItem
1. 1. 1. Return a new empty List.
ExportDeclaration : export ExportFromClause FromClause ;
1. 1. 1. Return a new empty List.
ExportDeclaration : export NamedExports ;
1. 1. 1. Return the ExportedBindings of NamedExports.
ExportDeclaration : export VariableStatement
1. 1. 1. Return the BoundNames of VariableStatement.
ExportDeclaration : export Declaration
1. 1. 1. Return the BoundNames of Declaration.
ExportDeclaration : export default HoistableDeclaration export default
ClassDeclaration export default AssignmentExpression ;
1. 1. 1. Return the BoundNames of this ExportDeclaration.
NamedExports : { }
1. 1. 1. Return a new empty List.
ExportsList : ExportsList , ExportSpecifier
1. 1. 1. Let names1 be the ExportedBindings of ExportsList.
2. 2. 2. Let names2 be the ExportedBindings of ExportSpecifier.
3. 3. 3. Return the list-concatenation of names1 and names2.
ExportSpecifier : ModuleExportName
1. 1. 1. Return a List whose sole element is the StringValue of
ModuleExportName.
ExportSpecifier : ModuleExportName as ModuleExportName
1. 1. 1. Return a List whose sole element is the StringValue of the first
ModuleExportName.
16.2.3.3 STATIC SEMANTICS: EXPORTEDNAMES
The syntax-directed operation ExportedNames takes no arguments and returns a
List of Strings.
Note
ExportedNames are the externally visible names that a Module explicitly maps to
one of its local name bindings.
It is defined piecewise over the following productions:
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let names1 be ExportedNames of ModuleItemList.
2. 2. 2. Let names2 be ExportedNames of ModuleItem.
3. 3. 3. Return the list-concatenation of names1 and names2.
ModuleItem : ExportDeclaration
1. 1. 1. Return the ExportedNames of ExportDeclaration.
ModuleItem : ImportDeclaration StatementListItem
1. 1. 1. Return a new empty List.
ExportDeclaration : export ExportFromClause FromClause ;
1. 1. 1. Return the ExportedNames of ExportFromClause.
ExportFromClause : *
1. 1. 1. Return a new empty List.
ExportFromClause : * as ModuleExportName
1. 1. 1. Return a List whose sole element is the StringValue of
ModuleExportName.
ExportFromClause : NamedExports
1. 1. 1. Return the ExportedNames of NamedExports.
ExportDeclaration : export VariableStatement
1. 1. 1. Return the BoundNames of VariableStatement.
ExportDeclaration : export Declaration
1. 1. 1. Return the BoundNames of Declaration.
ExportDeclaration : export default HoistableDeclaration export default
ClassDeclaration export default AssignmentExpression ;
1. 1. 1. Return « "default" ».
NamedExports : { }
1. 1. 1. Return a new empty List.
ExportsList : ExportsList , ExportSpecifier
1. 1. 1. Let names1 be the ExportedNames of ExportsList.
2. 2. 2. Let names2 be the ExportedNames of ExportSpecifier.
3. 3. 3. Return the list-concatenation of names1 and names2.
ExportSpecifier : ModuleExportName
1. 1. 1. Return a List whose sole element is the StringValue of
ModuleExportName.
ExportSpecifier : ModuleExportName as ModuleExportName
1. 1. 1. Return a List whose sole element is the StringValue of the second
ModuleExportName.
16.2.3.4 STATIC SEMANTICS: EXPORTENTRIES
The syntax-directed operation ExportEntries takes no arguments and returns a
List of ExportEntry Records. It is defined piecewise over the following
productions:
Module : [empty]
1. 1. 1. Return a new empty List.
ModuleItemList : ModuleItemList ModuleItem
1. 1. 1. Let entries1 be ExportEntries of ModuleItemList.
2. 2. 2. Let entries2 be ExportEntries of ModuleItem.
3. 3. 3. Return the list-concatenation of entries1 and entries2.
ModuleItem : ImportDeclaration StatementListItem
1. 1. 1. Return a new empty List.
ExportDeclaration : export ExportFromClause FromClause ;
1. 1. 1. Let module be the sole element of ModuleRequests of FromClause.
2. 2. 2. Return ExportEntriesForModule of ExportFromClause with argument
module.
ExportDeclaration : export NamedExports ;
1. 1. 1. Return ExportEntriesForModule of NamedExports with argument null.
ExportDeclaration : export VariableStatement
1. 1. 1. Let entries be a new empty List.
2. 2. 2. Let names be the BoundNames of VariableStatement.
3. 3. 3. For each element name of names, do
1. a. a. Append the ExportEntry Record { [[ModuleRequest]]: null,
[[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to
entries.
4. 4. 4. Return entries.
ExportDeclaration : export Declaration
1. 1. 1. Let entries be a new empty List.
2. 2. 2. Let names be the BoundNames of Declaration.
3. 3. 3. For each element name of names, do
1. a. a. Append the ExportEntry Record { [[ModuleRequest]]: null,
[[ImportName]]: null, [[LocalName]]: name, [[ExportName]]: name } to
entries.
4. 4. 4. Return entries.
ExportDeclaration : export default HoistableDeclaration
1. 1. 1. Let names be BoundNames of HoistableDeclaration.
2. 2. 2. Let localName be the sole element of names.
3. 3. 3. Return a List whose sole element is a new ExportEntry Record {
[[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName,
[[ExportName]]: "default" }.
ExportDeclaration : export default ClassDeclaration
1. 1. 1. Let names be BoundNames of ClassDeclaration.
2. 2. 2. Let localName be the sole element of names.
3. 3. 3. Return a List whose sole element is a new ExportEntry Record {
[[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName,
[[ExportName]]: "default" }.
ExportDeclaration : export default AssignmentExpression ;
1. 1. 1. Let entry be the ExportEntry Record { [[ModuleRequest]]: null,
[[ImportName]]: null, [[LocalName]]: "*default*", [[ExportName]]: "default"
}.
2. 2. 2. Return « entry ».
Note
"*default*" is used within this specification as a synthetic name for anonymous
default export values. See this note for more details.
16.2.3.5 STATIC SEMANTICS: EXPORTENTRIESFORMODULE
The syntax-directed operation ExportEntriesForModule takes argument module (a
String or null) and returns a List of ExportEntry Records. It is defined
piecewise over the following productions:
ExportFromClause : *
1. 1. 1. Let entry be the ExportEntry Record { [[ModuleRequest]]: module,
[[ImportName]]: all-but-default, [[LocalName]]: null, [[ExportName]]: null
}.
2. 2. 2. Return « entry ».
ExportFromClause : * as ModuleExportName
1. 1. 1. Let exportName be the StringValue of ModuleExportName.
2. 2. 2. Let entry be the ExportEntry Record { [[ModuleRequest]]: module,
[[ImportName]]: all, [[LocalName]]: null, [[ExportName]]: exportName }.
3. 3. 3. Return « entry ».
NamedExports : { }
1. 1. 1. Return a new empty List.
ExportsList : ExportsList , ExportSpecifier
1. 1. 1. Let specs1 be the ExportEntriesForModule of ExportsList with argument
module.
2. 2. 2. Let specs2 be the ExportEntriesForModule of ExportSpecifier with
argument module.
3. 3. 3. Return the list-concatenation of specs1 and specs2.
ExportSpecifier : ModuleExportName
1. 1. 1. Let sourceName be the StringValue of ModuleExportName.
2. 2. 2. If module is null, then
1. a. a. Let localName be sourceName.
2. b. b. Let importName be null.
3. 3. 3. Else,
1. a. a. Let localName be null.
2. b. b. Let importName be sourceName.
4. 4. 4. Return a List whose sole element is a new ExportEntry Record {
[[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]:
localName, [[ExportName]]: sourceName }.
ExportSpecifier : ModuleExportName as ModuleExportName
1. 1. 1. Let sourceName be the StringValue of the first ModuleExportName.
2. 2. 2. Let exportName be the StringValue of the second ModuleExportName.
3. 3. 3. If module is null, then
1. a. a. Let localName be sourceName.
2. b. b. Let importName be null.
4. 4. 4. Else,
1. a. a. Let localName be null.
2. b. b. Let importName be sourceName.
5. 5. 5. Return a List whose sole element is a new ExportEntry Record {
[[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]:
localName, [[ExportName]]: exportName }.
16.2.3.6 STATIC SEMANTICS: REFERENCEDBINDINGS
The syntax-directed operation ReferencedBindings takes no arguments and returns
a List of Parse Nodes. It is defined piecewise over the following productions:
NamedExports : { }
1. 1. 1. Return a new empty List.
ExportsList : ExportsList , ExportSpecifier
1. 1. 1. Let names1 be the ReferencedBindings of ExportsList.
2. 2. 2. Let names2 be the ReferencedBindings of ExportSpecifier.
3. 3. 3. Return the list-concatenation of names1 and names2.
ExportSpecifier : ModuleExportName as ModuleExportName
1. 1. 1. Return the ReferencedBindings of the first ModuleExportName.
ModuleExportName : IdentifierName
1. 1. 1. Return a List whose sole element is the IdentifierName.
ModuleExportName : StringLiteral
1. 1. 1. Return a List whose sole element is the StringLiteral.
16.2.3.7 RUNTIME SEMANTICS: EVALUATION
ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
1. 1. 1. Return empty.
ExportDeclaration : export VariableStatement
1. 1. 1. Return ? Evaluation of VariableStatement.
ExportDeclaration : export Declaration
1. 1. 1. Return ? Evaluation of Declaration.
ExportDeclaration : export default HoistableDeclaration
1. 1. 1. Return ? Evaluation of HoistableDeclaration.
ExportDeclaration : export default ClassDeclaration
1. 1. 1. Let value be ? BindingClassDeclarationEvaluation of ClassDeclaration.
2. 2. 2. Let className be the sole element of BoundNames of ClassDeclaration.
3. 3. 3. If className is "*default*", then
1. a. a. Let env be the running execution context's LexicalEnvironment.
2. b. b. Perform ? InitializeBoundName("*default*", value, env).
4. 4. 4. Return empty.
ExportDeclaration : export default AssignmentExpression ;
1. 1. 1. If IsAnonymousFunctionDefinition(AssignmentExpression) is true, then
1. a. a. Let value be ? NamedEvaluation of AssignmentExpression with
argument "default".
2. 2. 2. Else,
1. a. a. Let rhs be ? Evaluation of AssignmentExpression.
2. b. b. Let value be ? GetValue(rhs).
3. 3. 3. Let env be the running execution context's LexicalEnvironment.
4. 4. 4. Perform ? InitializeBoundName("*default*", value, env).
5. 5. 5. Return empty.
17 ERROR HANDLING AND LANGUAGE EXTENSIONS
An implementation must report most errors at the time the relevant ECMAScript
language construct is evaluated. An early error is an error that can be detected
and reported prior to the evaluation of any construct in the Script containing
the error. The presence of an early error prevents the evaluation of the
construct. An implementation must report early errors in a Script as part of
parsing that Script in ParseScript. Early errors in a Module are reported at the
point when the Module would be evaluated and the Module is never initialized.
Early errors in eval code are reported at the time eval is called and prevent
evaluation of the eval code. All errors that are not early errors are runtime
errors.
An implementation must report as an early error any occurrence of a condition
that is listed in a “Static Semantics: Early Errors” subclause of this
specification.
An implementation shall not treat other kinds of errors as early errors even if
the compiler can prove that a construct cannot execute without error under any
circumstances. An implementation may issue an early warning in such a case, but
it should not report the error until the relevant construct is actually
executed.
An implementation shall report all errors as specified, except for the
following:
* Except as restricted in 17.1, a host or implementation may extend Script
syntax, Module syntax, and regular expression pattern or flag syntax. To
permit this, all operations (such as calling eval, using a regular expression
literal, or using the Function or RegExp constructor) that are allowed to
throw SyntaxError are permitted to exhibit host-defined behaviour instead of
throwing SyntaxError when they encounter a host-defined extension to the
script syntax or regular expression pattern or flag syntax.
* Except as restricted in 17.1, a host or implementation may provide additional
types, values, objects, properties, and functions beyond those described in
this specification. This may cause constructs (such as looking up a variable
in the global scope) to have host-defined behaviour instead of throwing an
error (such as ReferenceError).
17.1 FORBIDDEN EXTENSIONS
An implementation must not extend this specification in the following ways:
* ECMAScript function objects defined using syntactic constructors in strict
mode code must not be created with own properties named "caller" or
"arguments". Such own properties also must not be created for function
objects defined using an ArrowFunction, MethodDefinition,
GeneratorDeclaration, GeneratorExpression, AsyncGeneratorDeclaration,
AsyncGeneratorExpression, ClassDeclaration, ClassExpression,
AsyncFunctionDeclaration, AsyncFunctionExpression, or AsyncArrowFunction
regardless of whether the definition is contained in strict mode code.
Built-in functions, strict functions created using the Function constructor,
generator functions created using the Generator constructor, async functions
created using the AsyncFunction constructor, and functions created using the
bind method also must not be created with such own properties.
* If an implementation extends any function object with an own property named
"caller" the value of that property, as observed using [[Get]] or
[[GetOwnProperty]], must not be a strict function object. If it is an
accessor property, the function that is the value of the property's [[Get]]
attribute must never return a strict function when called.
* Neither mapped nor unmapped arguments objects may be created with an own
property named "caller".
* The behaviour of built-in methods which are specified in ECMA-402, such as
those named toLocaleString, must not be extended except as specified in
ECMA-402.
* The RegExp pattern grammars in 22.2.1 and B.1.2 must not be extended to
recognize any of the source characters A-Z or a-z as
IdentityEscape[+UnicodeMode] when the [UnicodeMode] grammar parameter is
present.
* The Syntactic Grammar must not be extended in any manner that allows the
token : to immediately follow source text that is matched by the
BindingIdentifier nonterminal symbol.
* When processing strict mode code, an implementation must not relax the early
error rules of 12.9.3.1.
* TemplateEscapeSequence must not be extended to include
LegacyOctalEscapeSequence or NonOctalDecimalEscapeSequence as defined in
12.9.4.
* When processing strict mode code, the extensions defined in B.3.1, B.3.2,
B.3.3, and B.3.5 must not be supported.
* When parsing for the Module goal symbol, the lexical grammar extensions
defined in B.1.1 must not be supported.
* ImportCall must not be extended.
18 ECMASCRIPT STANDARD BUILT-IN OBJECTS
There are certain built-in objects available whenever an ECMAScript Script or
Module begins execution. One, the global object, is part of the global
environment of the executing program. Others are accessible as initial
properties of the global object or indirectly as properties of accessible
built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is
a built-in function object with the characteristics described in 10.3. Unless
specified otherwise, the [[Extensible]] internal slot of a built-in object
initially has the value true. Every built-in function object has a [[Realm]]
internal slot whose value is the Realm Record of the realm for which the object
was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of
them furthermore are constructors: they are functions intended for use with the
new operator. For each built-in function, this specification describes the
arguments required by that function and the properties of that function object.
For each built-in constructor, this specification furthermore describes
properties of the prototype object of that constructor and properties of
specific object instances returned by a new expression that invokes that
constructor.
Unless otherwise specified in the description of a particular function, if a
built-in function or constructor is given fewer arguments than the function is
specified to require, the function or constructor shall behave exactly as if it
had been given sufficient additional arguments, each such argument being the
undefined value. Such missing arguments are considered to be “not present” and
may be identified in that manner by specification algorithms. In the description
of a particular function, the terms “this value” and “NewTarget” have the
meanings given in 10.3.
Unless otherwise specified in the description of a particular function, if a
built-in function or constructor described is given more arguments than the
function is specified to allow, the extra arguments are evaluated by the call
and then ignored by the function. However, an implementation may define
implementation specific behaviour relating to such arguments as long as the
behaviour is not the throwing of a TypeError exception that is predicated simply
on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in
functions are encouraged to do so by adding new functions rather than adding new
parameters to existing functions.
Unless otherwise specified every built-in function and every built-in
constructor has the Function prototype object, which is the initial value of the
expression Function.prototype (20.2.3), as the value of its [[Prototype]]
internal slot.
Unless otherwise specified every built-in prototype object has the Object
prototype object, which is the initial value of the expression Object.prototype
(20.1.3), as the value of its [[Prototype]] internal slot, except the Object
prototype object itself.
Built-in function objects that are not identified as constructors do not
implement the [[Construct]] internal method unless otherwise specified in the
description of a particular function.
Each built-in function defined in this specification is created by calling the
CreateBuiltinFunction abstract operation (10.3.3). The values of the length and
name parameters are the initial values of the "length" and "name" properties as
discussed below. The values of the prefix parameter are similarly discussed
below.
Every built-in function object, including constructors, has a "length" property
whose value is a non-negative integral Number. Unless otherwise specified, this
value is the number of required parameters shown in the subclause heading for
the function description. Optional parameters and rest parameters are not
included in the parameter count.
Note 2
For example, the function object that is the initial value of the "map" property
of the Array prototype object is described under the subclause heading
«Array.prototype.map (callbackFn [ , thisArg])» which shows the two named
arguments callbackFn and thisArg, the latter being optional; therefore the value
of the "length" property of that function object is 1𝔽.
Unless otherwise specified, the "length" property of a built-in function object
has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
Every built-in function object, including constructors, has a "name" property
whose value is a String. Unless otherwise specified, this value is the name that
is given to the function in this specification. Functions that are identified as
anonymous functions use the empty String as the value of the "name" property.
For functions that are specified as properties of objects, the name value is the
property name string used to access the function. Functions that are specified
as get or set accessor functions of built-in properties have "get" or "set"
(respectively) passed to the prefix parameter when calling
CreateBuiltinFunction.
The value of the "name" property is explicitly specified for each built-in
functions whose property key is a Symbol value. If such an explicitly specified
value starts with the prefix "get " or "set " and the function for which it is
specified is a get or set accessor function of a built-in property, the value
without the prefix is passed to the name parameter, and the value "get" or "set"
(respectively) is passed to the prefix parameter when calling
CreateBuiltinFunction.
Unless otherwise specified, the "name" property of a built-in function object
has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
Every other data property described in clauses 19 through 28 and in Annex B.2
has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true } unless otherwise specified.
Every accessor property described in clauses 19 through 28 and in Annex B.2 has
the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless
otherwise specified. If only a get accessor function is described, the set
accessor function is the default value, undefined. If only a set accessor is
described the get accessor is the default value, undefined.
19 THE GLOBAL OBJECT
The global object:
* is created before control enters any execution context.
* does not have a [[Construct]] internal method; it cannot be used as a
constructor with the new operator.
* does not have a [[Call]] internal method; it cannot be invoked as a function.
* has a [[Prototype]] internal slot whose value is host-defined.
* may have host-defined properties in addition to the properties defined in
this specification. This may include a property whose value is the global
object itself.
19.1 VALUE PROPERTIES OF THE GLOBAL OBJECT
19.1.1 GLOBALTHIS
The initial value of the "globalThis" property of the global object in a Realm
Record realm is realm.[[GlobalEnv]].[[GlobalThisValue]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true }.
19.1.2 INFINITY
The value of Infinity is +∞𝔽 (see 6.1.6.1). This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.1.3 NAN
The value of NaN is NaN (see 6.1.6.1). This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.1.4 UNDEFINED
The value of undefined is undefined (see 6.1.1). This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false
}.
19.2 FUNCTION PROPERTIES OF THE GLOBAL OBJECT
19.2.1 EVAL ( X )
This function is the %eval% intrinsic object.
It performs the following steps when called:
1. 1. 1. Return ? PerformEval(x, false, false).
19.2.1.1 PERFORMEVAL ( X, STRICTCALLER, DIRECT )
The abstract operation PerformEval takes arguments x (an ECMAScript language
value), strictCaller (a Boolean), and direct (a Boolean) and returns either a
normal completion containing an ECMAScript language value or a throw completion.
It performs the following steps when called:
1. 1. 1. Assert: If direct is false, then strictCaller is also false.
2. 2. 2. If x is not a String, return x.
3. 3. 3. Let evalRealm be the current Realm Record.
4. 4. 4. NOTE: In the case of a direct eval, evalRealm is the realm of both
the caller of eval and of the eval function itself.
5. 5. 5. Perform ? HostEnsureCanCompileStrings(evalRealm).
6. 6. 6. Let inFunction be false.
7. 7. 7. Let inMethod be false.
8. 8. 8. Let inDerivedConstructor be false.
9. 9. 9. Let inClassFieldInitializer be false.
10. 10. 10. If direct is true, then
1. a. a. Let thisEnvRec be GetThisEnvironment().
2. b. b. If thisEnvRec is a Function Environment Record, then
1. i. i. Let F be thisEnvRec.[[FunctionObject]].
2. ii. ii. Set inFunction to true.
3. iii. iii. Set inMethod to thisEnvRec.HasSuperBinding().
4. iv. iv. If F.[[ConstructorKind]] is derived, set inDerivedConstructor
to true.
5. v. v. Let classFieldInitializerName be
F.[[ClassFieldInitializerName]].
6. vi. vi. If classFieldInitializerName is not empty, set
inClassFieldInitializer to true.
11. 11. 11. Perform the following substeps in an implementation-defined order,
possibly interleaving parsing and error detection:
1. a. a. Let script be ParseText(StringToCodePoints(x), Script).
2. b. b. If script is a List of errors, throw a SyntaxError exception.
3. c. c. If script Contains ScriptBody is false, return undefined.
4. d. d. Let body be the ScriptBody of script.
5. e. e. If inFunction is false and body Contains NewTarget, throw a
SyntaxError exception.
6. f. f. If inMethod is false and body Contains SuperProperty, throw a
SyntaxError exception.
7. g. g. If inDerivedConstructor is false and body Contains SuperCall,
throw a SyntaxError exception.
8. h. h. If inClassFieldInitializer is true and ContainsArguments of body
is true, throw a SyntaxError exception.
12. 12. 12. If strictCaller is true, let strictEval be true.
13. 13. 13. Else, let strictEval be IsStrict of script.
14. 14. 14. Let runningContext be the running execution context.
15. 15. 15. NOTE: If direct is true, runningContext will be the execution
context that performed the direct eval. If direct is false, runningContext
will be the execution context for the invocation of the eval function.
16. 16. 16. If direct is true, then
1. a. a. Let lexEnv be NewDeclarativeEnvironment(runningContext's
LexicalEnvironment).
2. b. b. Let varEnv be runningContext's VariableEnvironment.
3. c. c. Let privateEnv be runningContext's PrivateEnvironment.
17. 17. 17. Else,
1. a. a. Let lexEnv be NewDeclarativeEnvironment(evalRealm.[[GlobalEnv]]).
2. b. b. Let varEnv be evalRealm.[[GlobalEnv]].
3. c. c. Let privateEnv be null.
18. 18. 18. If strictEval is true, set varEnv to lexEnv.
19. 19. 19. If runningContext is not already suspended, suspend runningContext.
20. 20. 20. Let evalContext be a new ECMAScript code execution context.
21. 21. 21. Set evalContext's Function to null.
22. 22. 22. Set evalContext's Realm to evalRealm.
23. 23. 23. Set evalContext's ScriptOrModule to runningContext's
ScriptOrModule.
24. 24. 24. Set evalContext's VariableEnvironment to varEnv.
25. 25. 25. Set evalContext's LexicalEnvironment to lexEnv.
26. 26. 26. Set evalContext's PrivateEnvironment to privateEnv.
27. 27. 27. Push evalContext onto the execution context stack; evalContext is
now the running execution context.
28. 28. 28. Let result be Completion(EvalDeclarationInstantiation(body, varEnv,
lexEnv, privateEnv, strictEval)).
29. 29. 29. If result.[[Type]] is normal, then
1. a. a. Set result to Completion(Evaluation of body).
30. 30. 30. If result.[[Type]] is normal and result.[[Value]] is empty, then
1. a. a. Set result to NormalCompletion(undefined).
31. 31. 31. Suspend evalContext and remove it from the execution context stack.
32. 32. 32. Resume the context that is now on the top of the execution context
stack as the running execution context.
33. 33. 33. Return ? result.
Note
The eval code cannot instantiate variable or function bindings in the variable
environment of the calling context that invoked the eval if either the code of
the calling context or the eval code is strict mode code. Instead such bindings
are instantiated in a new VariableEnvironment that is only accessible to the
eval code. Bindings introduced by let, const, or class declarations are always
instantiated in a new LexicalEnvironment.
19.2.1.2 HOSTENSURECANCOMPILESTRINGS ( CALLEEREALM )
The host-defined abstract operation HostEnsureCanCompileStrings takes argument
calleeRealm (a Realm Record) and returns either a normal completion containing
unused or a throw completion. It allows host environments to block certain
ECMAScript functions which allow developers to interpret and evaluate strings as
ECMAScript code.
An implementation of HostEnsureCanCompileStrings must conform to the following
requirements:
* If the returned Completion Record is a normal completion, it must be a normal
completion containing unused.
The default implementation of HostEnsureCanCompileStrings is to return
NormalCompletion(unused).
19.2.1.3 EVALDECLARATIONINSTANTIATION ( BODY, VARENV, LEXENV, PRIVATEENV, STRICT
)
The abstract operation EvalDeclarationInstantiation takes arguments body (a
ScriptBody Parse Node), varEnv (an Environment Record), lexEnv (a Declarative
Environment Record), privateEnv (a PrivateEnvironment Record or null), and
strict (a Boolean) and returns either a normal completion containing unused or a
throw completion. It performs the following steps when called:
1. 1. 1. Let varNames be the VarDeclaredNames of body.
2. 2. 2. Let varDeclarations be the VarScopedDeclarations of body.
3. 3. 3. If strict is false, then
1. a. a. If varEnv is a Global Environment Record, then
1. i. i. For each element name of varNames, do
1. 1. 1. If varEnv.HasLexicalDeclaration(name) is true, throw a
SyntaxError exception.
2. 2. 2. NOTE: eval will not create a global var declaration that
would be shadowed by a global lexical declaration.
2. b. b. Let thisEnv be lexEnv.
3. c. c. Assert: The following loop will terminate.
4. d. d. Repeat, while thisEnv is not varEnv,
1. i. i. If thisEnv is not an Object Environment Record, then
1. 1. 1. NOTE: The environment of with statements cannot contain any
lexical declaration so it doesn't need to be checked for var/let
hoisting conflicts.
2. 2. 2. For each element name of varNames, do
1. a. a. If ! thisEnv.HasBinding(name) is true, then
1. i. i. Throw a SyntaxError exception.
2. ii. ii. NOTE: Annex B.3.4 defines alternate semantics for
the above step.
2. b. b. NOTE: A direct eval will not hoist var declaration over a
like-named lexical declaration.
2. ii. ii. Set thisEnv to thisEnv.[[OuterEnv]].
4. 4. 4. Let privateIdentifiers be a new empty List.
5. 5. 5. Let pointer be privateEnv.
6. 6. 6. Repeat, while pointer is not null,
1. a. a. For each Private Name binding of pointer.[[Names]], do
1. i. i. If privateIdentifiers does not contain binding.[[Description]],
append binding.[[Description]] to privateIdentifiers.
2. b. b. Set pointer to pointer.[[OuterPrivateEnvironment]].
7. 7. 7. If AllPrivateIdentifiersValid of body with argument
privateIdentifiers is false, throw a SyntaxError exception.
8. 8. 8. Let functionsToInitialize be a new empty List.
9. 9. 9. Let declaredFunctionNames be a new empty List.
10. 10. 10. For each element d of varDeclarations, in reverse List order, do
1. a. a. If d is not either a VariableDeclaration, a ForBinding, or a
BindingIdentifier, then
1. i. i. Assert: d is either a FunctionDeclaration, a
GeneratorDeclaration, an AsyncFunctionDeclaration, or an
AsyncGeneratorDeclaration.
2. ii. ii. NOTE: If there are multiple function declarations for the
same name, the last declaration is used.
3. iii. iii. Let fn be the sole element of the BoundNames of d.
4. iv. iv. If declaredFunctionNames does not contain fn, then
1. 1. 1. If varEnv is a Global Environment Record, then
1. a. a. Let fnDefinable be ? varEnv.CanDeclareGlobalFunction(fn).
2. b. b. If fnDefinable is false, throw a TypeError exception.
2. 2. 2. Append fn to declaredFunctionNames.
3. 3. 3. Insert d as the first element of functionsToInitialize.
11. 11. 11. NOTE: Annex B.3.2.3 adds additional steps at this point.
12. 12. 12. Let declaredVarNames be a new empty List.
13. 13. 13. For each element d of varDeclarations, do
1. a. a. If d is either a VariableDeclaration, a ForBinding, or a
BindingIdentifier, then
1. i. i. For each String vn of the BoundNames of d, do
1. 1. 1. If declaredFunctionNames does not contain vn, then
1. a. a. If varEnv is a Global Environment Record, then
1. i. i. Let vnDefinable be ? varEnv.CanDeclareGlobalVar(vn).
2. ii. ii. If vnDefinable is false, throw a TypeError
exception.
2. b. b. If declaredVarNames does not contain vn, then
1. i. i. Append vn to declaredVarNames.
14. 14. 14. NOTE: No abnormal terminations occur after this algorithm step
unless varEnv is a Global Environment Record and the global object is a
Proxy exotic object.
15. 15. 15. Let lexDeclarations be the LexicallyScopedDeclarations of body.
16. 16. 16. For each element d of lexDeclarations, do
1. a. a. NOTE: Lexically declared names are only instantiated here but not
initialized.
2. b. b. For each element dn of the BoundNames of d, do
1. i. i. If IsConstantDeclaration of d is true, then
1. 1. 1. Perform ? lexEnv.CreateImmutableBinding(dn, true).
2. ii. ii. Else,
1. 1. 1. Perform ? lexEnv.CreateMutableBinding(dn, false).
17. 17. 17. For each Parse Node f of functionsToInitialize, do
1. a. a. Let fn be the sole element of the BoundNames of f.
2. b. b. Let fo be InstantiateFunctionObject of f with arguments lexEnv and
privateEnv.
3. c. c. If varEnv is a Global Environment Record, then
1. i. i. Perform ? varEnv.CreateGlobalFunctionBinding(fn, fo, true).
4. d. d. Else,
1. i. i. Let bindingExists be ! varEnv.HasBinding(fn).
2. ii. ii. If bindingExists is false, then
1. 1. 1. NOTE: The following invocation cannot return an abrupt
completion because of the validation preceding step 14.
2. 2. 2. Perform ! varEnv.CreateMutableBinding(fn, true).
3. 3. 3. Perform ! varEnv.InitializeBinding(fn, fo).
3. iii. iii. Else,
1. 1. 1. Perform ! varEnv.SetMutableBinding(fn, fo, false).
18. 18. 18. For each String vn of declaredVarNames, do
1. a. a. If varEnv is a Global Environment Record, then
1. i. i. Perform ? varEnv.CreateGlobalVarBinding(vn, true).
2. b. b. Else,
1. i. i. Let bindingExists be ! varEnv.HasBinding(vn).
2. ii. ii. If bindingExists is false, then
1. 1. 1. NOTE: The following invocation cannot return an abrupt
completion because of the validation preceding step 14.
2. 2. 2. Perform ! varEnv.CreateMutableBinding(vn, true).
3. 3. 3. Perform ! varEnv.InitializeBinding(vn, undefined).
19. 19. 19. Return unused.
Note
An alternative version of this algorithm is described in B.3.4.
19.2.2 ISFINITE ( NUMBER )
This function is the %isFinite% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let num be ? ToNumber(number).
2. 2. 2. If num is not finite, return false.
3. 3. 3. Otherwise, return true.
19.2.3 ISNAN ( NUMBER )
This function is the %isNaN% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let num be ? ToNumber(number).
2. 2. 2. If num is NaN, return true.
3. 3. 3. Otherwise, return false.
Note
A reliable way for ECMAScript code to test if a value X is NaN is an expression
of the form X !== X. The result will be true if and only if X is NaN.
19.2.4 PARSEFLOAT ( STRING )
This function produces a Number value dictated by interpretation of the contents
of the string argument as a decimal literal.
It is the %parseFloat% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let inputString be ? ToString(string).
2. 2. 2. Let trimmedString be ! TrimString(inputString, start).
3. 3. 3. Let trimmed be StringToCodePoints(trimmedString).
4. 4. 4. Let trimmedPrefix be the longest prefix of trimmed that satisfies the
syntax of a StrDecimalLiteral, which might be trimmed itself. If there is no
such prefix, return NaN.
5. 5. 5. Let parsedNumber be ParseText(trimmedPrefix, StrDecimalLiteral).
6. 6. 6. Assert: parsedNumber is a Parse Node.
7. 7. 7. Return StringNumericValue of parsedNumber.
Note
This function may interpret only a leading portion of string as a Number value;
it ignores any code units that cannot be interpreted as part of the notation of
a decimal literal, and no indication is given that any such code units were
ignored.
19.2.5 PARSEINT ( STRING, RADIX )
This function produces an integral Number dictated by interpretation of the
contents of string according to the specified radix. Leading white space in
string is ignored. If radix coerces to 0 (such as when it is undefined), it is
assumed to be 10 except when the number representation begins with "0x" or "0X",
in which case it is assumed to be 16. If radix is 16, the number representation
may optionally begin with "0x" or "0X".
It is the %parseInt% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let inputString be ? ToString(string).
2. 2. 2. Let S be ! TrimString(inputString, start).
3. 3. 3. Let sign be 1.
4. 4. 4. If S is not empty and the first code unit of S is the code unit
0x002D (HYPHEN-MINUS), set sign to -1.
5. 5. 5. If S is not empty and the first code unit of S is either the code
unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), set S to
the substring of S from index 1.
6. 6. 6. Let R be ℝ(? ToInt32(radix)).
7. 7. 7. Let stripPrefix be true.
8. 8. 8. If R ≠ 0, then
1. a. a. If R < 2 or R > 36, return NaN.
2. b. b. If R ≠ 16, set stripPrefix to false.
9. 9. 9. Else,
1. a. a. Set R to 10.
10. 10. 10. If stripPrefix is true, then
1. a. a. If the length of S is at least 2 and the first two code units of S
are either "0x" or "0X", then
1. i. i. Set S to the substring of S from index 2.
2. ii. ii. Set R to 16.
11. 11. 11. If S contains a code unit that is not a radix-R digit, let end be
the index within S of the first such code unit; otherwise, let end be the
length of S.
12. 12. 12. Let Z be the substring of S from 0 to end.
13. 13. 13. If Z is empty, return NaN.
14. 14. 14. Let mathInt be the integer value that is represented by Z in
radix-R notation, using the letters A-Z and a-z for digits with values 10
through 35. (However, if R = 10 and Z contains more than 20 significant
digits, every significant digit after the 20th may be replaced by a 0
digit, at the option of the implementation; and if R is not one of 2, 4, 8,
10, 16, or 32, then mathInt may be an implementation-approximated integer
representing the integer value denoted by Z in radix-R notation.)
15. 15. 15. If mathInt = 0, then
1. a. a. If sign = -1, return -0𝔽.
2. b. b. Return +0𝔽.
16. 16. 16. Return 𝔽(sign × mathInt).
Note
This function may interpret only a leading portion of string as an integer
value; it ignores any code units that cannot be interpreted as part of the
notation of an integer, and no indication is given that any such code units were
ignored.
19.2.6 URI HANDLING FUNCTIONS
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g.
web pages or files) and transport protocols by which to access them (e.g. HTTP
or FTP) on the Internet. The ECMAScript language itself does not provide any
support for using URIs except for functions that encode and decode URIs as
described in this section. encodeURI and decodeURI are intended to work with
complete URIs; they assume that any reserved characters are intended to have
special meaning (e.g., as delimiters) and so are not encoded. encodeURIComponent
and decodeURIComponent are intended to work with the individual components of a
URI; they assume that any reserved characters represent text and must be encoded
to avoid special meaning when the component is part of a complete URI.
Note 1
The set of reserved characters is based upon RFC 2396 and does not reflect
changes introduced by the more recent RFC 3986.
Note 2
Many implementations of ECMAScript provide additional functions and methods that
manipulate web pages; these functions are beyond the scope of this standard.
19.2.6.1 DECODEURI ( ENCODEDURI )
This function computes a new version of a URI in which each escape sequence and
UTF-8 encoding of the sort that might be introduced by the encodeURI function is
replaced with the UTF-16 encoding of the code point that it represents. Escape
sequences that could not have been introduced by encodeURI are not replaced.
It is the %decodeURI% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let uriString be ? ToString(encodedURI).
2. 2. 2. Let preserveEscapeSet be ";/?:@&=+$,#".
3. 3. 3. Return ? Decode(uriString, preserveEscapeSet).
19.2.6.2 DECODEURICOMPONENT ( ENCODEDURICOMPONENT )
This function computes a new version of a URI in which each escape sequence and
UTF-8 encoding of the sort that might be introduced by the encodeURIComponent
function is replaced with the UTF-16 encoding of the code point that it
represents.
It is the %decodeURIComponent% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let componentString be ? ToString(encodedURIComponent).
2. 2. 2. Let preserveEscapeSet be the empty String.
3. 3. 3. Return ? Decode(componentString, preserveEscapeSet).
19.2.6.3 ENCODEURI ( URI )
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which
each instance of certain code points is replaced by one, two, three, or four
escape sequences representing the UTF-8 encoding of the code point.
It is the %encodeURI% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let uriString be ? ToString(uri).
2. 2. 2. Let extraUnescaped be ";/?:@&=+$,#".
3. 3. 3. Return ? Encode(uriString, extraUnescaped).
19.2.6.4 ENCODEURICOMPONENT ( URICOMPONENT )
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which
each instance of certain code points is replaced by one, two, three, or four
escape sequences representing the UTF-8 encoding of the code point.
It is the %encodeURIComponent% intrinsic object.
It performs the following steps when called:
1. 1. 1. Let componentString be ? ToString(uriComponent).
2. 2. 2. Let extraUnescaped be the empty String.
3. 3. 3. Return ? Encode(componentString, extraUnescaped).
19.2.6.5 ENCODE ( STRING, EXTRAUNESCAPED )
The abstract operation Encode takes arguments string (a String) and
extraUnescaped (a String) and returns either a normal completion containing a
String or a throw completion. It performs URI encoding and escaping,
interpreting string as a sequence of UTF-16 encoded code points as described in
6.1.4. If a character is identified as unreserved in RFC 2396 or appears in
extraUnescaped, it is not escaped. It performs the following steps when called:
1. 1. 1. Let len be the length of string.
2. 2. 2. Let R be the empty String.
3. 3. 3. Let alwaysUnescaped be the string-concatenation of the ASCII word
characters and "-.!~*'()".
4. 4. 4. Let unescapedSet be the string-concatenation of alwaysUnescaped and
extraUnescaped.
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let C be the code unit at index k within string.
2. b. b. If unescapedSet contains C, then
1. i. i. Set k to k + 1.
2. ii. ii. Set R to the string-concatenation of R and C.
3. c. c. Else,
1. i. i. Let cp be CodePointAt(string, k).
2. ii. ii. If cp.[[IsUnpairedSurrogate]] is true, throw a URIError
exception.
3. iii. iii. Set k to k + cp.[[CodeUnitCount]].
4. iv. iv. Let Octets be the List of octets resulting by applying the
UTF-8 transformation to cp.[[CodePoint]].
5. v. v. For each element octet of Octets, do
1. 1. 1. Let hex be the String representation of octet, formatted as
an uppercase hexadecimal number.
2. 2. 2. Set R to the string-concatenation of R, "%", and
! StringPad(hex, 2𝔽, "0", start).
7. 7. 7. Return R.
Note
Because percent-encoding is used to represent individual octets, a single code
point may be expressed as multiple consecutive escape sequences (one for each of
its 8-bit UTF-8 code units).
19.2.6.6 DECODE ( STRING, PRESERVEESCAPESET )
The abstract operation Decode takes arguments string (a String) and
preserveEscapeSet (a String) and returns either a normal completion containing a
String or a throw completion. It performs URI unescaping and decoding,
preserving any escape sequences that correspond to Basic Latin characters in
preserveEscapeSet. It performs the following steps when called:
1. 1. 1. Let len be the length of string.
2. 2. 2. Let R be the empty String.
3. 3. 3. Let k be 0.
4. 4. 4. Repeat, while k < len,
1. a. a. Let C be the code unit at index k within string.
2. b. b. Let S be C.
3. c. c. If C is the code unit 0x0025 (PERCENT SIGN), then
1. i. i. If k + 3 > len, throw a URIError exception.
2. ii. ii. Let escape be the substring of string from k to k + 3.
3. iii. iii. Let B be ParseHexOctet(string, k + 1).
4. iv. iv. If B is not an integer, throw a URIError exception.
5. v. v. Set k to k + 2.
6. vi. vi. Let n be the number of leading 1 bits in B.
7. vii. vii. If n = 0, then
1. 1. 1. Let asciiChar be the code unit whose numeric value is B.
2. 2. 2. If preserveEscapeSet contains asciiChar, set S to escape.
Otherwise, set S to asciiChar.
8. viii. viii. Else,
1. 1. 1. If n = 1 or n > 4, throw a URIError exception.
2. 2. 2. Let Octets be « B ».
3. 3. 3. Let j be 1.
4. 4. 4. Repeat, while j < n,
1. a. a. Set k to k + 1.
2. b. b. If k + 3 > len, throw a URIError exception.
3. c. c. If the code unit at index k within string is not the code
unit 0x0025 (PERCENT SIGN), throw a URIError exception.
4. d. d. Let continuationByte be ParseHexOctet(string, k + 1).
5. e. e. If continuationByte is not an integer, throw a URIError
exception.
6. f. f. Append continuationByte to Octets.
7. g. g. Set k to k + 2.
8. h. h. Set j to j + 1.
5. 5. 5. Assert: The length of Octets is n.
6. 6. 6. If Octets does not contain a valid UTF-8 encoding of a
Unicode code point, throw a URIError exception.
7. 7. 7. Let V be the code point obtained by applying the UTF-8
transformation to Octets, that is, from a List of octets into a
21-bit value.
8. 8. 8. Set S to UTF16EncodeCodePoint(V).
4. d. d. Set R to the string-concatenation of R and S.
5. e. e. Set k to k + 1.
5. 5. 5. Return R.
Note
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example,
the invalid sequence 0xC0 0x80 must not decode into the code unit 0x0000.
Implementations of the Decode algorithm are required to throw a URIError when
encountering such invalid sequences.
19.2.6.7 PARSEHEXOCTET ( STRING, POSITION )
The abstract operation ParseHexOctet takes arguments string (a String) and
position (a non-negative integer) and returns either a non-negative integer or a
non-empty List of SyntaxError objects. It parses a sequence of two hexadecimal
characters at the specified position in string into an unsigned 8-bit integer.
It performs the following steps when called:
1. 1. 1. Let len be the length of string.
2. 2. 2. Assert: position + 2 ≤ len.
3. 3. 3. Let hexDigits be the substring of string from position to position +
2.
4. 4. 4. Let parseResult be ParseText(StringToCodePoints(hexDigits),
HexDigits[~Sep]).
5. 5. 5. If parseResult is not a Parse Node, return parseResult.
6. 6. 6. Let n be the MV of parseResult.
7. 7. 7. Assert: n is in the inclusive interval from 0 to 255.
8. 8. 8. Return n.
19.3 CONSTRUCTOR PROPERTIES OF THE GLOBAL OBJECT
19.3.1 AGGREGATEERROR ( . . . )
See 20.5.7.1.
19.3.2 ARRAY ( . . . )
See 23.1.1.
19.3.3 ARRAYBUFFER ( . . . )
See 25.1.3.
19.3.4 BIGINT ( . . . )
See 21.2.1.
19.3.5 BIGINT64ARRAY ( . . . )
See 23.2.5.
19.3.6 BIGUINT64ARRAY ( . . . )
See 23.2.5.
19.3.7 BOOLEAN ( . . . )
See 20.3.1.
19.3.8 DATAVIEW ( . . . )
See 25.3.2.
19.3.9 DATE ( . . . )
See 21.4.2.
19.3.10 ERROR ( . . . )
See 20.5.1.
19.3.11 EVALERROR ( . . . )
See 20.5.5.1.
19.3.12 FINALIZATIONREGISTRY ( . . . )
See 26.2.1.
19.3.13 FLOAT32ARRAY ( . . . )
See 23.2.5.
19.3.14 FLOAT64ARRAY ( . . . )
See 23.2.5.
19.3.15 FUNCTION ( . . . )
See 20.2.1.
19.3.16 INT8ARRAY ( . . . )
See 23.2.5.
19.3.17 INT16ARRAY ( . . . )
See 23.2.5.
19.3.18 INT32ARRAY ( . . . )
See 23.2.5.
19.3.19 MAP ( . . . )
See 24.1.1.
19.3.20 NUMBER ( . . . )
See 21.1.1.
19.3.21 OBJECT ( . . . )
See 20.1.1.
19.3.22 PROMISE ( . . . )
See 27.2.3.
19.3.23 PROXY ( . . . )
See 28.2.1.
19.3.24 RANGEERROR ( . . . )
See 20.5.5.2.
19.3.25 REFERENCEERROR ( . . . )
See 20.5.5.3.
19.3.26 REGEXP ( . . . )
See 22.2.4.
19.3.27 SET ( . . . )
See 24.2.1.
19.3.28 SHAREDARRAYBUFFER ( . . . )
See 25.2.2.
19.3.29 STRING ( . . . )
See 22.1.1.
19.3.30 SYMBOL ( . . . )
See 20.4.1.
19.3.31 SYNTAXERROR ( . . . )
See 20.5.5.4.
19.3.32 TYPEERROR ( . . . )
See 20.5.5.5.
19.3.33 UINT8ARRAY ( . . . )
See 23.2.5.
19.3.34 UINT8CLAMPEDARRAY ( . . . )
See 23.2.5.
19.3.35 UINT16ARRAY ( . . . )
See 23.2.5.
19.3.36 UINT32ARRAY ( . . . )
See 23.2.5.
19.3.37 URIERROR ( . . . )
See 20.5.5.6.
19.3.38 WEAKMAP ( . . . )
See 24.3.1.
19.3.39 WEAKREF ( . . . )
See 26.1.1.
19.3.40 WEAKSET ( . . . )
See 24.4.
19.4 OTHER PROPERTIES OF THE GLOBAL OBJECT
19.4.1 ATOMICS
See 25.4.
19.4.2 JSON
See 25.5.
19.4.3 MATH
See 21.3.
19.4.4 REFLECT
See 28.1.
20 FUNDAMENTAL OBJECTS
20.1 OBJECT OBJECTS
20.1.1 THE OBJECT CONSTRUCTOR
The Object constructor:
* is %Object%.
* is the initial value of the "Object" property of the global object.
* creates a new ordinary object when called as a constructor.
* performs a type conversion when called as a function rather than as a
constructor.
* may be used as the value of an extends clause of a class definition.
20.1.1.1 OBJECT ( [ VALUE ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is neither undefined nor the active function object, then
1. a. a. Return ? OrdinaryCreateFromConstructor(NewTarget,
"%Object.prototype%").
2. 2. 2. If value is either undefined or null, return
OrdinaryObjectCreate(%Object.prototype%).
3. 3. 3. Return ! ToObject(value).
The "length" property of this function is 1𝔽.
20.1.2 PROPERTIES OF THE OBJECT CONSTRUCTOR
The Object constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has a "length" property.
* has the following additional properties:
20.1.2.1 OBJECT.ASSIGN ( TARGET, ...SOURCES )
This function copies the values of all of the enumerable own properties from one
or more source objects to a target object.
It performs the following steps when called:
1. 1. 1. Let to be ? ToObject(target).
2. 2. 2. If only one argument was passed, return to.
3. 3. 3. For each element nextSource of sources, do
1. a. a. If nextSource is neither undefined nor null, then
1. i. i. Let from be ! ToObject(nextSource).
2. ii. ii. Let keys be ? from.[[OwnPropertyKeys]]().
3. iii. iii. For each element nextKey of keys, do
1. 1. 1. Let desc be ? from.[[GetOwnProperty]](nextKey).
2. 2. 2. If desc is not undefined and desc.[[Enumerable]] is true,
then
1. a. a. Let propValue be ? Get(from, nextKey).
2. b. b. Perform ? Set(to, nextKey, propValue, true).
4. 4. 4. Return to.
The "length" property of this function is 2𝔽.
20.1.2.2 OBJECT.CREATE ( O, PROPERTIES )
This function creates a new object with a specified prototype.
It performs the following steps when called:
1. 1. 1. If O is not an Object and O is not null, throw a TypeError exception.
2. 2. 2. Let obj be OrdinaryObjectCreate(O).
3. 3. 3. If Properties is not undefined, then
1. a. a. Return ? ObjectDefineProperties(obj, Properties).
4. 4. 4. Return obj.
20.1.2.3 OBJECT.DEFINEPROPERTIES ( O, PROPERTIES )
This function adds own properties and/or updates the attributes of existing own
properties of an object.
It performs the following steps when called:
1. 1. 1. If O is not an Object, throw a TypeError exception.
2. 2. 2. Return ? ObjectDefineProperties(O, Properties).
20.1.2.3.1 OBJECTDEFINEPROPERTIES ( O, PROPERTIES )
The abstract operation ObjectDefineProperties takes arguments O (an Object) and
Properties (an ECMAScript language value) and returns either a normal completion
containing an Object or a throw completion. It performs the following steps when
called:
1. 1. 1. Let props be ? ToObject(Properties).
2. 2. 2. Let keys be ? props.[[OwnPropertyKeys]]().
3. 3. 3. Let descriptors be a new empty List.
4. 4. 4. For each element nextKey of keys, do
1. a. a. Let propDesc be ? props.[[GetOwnProperty]](nextKey).
2. b. b. If propDesc is not undefined and propDesc.[[Enumerable]] is true,
then
1. i. i. Let descObj be ? Get(props, nextKey).
2. ii. ii. Let desc be ? ToPropertyDescriptor(descObj).
3. iii. iii. Append the pair (a two element List) consisting of nextKey
and desc to the end of descriptors.
5. 5. 5. For each element pair of descriptors, do
1. a. a. Let P be the first element of pair.
2. b. b. Let desc be the second element of pair.
3. c. c. Perform ? DefinePropertyOrThrow(O, P, desc).
6. 6. 6. Return O.
20.1.2.4 OBJECT.DEFINEPROPERTY ( O, P, ATTRIBUTES )
This function adds an own property and/or updates the attributes of an existing
own property of an object.
It performs the following steps when called:
1. 1. 1. If O is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(P).
3. 3. 3. Let desc be ? ToPropertyDescriptor(Attributes).
4. 4. 4. Perform ? DefinePropertyOrThrow(O, key, desc).
5. 5. 5. Return O.
20.1.2.5 OBJECT.ENTRIES ( O )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let entryList be ? EnumerableOwnProperties(obj, key+value).
3. 3. 3. Return CreateArrayFromList(entryList).
20.1.2.6 OBJECT.FREEZE ( O )
This function performs the following steps when called:
1. 1. 1. If O is not an Object, return O.
2. 2. 2. Let status be ? SetIntegrityLevel(O, frozen).
3. 3. 3. If status is false, throw a TypeError exception.
4. 4. 4. Return O.
20.1.2.7 OBJECT.FROMENTRIES ( ITERABLE )
This function performs the following steps when called:
1. 1. 1. Perform ? RequireObjectCoercible(iterable).
2. 2. 2. Let obj be OrdinaryObjectCreate(%Object.prototype%).
3. 3. 3. Assert: obj is an extensible ordinary object with no own properties.
4. 4. 4. Let closure be a new Abstract Closure with parameters (key, value)
that captures obj and performs the following steps when called:
1. a. a. Let propertyKey be ? ToPropertyKey(key).
2. b. b. Perform ! CreateDataPropertyOrThrow(obj, propertyKey, value).
3. c. c. Return undefined.
5. 5. 5. Let adder be CreateBuiltinFunction(closure, 2, "", « »).
6. 6. 6. Return ? AddEntriesFromIterable(obj, iterable, adder).
Note
The function created for adder is never directly accessible to ECMAScript code.
20.1.2.8 OBJECT.GETOWNPROPERTYDESCRIPTOR ( O, P )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let key be ? ToPropertyKey(P).
3. 3. 3. Let desc be ? obj.[[GetOwnProperty]](key).
4. 4. 4. Return FromPropertyDescriptor(desc).
20.1.2.9 OBJECT.GETOWNPROPERTYDESCRIPTORS ( O )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let ownKeys be ? obj.[[OwnPropertyKeys]]().
3. 3. 3. Let descriptors be OrdinaryObjectCreate(%Object.prototype%).
4. 4. 4. For each element key of ownKeys, do
1. a. a. Let desc be ? obj.[[GetOwnProperty]](key).
2. b. b. Let descriptor be FromPropertyDescriptor(desc).
3. c. c. If descriptor is not undefined, perform
! CreateDataPropertyOrThrow(descriptors, key, descriptor).
5. 5. 5. Return descriptors.
20.1.2.10 OBJECT.GETOWNPROPERTYNAMES ( O )
This function performs the following steps when called:
1. 1. 1. Return CreateArrayFromList(? GetOwnPropertyKeys(O, string)).
20.1.2.11 OBJECT.GETOWNPROPERTYSYMBOLS ( O )
This function performs the following steps when called:
1. 1. 1. Return CreateArrayFromList(? GetOwnPropertyKeys(O, symbol)).
20.1.2.11.1 GETOWNPROPERTYKEYS ( O, TYPE )
The abstract operation GetOwnPropertyKeys takes arguments O (an ECMAScript
language value) and type (string or symbol) and returns either a normal
completion containing a List of property keys or a throw completion. It performs
the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let keys be ? obj.[[OwnPropertyKeys]]().
3. 3. 3. Let nameList be a new empty List.
4. 4. 4. For each element nextKey of keys, do
1. a. a. If nextKey is a Symbol and type is symbol, or if nextKey is a
String and type is string, then
1. i. i. Append nextKey to nameList.
5. 5. 5. Return nameList.
20.1.2.12 OBJECT.GETPROTOTYPEOF ( O )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Return ? obj.[[GetPrototypeOf]]().
20.1.2.13 OBJECT.HASOWN ( O, P )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let key be ? ToPropertyKey(P).
3. 3. 3. Return ? HasOwnProperty(obj, key).
20.1.2.14 OBJECT.IS ( VALUE1, VALUE2 )
This function performs the following steps when called:
1. 1. 1. Return SameValue(value1, value2).
20.1.2.15 OBJECT.ISEXTENSIBLE ( O )
This function performs the following steps when called:
1. 1. 1. If O is not an Object, return false.
2. 2. 2. Return ? IsExtensible(O).
20.1.2.16 OBJECT.ISFROZEN ( O )
This function performs the following steps when called:
1. 1. 1. If O is not an Object, return true.
2. 2. 2. Return ? TestIntegrityLevel(O, frozen).
20.1.2.17 OBJECT.ISSEALED ( O )
This function performs the following steps when called:
1. 1. 1. If O is not an Object, return true.
2. 2. 2. Return ? TestIntegrityLevel(O, sealed).
20.1.2.18 OBJECT.KEYS ( O )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let keyList be ? EnumerableOwnProperties(obj, key).
3. 3. 3. Return CreateArrayFromList(keyList).
20.1.2.19 OBJECT.PREVENTEXTENSIONS ( O )
This function performs the following steps when called:
1. 1. 1. If O is not an Object, return O.
2. 2. 2. Let status be ? O.[[PreventExtensions]]().
3. 3. 3. If status is false, throw a TypeError exception.
4. 4. 4. Return O.
20.1.2.20 OBJECT.PROTOTYPE
The initial value of Object.prototype is the Object prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.1.2.21 OBJECT.SEAL ( O )
This function performs the following steps when called:
1. 1. 1. If O is not an Object, return O.
2. 2. 2. Let status be ? SetIntegrityLevel(O, sealed).
3. 3. 3. If status is false, throw a TypeError exception.
4. 4. 4. Return O.
20.1.2.22 OBJECT.SETPROTOTYPEOF ( O, PROTO )
This function performs the following steps when called:
1. 1. 1. Set O to ? RequireObjectCoercible(O).
2. 2. 2. If proto is not an Object and proto is not null, throw a TypeError
exception.
3. 3. 3. If O is not an Object, return O.
4. 4. 4. Let status be ? O.[[SetPrototypeOf]](proto).
5. 5. 5. If status is false, throw a TypeError exception.
6. 6. 6. Return O.
20.1.2.23 OBJECT.VALUES ( O )
This function performs the following steps when called:
1. 1. 1. Let obj be ? ToObject(O).
2. 2. 2. Let valueList be ? EnumerableOwnProperties(obj, value).
3. 3. 3. Return CreateArrayFromList(valueList).
20.1.3 PROPERTIES OF THE OBJECT PROTOTYPE OBJECT
The Object prototype object:
* is %Object.prototype%.
* has an [[Extensible]] internal slot whose value is true.
* has the internal methods defined for ordinary objects, except for the
[[SetPrototypeOf]] method, which is as defined in 10.4.7.1. (Thus, it is an
immutable prototype exotic object.)
* has a [[Prototype]] internal slot whose value is null.
20.1.3.1 OBJECT.PROTOTYPE.CONSTRUCTOR
The initial value of Object.prototype.constructor is %Object%.
20.1.3.2 OBJECT.PROTOTYPE.HASOWNPROPERTY ( V )
This method performs the following steps when called:
1. 1. 1. Let P be ? ToPropertyKey(V).
2. 2. 2. Let O be ? ToObject(this value).
3. 3. 3. Return ? HasOwnProperty(O, P).
Note
The ordering of steps 1 and 2 is chosen to ensure that any exception that would
have been thrown by step 1 in previous editions of this specification will
continue to be thrown even if the this value is undefined or null.
20.1.3.3 OBJECT.PROTOTYPE.ISPROTOTYPEOF ( V )
This method performs the following steps when called:
1. 1. 1. If V is not an Object, return false.
2. 2. 2. Let O be ? ToObject(this value).
3. 3. 3. Repeat,
1. a. a. Set V to ? V.[[GetPrototypeOf]]().
2. b. b. If V is null, return false.
3. c. c. If SameValue(O, V) is true, return true.
Note
The ordering of steps 1 and 2 preserves the behaviour specified by previous
editions of this specification for the case where V is not an object and the
this value is undefined or null.
20.1.3.4 OBJECT.PROTOTYPE.PROPERTYISENUMERABLE ( V )
This method performs the following steps when called:
1. 1. 1. Let P be ? ToPropertyKey(V).
2. 2. 2. Let O be ? ToObject(this value).
3. 3. 3. Let desc be ? O.[[GetOwnProperty]](P).
4. 4. 4. If desc is undefined, return false.
5. 5. 5. Return desc.[[Enumerable]].
Note 1
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any exception that would
have been thrown by step 1 in previous editions of this specification will
continue to be thrown even if the this value is undefined or null.
20.1.3.5 OBJECT.PROTOTYPE.TOLOCALESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Return ? Invoke(O, "toString").
The optional parameters to this method are not used but are intended to
correspond to the parameter pattern used by ECMA-402 toLocaleString methods.
Implementations that do not include ECMA-402 support must not use those
parameter positions for other purposes.
Note 1
This method provides a generic toLocaleString implementation for objects that
have no locale-sensitive toString behaviour. Array, Number, Date, and
%TypedArray% provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default
implementation.
20.1.3.6 OBJECT.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. If the this value is undefined, return "[object Undefined]".
2. 2. 2. If the this value is null, return "[object Null]".
3. 3. 3. Let O be ! ToObject(this value).
4. 4. 4. Let isArray be ? IsArray(O).
5. 5. 5. If isArray is true, let builtinTag be "Array".
6. 6. 6. Else if O has a [[ParameterMap]] internal slot, let builtinTag be
"Arguments".
7. 7. 7. Else if O has a [[Call]] internal method, let builtinTag be
"Function".
8. 8. 8. Else if O has an [[ErrorData]] internal slot, let builtinTag be
"Error".
9. 9. 9. Else if O has a [[BooleanData]] internal slot, let builtinTag be
"Boolean".
10. 10. 10. Else if O has a [[NumberData]] internal slot, let builtinTag be
"Number".
11. 11. 11. Else if O has a [[StringData]] internal slot, let builtinTag be
"String".
12. 12. 12. Else if O has a [[DateValue]] internal slot, let builtinTag be
"Date".
13. 13. 13. Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be
"RegExp".
14. 14. 14. Else, let builtinTag be "Object".
15. 15. 15. Let tag be ? Get(O, @@toStringTag).
16. 16. 16. If tag is not a String, set tag to builtinTag.
17. 17. 17. Return the string-concatenation of "[object ", tag, and "]".
Note
Historically, this method was occasionally used to access the String value of
the [[Class]] internal slot that was used in previous editions of this
specification as a nominal type tag for various built-in objects. The above
definition of toString preserves compatibility for legacy code that uses
toString as a test for those specific kinds of built-in objects. It does not
provide a reliable type testing mechanism for other kinds of built-in or program
defined objects. In addition, programs can use @@toStringTag in ways that will
invalidate the reliability of such legacy type tests.
20.1.3.7 OBJECT.PROTOTYPE.VALUEOF ( )
This method performs the following steps when called:
1. 1. 1. Return ? ToObject(this value).
Normative Optional, Legacy
20.1.3.8 OBJECT.PROTOTYPE.__PROTO__
Object.prototype.__proto__ is an accessor property with attributes {
[[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]]
attributes are defined as follows:
20.1.3.8.1 GET OBJECT.PROTOTYPE.__PROTO__
The value of the [[Get]] attribute is a built-in function that requires no
arguments. It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Return ? O.[[GetPrototypeOf]]().
20.1.3.8.2 SET OBJECT.PROTOTYPE.__PROTO__
The value of the [[Set]] attribute is a built-in function that takes an argument
proto. It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If proto is not an Object and proto is not null, return undefined.
3. 3. 3. If O is not an Object, return undefined.
4. 4. 4. Let status be ? O.[[SetPrototypeOf]](proto).
5. 5. 5. If status is false, throw a TypeError exception.
6. 6. 6. Return undefined.
Normative Optional, Legacy
20.1.3.9 LEGACY OBJECT.PROTOTYPE ACCESSOR METHODS
20.1.3.9.1 OBJECT.PROTOTYPE.__DEFINEGETTER__ ( P, GETTER )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. If IsCallable(getter) is false, throw a TypeError exception.
3. 3. 3. Let desc be PropertyDescriptor { [[Get]]: getter, [[Enumerable]]:
true, [[Configurable]]: true }.
4. 4. 4. Let key be ? ToPropertyKey(P).
5. 5. 5. Perform ? DefinePropertyOrThrow(O, key, desc).
6. 6. 6. Return undefined.
20.1.3.9.2 OBJECT.PROTOTYPE.__DEFINESETTER__ ( P, SETTER )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. If IsCallable(setter) is false, throw a TypeError exception.
3. 3. 3. Let desc be PropertyDescriptor { [[Set]]: setter, [[Enumerable]]:
true, [[Configurable]]: true }.
4. 4. 4. Let key be ? ToPropertyKey(P).
5. 5. 5. Perform ? DefinePropertyOrThrow(O, key, desc).
6. 6. 6. Return undefined.
20.1.3.9.3 OBJECT.PROTOTYPE.__LOOKUPGETTER__ ( P )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let key be ? ToPropertyKey(P).
3. 3. 3. Repeat,
1. a. a. Let desc be ? O.[[GetOwnProperty]](key).
2. b. b. If desc is not undefined, then
1. i. i. If IsAccessorDescriptor(desc) is true, return desc.[[Get]].
2. ii. ii. Return undefined.
3. c. c. Set O to ? O.[[GetPrototypeOf]]().
4. d. d. If O is null, return undefined.
20.1.3.9.4 OBJECT.PROTOTYPE.__LOOKUPSETTER__ ( P )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let key be ? ToPropertyKey(P).
3. 3. 3. Repeat,
1. a. a. Let desc be ? O.[[GetOwnProperty]](key).
2. b. b. If desc is not undefined, then
1. i. i. If IsAccessorDescriptor(desc) is true, return desc.[[Set]].
2. ii. ii. Return undefined.
3. c. c. Set O to ? O.[[GetPrototypeOf]]().
4. d. d. If O is null, return undefined.
20.1.4 PROPERTIES OF OBJECT INSTANCES
Object instances have no special properties beyond those inherited from the
Object prototype object.
20.2 FUNCTION OBJECTS
20.2.1 THE FUNCTION CONSTRUCTOR
The Function constructor:
* is %Function%.
* is the initial value of the "Function" property of the global object.
* creates and initializes a new function object when called as a function
rather than as a constructor. Thus the function call Function(…) is
equivalent to the object creation expression new Function(…) with the same
arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Function behaviour must
include a super call to the Function constructor to create and initialize a
subclass instance with the internal slots necessary for built-in function
behaviour. All ECMAScript syntactic forms for defining function objects
create instances of Function. There is no syntactic means to create instances
of Function subclasses except for the built-in GeneratorFunction,
AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 FUNCTION ( ...PARAMETERARGS, BODYARG )
The last argument (if any) specifies the body (executable code) of a function;
any preceding arguments specify formal parameters.
This function performs the following steps when called:
1. 1. 1. Let C be the active function object.
2. 2. 2. If bodyArg is not present, set bodyArg to the empty String.
3. 3. 3. Return ? CreateDynamicFunction(C, NewTarget, normal, parameterArgs,
bodyArg).
Note
It is permissible but not necessary to have one argument for each formal
parameter to be specified. For example, all three of the following expressions
produce the same result:
new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")
20.2.1.1.1 CREATEDYNAMICFUNCTION ( CONSTRUCTOR, NEWTARGET, KIND, PARAMETERARGS,
BODYARG )
The abstract operation CreateDynamicFunction takes arguments constructor (a
constructor), newTarget (a constructor), kind (normal, generator, async, or
asyncGenerator), parameterArgs (a List of ECMAScript language values), and
bodyArg (an ECMAScript language value) and returns either a normal completion
containing a function object or a throw completion. constructor is the
constructor function that is performing this action. newTarget is the
constructor that new was initially applied to. parameterArgs and bodyArg reflect
the argument values that were passed to constructor. It performs the following
steps when called:
1. 1. 1. Let currentRealm be the current Realm Record.
2. 2. 2. Perform ? HostEnsureCanCompileStrings(currentRealm).
3. 3. 3. If newTarget is undefined, set newTarget to constructor.
4. 4. 4. If kind is normal, then
1. a. a. Let prefix be "function".
2. b. b. Let exprSym be the grammar symbol FunctionExpression.
3. c. c. Let bodySym be the grammar symbol FunctionBody[~Yield, ~Await].
4. d. d. Let parameterSym be the grammar symbol FormalParameters[~Yield,
~Await].
5. e. e. Let fallbackProto be "%Function.prototype%".
5. 5. 5. Else if kind is generator, then
1. a. a. Let prefix be "function*".
2. b. b. Let exprSym be the grammar symbol GeneratorExpression.
3. c. c. Let bodySym be the grammar symbol GeneratorBody.
4. d. d. Let parameterSym be the grammar symbol FormalParameters[+Yield,
~Await].
5. e. e. Let fallbackProto be "%GeneratorFunction.prototype%".
6. 6. 6. Else if kind is async, then
1. a. a. Let prefix be "async function".
2. b. b. Let exprSym be the grammar symbol AsyncFunctionExpression.
3. c. c. Let bodySym be the grammar symbol AsyncFunctionBody.
4. d. d. Let parameterSym be the grammar symbol FormalParameters[~Yield,
+Await].
5. e. e. Let fallbackProto be "%AsyncFunction.prototype%".
7. 7. 7. Else,
1. a. a. Assert: kind is asyncGenerator.
2. b. b. Let prefix be "async function*".
3. c. c. Let exprSym be the grammar symbol AsyncGeneratorExpression.
4. d. d. Let bodySym be the grammar symbol AsyncGeneratorBody.
5. e. e. Let parameterSym be the grammar symbol FormalParameters[+Yield,
+Await].
6. f. f. Let fallbackProto be "%AsyncGeneratorFunction.prototype%".
8. 8. 8. Let argCount be the number of elements in parameterArgs.
9. 9. 9. Let P be the empty String.
10. 10. 10. If argCount > 0, then
1. a. a. Let firstArg be parameterArgs[0].
2. b. b. Set P to ? ToString(firstArg).
3. c. c. Let k be 1.
4. d. d. Repeat, while k < argCount,
1. i. i. Let nextArg be parameterArgs[k].
2. ii. ii. Let nextArgString be ? ToString(nextArg).
3. iii. iii. Set P to the string-concatenation of P, "," (a comma), and
nextArgString.
4. iv. iv. Set k to k + 1.
11. 11. 11. Let bodyString be the string-concatenation of 0x000A (LINE FEED),
? ToString(bodyArg), and 0x000A (LINE FEED).
12. 12. 12. Let sourceString be the string-concatenation of prefix, "
anonymous(", P, 0x000A (LINE FEED), ") {", bodyString, and "}".
13. 13. 13. Let sourceText be StringToCodePoints(sourceString).
14. 14. 14. Let parameters be ParseText(StringToCodePoints(P), parameterSym).
15. 15. 15. If parameters is a List of errors, throw a SyntaxError exception.
16. 16. 16. Let body be ParseText(StringToCodePoints(bodyString), bodySym).
17. 17. 17. If body is a List of errors, throw a SyntaxError exception.
18. 18. 18. NOTE: The parameters and body are parsed separately to ensure that
each is valid alone. For example, new Function("/*", "*/ ) {") does not
evaluate to a function.
19. 19. 19. NOTE: If this step is reached, sourceText must have the syntax of
exprSym (although the reverse implication does not hold). The purpose of
the next two steps is to enforce any Early Error rules which apply to
exprSym directly.
20. 20. 20. Let expr be ParseText(sourceText, exprSym).
21. 21. 21. If expr is a List of errors, throw a SyntaxError exception.
22. 22. 22. Let proto be ? GetPrototypeFromConstructor(newTarget,
fallbackProto).
23. 23. 23. Let realmF be the current Realm Record.
24. 24. 24. Let env be realmF.[[GlobalEnv]].
25. 25. 25. Let privateEnv be null.
26. 26. 26. Let F be OrdinaryFunctionCreate(proto, sourceText, parameters,
body, non-lexical-this, env, privateEnv).
27. 27. 27. Perform SetFunctionName(F, "anonymous").
28. 28. 28. If kind is generator, then
1. a. a. Let prototype be
OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
2. b. b. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor
{ [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
29. 29. 29. Else if kind is asyncGenerator, then
1. a. a. Let prototype be
OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
2. b. b. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor
{ [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }).
30. 30. 30. Else if kind is normal, perform MakeConstructor(F).
31. 31. 31. NOTE: Functions whose kind is async are not constructible and do
not have a [[Construct]] internal method or a "prototype" property.
32. 32. 32. Return F.
Note
CreateDynamicFunction defines a "prototype" property on any function it creates
whose kind is not async to provide for the possibility that the function will be
used as a constructor.
20.2.2 PROPERTIES OF THE FUNCTION CONSTRUCTOR
The Function constructor:
* is itself a built-in function object.
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
20.2.2.1 FUNCTION.LENGTH
This is a data property with a value of 1. This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
20.2.2.2 FUNCTION.PROTOTYPE
The value of Function.prototype is the Function prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.2.3 PROPERTIES OF THE FUNCTION PROTOTYPE OBJECT
The Function prototype object:
* is %Function.prototype%.
* is itself a built-in function object.
* accepts any arguments and returns undefined when invoked.
* does not have a [[Construct]] internal method; it cannot be used as a
constructor with the new operator.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* does not have a "prototype" property.
* has a "length" property whose value is +0𝔽.
* has a "name" property whose value is the empty String.
Note
The Function prototype object is specified to be a function object to ensure
compatibility with ECMAScript code that was created prior to the ECMAScript 2015
specification.
20.2.3.1 FUNCTION.PROTOTYPE.APPLY ( THISARG, ARGARRAY )
This method performs the following steps when called:
1. 1. 1. Let func be the this value.
2. 2. 2. If IsCallable(func) is false, throw a TypeError exception.
3. 3. 3. If argArray is either undefined or null, then
1. a. a. Perform PrepareForTailCall().
2. b. b. Return ? Call(func, thisArg).
4. 4. 4. Let argList be ? CreateListFromArrayLike(argArray).
5. 5. 5. Perform PrepareForTailCall().
6. 6. 6. Return ? Call(func, thisArg, argList).
Note 1
The thisArg value is passed without modification as the this value. This is a
change from Edition 3, where an undefined or null thisArg is replaced with the
global object and ToObject is applied to all other values and that result is
passed as the this value. Even though the thisArg is passed without
modification, non-strict functions still perform these transformations upon
entry to the function.
Note 2
If func is either an arrow function or a bound function exotic object, then the
thisArg will be ignored by the function [[Call]] in step 6.
20.2.3.2 FUNCTION.PROTOTYPE.BIND ( THISARG, ...ARGS )
This method performs the following steps when called:
1. 1. 1. Let Target be the this value.
2. 2. 2. If IsCallable(Target) is false, throw a TypeError exception.
3. 3. 3. Let F be ? BoundFunctionCreate(Target, thisArg, args).
4. 4. 4. Let L be 0.
5. 5. 5. Let targetHasLength be ? HasOwnProperty(Target, "length").
6. 6. 6. If targetHasLength is true, then
1. a. a. Let targetLen be ? Get(Target, "length").
2. b. b. If targetLen is a Number, then
1. i. i. If targetLen is +∞𝔽, set L to +∞.
2. ii. ii. Else if targetLen is -∞𝔽, set L to 0.
3. iii. iii. Else,
1. 1. 1. Let targetLenAsInt be ! ToIntegerOrInfinity(targetLen).
2. 2. 2. Assert: targetLenAsInt is finite.
3. 3. 3. Let argCount be the number of elements in args.
4. 4. 4. Set L to max(targetLenAsInt - argCount, 0).
7. 7. 7. Perform SetFunctionLength(F, L).
8. 8. 8. Let targetName be ? Get(Target, "name").
9. 9. 9. If targetName is not a String, set targetName to the empty String.
10. 10. 10. Perform SetFunctionName(F, targetName, "bound").
11. 11. 11. Return F.
Note 1
Function objects created using Function.prototype.bind are exotic objects. They
also do not have a "prototype" property.
Note 2
If Target is either an arrow function or a bound function exotic object, then
the thisArg passed to this method will not be used by subsequent calls to F.
20.2.3.3 FUNCTION.PROTOTYPE.CALL ( THISARG, ...ARGS )
This method performs the following steps when called:
1. 1. 1. Let func be the this value.
2. 2. 2. If IsCallable(func) is false, throw a TypeError exception.
3. 3. 3. Perform PrepareForTailCall().
4. 4. 4. Return ? Call(func, thisArg, args).
Note 1
The thisArg value is passed without modification as the this value. This is a
change from Edition 3, where an undefined or null thisArg is replaced with the
global object and ToObject is applied to all other values and that result is
passed as the this value. Even though the thisArg is passed without
modification, non-strict functions still perform these transformations upon
entry to the function.
Note 2
If func is either an arrow function or a bound function exotic object, then the
thisArg will be ignored by the function [[Call]] in step 4.
20.2.3.4 FUNCTION.PROTOTYPE.CONSTRUCTOR
The initial value of Function.prototype.constructor is %Function%.
20.2.3.5 FUNCTION.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Let func be the this value.
2. 2. 2. If func is an Object, func has a [[SourceText]] internal slot,
func.[[SourceText]] is a sequence of Unicode code points, and
HostHasSourceTextAvailable(func) is true, then
1. a. a. Return CodePointsToString(func.[[SourceText]]).
3. 3. 3. If func is a built-in function object, return an
implementation-defined String source code representation of func. The
representation must have the syntax of a NativeFunction. Additionally, if
func has an [[InitialName]] internal slot and func.[[InitialName]] is a
String, the portion of the returned String that would be matched by
NativeFunctionAccessoropt PropertyName must be the value of
func.[[InitialName]].
4. 4. 4. If func is an Object and IsCallable(func) is true, return an
implementation-defined String source code representation of func. The
representation must have the syntax of a NativeFunction.
5. 5. 5. Throw a TypeError exception.
NativeFunction : function NativeFunctionAccessoropt PropertyName[~Yield,
~Await]opt ( FormalParameters[~Yield, ~Await] ) { [ native code ] }
NativeFunctionAccessor : get set
20.2.3.6 FUNCTION.PROTOTYPE [ @@HASINSTANCE ] ( V )
This method performs the following steps when called:
1. 1. 1. Let F be the this value.
2. 2. 2. Return ? OrdinaryHasInstance(F, V).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
This is the default implementation of @@hasInstance that most functions inherit.
@@hasInstance is called by the instanceof operator to determine whether a value
is an instance of a specific constructor. An expression such as
v instanceof F
evaluates as
F[@@hasInstance](v)
A constructor function can control which objects are recognized as its instances
by instanceof by exposing a different @@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that
could be used to globally expose the target function of a bound function.
The value of the "name" property of this method is "[Symbol.hasInstance]".
20.2.4 FUNCTION INSTANCES
Every Function instance is an ECMAScript function object and has the internal
slots listed in Table 30. Function objects created using the
Function.prototype.bind method (20.2.3.2) have the internal slots listed in
Table 31.
Function instances have the following properties:
20.2.4.1 LENGTH
The value of the "length" property is an integral Number that indicates the
typical number of arguments expected by the function. However, the language
permits the function to be invoked with some other number of arguments. The
behaviour of a function when invoked on a number of arguments other than the
number specified by its "length" property depends on the function. This property
has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
20.2.4.2 NAME
The value of the "name" property is a String that is descriptive of the
function. The name has no semantic significance but is typically a variable or
property name that is used to refer to the function at its point of definition
in ECMAScript source text. This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false, [[Configurable]]: true }.
Anonymous functions objects that do not have a contextual name associated with
them by this specification use the empty String as the value of the "name"
property.
20.2.4.3 PROTOTYPE
Function instances that can be used as a constructor have a "prototype"
property. Whenever such a Function instance is created another ordinary object
is also created and is the initial value of the function's "prototype" property.
Unless otherwise specified, the value of the "prototype" property is used to
initialize the [[Prototype]] internal slot of the object created when that
function is invoked as a constructor.
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Function objects created using Function.prototype.bind, or by evaluating a
MethodDefinition (that is not a GeneratorMethod or AsyncGeneratorMethod) or an
ArrowFunction do not have a "prototype" property.
20.2.5 HOSTHASSOURCETEXTAVAILABLE ( FUNC )
The host-defined abstract operation HostHasSourceTextAvailable takes argument
func (a function object) and returns a Boolean. It allows host environments to
prevent the source text from being provided for func.
An implementation of HostHasSourceTextAvailable must conform to the following
requirements:
* It must be deterministic with respect to its parameters. Each time it is
called with a specific func as its argument, it must return the same result.
The default implementation of HostHasSourceTextAvailable is to return true.
20.3 BOOLEAN OBJECTS
20.3.1 THE BOOLEAN CONSTRUCTOR
The Boolean constructor:
* is %Boolean%.
* is the initial value of the "Boolean" property of the global object.
* creates and initializes a new Boolean object when called as a constructor.
* performs a type conversion when called as a function rather than as a
constructor.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Boolean behaviour must
include a super call to the Boolean constructor to create and initialize the
subclass instance with a [[BooleanData]] internal slot.
20.3.1.1 BOOLEAN ( VALUE )
This function performs the following steps when called:
1. 1. 1. Let b be ToBoolean(value).
2. 2. 2. If NewTarget is undefined, return b.
3. 3. 3. Let O be ? OrdinaryCreateFromConstructor(NewTarget,
"%Boolean.prototype%", « [[BooleanData]] »).
4. 4. 4. Set O.[[BooleanData]] to b.
5. 5. 5. Return O.
20.3.2 PROPERTIES OF THE BOOLEAN CONSTRUCTOR
The Boolean constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
20.3.2.1 BOOLEAN.PROTOTYPE
The initial value of Boolean.prototype is the Boolean prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.3.3 PROPERTIES OF THE BOOLEAN PROTOTYPE OBJECT
The Boolean prototype object:
* is %Boolean.prototype%.
* is an ordinary object.
* is itself a Boolean object; it has a [[BooleanData]] internal slot with the
value false.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
The abstract operation thisBooleanValue takes argument value. It performs the
following steps when called:
1. 1. 1. If value is a Boolean, return value.
2. 2. 2. If value is an Object and value has a [[BooleanData]] internal slot,
then
1. a. a. Let b be value.[[BooleanData]].
2. b. b. Assert: b is a Boolean.
3. c. c. Return b.
3. 3. 3. Throw a TypeError exception.
20.3.3.1 BOOLEAN.PROTOTYPE.CONSTRUCTOR
The initial value of Boolean.prototype.constructor is %Boolean%.
20.3.3.2 BOOLEAN.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Let b be ? thisBooleanValue(this value).
2. 2. 2. If b is true, return "true"; else return "false".
20.3.3.3 BOOLEAN.PROTOTYPE.VALUEOF ( )
This method performs the following steps when called:
1. 1. 1. Return ? thisBooleanValue(this value).
20.3.4 PROPERTIES OF BOOLEAN INSTANCES
Boolean instances are ordinary objects that inherit properties from the Boolean
prototype object. Boolean instances have a [[BooleanData]] internal slot. The
[[BooleanData]] internal slot is the Boolean value represented by this Boolean
object.
20.4 SYMBOL OBJECTS
20.4.1 THE SYMBOL CONSTRUCTOR
The Symbol constructor:
* is %Symbol%.
* is the initial value of the "Symbol" property of the global object.
* returns a new Symbol value when called as a function.
* is not intended to be used with the new operator.
* is not intended to be subclassed.
* may be used as the value of an extends clause of a class definition but a
super call to it will cause an exception.
20.4.1.1 SYMBOL ( [ DESCRIPTION ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is not undefined, throw a TypeError exception.
2. 2. 2. If description is undefined, let descString be undefined.
3. 3. 3. Else, let descString be ? ToString(description).
4. 4. 4. Return a new Symbol whose [[Description]] is descString.
20.4.2 PROPERTIES OF THE SYMBOL CONSTRUCTOR
The Symbol constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
20.4.2.1 SYMBOL.ASYNCITERATOR
The initial value of Symbol.asyncIterator is the well known symbol
@@asyncIterator (Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.2 SYMBOL.FOR ( KEY )
This function performs the following steps when called:
1. 1. 1. Let stringKey be ? ToString(key).
2. 2. 2. For each element e of the GlobalSymbolRegistry List, do
1. a. a. If SameValue(e.[[Key]], stringKey) is true, return e.[[Symbol]].
3. 3. 3. Assert: GlobalSymbolRegistry does not currently contain an entry for
stringKey.
4. 4. 4. Let newSymbol be a new Symbol whose [[Description]] is stringKey.
5. 5. 5. Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the
GlobalSymbolRegistry List.
6. 6. 6. Return newSymbol.
The GlobalSymbolRegistry is an append-only List that is globally available. It
is shared by all realms. Prior to the evaluation of any ECMAScript code, it is
initialized as a new empty List. Elements of the GlobalSymbolRegistry are
Records with the structure defined in Table 59.
Table 59: GlobalSymbolRegistry Record Fields
Field Name Value Usage [[Key]] a String A string key used to globally identify a
Symbol. [[Symbol]] a Symbol A symbol that can be retrieved from any realm.
20.4.2.3 SYMBOL.HASINSTANCE
The initial value of Symbol.hasInstance is the well-known symbol @@hasInstance
(Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.4 SYMBOL.ISCONCATSPREADABLE
The initial value of Symbol.isConcatSpreadable is the well-known symbol
@@isConcatSpreadable (Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.5 SYMBOL.ITERATOR
The initial value of Symbol.iterator is the well-known symbol @@iterator (Table
1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.6 SYMBOL.KEYFOR ( SYM )
This function performs the following steps when called:
1. 1. 1. If sym is not a Symbol, throw a TypeError exception.
2. 2. 2. Return KeyForSymbol(sym).
20.4.2.7 SYMBOL.MATCH
The initial value of Symbol.match is the well-known symbol @@match (Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.8 SYMBOL.MATCHALL
The initial value of Symbol.matchAll is the well-known symbol @@matchAll (Table
1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.9 SYMBOL.PROTOTYPE
The initial value of Symbol.prototype is the Symbol prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.10 SYMBOL.REPLACE
The initial value of Symbol.replace is the well-known symbol @@replace (Table
1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.11 SYMBOL.SEARCH
The initial value of Symbol.search is the well-known symbol @@search (Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.12 SYMBOL.SPECIES
The initial value of Symbol.species is the well-known symbol @@species (Table
1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.13 SYMBOL.SPLIT
The initial value of Symbol.split is the well-known symbol @@split (Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.14 SYMBOL.TOPRIMITIVE
The initial value of Symbol.toPrimitive is the well-known symbol @@toPrimitive
(Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.15 SYMBOL.TOSTRINGTAG
The initial value of Symbol.toStringTag is the well-known symbol @@toStringTag
(Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.2.16 SYMBOL.UNSCOPABLES
The initial value of Symbol.unscopables is the well-known symbol @@unscopables
(Table 1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.4.3 PROPERTIES OF THE SYMBOL PROTOTYPE OBJECT
The Symbol prototype object:
* is %Symbol.prototype%.
* is an ordinary object.
* is not a Symbol instance and does not have a [[SymbolData]] internal slot.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
The abstract operation thisSymbolValue takes argument value. It performs the
following steps when called:
1. 1. 1. If value is a Symbol, return value.
2. 2. 2. If value is an Object and value has a [[SymbolData]] internal slot,
then
1. a. a. Let s be value.[[SymbolData]].
2. b. b. Assert: s is a Symbol.
3. c. c. Return s.
3. 3. 3. Throw a TypeError exception.
20.4.3.1 SYMBOL.PROTOTYPE.CONSTRUCTOR
The initial value of Symbol.prototype.constructor is %Symbol%.
20.4.3.2 GET SYMBOL.PROTOTYPE.DESCRIPTION
Symbol.prototype.description is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
1. 1. 1. Let s be the this value.
2. 2. 2. Let sym be ? thisSymbolValue(s).
3. 3. 3. Return sym.[[Description]].
20.4.3.3 SYMBOL.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Let sym be ? thisSymbolValue(this value).
2. 2. 2. Return SymbolDescriptiveString(sym).
20.4.3.3.1 SYMBOLDESCRIPTIVESTRING ( SYM )
The abstract operation SymbolDescriptiveString takes argument sym (a Symbol) and
returns a String. It performs the following steps when called:
1. 1. 1. Let desc be sym's [[Description]] value.
2. 2. 2. If desc is undefined, set desc to the empty String.
3. 3. 3. Assert: desc is a String.
4. 4. 4. Return the string-concatenation of "Symbol(", desc, and ")".
20.4.3.4 SYMBOL.PROTOTYPE.VALUEOF ( )
This method performs the following steps when called:
1. 1. 1. Return ? thisSymbolValue(this value).
20.4.3.5 SYMBOL.PROTOTYPE [ @@TOPRIMITIVE ] ( HINT )
This method is called by ECMAScript language operators to convert a Symbol
object to a primitive value.
It performs the following steps when called:
1. 1. 1. Return ? thisSymbolValue(this value).
Note
The argument is ignored.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
The value of the "name" property of this method is "[Symbol.toPrimitive]".
20.4.3.6 SYMBOL.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Symbol".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
20.4.4 PROPERTIES OF SYMBOL INSTANCES
Symbol instances are ordinary objects that inherit properties from the Symbol
prototype object. Symbol instances have a [[SymbolData]] internal slot. The
[[SymbolData]] internal slot is the Symbol value represented by this Symbol
object.
20.4.5 ABSTRACT OPERATIONS FOR SYMBOLS
20.4.5.1 KEYFORSYMBOL ( SYM )
The abstract operation KeyForSymbol takes argument sym (a Symbol) and returns a
String or undefined. If sym is in the GlobalSymbolRegistry (see 20.4.2.2) the
String used to register sym will be returned. It performs the following steps
when called:
1. 1. 1. For each element e of the GlobalSymbolRegistry List, do
1. a. a. If SameValue(e.[[Symbol]], sym) is true, return e.[[Key]].
2. 2. 2. Assert: GlobalSymbolRegistry does not currently contain an entry for
sym.
3. 3. 3. Return undefined.
20.5 ERROR OBJECTS
Instances of Error objects are thrown as exceptions when runtime errors occur.
The Error objects may also serve as base objects for user-defined exception
classes.
When an ECMAScript implementation detects a runtime error, it throws a new
instance of one of the NativeError objects defined in 20.5.5 or a new instance
of AggregateError object defined in 20.5.7. Each of these objects has the
structure described below, differing only in the name used as the constructor
name instead of NativeError, in the "name" property of the prototype object, in
the implementation-defined "message" property of the prototype object, and in
the presence of the %AggregateError%-specific "errors" property.
20.5.1 THE ERROR CONSTRUCTOR
The Error constructor:
* is %Error%.
* is the initial value of the "Error" property of the global object.
* creates and initializes a new Error object when called as a function rather
than as a constructor. Thus the function call Error(…) is equivalent to the
object creation expression new Error(…) with the same arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Error behaviour must
include a super call to the Error constructor to create and initialize
subclass instances with an [[ErrorData]] internal slot.
20.5.1.1 ERROR ( MESSAGE [ , OPTIONS ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
2. 2. 2. Let O be ? OrdinaryCreateFromConstructor(newTarget,
"%Error.prototype%", « [[ErrorData]] »).
3. 3. 3. If message is not undefined, then
1. a. a. Let msg be ? ToString(message).
2. b. b. Perform CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).
4. 4. 4. Perform ? InstallErrorCause(O, options).
5. 5. 5. Return O.
20.5.2 PROPERTIES OF THE ERROR CONSTRUCTOR
The Error constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
20.5.2.1 ERROR.PROTOTYPE
The initial value of Error.prototype is the Error prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.5.3 PROPERTIES OF THE ERROR PROTOTYPE OBJECT
The Error prototype object:
* is %Error.prototype%.
* is an ordinary object.
* is not an Error instance and does not have an [[ErrorData]] internal slot.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
20.5.3.1 ERROR.PROTOTYPE.CONSTRUCTOR
The initial value of Error.prototype.constructor is %Error%.
20.5.3.2 ERROR.PROTOTYPE.MESSAGE
The initial value of Error.prototype.message is the empty String.
20.5.3.3 ERROR.PROTOTYPE.NAME
The initial value of Error.prototype.name is "Error".
20.5.3.4 ERROR.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. If O is not an Object, throw a TypeError exception.
3. 3. 3. Let name be ? Get(O, "name").
4. 4. 4. If name is undefined, set name to "Error"; otherwise set name to
? ToString(name).
5. 5. 5. Let msg be ? Get(O, "message").
6. 6. 6. If msg is undefined, set msg to the empty String; otherwise set msg to
? ToString(msg).
7. 7. 7. If name is the empty String, return msg.
8. 8. 8. If msg is the empty String, return name.
9. 9. 9. Return the string-concatenation of name, the code unit 0x003A (COLON),
the code unit 0x0020 (SPACE), and msg.
20.5.4 PROPERTIES OF ERROR INSTANCES
Error instances are ordinary objects that inherit properties from the Error
prototype object and have an [[ErrorData]] internal slot whose value is
undefined. The only specified uses of [[ErrorData]] is to identify Error,
AggregateError, and NativeError instances as Error objects within
Object.prototype.toString.
20.5.5 NATIVE ERROR TYPES USED IN THIS STANDARD
A new instance of one of the NativeError objects below or of the AggregateError
object is thrown when a runtime error is detected. All NativeError objects share
the same structure, as described in 20.5.6.
20.5.5.1 EVALERROR
The EvalError constructor is %EvalError%.
This exception is not currently used within this specification. This object
remains for compatibility with previous editions of this specification.
20.5.5.2 RANGEERROR
The RangeError constructor is %RangeError%.
Indicates a value that is not in the set or range of allowable values.
20.5.5.3 REFERENCEERROR
The ReferenceError constructor is %ReferenceError%.
Indicate that an invalid reference has been detected.
20.5.5.4 SYNTAXERROR
The SyntaxError constructor is %SyntaxError%.
Indicates that a parsing error has occurred.
20.5.5.5 TYPEERROR
The TypeError constructor is %TypeError%.
TypeError is used to indicate an unsuccessful operation when none of the other
NativeError objects are an appropriate indication of the failure cause.
20.5.5.6 URIERROR
The URIError constructor is %URIError%.
Indicates that one of the global URI handling functions was used in a way that
is incompatible with its definition.
20.5.6 NATIVEERROR OBJECT STRUCTURE
When an ECMAScript implementation detects a runtime error, it throws a new
instance of one of the NativeError objects defined in 20.5.5. Each of these
objects has the structure described below, differing only in the name used as
the constructor name instead of NativeError, in the "name" property of the
prototype object, and in the implementation-defined "message" property of the
prototype object.
For each error object, references to NativeError in the definition should be
replaced with the appropriate error object name from 20.5.5.
20.5.6.1 THE NATIVEERROR CONSTRUCTORS
Each NativeError constructor:
* creates and initializes a new NativeError object when called as a function
rather than as a constructor. A call of the object as a function is
equivalent to calling it as a constructor with the same arguments. Thus the
function call NativeError(…) is equivalent to the object creation expression
new NativeError(…) with the same arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified NativeError behaviour must
include a super call to the NativeError constructor to create and initialize
subclass instances with an [[ErrorData]] internal slot.
20.5.6.1.1 NATIVEERROR ( MESSAGE [ , OPTIONS ] )
Each NativeError function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
2. 2. 2. Let O be ? OrdinaryCreateFromConstructor(newTarget,
"%NativeError.prototype%", « [[ErrorData]] »).
3. 3. 3. If message is not undefined, then
1. a. a. Let msg be ? ToString(message).
2. b. b. Perform CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).
4. 4. 4. Perform ? InstallErrorCause(O, options).
5. 5. 5. Return O.
The actual value of the string passed in step 2 is either
"%EvalError.prototype%", "%RangeError.prototype%", "%ReferenceError.prototype%",
"%SyntaxError.prototype%", "%TypeError.prototype%", or "%URIError.prototype%"
corresponding to which NativeError constructor is being defined.
20.5.6.2 PROPERTIES OF THE NATIVEERROR CONSTRUCTORS
Each NativeError constructor:
* has a [[Prototype]] internal slot whose value is %Error%.
* has a "name" property whose value is the String value "NativeError".
* has the following properties:
20.5.6.2.1 NATIVEERROR.PROTOTYPE
The initial value of NativeError.prototype is a NativeError prototype object
(20.5.6.3). Each NativeError constructor has a distinct prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.5.6.3 PROPERTIES OF THE NATIVEERROR PROTOTYPE OBJECTS
Each NativeError prototype object:
* is an ordinary object.
* is not an Error instance and does not have an [[ErrorData]] internal slot.
* has a [[Prototype]] internal slot whose value is %Error.prototype%.
20.5.6.3.1 NATIVEERROR.PROTOTYPE.CONSTRUCTOR
The initial value of the "constructor" property of the prototype for a given
NativeError constructor is the corresponding intrinsic object %NativeError%
(20.5.6.1).
20.5.6.3.2 NATIVEERROR.PROTOTYPE.MESSAGE
The initial value of the "message" property of the prototype for a given
NativeError constructor is the empty String.
20.5.6.3.3 NATIVEERROR.PROTOTYPE.NAME
The initial value of the "name" property of the prototype for a given
NativeError constructor is the String value consisting of the name of the
constructor (the name used instead of NativeError).
20.5.6.4 PROPERTIES OF NATIVEERROR INSTANCES
NativeError instances are ordinary objects that inherit properties from their
NativeError prototype object and have an [[ErrorData]] internal slot whose value
is undefined. The only specified use of [[ErrorData]] is by
Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or
NativeError instances.
20.5.7 AGGREGATEERROR OBJECTS
20.5.7.1 THE AGGREGATEERROR CONSTRUCTOR
The AggregateError constructor:
* is %AggregateError%.
* is the initial value of the "AggregateError" property of the global object.
* creates and initializes a new AggregateError object when called as a function
rather than as a constructor. Thus the function call AggregateError(…) is
equivalent to the object creation expression new AggregateError(…) with the
same arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified AggregateError behaviour
must include a super call to the AggregateError constructor to create and
initialize subclass instances with an [[ErrorData]] internal slot.
20.5.7.1.1 AGGREGATEERROR ( ERRORS, MESSAGE [ , OPTIONS ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
2. 2. 2. Let O be ? OrdinaryCreateFromConstructor(newTarget,
"%AggregateError.prototype%", « [[ErrorData]] »).
3. 3. 3. If message is not undefined, then
1. a. a. Let msg be ? ToString(message).
2. b. b. Perform CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).
4. 4. 4. Perform ? InstallErrorCause(O, options).
5. 5. 5. Let errorsList be ? IteratorToList(? GetIterator(errors, sync)).
6. 6. 6. Perform ! DefinePropertyOrThrow(O, "errors", PropertyDescriptor {
[[Configurable]]: true, [[Enumerable]]: false, [[Writable]]: true,
[[Value]]: CreateArrayFromList(errorsList) }).
7. 7. 7. Return O.
20.5.7.2 PROPERTIES OF THE AGGREGATEERROR CONSTRUCTOR
The AggregateError constructor:
* has a [[Prototype]] internal slot whose value is %Error%.
* has the following properties:
20.5.7.2.1 AGGREGATEERROR.PROTOTYPE
The initial value of AggregateError.prototype is %AggregateError.prototype%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
20.5.7.3 PROPERTIES OF THE AGGREGATEERROR PROTOTYPE OBJECT
The AggregateError prototype object:
* is %AggregateError.prototype%.
* is an ordinary object.
* is not an Error instance or an AggregateError instance and does not have an
[[ErrorData]] internal slot.
* has a [[Prototype]] internal slot whose value is %Error.prototype%.
20.5.7.3.1 AGGREGATEERROR.PROTOTYPE.CONSTRUCTOR
The initial value of AggregateError.prototype.constructor is %AggregateError%.
20.5.7.3.2 AGGREGATEERROR.PROTOTYPE.MESSAGE
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AGGREGATEERROR.PROTOTYPE.NAME
The initial value of AggregateError.prototype.name is "AggregateError".
20.5.7.4 PROPERTIES OF AGGREGATEERROR INSTANCES
AggregateError instances are ordinary objects that inherit properties from their
AggregateError prototype object and have an [[ErrorData]] internal slot whose
value is undefined. The only specified use of [[ErrorData]] is by
Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or
NativeError instances.
20.5.8 ABSTRACT OPERATIONS FOR ERROR OBJECTS
20.5.8.1 INSTALLERRORCAUSE ( O, OPTIONS )
The abstract operation InstallErrorCause takes arguments O (an Object) and
options (an ECMAScript language value) and returns either a normal completion
containing unused or a throw completion. It is used to create a "cause" property
on O when a "cause" property is present on options. It performs the following
steps when called:
1. 1. 1. If options is an Object and ? HasProperty(options, "cause") is true,
then
1. a. a. Let cause be ? Get(options, "cause").
2. b. b. Perform CreateNonEnumerableDataPropertyOrThrow(O, "cause", cause).
2. 2. 2. Return unused.
21 NUMBERS AND DATES
21.1 NUMBER OBJECTS
21.1.1 THE NUMBER CONSTRUCTOR
The Number constructor:
* is %Number%.
* is the initial value of the "Number" property of the global object.
* creates and initializes a new Number object when called as a constructor.
* performs a type conversion when called as a function rather than as a
constructor.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Number behaviour must
include a super call to the Number constructor to create and initialize the
subclass instance with a [[NumberData]] internal slot.
21.1.1.1 NUMBER ( VALUE )
This function performs the following steps when called:
1. 1. 1. If value is present, then
1. a. a. Let prim be ? ToNumeric(value).
2. b. b. If prim is a BigInt, let n be 𝔽(ℝ(prim)).
3. c. c. Otherwise, let n be prim.
2. 2. 2. Else,
1. a. a. Let n be +0𝔽.
3. 3. 3. If NewTarget is undefined, return n.
4. 4. 4. Let O be ? OrdinaryCreateFromConstructor(NewTarget,
"%Number.prototype%", « [[NumberData]] »).
5. 5. 5. Set O.[[NumberData]] to n.
6. 6. 6. Return O.
21.1.2 PROPERTIES OF THE NUMBER CONSTRUCTOR
The Number constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
21.1.2.1 NUMBER.EPSILON
The value of Number.EPSILON is the Number value for the magnitude of the
difference between 1 and the smallest value greater than 1 that is representable
as a Number value, which is approximately 2.2204460492503130808472633361816 ×
10-16.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.2 NUMBER.ISFINITE ( NUMBER )
This function performs the following steps when called:
1. 1. 1. If number is not a Number, return false.
2. 2. 2. If number is not finite, return false.
3. 3. 3. Otherwise, return true.
21.1.2.3 NUMBER.ISINTEGER ( NUMBER )
This function performs the following steps when called:
1. 1. 1. Return IsIntegralNumber(number).
21.1.2.4 NUMBER.ISNAN ( NUMBER )
This function performs the following steps when called:
1. 1. 1. If number is not a Number, return false.
2. 2. 2. If number is NaN, return true.
3. 3. 3. Otherwise, return false.
Note
This function differs from the global isNaN function (19.2.3) in that it does
not convert its argument to a Number before determining whether it is NaN.
21.1.2.5 NUMBER.ISSAFEINTEGER ( NUMBER )
Note
An integer n is considered "safe" if and only if the Number value for n is not
the Number value for any other integer.
This function performs the following steps when called:
1. 1. 1. If IsIntegralNumber(number) is true, then
1. a. a. If abs(ℝ(number)) ≤ 253 - 1, return true.
2. 2. 2. Return false.
21.1.2.6 NUMBER.MAX_SAFE_INTEGER
Note
Due to rounding behaviour necessitated by precision limitations of IEEE
754-2019, the Number value for every integer greater than
Number.MAX_SAFE_INTEGER is shared with at least one other integer. Such
large-magnitude integers are therefore not safe, and are not guaranteed to be
exactly representable as Number values or even to be distinguishable from each
other. For example, both 9007199254740992 and 9007199254740993 evaluate to the
Number value 9007199254740992𝔽.
The value of Number.MAX_SAFE_INTEGER is 9007199254740991𝔽 (𝔽(253 - 1)).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.7 NUMBER.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the Number
type, which is approximately 1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.8 NUMBER.MIN_SAFE_INTEGER
Note
Due to rounding behaviour necessitated by precision limitations of IEEE
754-2019, the Number value for every integer less than Number.MIN_SAFE_INTEGER
is shared with at least one other integer. Such large-magnitude integers are
therefore not safe, and are not guaranteed to be exactly representable as Number
values or even to be distinguishable from each other. For example, both
-9007199254740992 and -9007199254740993 evaluate to the Number value
-9007199254740992𝔽.
The value of Number.MIN_SAFE_INTEGER is -9007199254740991𝔽 (𝔽(-(253 - 1))).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.9 NUMBER.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number type,
which is approximately 5 × 10-324.
In the IEEE 754-2019 double precision binary representation, the smallest
possible value is a denormalized number. If an implementation does not support
denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero
positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.10 NUMBER.NAN
The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.11 NUMBER.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is -∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.12 NUMBER.PARSEFLOAT ( STRING )
The initial value of the "parseFloat" property is %parseFloat%.
21.1.2.13 NUMBER.PARSEINT ( STRING, RADIX )
The initial value of the "parseInt" property is %parseInt%.
21.1.2.14 NUMBER.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is +∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.15 NUMBER.PROTOTYPE
The initial value of Number.prototype is the Number prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.3 PROPERTIES OF THE NUMBER PROTOTYPE OBJECT
The Number prototype object:
* is %Number.prototype%.
* is an ordinary object.
* is itself a Number object; it has a [[NumberData]] internal slot with the
value +0𝔽.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
Unless explicitly stated otherwise, the methods of the Number prototype object
defined below are not generic and the this value passed to them must be either a
Number value or an object that has a [[NumberData]] internal slot that has been
initialized to a Number value.
The abstract operation thisNumberValue takes argument value. It performs the
following steps when called:
1. 1. 1. If value is a Number, return value.
2. 2. 2. If value is an Object and value has a [[NumberData]] internal slot,
then
1. a. a. Let n be value.[[NumberData]].
2. b. b. Assert: n is a Number.
3. c. c. Return n.
3. 3. 3. Throw a TypeError exception.
The phrase “this Number value” within the specification of a method refers to
the result returned by calling the abstract operation thisNumberValue with the
this value of the method invocation passed as the argument.
21.1.3.1 NUMBER.PROTOTYPE.CONSTRUCTOR
The initial value of Number.prototype.constructor is %Number%.
21.1.3.2 NUMBER.PROTOTYPE.TOEXPONENTIAL ( FRACTIONDIGITS )
This method returns a String containing this Number value represented in decimal
exponential notation with one digit before the significand's decimal point and
fractionDigits digits after the significand's decimal point. If fractionDigits
is undefined, it includes as many significand digits as necessary to uniquely
specify the Number (just like in ToString except that in this case the Number is
always output in exponential notation).
It performs the following steps when called:
1. 1. 1. Let x be ? thisNumberValue(this value).
2. 2. 2. Let f be ? ToIntegerOrInfinity(fractionDigits).
3. 3. 3. Assert: If fractionDigits is undefined, then f is 0.
4. 4. 4. If x is not finite, return Number::toString(x, 10).
5. 5. 5. If f < 0 or f > 100, throw a RangeError exception.
6. 6. 6. Set x to ℝ(x).
7. 7. 7. Let s be the empty String.
8. 8. 8. If x < 0, then
1. a. a. Set s to "-".
2. b. b. Set x to -x.
9. 9. 9. If x = 0, then
1. a. a. Let m be the String value consisting of f + 1 occurrences of the
code unit 0x0030 (DIGIT ZERO).
2. b. b. Let e be 0.
10. 10. 10. Else,
1. a. a. If fractionDigits is not undefined, then
1. i. i. Let e and n be integers such that 10f ≤ n < 10f + 1 and for
which n × 10e - f - x is as close to zero as possible. If there are
two such sets of e and n, pick the e and n for which n × 10e - f is
larger.
2. b. b. Else,
1. i. i. Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1,
𝔽(n × 10e - f) is 𝔽(x), and f is as small as possible. Note that
the decimal representation of n has f + 1 digits, n is not divisible
by 10, and the least significant digit of n is not necessarily
uniquely determined by these criteria.
3. c. c. Let m be the String value consisting of the digits of the decimal
representation of n (in order, with no leading zeroes).
11. 11. 11. If f ≠ 0, then
1. a. a. Let a be the first code unit of m.
2. b. b. Let b be the other f code units of m.
3. c. c. Set m to the string-concatenation of a, ".", and b.
12. 12. 12. If e = 0, then
1. a. a. Let c be "+".
2. b. b. Let d be "0".
13. 13. 13. Else,
1. a. a. If e > 0, let c be "+".
2. b. b. Else,
1. i. i. Assert: e < 0.
2. ii. ii. Let c be "-".
3. iii. iii. Set e to -e.
3. c. c. Let d be the String value consisting of the digits of the decimal
representation of e (in order, with no leading zeroes).
14. 14. 14. Set m to the string-concatenation of m, "e", c, and d.
15. 15. 15. Return the string-concatenation of s and m.
Note
For implementations that provide more accurate conversions than required by the
rules above, it is recommended that the following alternative version of step
10.b.i be used as a guideline:
1. i. i. Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1, 𝔽(n ×
10e - f) is 𝔽(x), and f is as small as possible. If there are multiple
possibilities for n, choose the value of n for which 𝔽(n × 10e - f) is
closest in value to 𝔽(x). If there are two such possible values of n,
choose the one that is even.
21.1.3.3 NUMBER.PROTOTYPE.TOFIXED ( FRACTIONDIGITS )
Note 1
This method returns a String containing this Number value represented in decimal
fixed-point notation with fractionDigits digits after the decimal point. If
fractionDigits is undefined, 0 is assumed.
It performs the following steps when called:
1. 1. 1. Let x be ? thisNumberValue(this value).
2. 2. 2. Let f be ? ToIntegerOrInfinity(fractionDigits).
3. 3. 3. Assert: If fractionDigits is undefined, then f is 0.
4. 4. 4. If f is not finite, throw a RangeError exception.
5. 5. 5. If f < 0 or f > 100, throw a RangeError exception.
6. 6. 6. If x is not finite, return Number::toString(x, 10).
7. 7. 7. Set x to ℝ(x).
8. 8. 8. Let s be the empty String.
9. 9. 9. If x < 0, then
1. a. a. Set s to "-".
2. b. b. Set x to -x.
10. 10. 10. If x ≥ 1021, then
1. a. a. Let m be ! ToString(𝔽(x)).
11. 11. 11. Else,
1. a. a. Let n be an integer for which n / 10f - x is as close to zero as
possible. If there are two such n, pick the larger n.
2. b. b. If n = 0, let m be "0". Otherwise, let m be the String value
consisting of the digits of the decimal representation of n (in order,
with no leading zeroes).
3. c. c. If f ≠ 0, then
1. i. i. Let k be the length of m.
2. ii. ii. If k ≤ f, then
1. 1. 1. Let z be the String value consisting of f + 1 - k
occurrences of the code unit 0x0030 (DIGIT ZERO).
2. 2. 2. Set m to the string-concatenation of z and m.
3. 3. 3. Set k to f + 1.
3. iii. iii. Let a be the first k - f code units of m.
4. iv. iv. Let b be the other f code units of m.
5. v. v. Set m to the string-concatenation of a, ".", and b.
12. 12. 12. Return the string-concatenation of s and m.
Note 2
The output of toFixed may be more precise than toString for some values because
toString only prints enough significant digits to distinguish the number from
adjacent Number values. For example,
(1000000000000000128).toString() returns "1000000000000000100", while
(1000000000000000128).toFixed(0) returns "1000000000000000128".
21.1.3.4 NUMBER.PROTOTYPE.TOLOCALESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method produces a String value that represents this Number value formatted
according to the conventions of the host environment's current locale. This
method is implementation-defined, and it is permissible, but not encouraged, for
it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
21.1.3.5 NUMBER.PROTOTYPE.TOPRECISION ( PRECISION )
This method returns a String containing this Number value represented either in
decimal exponential notation with one digit before the significand's decimal
point and precision - 1 digits after the significand's decimal point or in
decimal fixed notation with precision significant digits. If precision is
undefined, it calls ToString instead.
It performs the following steps when called:
1. 1. 1. Let x be ? thisNumberValue(this value).
2. 2. 2. If precision is undefined, return ! ToString(x).
3. 3. 3. Let p be ? ToIntegerOrInfinity(precision).
4. 4. 4. If x is not finite, return Number::toString(x, 10).
5. 5. 5. If p < 1 or p > 100, throw a RangeError exception.
6. 6. 6. Set x to ℝ(x).
7. 7. 7. Let s be the empty String.
8. 8. 8. If x < 0, then
1. a. a. Set s to the code unit 0x002D (HYPHEN-MINUS).
2. b. b. Set x to -x.
9. 9. 9. If x = 0, then
1. a. a. Let m be the String value consisting of p occurrences of the code
unit 0x0030 (DIGIT ZERO).
2. b. b. Let e be 0.
10. 10. 10. Else,
1. a. a. Let e and n be integers such that 10p - 1 ≤ n < 10p and for which
n × 10e - p + 1 - x is as close to zero as possible. If there are two
such sets of e and n, pick the e and n for which n × 10e - p + 1 is
larger.
2. b. b. Let m be the String value consisting of the digits of the decimal
representation of n (in order, with no leading zeroes).
3. c. c. If e < -6 or e ≥ p, then
1. i. i. Assert: e ≠ 0.
2. ii. ii. If p ≠ 1, then
1. 1. 1. Let a be the first code unit of m.
2. 2. 2. Let b be the other p - 1 code units of m.
3. 3. 3. Set m to the string-concatenation of a, ".", and b.
3. iii. iii. If e > 0, then
1. 1. 1. Let c be the code unit 0x002B (PLUS SIGN).
4. iv. iv. Else,
1. 1. 1. Assert: e < 0.
2. 2. 2. Let c be the code unit 0x002D (HYPHEN-MINUS).
3. 3. 3. Set e to -e.
5. v. v. Let d be the String value consisting of the digits of the
decimal representation of e (in order, with no leading zeroes).
6. vi. vi. Return the string-concatenation of s, m, the code unit 0x0065
(LATIN SMALL LETTER E), c, and d.
11. 11. 11. If e = p - 1, return the string-concatenation of s and m.
12. 12. 12. If e ≥ 0, then
1. a. a. Set m to the string-concatenation of the first e + 1 code units of
m, the code unit 0x002E (FULL STOP), and the remaining p - (e + 1) code
units of m.
13. 13. 13. Else,
1. a. a. Set m to the string-concatenation of the code unit 0x0030 (DIGIT
ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the
code unit 0x0030 (DIGIT ZERO), and the String m.
14. 14. 14. Return the string-concatenation of s and m.
21.1.3.6 NUMBER.PROTOTYPE.TOSTRING ( [ RADIX ] )
Note
The optional radix should be an integral Number value in the inclusive interval
from 2𝔽 to 36𝔽. If radix is undefined then 10𝔽 is used as the value of radix.
This method performs the following steps when called:
1. 1. 1. Let x be ? thisNumberValue(this value).
2. 2. 2. If radix is undefined, let radixMV be 10.
3. 3. 3. Else, let radixMV be ? ToIntegerOrInfinity(radix).
4. 4. 4. If radixMV is not in the inclusive interval from 2 to 36, throw a
RangeError exception.
5. 5. 5. Return Number::toString(x, radixMV).
This method is not generic; it throws a TypeError exception if its this value is
not a Number or a Number object. Therefore, it cannot be transferred to other
kinds of objects for use as a method.
The "length" property of this method is 1𝔽.
21.1.3.7 NUMBER.PROTOTYPE.VALUEOF ( )
1. 1. 1. Return ? thisNumberValue(this value).
21.1.4 PROPERTIES OF NUMBER INSTANCES
Number instances are ordinary objects that inherit properties from the Number
prototype object. Number instances also have a [[NumberData]] internal slot. The
[[NumberData]] internal slot is the Number value represented by this Number
object.
21.2 BIGINT OBJECTS
21.2.1 THE BIGINT CONSTRUCTOR
The BigInt constructor:
* is %BigInt%.
* is the initial value of the "BigInt" property of the global object.
* performs a type conversion when called as a function rather than as a
constructor.
* is not intended to be used with the new operator or to be subclassed. It may
be used as the value of an extends clause of a class definition but a super
call to the BigInt constructor will cause an exception.
21.2.1.1 BIGINT ( VALUE )
This function performs the following steps when called:
1. 1. 1. If NewTarget is not undefined, throw a TypeError exception.
2. 2. 2. Let prim be ? ToPrimitive(value, number).
3. 3. 3. If prim is a Number, return ? NumberToBigInt(prim).
4. 4. 4. Otherwise, return ? ToBigInt(prim).
21.2.1.1.1 NUMBERTOBIGINT ( NUMBER )
The abstract operation NumberToBigInt takes argument number (a Number) and
returns either a normal completion containing a BigInt or a throw completion. It
performs the following steps when called:
1. 1. 1. If IsIntegralNumber(number) is false, throw a RangeError exception.
2. 2. 2. Return the BigInt value that represents ℝ(number).
21.2.2 PROPERTIES OF THE BIGINT CONSTRUCTOR
The BigInt constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
21.2.2.1 BIGINT.ASINTN ( BITS, BIGINT )
This function performs the following steps when called:
1. 1. 1. Set bits to ? ToIndex(bits).
2. 2. 2. Set bigint to ? ToBigInt(bigint).
3. 3. 3. Let mod be ℝ(bigint) modulo 2bits.
4. 4. 4. If mod ≥ 2bits - 1, return ℤ(mod - 2bits); otherwise, return ℤ(mod).
21.2.2.2 BIGINT.ASUINTN ( BITS, BIGINT )
This function performs the following steps when called:
1. 1. 1. Set bits to ? ToIndex(bits).
2. 2. 2. Set bigint to ? ToBigInt(bigint).
3. 3. 3. Return the BigInt value that represents ℝ(bigint) modulo 2bits.
21.2.2.3 BIGINT.PROTOTYPE
The initial value of BigInt.prototype is the BigInt prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.2.3 PROPERTIES OF THE BIGINT PROTOTYPE OBJECT
The BigInt prototype object:
* is %BigInt.prototype%.
* is an ordinary object.
* is not a BigInt object; it does not have a [[BigIntData]] internal slot.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
The abstract operation thisBigIntValue takes argument value. It performs the
following steps when called:
1. 1. 1. If value is a BigInt, return value.
2. 2. 2. If value is an Object and value has a [[BigIntData]] internal slot,
then
1. a. a. Assert: value.[[BigIntData]] is a BigInt.
2. b. b. Return value.[[BigIntData]].
3. 3. 3. Throw a TypeError exception.
The phrase “this BigInt value” within the specification of a method refers to
the result returned by calling the abstract operation thisBigIntValue with the
this value of the method invocation passed as the argument.
21.2.3.1 BIGINT.PROTOTYPE.CONSTRUCTOR
The initial value of BigInt.prototype.constructor is %BigInt%.
21.2.3.2 BIGINT.PROTOTYPE.TOLOCALESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method produces a String value that represents this BigInt value formatted
according to the conventions of the host environment's current locale. This
method is implementation-defined, and it is permissible, but not encouraged, for
it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
21.2.3.3 BIGINT.PROTOTYPE.TOSTRING ( [ RADIX ] )
Note
The optional radix should be an integral Number value in the inclusive interval
from 2𝔽 to 36𝔽. If radix is undefined then 10𝔽 is used as the value of radix.
This method performs the following steps when called:
1. 1. 1. Let x be ? thisBigIntValue(this value).
2. 2. 2. If radix is undefined, let radixMV be 10.
3. 3. 3. Else, let radixMV be ? ToIntegerOrInfinity(radix).
4. 4. 4. If radixMV is not in the inclusive interval from 2 to 36, throw a
RangeError exception.
5. 5. 5. Return BigInt::toString(x, radixMV).
This method is not generic; it throws a TypeError exception if its this value is
not a BigInt or a BigInt object. Therefore, it cannot be transferred to other
kinds of objects for use as a method.
21.2.3.4 BIGINT.PROTOTYPE.VALUEOF ( )
1. 1. 1. Return ? thisBigIntValue(this value).
21.2.3.5 BIGINT.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "BigInt".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
21.3 THE MATH OBJECT
The Math object:
* is %Math%.
* is the initial value of the "Math" property of the global object.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is not a function object.
* does not have a [[Construct]] internal method; it cannot be used as a
constructor with the new operator.
* does not have a [[Call]] internal method; it cannot be invoked as a function.
Note
In this specification, the phrase “the Number value for x” has a technical
meaning defined in 6.1.6.1.
21.3.1 VALUE PROPERTIES OF THE MATH OBJECT
21.3.1.1 MATH.E
The Number value for e, the base of the natural logarithms, which is
approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.2 MATH.LN10
The Number value for the natural logarithm of 10, which is approximately
2.302585092994046.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.3 MATH.LN2
The Number value for the natural logarithm of 2, which is approximately
0.6931471805599453.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.4 MATH.LOG10E
The Number value for the base-10 logarithm of e, the base of the natural
logarithms; this value is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
The value of Math.LOG10E is approximately the reciprocal of the value of
Math.LN10.
21.3.1.5 MATH.LOG2E
The Number value for the base-2 logarithm of e, the base of the natural
logarithms; this value is approximately 1.4426950408889634.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
The value of Math.LOG2E is approximately the reciprocal of the value of
Math.LN2.
21.3.1.6 MATH.PI
The Number value for π, the ratio of the circumference of a circle to its
diameter, which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.7 MATH.SQRT1_2
The Number value for the square root of ½, which is approximately
0.7071067811865476.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
The value of Math.SQRT1_2 is approximately the reciprocal of the value of
Math.SQRT2.
21.3.1.8 MATH.SQRT2
The Number value for the square root of 2, which is approximately
1.4142135623730951.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.9 MATH [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Math".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
21.3.2 FUNCTION PROPERTIES OF THE MATH OBJECT
Note
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2,
cbrt, cos, cosh, exp, expm1, hypot, log, log1p, log2, log10, pow, random, sin,
sinh, sqrt, tan, and tanh is not precisely specified here except to require
specific results for certain argument values that represent boundary cases of
interest. For other argument values, these functions are intended to compute
approximations to the results of familiar mathematical functions, but some
latitude is allowed in the choice of approximation algorithms. The general
intent is that an implementer should be able to use the same mathematical
library for ECMAScript on a given hardware platform that is available to C
programmers on that platform.
Although the choice of algorithms is left to the implementation, it is
recommended (but not specified by this standard) that implementations use the
approximation algorithms for IEEE 754-2019 arithmetic contained in fdlibm, the
freely distributable mathematical library from Sun Microsystems
(http://www.netlib.org/fdlibm).
21.3.2.1 MATH.ABS ( X )
This function returns the absolute value of x; the result has the same magnitude
as x but has positive sign.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is NaN, return NaN.
3. 3. 3. If n is -0𝔽, return +0𝔽.
4. 4. 4. If n is -∞𝔽, return +∞𝔽.
5. 5. 5. If n < -0𝔽, return -n.
6. 6. 6. Return n.
21.3.2.2 MATH.ACOS ( X )
This function returns the inverse cosine of x. The result is expressed in
radians and is in the inclusive interval from +0𝔽 to 𝔽(π).
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is NaN, n > 1𝔽, or n < -1𝔽, return NaN.
3. 3. 3. If n is 1𝔽, return +0𝔽.
4. 4. 4. Return an implementation-approximated Number value representing the
result of the inverse cosine of ℝ(n).
21.3.2.3 MATH.ACOSH ( X )
This function returns the inverse hyperbolic cosine of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is either NaN or +∞𝔽, return n.
3. 3. 3. If n is 1𝔽, return +0𝔽.
4. 4. 4. If n < 1𝔽, return NaN.
5. 5. 5. Return an implementation-approximated Number value representing the
result of the inverse hyperbolic cosine of ℝ(n).
21.3.2.4 MATH.ASIN ( X )
This function returns the inverse sine of x. The result is expressed in radians
and is in the inclusive interval from 𝔽(-π / 2) to 𝔽(π / 2).
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n > 1𝔽 or n < -1𝔽, return NaN.
4. 4. 4. Return an implementation-approximated Number value representing the
result of the inverse sine of ℝ(n).
21.3.2.5 MATH.ASINH ( X )
This function returns the inverse hyperbolic sine of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is either +0𝔽 or -0𝔽, return n.
3. 3. 3. Return an implementation-approximated Number value representing the
result of the inverse hyperbolic sine of ℝ(n).
21.3.2.6 MATH.ATAN ( X )
This function returns the inverse tangent of x. The result is expressed in
radians and is in the inclusive interval from 𝔽(-π / 2) to 𝔽(π / 2).
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n is +∞𝔽, return an implementation-approximated Number value
representing π / 2.
4. 4. 4. If n is -∞𝔽, return an implementation-approximated Number value
representing -π / 2.
5. 5. 5. Return an implementation-approximated Number value representing the
result of the inverse tangent of ℝ(n).
21.3.2.7 MATH.ATANH ( X )
This function returns the inverse hyperbolic tangent of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n > 1𝔽 or n < -1𝔽, return NaN.
4. 4. 4. If n is 1𝔽, return +∞𝔽.
5. 5. 5. If n is -1𝔽, return -∞𝔽.
6. 6. 6. Return an implementation-approximated Number value representing the
result of the inverse hyperbolic tangent of ℝ(n).
21.3.2.8 MATH.ATAN2 ( Y, X )
This function returns the inverse tangent of the quotient y / x of the arguments
y and x, where the signs of y and x are used to determine the quadrant of the
result. Note that it is intentional and traditional for the two-argument inverse
tangent function that the argument named y be first and the argument named x be
second. The result is expressed in radians and is in the inclusive interval from
-π to +π.
It performs the following steps when called:
1. 1. 1. Let ny be ? ToNumber(y).
2. 2. 2. Let nx be ? ToNumber(x).
3. 3. 3. If ny is NaN or nx is NaN, return NaN.
4. 4. 4. If ny is +∞𝔽, then
1. a. a. If nx is +∞𝔽, return an implementation-approximated Number value
representing π / 4.
2. b. b. If nx is -∞𝔽, return an implementation-approximated Number value
representing 3π / 4.
3. c. c. Return an implementation-approximated Number value representing π
/ 2.
5. 5. 5. If ny is -∞𝔽, then
1. a. a. If nx is +∞𝔽, return an implementation-approximated Number value
representing -π / 4.
2. b. b. If nx is -∞𝔽, return an implementation-approximated Number value
representing -3π / 4.
3. c. c. Return an implementation-approximated Number value representing -π
/ 2.
6. 6. 6. If ny is +0𝔽, then
1. a. a. If nx > +0𝔽 or nx is +0𝔽, return +0𝔽.
2. b. b. Return an implementation-approximated Number value representing π.
7. 7. 7. If ny is -0𝔽, then
1. a. a. If nx > +0𝔽 or nx is +0𝔽, return -0𝔽.
2. b. b. Return an implementation-approximated Number value representing
-π.
8. 8. 8. Assert: ny is finite and is neither +0𝔽 nor -0𝔽.
9. 9. 9. If ny > +0𝔽, then
1. a. a. If nx is +∞𝔽, return +0𝔽.
2. b. b. If nx is -∞𝔽, return an implementation-approximated Number value
representing π.
3. c. c. If nx is either +0𝔽 or -0𝔽, return an
implementation-approximated Number value representing π / 2.
10. 10. 10. If ny < -0𝔽, then
1. a. a. If nx is +∞𝔽, return -0𝔽.
2. b. b. If nx is -∞𝔽, return an implementation-approximated Number value
representing -π.
3. c. c. If nx is either +0𝔽 or -0𝔽, return an
implementation-approximated Number value representing -π / 2.
11. 11. 11. Assert: nx is finite and is neither +0𝔽 nor -0𝔽.
12. 12. 12. Return an implementation-approximated Number value representing the
result of the inverse tangent of the quotient ℝ(ny) / ℝ(nx).
21.3.2.9 MATH.CBRT ( X )
This function returns the cube root of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is either +0𝔽 or -0𝔽, return n.
3. 3. 3. Return an implementation-approximated Number value representing the
result of the cube root of ℝ(n).
21.3.2.10 MATH.CEIL ( X )
This function returns the smallest (closest to -∞) integral Number value that is
not less than x. If x is already an integral Number, the result is x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is either +0𝔽 or -0𝔽, return n.
3. 3. 3. If n < -0𝔽 and n > -1𝔽, return -0𝔽.
4. 4. 4. If n is an integral Number, return n.
5. 5. 5. Return the smallest (closest to -∞) integral Number value that is not
less than n.
Note
The value of Math.ceil(x) is the same as the value of -Math.floor(-x).
21.3.2.11 MATH.CLZ32 ( X )
This function performs the following steps when called:
1. 1. 1. Let n be ? ToUint32(x).
2. 2. 2. Let p be the number of leading zero bits in the unsigned 32-bit binary
representation of n.
3. 3. 3. Return 𝔽(p).
Note
If n is either +0𝔽 or -0𝔽, this method returns 32𝔽. If the most significant
bit of the 32-bit binary encoding of n is 1, this method returns +0𝔽.
21.3.2.12 MATH.COS ( X )
This function returns the cosine of x. The argument is expressed in radians.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite, return NaN.
3. 3. 3. If n is either +0𝔽 or -0𝔽, return 1𝔽.
4. 4. 4. Return an implementation-approximated Number value representing the
result of the cosine of ℝ(n).
21.3.2.13 MATH.COSH ( X )
This function returns the hyperbolic cosine of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is NaN, return NaN.
3. 3. 3. If n is either +∞𝔽 or -∞𝔽, return +∞𝔽.
4. 4. 4. If n is either +0𝔽 or -0𝔽, return 1𝔽.
5. 5. 5. Return an implementation-approximated Number value representing the
result of the hyperbolic cosine of ℝ(n).
Note
The value of Math.cosh(x) is the same as the value of (Math.exp(x) +
Math.exp(-x)) / 2.
21.3.2.14 MATH.EXP ( X )
This function returns the exponential function of x (e raised to the power of x,
where e is the base of the natural logarithms).
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is either NaN or +∞𝔽, return n.
3. 3. 3. If n is either +0𝔽 or -0𝔽, return 1𝔽.
4. 4. 4. If n is -∞𝔽, return +0𝔽.
5. 5. 5. Return an implementation-approximated Number value representing the
result of the exponential function of ℝ(n).
21.3.2.15 MATH.EXPM1 ( X )
This function returns the result of subtracting 1 from the exponential function
of x (e raised to the power of x, where e is the base of the natural
logarithms). The result is computed in a way that is accurate even when the
value of x is close to 0.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, -0𝔽, or +∞𝔽, return n.
3. 3. 3. If n is -∞𝔽, return -1𝔽.
4. 4. 4. Return an implementation-approximated Number value representing the
result of subtracting 1 from the exponential function of ℝ(n).
21.3.2.16 MATH.FLOOR ( X )
This function returns the greatest (closest to +∞) integral Number value that is
not greater than x. If x is already an integral Number, the result is x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is either +0𝔽 or -0𝔽, return n.
3. 3. 3. If n < 1𝔽 and n > +0𝔽, return +0𝔽.
4. 4. 4. If n is an integral Number, return n.
5. 5. 5. Return the greatest (closest to +∞) integral Number value that is not
greater than n.
Note
The value of Math.floor(x) is the same as the value of -Math.ceil(-x).
21.3.2.17 MATH.FROUND ( X )
This function performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is NaN, return NaN.
3. 3. 3. If n is one of +0𝔽, -0𝔽, +∞𝔽, or -∞𝔽, return n.
4. 4. 4. Let n32 be the result of converting n to a value in IEEE 754-2019
binary32 format using roundTiesToEven mode.
5. 5. 5. Let n64 be the result of converting n32 to a value in IEEE 754-2019
binary64 format.
6. 6. 6. Return the ECMAScript Number value corresponding to n64.
21.3.2.18 MATH.HYPOT ( ...ARGS )
Given zero or more arguments, this function returns the square root of the sum
of squares of its arguments.
It performs the following steps when called:
1. 1. 1. Let coerced be a new empty List.
2. 2. 2. For each element arg of args, do
1. a. a. Let n be ? ToNumber(arg).
2. b. b. Append n to coerced.
3. 3. 3. For each element number of coerced, do
1. a. a. If number is either +∞𝔽 or -∞𝔽, return +∞𝔽.
4. 4. 4. Let onlyZero be true.
5. 5. 5. For each element number of coerced, do
1. a. a. If number is NaN, return NaN.
2. b. b. If number is neither +0𝔽 nor -0𝔽, set onlyZero to false.
6. 6. 6. If onlyZero is true, return +0𝔽.
7. 7. 7. Return an implementation-approximated Number value representing the
square root of the sum of squares of the mathematical values of the elements
of coerced.
The "length" property of this function is 2𝔽.
Note
Implementations should take care to avoid the loss of precision from overflows
and underflows that are prone to occur in naive implementations when this
function is called with two or more arguments.
21.3.2.19 MATH.IMUL ( X, Y )
This function performs the following steps when called:
1. 1. 1. Let a be ℝ(? ToUint32(x)).
2. 2. 2. Let b be ℝ(? ToUint32(y)).
3. 3. 3. Let product be (a × b) modulo 232.
4. 4. 4. If product ≥ 231, return 𝔽(product - 232); otherwise return
𝔽(product).
21.3.2.20 MATH.LOG ( X )
This function returns the natural logarithm of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is either NaN or +∞𝔽, return n.
3. 3. 3. If n is 1𝔽, return +0𝔽.
4. 4. 4. If n is either +0𝔽 or -0𝔽, return -∞𝔽.
5. 5. 5. If n < -0𝔽, return NaN.
6. 6. 6. Return an implementation-approximated Number value representing the
result of the natural logarithm of ℝ(n).
21.3.2.21 MATH.LOG1P ( X )
This function returns the natural logarithm of 1 + x. The result is computed in
a way that is accurate even when the value of x is close to zero.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, -0𝔽, or +∞𝔽, return n.
3. 3. 3. If n is -1𝔽, return -∞𝔽.
4. 4. 4. If n < -1𝔽, return NaN.
5. 5. 5. Return an implementation-approximated Number value representing the
result of the natural logarithm of 1 + ℝ(n).
21.3.2.22 MATH.LOG10 ( X )
This function returns the base 10 logarithm of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is either NaN or +∞𝔽, return n.
3. 3. 3. If n is 1𝔽, return +0𝔽.
4. 4. 4. If n is either +0𝔽 or -0𝔽, return -∞𝔽.
5. 5. 5. If n < -0𝔽, return NaN.
6. 6. 6. Return an implementation-approximated Number value representing the
result of the base 10 logarithm of ℝ(n).
21.3.2.23 MATH.LOG2 ( X )
This function returns the base 2 logarithm of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is either NaN or +∞𝔽, return n.
3. 3. 3. If n is 1𝔽, return +0𝔽.
4. 4. 4. If n is either +0𝔽 or -0𝔽, return -∞𝔽.
5. 5. 5. If n < -0𝔽, return NaN.
6. 6. 6. Return an implementation-approximated Number value representing the
result of the base 2 logarithm of ℝ(n).
21.3.2.24 MATH.MAX ( ...ARGS )
Given zero or more arguments, this function calls ToNumber on each of the
arguments and returns the largest of the resulting values.
It performs the following steps when called:
1. 1. 1. Let coerced be a new empty List.
2. 2. 2. For each element arg of args, do
1. a. a. Let n be ? ToNumber(arg).
2. b. b. Append n to coerced.
3. 3. 3. Let highest be -∞𝔽.
4. 4. 4. For each element number of coerced, do
1. a. a. If number is NaN, return NaN.
2. b. b. If number is +0𝔽 and highest is -0𝔽, set highest to +0𝔽.
3. c. c. If number > highest, set highest to number.
5. 5. 5. Return highest.
Note
The comparison of values to determine the largest value is done using the
IsLessThan algorithm except that +0𝔽 is considered to be larger than -0𝔽.
The "length" property of this function is 2𝔽.
21.3.2.25 MATH.MIN ( ...ARGS )
Given zero or more arguments, this function calls ToNumber on each of the
arguments and returns the smallest of the resulting values.
It performs the following steps when called:
1. 1. 1. Let coerced be a new empty List.
2. 2. 2. For each element arg of args, do
1. a. a. Let n be ? ToNumber(arg).
2. b. b. Append n to coerced.
3. 3. 3. Let lowest be +∞𝔽.
4. 4. 4. For each element number of coerced, do
1. a. a. If number is NaN, return NaN.
2. b. b. If number is -0𝔽 and lowest is +0𝔽, set lowest to -0𝔽.
3. c. c. If number < lowest, set lowest to number.
5. 5. 5. Return lowest.
Note
The comparison of values to determine the largest value is done using the
IsLessThan algorithm except that +0𝔽 is considered to be larger than -0𝔽.
The "length" property of this function is 2𝔽.
21.3.2.26 MATH.POW ( BASE, EXPONENT )
This function performs the following steps when called:
1. 1. 1. Set base to ? ToNumber(base).
2. 2. 2. Set exponent to ? ToNumber(exponent).
3. 3. 3. Return Number::exponentiate(base, exponent).
21.3.2.27 MATH.RANDOM ( )
This function returns a Number value with positive sign, greater than or equal
to +0𝔽 but strictly less than 1𝔽, chosen randomly or pseudo randomly with
approximately uniform distribution over that range, using an
implementation-defined algorithm or strategy.
Each Math.random function created for distinct realms must produce a distinct
sequence of values from successive calls.
21.3.2.28 MATH.ROUND ( X )
This function returns the Number value that is closest to x and is integral. If
two integral Numbers are equally close to x, then the result is the Number value
that is closer to +∞. If x is already integral, the result is x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is an integral Number, return n.
3. 3. 3. If n < 0.5𝔽 and n > +0𝔽, return +0𝔽.
4. 4. 4. If n < -0𝔽 and n ≥ -0.5𝔽, return -0𝔽.
5. 5. 5. Return the integral Number closest to n, preferring the Number closer
to +∞ in the case of a tie.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of Math.floor(x +
0.5). When x is -0𝔽 or x is less than +0𝔽 but greater than or equal to -0.5𝔽,
Math.round(x) returns -0𝔽, but Math.floor(x + 0.5) returns +0𝔽. Math.round(x)
may also differ from the value of Math.floor(x + 0.5)because of internal
rounding when computing x + 0.5.
21.3.2.29 MATH.SIGN ( X )
This function returns the sign of x, indicating whether x is positive, negative,
or zero.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n < -0𝔽, return -1𝔽.
4. 4. 4. Return 1𝔽.
21.3.2.30 MATH.SIN ( X )
This function returns the sine of x. The argument is expressed in radians.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n is either +∞𝔽 or -∞𝔽, return NaN.
4. 4. 4. Return an implementation-approximated Number value representing the
result of the sine of ℝ(n).
21.3.2.31 MATH.SINH ( X )
This function returns the hyperbolic sine of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is either +0𝔽 or -0𝔽, return n.
3. 3. 3. Return an implementation-approximated Number value representing the
result of the hyperbolic sine of ℝ(n).
Note
The value of Math.sinh(x) is the same as the value of (Math.exp(x) -
Math.exp(-x)) / 2.
21.3.2.32 MATH.SQRT ( X )
This function returns the square root of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, -0𝔽, or +∞𝔽, return n.
3. 3. 3. If n < -0𝔽, return NaN.
4. 4. 4. Return an implementation-approximated Number value representing the
result of the square root of ℝ(n).
21.3.2.33 MATH.TAN ( X )
This function returns the tangent of x. The argument is expressed in radians.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n is either +∞𝔽 or -∞𝔽, return NaN.
4. 4. 4. Return an implementation-approximated Number value representing the
result of the tangent of ℝ(n).
21.3.2.34 MATH.TANH ( X )
This function returns the hyperbolic tangent of x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is one of NaN, +0𝔽, or -0𝔽, return n.
3. 3. 3. If n is +∞𝔽, return 1𝔽.
4. 4. 4. If n is -∞𝔽, return -1𝔽.
5. 5. 5. Return an implementation-approximated Number value representing the
result of the hyperbolic tangent of ℝ(n).
Note
The value of Math.tanh(x) is the same as the value of (Math.exp(x) -
Math.exp(-x)) / (Math.exp(x) + Math.exp(-x)).
21.3.2.35 MATH.TRUNC ( X )
This function returns the integral part of the number x, removing any fractional
digits. If x is already integral, the result is x.
It performs the following steps when called:
1. 1. 1. Let n be ? ToNumber(x).
2. 2. 2. If n is not finite or n is either +0𝔽 or -0𝔽, return n.
3. 3. 3. If n < 1𝔽 and n > +0𝔽, return +0𝔽.
4. 4. 4. If n < -0𝔽 and n > -1𝔽, return -0𝔽.
5. 5. 5. Return the integral Number nearest n in the direction of +0𝔽.
21.4 DATE OBJECTS
21.4.1 OVERVIEW OF DATE OBJECTS AND DEFINITIONS OF ABSTRACT OPERATIONS
The following abstract operations operate on time values (defined in 21.4.1.1).
Note that, in every case, if any argument to one of these functions is NaN, the
result will be NaN.
21.4.1.1 TIME VALUES AND TIME RANGE
Time measurement in ECMAScript is analogous to time measurement in POSIX, in
particular sharing definition in terms of the proleptic Gregorian calendar, an
epoch of midnight at the beginning of 1 January 1970 UTC, and an accounting of
every day as comprising exactly 86,400 seconds (each of which is 1000
milliseconds long).
An ECMAScript time value is a Number, either a finite integral Number
representing an instant in time to millisecond precision or NaN representing no
specific instant. A time value that is a multiple of 24 × 60 × 60 × 1000 =
86,400,000 (i.e., is 86,400,000 × d for some integer d) represents the instant
at the start of the UTC day that follows the epoch by d whole UTC days
(preceding the epoch for negative d). Every other finite time value t is defined
relative to the greatest preceding time value s that is such a multiple, and
represents the instant that occurs within the same UTC day as s but follows it
by (t - s) milliseconds.
Time values do not account for UTC leap seconds—there are no time values
representing instants within positive leap seconds, and there are time values
representing instants removed from the UTC timeline by negative leap seconds.
However, the definition of time values nonetheless yields piecewise alignment
with UTC, with discontinuities only at leap second boundaries and zero
difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to
9,007,199,254,740,992 (21.1.2.8 and 21.1.2.6). A time value supports a slightly
smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds.
This yields a supported time value range of exactly -100,000,000 days to
100,000,000 days relative to midnight at the beginning of 1 January 1970 UTC.
The exact moment of midnight at the beginning of 1 January 1970 UTC is
represented by the time value +0𝔽.
Note
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years.
This yields an average of 365.2425 days per year, which is 31,556,952,000
milliseconds. Therefore, the maximum range a Number could represent exactly with
millisecond precision is approximately -285,426 to 285,426 years relative to
1970. The smaller range supported by a time value as specified in this section
is approximately -273,790 to 273,790 years relative to 1970.
21.4.1.2 DAY NUMBER AND TIME WITHIN DAY
A given time value t belongs to day number
Day(t) = 𝔽(floor(ℝ(t / msPerDay)))
where the number of milliseconds per day is
msPerDay = 86400000𝔽
The remainder is called the time within the day:
TimeWithinDay(t) = 𝔽(ℝ(t) modulo ℝ(msPerDay))
21.4.1.3 YEAR NUMBER
ECMAScript uses a proleptic Gregorian calendar to map a day number to a year
number and to determine the month and date within that year. In this calendar,
leap years are precisely those which are (divisible by 4) and ((not divisible by
100) or (divisible by 400)). The number of days in year number y is therefore
defined by
DaysInYear(y)
= 365𝔽 if (ℝ(y) modulo 4) ≠ 0
= 366𝔽 if (ℝ(y) modulo 4) = 0 and (ℝ(y) modulo 100) ≠ 0
= 365𝔽 if (ℝ(y) modulo 100) = 0 and (ℝ(y) modulo 400) ≠ 0
= 366𝔽 if (ℝ(y) modulo 400) = 0
All non-leap years have 365 days with the usual number of days per month and
leap years have an extra day in February. The day number of the first day of
year y is given by:
DayFromYear(y) = 𝔽(365 × (ℝ(y) - 1970) + floor((ℝ(y) - 1969) / 4) - floor((ℝ(y)
- 1901) / 100) + floor((ℝ(y) - 1601) / 400))
The time value of the start of a year is:
TimeFromYear(y) = msPerDay × DayFromYear(y)
A time value determines a year by:
YearFromTime(t) = the largest integral Number y (closest to +∞) such that
TimeFromYear(y) ≤ t
The leap-year function is 1𝔽 for a time within a leap year and otherwise is
+0𝔽:
InLeapYear(t)
= +0𝔽 if DaysInYear(YearFromTime(t)) is 365𝔽
= 1𝔽 if DaysInYear(YearFromTime(t)) is 366𝔽
21.4.1.4 MONTH NUMBER
Months are identified by an integral Number in the inclusive interval from +0𝔽
to 11𝔽. The mapping MonthFromTime(t) from a time value t to a month number is
defined by:
MonthFromTime(t)
= +0𝔽 if +0𝔽 ≤ DayWithinYear(t) < 31𝔽
= 1𝔽 if 31𝔽 ≤ DayWithinYear(t) < 59𝔽 + InLeapYear(t)
= 2𝔽 if 59𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 90𝔽 + InLeapYear(t)
= 3𝔽 if 90𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 120𝔽 + InLeapYear(t)
= 4𝔽 if 120𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 151𝔽 + InLeapYear(t)
= 5𝔽 if 151𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 181𝔽 + InLeapYear(t)
= 6𝔽 if 181𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 212𝔽 + InLeapYear(t)
= 7𝔽 if 212𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 243𝔽 + InLeapYear(t)
= 8𝔽 if 243𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 273𝔽 + InLeapYear(t)
= 9𝔽 if 273𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 304𝔽 + InLeapYear(t)
= 10𝔽 if 304𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 334𝔽 + InLeapYear(t)
= 11𝔽 if 334𝔽 + InLeapYear(t) ≤ DayWithinYear(t) < 365𝔽 + InLeapYear(t)
where
DayWithinYear(t) = Day(t) - DayFromYear(YearFromTime(t))
A month value of +0𝔽 specifies January; 1𝔽 specifies February; 2𝔽 specifies
March; 3𝔽 specifies April; 4𝔽 specifies May; 5𝔽 specifies June; 6𝔽 specifies
July; 7𝔽 specifies August; 8𝔽 specifies September; 9𝔽 specifies October; 10𝔽
specifies November; and 11𝔽 specifies December. Note that MonthFromTime(+0𝔽) =
+0𝔽, corresponding to Thursday, 1 January 1970.
21.4.1.5 DATE NUMBER
A date number is identified by an integral Number in the inclusive interval from
1𝔽 to 31𝔽. The mapping DateFromTime(t) from a time value t to a date number is
defined by:
DateFromTime(t)
= DayWithinYear(t) + 1𝔽 if MonthFromTime(t) is +0𝔽
= DayWithinYear(t) - 30𝔽 if MonthFromTime(t) is 1𝔽
= DayWithinYear(t) - 58𝔽 - InLeapYear(t) if MonthFromTime(t) is 2𝔽
= DayWithinYear(t) - 89𝔽 - InLeapYear(t) if MonthFromTime(t) is 3𝔽
= DayWithinYear(t) - 119𝔽 - InLeapYear(t) if MonthFromTime(t) is 4𝔽
= DayWithinYear(t) - 150𝔽 - InLeapYear(t) if MonthFromTime(t) is 5𝔽
= DayWithinYear(t) - 180𝔽 - InLeapYear(t) if MonthFromTime(t) is 6𝔽
= DayWithinYear(t) - 211𝔽 - InLeapYear(t) if MonthFromTime(t) is 7𝔽
= DayWithinYear(t) - 242𝔽 - InLeapYear(t) if MonthFromTime(t) is 8𝔽
= DayWithinYear(t) - 272𝔽 - InLeapYear(t) if MonthFromTime(t) is 9𝔽
= DayWithinYear(t) - 303𝔽 - InLeapYear(t) if MonthFromTime(t) is 10𝔽
= DayWithinYear(t) - 333𝔽 - InLeapYear(t) if MonthFromTime(t) is 11𝔽
21.4.1.6 WEEK DAY
The weekday for a particular time value t is defined as
WeekDay(t) = 𝔽(ℝ(Day(t) + 4𝔽) modulo 7)
A weekday value of +0𝔽 specifies Sunday; 1𝔽 specifies Monday; 2𝔽 specifies
Tuesday; 3𝔽 specifies Wednesday; 4𝔽 specifies Thursday; 5𝔽 specifies Friday;
and 6𝔽 specifies Saturday. Note that WeekDay(+0𝔽) = 4𝔽, corresponding to
Thursday, 1 January 1970.
21.4.1.7 GETUTCEPOCHNANOSECONDS ( YEAR, MONTH, DAY, HOUR, MINUTE, SECOND,
MILLISECOND, MICROSECOND, NANOSECOND )
The abstract operation GetUTCEpochNanoseconds takes arguments year (an integer),
month (an integer in the inclusive interval from 1 to 12), day (an integer in
the inclusive interval from 1 to 31), hour (an integer in the inclusive interval
from 0 to 23), minute (an integer in the inclusive interval from 0 to 59),
second (an integer in the inclusive interval from 0 to 59), millisecond (an
integer in the inclusive interval from 0 to 999), microsecond (an integer in the
inclusive interval from 0 to 999), and nanosecond (an integer in the inclusive
interval from 0 to 999) and returns a BigInt. The returned value represents a
number of nanoseconds since the epoch that corresponds to the given ISO 8601
calendar date and wall-clock time in UTC. It performs the following steps when
called:
1. 1. 1. Let date be MakeDay(𝔽(year), 𝔽(month - 1), 𝔽(day)).
2. 2. 2. Let time be MakeTime(𝔽(hour), 𝔽(minute), 𝔽(second),
𝔽(millisecond)).
3. 3. 3. Let ms be MakeDate(date, time).
4. 4. 4. Assert: ms is an integral Number.
5. 5. 5. Return ℤ(ℝ(ms) × 106 + microsecond × 103 + nanosecond).
21.4.1.8 GETNAMEDTIMEZONEEPOCHNANOSECONDS ( TIMEZONEIDENTIFIER, YEAR, MONTH,
DAY, HOUR, MINUTE, SECOND, MILLISECOND, MICROSECOND, NANOSECOND )
The implementation-defined abstract operation GetNamedTimeZoneEpochNanoseconds
takes arguments timeZoneIdentifier (a String), year (an integer), month (an
integer in the inclusive interval from 1 to 12), day (an integer in the
inclusive interval from 1 to 31), hour (an integer in the inclusive interval
from 0 to 23), minute (an integer in the inclusive interval from 0 to 59),
second (an integer in the inclusive interval from 0 to 59), millisecond (an
integer in the inclusive interval from 0 to 999), microsecond (an integer in the
inclusive interval from 0 to 999), and nanosecond (an integer in the inclusive
interval from 0 to 999) and returns a List of BigInts. Each value in the
returned List represents a number of nanoseconds since the epoch that
corresponds to the given ISO 8601 calendar date and wall-clock time in the named
time zone identified by timeZoneIdentifier.
When the input represents a local time occurring more than once because of a
negative time zone transition (e.g. when daylight saving time ends or the time
zone offset is decreased due to a time zone rule change), the returned List will
have more than one element and will be sorted by ascending numerical value. When
the input represents a local time skipped because of a positive time zone
transition (e.g. when daylight saving time begins or the time zone offset is
increased due to a time zone rule change), the returned List will be empty.
Otherwise, the returned List will have one element.
The default implementation of GetNamedTimeZoneEpochNanoseconds, to be used for
ECMAScript implementations that do not include local political rules for any
time zones, performs the following steps when called:
1. 1. 1. Assert: timeZoneIdentifier is "UTC".
2. 2. 2. Let epochNanoseconds be GetUTCEpochNanoseconds(year, month, day, hour,
minute, second, millisecond, microsecond, nanosecond).
3. 3. 3. Return « epochNanoseconds ».
Note
It is recommended that implementations use the time zone information of the IANA
Time Zone Database https://www.iana.org/time-zones/.
1:30 AM on 5 November 2017 in America/New_York is repeated twice, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 11, 5, 1, 30, 0, 0,
0, 0) would return a List of length 2 in which the first element represents
05:30 UTC (corresponding with 01:30 US Eastern Daylight Time at UTC offset
-04:00) and the second element represents 06:30 UTC (corresponding with 01:30 US
Eastern Standard Time at UTC offset -05:00).
2:30 AM on 12 March 2017 in America/New_York does not exist, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 3, 12, 2, 30, 0, 0,
0, 0) would return an empty List.
21.4.1.9 GETNAMEDTIMEZONEOFFSETNANOSECONDS ( TIMEZONEIDENTIFIER,
EPOCHNANOSECONDS )
The implementation-defined abstract operation GetNamedTimeZoneOffsetNanoseconds
takes arguments timeZoneIdentifier (a String) and epochNanoseconds (a BigInt)
and returns an integer.
The returned integer represents the offset from UTC of the named time zone
identified by timeZoneIdentifier, at the instant corresponding with
epochNanoseconds relative to the epoch, both in nanoseconds.
The default implementation of GetNamedTimeZoneOffsetNanoseconds, to be used for
ECMAScript implementations that do not include local political rules for any
time zones, performs the following steps when called:
1. 1. 1. Assert: timeZoneIdentifier is "UTC".
2. 2. 2. Return 0.
Note
Time zone offset values may be positive or negative.
21.4.1.10 DEFAULTTIMEZONE ( )
The implementation-defined abstract operation DefaultTimeZone takes no arguments
and returns a String. It returns a String value representing the host
environment's current time zone, which is either a String representing a UTC
offset for which IsTimeZoneOffsetString returns true, or a String identifier
accepted by GetNamedTimeZoneEpochNanoseconds and
GetNamedTimeZoneOffsetNanoseconds.
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement the DefaultTimeZone abstract operation as specified in the
ECMA-402 specification.
The default implementation of DefaultTimeZone, to be used for ECMAScript
implementations that do not include local political rules for any time zones,
performs the following steps when called:
1. 1. 1. Return "UTC".
Note
To ensure the level of functionality that implementations commonly provide in
the methods of the Date object, it is recommended that DefaultTimeZone return an
IANA time zone name corresponding to the host environment's time zone setting,
if such a thing exists. GetNamedTimeZoneEpochNanoseconds and
GetNamedTimeZoneOffsetNanoseconds must reflect the local political rules for
standard time and daylight saving time in that time zone, if such rules exist.
For example, if the host environment is a browser on a system where the user has
chosen US Eastern Time as their time zone, DefaultTimeZone returns
"America/New_York".
21.4.1.11 LOCALTIME ( T )
The abstract operation LocalTime takes argument t (a finite time value) and
returns an integral Number. It converts t from UTC to local time. The local
political rules for standard time and daylight saving time in effect at t should
be used to determine the result in the way specified in this section. It
performs the following steps when called:
1. 1. 1. Let localTimeZone be DefaultTimeZone().
2. 2. 2. If IsTimeZoneOffsetString(localTimeZone) is true, then
1. a. a. Let offsetNs be ParseTimeZoneOffsetString(localTimeZone).
3. 3. 3. Else,
1. a. a. Let offsetNs be GetNamedTimeZoneOffsetNanoseconds(localTimeZone,
ℤ(ℝ(t) × 106)).
4. 4. 4. Let offsetMs be truncate(offsetNs / 106).
5. 5. 5. Return t + 𝔽(offsetMs).
If political rules for the local time t are not available within the
implementation, the result is t because DefaultTimeZone returns "UTC" and
GetNamedTimeZoneOffsetNanoseconds returns 0.
Note 1
It is recommended that implementations use the time zone information of the IANA
Time Zone Database https://www.iana.org/time-zones/.
Note 2
Two different input time values tUTC are converted to the same local time tlocal
at a negative time zone transition when there are repeated times (e.g. the
daylight saving time ends or the time zone adjustment is decreased.).
LocalTime(UTC(tlocal)) is not necessarily always equal to tlocal.
Correspondingly, UTC(LocalTime(tUTC)) is not necessarily always equal to tUTC.
21.4.1.12 UTC ( T )
The abstract operation UTC takes argument t (a Number) and returns a time value.
It converts t from local time to a UTC time value. The local political rules for
standard time and daylight saving time in effect at t should be used to
determine the result in the way specified in this section. It performs the
following steps when called:
1. 1. 1. Let localTimeZone be DefaultTimeZone().
2. 2. 2. If IsTimeZoneOffsetString(localTimeZone) is true, then
1. a. a. Let offsetNs be ParseTimeZoneOffsetString(localTimeZone).
3. 3. 3. Else,
1. a. a. Let possibleInstants be
GetNamedTimeZoneEpochNanoseconds(localTimeZone, ℝ(YearFromTime(t)),
ℝ(MonthFromTime(t)) + 1, ℝ(DateFromTime(t)), ℝ(HourFromTime(t)),
ℝ(MinFromTime(t)), ℝ(SecFromTime(t)), ℝ(msFromTime(t)), 0, 0).
2. b. b. NOTE: The following steps ensure that when t represents local time
repeating multiple times at a negative time zone transition (e.g. when
the daylight saving time ends or the time zone offset is decreased due to
a time zone rule change) or skipped local time at a positive time zone
transition (e.g. when the daylight saving time starts or the time zone
offset is increased due to a time zone rule change), t is interpreted
using the time zone offset before the transition.
3. c. c. If possibleInstants is not empty, then
1. i. i. Let disambiguatedInstant be possibleInstants[0].
4. d. d. Else,
1. i. i. NOTE: t represents a local time skipped at a positive time zone
transition (e.g. due to daylight saving time starting or a time zone
rule change increasing the UTC offset).
2. ii. ii. Let possibleInstantsBefore be
GetNamedTimeZoneEpochNanoseconds(localTimeZone,
ℝ(YearFromTime(tBefore)), ℝ(MonthFromTime(tBefore)) + 1,
ℝ(DateFromTime(tBefore)), ℝ(HourFromTime(tBefore)),
ℝ(MinFromTime(tBefore)), ℝ(SecFromTime(tBefore)),
ℝ(msFromTime(tBefore)), 0, 0), where tBefore is the largest integral
Number < t for which possibleInstantsBefore is not empty (i.e.,
tBefore represents the last local time before the transition).
3. iii. iii. Let disambiguatedInstant be the last element of
possibleInstantsBefore.
5. e. e. Let offsetNs be GetNamedTimeZoneOffsetNanoseconds(localTimeZone,
disambiguatedInstant).
4. 4. 4. Let offsetMs be truncate(offsetNs / 106).
5. 5. 5. Return t - 𝔽(offsetMs).
Input t is nominally a time value but may be any Number value. The algorithm
must not limit t to the time value range, so that inputs corresponding with a
boundary of the time value range can be supported regardless of local UTC
offset. For example, the maximum time value is 8.64 × 1015, corresponding with
"+275760-09-13T00:00:00Z". In an environment where the local time zone offset is
ahead of UTC by 1 hour at that instant, it is represented by the larger input of
8.64 × 1015 + 3.6 × 106, corresponding with "+275760-09-13T01:00:00+01:00".
If political rules for the local time t are not available within the
implementation, the result is t because DefaultTimeZone returns "UTC" and
GetNamedTimeZoneOffsetNanoseconds returns 0.
Note 1
It is recommended that implementations use the time zone information of the IANA
Time Zone Database https://www.iana.org/time-zones/.
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall
backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM
UTC-05. In UTC(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0)))),
the value of offsetMs is -4 × msPerHour.
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be
interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). In
UTC(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0)))), the value
of offsetMs is -5 × msPerHour.
Note 2
UTC(LocalTime(tUTC)) is not necessarily always equal to tUTC. Correspondingly,
LocalTime(UTC(tlocal)) is not necessarily always equal to tlocal.
21.4.1.13 HOURS, MINUTES, SECOND, AND MILLISECONDS
The following abstract operations are useful in decomposing time values:
HourFromTime(t) = 𝔽(floor(ℝ(t / msPerHour)) modulo HoursPerDay)
MinFromTime(t) = 𝔽(floor(ℝ(t / msPerMinute)) modulo MinutesPerHour)
SecFromTime(t) = 𝔽(floor(ℝ(t / msPerSecond)) modulo SecondsPerMinute)
msFromTime(t) = 𝔽(ℝ(t) modulo ℝ(msPerSecond))
where
HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond = 1000𝔽
msPerMinute = 60000𝔽 = msPerSecond × 𝔽(SecondsPerMinute)
msPerHour = 3600000𝔽 = msPerMinute × 𝔽(MinutesPerHour)
21.4.1.14 MAKETIME ( HOUR, MIN, SEC, MS )
The abstract operation MakeTime takes arguments hour (a Number), min (a Number),
sec (a Number), and ms (a Number) and returns a Number. It calculates a number
of milliseconds. It performs the following steps when called:
1. 1. 1. If hour is not finite, min is not finite, sec is not finite, or ms is
not finite, return NaN.
2. 2. 2. Let h be 𝔽(! ToIntegerOrInfinity(hour)).
3. 3. 3. Let m be 𝔽(! ToIntegerOrInfinity(min)).
4. 4. 4. Let s be 𝔽(! ToIntegerOrInfinity(sec)).
5. 5. 5. Let milli be 𝔽(! ToIntegerOrInfinity(ms)).
6. 6. 6. Let t be ((h * msPerHour + m * msPerMinute) + s * msPerSecond) +
milli, performing the arithmetic according to IEEE 754-2019 rules (that is,
as if using the ECMAScript operators * and +).
7. 7. 7. Return t.
21.4.1.15 MAKEDAY ( YEAR, MONTH, DATE )
The abstract operation MakeDay takes arguments year (a Number), month (a
Number), and date (a Number) and returns a Number. It calculates a number of
days. It performs the following steps when called:
1. 1. 1. If year is not finite, month is not finite, or date is not finite,
return NaN.
2. 2. 2. Let y be 𝔽(! ToIntegerOrInfinity(year)).
3. 3. 3. Let m be 𝔽(! ToIntegerOrInfinity(month)).
4. 4. 4. Let dt be 𝔽(! ToIntegerOrInfinity(date)).
5. 5. 5. Let ym be y + 𝔽(floor(ℝ(m) / 12)).
6. 6. 6. If ym is not finite, return NaN.
7. 7. 7. Let mn be 𝔽(ℝ(m) modulo 12).
8. 8. 8. Find a finite time value t such that YearFromTime(t) is ym,
MonthFromTime(t) is mn, and DateFromTime(t) is 1𝔽; but if this is not
possible (because some argument is out of range), return NaN.
9. 9. 9. Return Day(t) + dt - 1𝔽.
21.4.1.16 MAKEDATE ( DAY, TIME )
The abstract operation MakeDate takes arguments day (a Number) and time (a
Number) and returns a Number. It calculates a number of milliseconds. It
performs the following steps when called:
1. 1. 1. If day is not finite or time is not finite, return NaN.
2. 2. 2. Let tv be day × msPerDay + time.
3. 3. 3. If tv is not finite, return NaN.
4. 4. 4. Return tv.
21.4.1.17 TIMECLIP ( TIME )
The abstract operation TimeClip takes argument time (a Number) and returns a
Number. It calculates a number of milliseconds. It performs the following steps
when called:
1. 1. 1. If time is not finite, return NaN.
2. 2. 2. If abs(ℝ(time)) > 8.64 × 1015, return NaN.
3. 3. 3. Return 𝔽(! ToIntegerOrInfinity(time)).
21.4.1.18 DATE TIME STRING FORMAT
ECMAScript defines a string interchange format for date-times based upon a
simplification of the ISO 8601 calendar date extended format. The format is as
follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY is the year in the proleptic Gregorian calendar as four decimal digits from
0000 to 9999, or as an expanded year of "+" or "-" followed by six decimal
digits. - "-" (hyphen) appears literally twice in the string. MM is the month of
the year as two decimal digits from 01 (January) to 12 (December). DD is the day
of the month as two decimal digits from 01 to 31. T "T" appears literally in the
string, to indicate the beginning of the time element. HH is the number of
complete hours that have passed since midnight as two decimal digits from 00 to
24. : ":" (colon) appears literally twice in the string. mm is the number of
complete minutes since the start of the hour as two decimal digits from 00 to
59. ss is the number of complete seconds since the start of the minute as two
decimal digits from 00 to 59. . "." (dot) appears literally in the string. sss
is the number of complete milliseconds since the start of the second as three
decimal digits. Z is the UTC offset representation specified as "Z" (for UTC
with no offset) or as either "+" or "-" followed by a time expression HH:mm (a
subset of the time zone offset string format for indicating local time ahead of
or behind UTC, respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only
forms immediately followed by one of the following time forms with an optional
UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid
instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and
24:00 are available to distinguish the two midnights that can be associated with
one date. This means that the following two notations refer to exactly the same
point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the
latter form as "end of a calendar day" is consistent with ISO 8601, even though
that specification reserves it for describing time intervals and does not permit
it within representations of single points in time.
Note 2
There exists no international standard that specifies abbreviations for civil
time zones like CET, EST, etc. and sometimes the same abbreviation is even used
for two very different time zones. For this reason, both ISO 8601 and this
format specify numeric representations of time zone offsets.
21.4.1.18.1 EXPANDED YEARS
Covering the full time value range of approximately 273,790 years forward or
backward from 1 January 1970 (21.4.1.1) requires representing years before 0 or
after 9999. ISO 8601 permits expansion of the year representation, but only by
mutual agreement of the partners in information interchange. In the simplified
ECMAScript format, such an expanded year representation shall have 6 digits and
is always prefixed with a + or - sign. The year 0 is considered positive and
must be prefixed with a + sign. The representation of the year 0 as -000000 is
invalid. Strings matching the Date Time String Format with expanded years
representing instants in time outside the range of a time value are treated as
unrecognizable by Date.parse and cause that function to return NaN without
falling back to implementation-specific behaviour or heuristics.
Note
Examples of date-time values with expanded years:
-271821-04-20T00:00:00Z 271822 B.C. -000001-01-01T00:00:00Z 2 B.C.
+000000-01-01T00:00:00Z 1 B.C. +000001-01-01T00:00:00Z 1 A.D.
+001970-01-01T00:00:00Z 1970 A.D. +002009-12-15T00:00:00Z 2009 A.D.
+275760-09-13T00:00:00Z 275760 A.D.
21.4.1.19 TIME ZONE OFFSET STRING FORMAT
ECMAScript defines a string interchange format for UTC offsets, derived from ISO
8601. The format is described by the following grammar. The usage of Unicode
code points in this grammar is listed in Table 60.
Table 60: Time Zone Offset String Code Points
Code Point Unicode Name Abbreviation U+2212 MINUS SIGN <MINUS>
SYNTAX
UTCOffset ::: TemporalSign Hour TemporalSign Hour HourSubcomponents[+Extended]
TemporalSign Hour HourSubcomponents[~Extended] TemporalSign ::: ASCIISign
<MINUS> ASCIISign ::: one of + - Hour ::: 0 DecimalDigit 1 DecimalDigit 20 21 22
23 HourSubcomponents[Extended] ::: TimeSeparator[?Extended] MinuteSecond
TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond
TemporalDecimalFractionopt TimeSeparator[Extended] ::: [+Extended] : [~Extended]
[empty] MinuteSecond ::: 0 DecimalDigit 1 DecimalDigit 2 DecimalDigit 3
DecimalDigit 4 DecimalDigit 5 DecimalDigit TemporalDecimalFraction :::
TemporalDecimalSeparator DecimalDigit TemporalDecimalSeparator DecimalDigit
DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit
TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit
TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit
DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit
DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit
DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit
TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit
DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator
DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit
DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator ::: one of . ,
21.4.1.19.1 ISTIMEZONEOFFSETSTRING ( OFFSETSTRING )
The abstract operation IsTimeZoneOffsetString takes argument offsetString (a
String) and returns a Boolean. The return value indicates whether offsetString
conforms to the grammar given by UTCOffset. It performs the following steps when
called:
1. 1. 1. Let parseResult be ParseText(StringToCodePoints(offsetString),
UTCOffset).
2. 2. 2. If parseResult is a List of errors, return false.
3. 3. 3. Return true.
21.4.1.19.2 PARSETIMEZONEOFFSETSTRING ( OFFSETSTRING )
The abstract operation ParseTimeZoneOffsetString takes argument offsetString (a
String) and returns an integer. The return value is the UTC offset, as a number
of nanoseconds, that corresponds to the String offsetString. It performs the
following steps when called:
1. 1. 1. Let parseResult be ParseText(StringToCodePoints(offsetString),
UTCOffset).
2. 2. 2. Assert: parseResult is not a List of errors.
3. 3. 3. Assert: parseResult contains a TemporalSign Parse Node.
4. 4. 4. Let parsedSign be the source text matched by the TemporalSign Parse
Node contained within parseResult.
5. 5. 5. If parsedSign is the single code point U+002D (HYPHEN-MINUS) or
U+2212 (MINUS SIGN), then
1. a. a. Let sign be -1.
6. 6. 6. Else,
1. a. a. Let sign be 1.
7. 7. 7. NOTE: Applications of StringToNumber below do not lose precision,
since each of the parsed values is guaranteed to be a sufficiently short
string of decimal digits.
8. 8. 8. Assert: parseResult contains an Hour Parse Node.
9. 9. 9. Let parsedHours be the source text matched by the Hour Parse Node
contained within parseResult.
10. 10. 10. Let hours be ℝ(StringToNumber(CodePointsToString(parsedHours))).
11. 11. 11. If parseResult does not contain a MinuteSecond Parse Node, then
1. a. a. Let minutes be 0.
12. 12. 12. Else,
1. a. a. Let parsedMinutes be the source text matched by the first
MinuteSecond Parse Node contained within parseResult.
2. b. b. Let minutes be
ℝ(StringToNumber(CodePointsToString(parsedMinutes))).
13. 13. 13. If parseResult does not contain two MinuteSecond Parse Nodes, then
1. a. a. Let seconds be 0.
14. 14. 14. Else,
1. a. a. Let parsedSeconds be the source text matched by the second
MinuteSecond Parse Node contained within parseResult.
2. b. b. Let seconds be
ℝ(StringToNumber(CodePointsToString(parsedSeconds))).
15. 15. 15. If parseResult does not contain a TemporalDecimalFraction Parse
Node, then
1. a. a. Let nanoseconds be 0.
16. 16. 16. Else,
1. a. a. Let parsedFraction be the source text matched by the
TemporalDecimalFraction Parse Node contained within parseResult.
2. b. b. Let fraction be the string-concatenation of
CodePointsToString(parsedFraction) and "000000000".
3. c. c. Let nanosecondsString be the substring of fraction from 1 to 10.
4. d. d. Let nanoseconds be ℝ(StringToNumber(nanosecondsString)).
17. 17. 17. Return sign × (((hours × 60 + minutes) × 60 + seconds) × 109 +
nanoseconds).
21.4.2 THE DATE CONSTRUCTOR
The Date constructor:
* is %Date%.
* is the initial value of the "Date" property of the global object.
* creates and initializes a new Date when called as a constructor.
* returns a String representing the current time (UTC) when called as a
function rather than as a constructor.
* is a function whose behaviour differs based upon the number and types of its
arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Date behaviour must include
a super call to the Date constructor to create and initialize the subclass
instance with a [[DateValue]] internal slot.
* has a "length" property whose value is 7𝔽.
21.4.2.1 DATE ( ...VALUES )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, then
1. a. a. Let now be the time value (UTC) identifying the current time.
2. b. b. Return ToDateString(now).
2. 2. 2. Let numberOfArgs be the number of elements in values.
3. 3. 3. If numberOfArgs = 0, then
1. a. a. Let dv be the time value (UTC) identifying the current time.
4. 4. 4. Else if numberOfArgs = 1, then
1. a. a. Let value be values[0].
2. b. b. If value is an Object and value has a [[DateValue]] internal slot,
then
1. i. i. Let tv be ! thisTimeValue(value).
3. c. c. Else,
1. i. i. Let v be ? ToPrimitive(value).
2. ii. ii. If v is a String, then
1. 1. 1. Assert: The next step never returns an abrupt completion
because v is a String.
2. 2. 2. Let tv be the result of parsing v as a date, in exactly the
same manner as for the parse method (21.4.3.2).
3. iii. iii. Else,
1. 1. 1. Let tv be ? ToNumber(v).
4. d. d. Let dv be TimeClip(tv).
5. 5. 5. Else,
1. a. a. Assert: numberOfArgs ≥ 2.
2. b. b. Let y be ? ToNumber(values[0]).
3. c. c. Let m be ? ToNumber(values[1]).
4. d. d. If numberOfArgs > 2, let dt be ? ToNumber(values[2]); else let dt
be 1𝔽.
5. e. e. If numberOfArgs > 3, let h be ? ToNumber(values[3]); else let h be
+0𝔽.
6. f. f. If numberOfArgs > 4, let min be ? ToNumber(values[4]); else let
min be +0𝔽.
7. g. g. If numberOfArgs > 5, let s be ? ToNumber(values[5]); else let s be
+0𝔽.
8. h. h. If numberOfArgs > 6, let milli be ? ToNumber(values[6]); else let
milli be +0𝔽.
9. i. i. If y is NaN, let yr be NaN.
10. j. j. Else,
1. i. i. Let yi be ! ToIntegerOrInfinity(y).
2. ii. ii. If 0 ≤ yi ≤ 99, let yr be 1900𝔽 + 𝔽(yi); otherwise, let yr
be y.
11. k. k. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s,
milli)).
12. l. l. Let dv be TimeClip(UTC(finalDate)).
6. 6. 6. Let O be ? OrdinaryCreateFromConstructor(NewTarget,
"%Date.prototype%", « [[DateValue]] »).
7. 7. 7. Set O.[[DateValue]] to dv.
8. 8. 8. Return O.
21.4.3 PROPERTIES OF THE DATE CONSTRUCTOR
The Date constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
21.4.3.1 DATE.NOW ( )
This function returns the time value designating the UTC date and time of the
occurrence of the call to it.
21.4.3.2 DATE.PARSE ( STRING )
This function applies the ToString operator to its argument. If ToString results
in an abrupt completion the Completion Record is immediately returned.
Otherwise, this function interprets the resulting String as a date and time; it
returns a Number, the UTC time value corresponding to the date and time. The
String may be interpreted as a local time, a UTC time, or a time in some other
time zone, depending on the contents of the String. The function first attempts
to parse the String according to the format described in Date Time String Format
(21.4.1.18), including expanded years. If the String does not conform to that
format the function may fall back to any implementation-specific heuristics or
implementation-specific date formats. Strings that are unrecognizable or contain
out-of-bounds format element values shall cause this function to return NaN.
If the String conforms to the Date Time String Format, substitute values take
the place of absent format elements. When the MM or DD elements are absent, "01"
is used. When the HH, mm, or ss elements are absent, "00" is used. When the sss
element is absent, "000" is used. When the UTC offset representation is absent,
date-only forms are interpreted as a UTC time and date-time forms are
interpreted as a local time.
If x is any Date whose milliseconds amount is zero within a particular
implementation of ECMAScript, then all of the following expressions should
produce the same numeric value in that implementation, if all the properties
referenced have their initial values:
x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())
However, the expression
Date.parse(x.toLocaleString())
is not required to produce the same Number value as the preceding three
expressions and, in general, the value produced by this function is
implementation-defined when given any String value that does not conform to the
Date Time String Format (21.4.1.18) and that could not be produced in that
implementation by the toString or toUTCString method.
21.4.3.3 DATE.PROTOTYPE
The initial value of Date.prototype is the Date prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.4.3.4 DATE.UTC ( YEAR [ , MONTH [ , DATE [ , HOURS [ , MINUTES [ , SECONDS [
, MS ] ] ] ] ] ] )
This function performs the following steps when called:
1. 1. 1. Let y be ? ToNumber(year).
2. 2. 2. If month is present, let m be ? ToNumber(month); else let m be +0𝔽.
3. 3. 3. If date is present, let dt be ? ToNumber(date); else let dt be 1𝔽.
4. 4. 4. If hours is present, let h be ? ToNumber(hours); else let h be +0𝔽.
5. 5. 5. If minutes is present, let min be ? ToNumber(minutes); else let min
be +0𝔽.
6. 6. 6. If seconds is present, let s be ? ToNumber(seconds); else let s be
+0𝔽.
7. 7. 7. If ms is present, let milli be ? ToNumber(ms); else let milli be
+0𝔽.
8. 8. 8. If y is NaN, let yr be NaN.
9. 9. 9. Else,
1. a. a. Let yi be ! ToIntegerOrInfinity(y).
2. b. b. If 0 ≤ yi ≤ 99, let yr be 1900𝔽 + 𝔽(yi); otherwise, let yr be y.
10. 10. 10. Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s,
milli))).
The "length" property of this function is 7𝔽.
Note
This function differs from the Date constructor in two ways: it returns a time
value as a Number, rather than creating a Date, and it interprets the arguments
in UTC rather than as local time.
21.4.4 PROPERTIES OF THE DATE PROTOTYPE OBJECT
The Date prototype object:
* is %Date.prototype%.
* is itself an ordinary object.
* is not a Date instance and does not have a [[DateValue]] internal slot.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
Unless explicitly defined otherwise, the methods of the Date prototype object
defined below are not generic and the this value passed to them must be an
object that has a [[DateValue]] internal slot that has been initialized to a
time value.
The abstract operation thisTimeValue takes argument value. It performs the
following steps when called:
1. 1. 1. If value is an Object and value has a [[DateValue]] internal slot,
then
1. a. a. Return value.[[DateValue]].
2. 2. 2. Throw a TypeError exception.
In following descriptions of functions that are properties of the Date prototype
object, the phrase “this Date object” refers to the object that is the this
value for the invocation of the function. If the Type of the this value is not
Object, a TypeError exception is thrown. The phrase “this time value” within the
specification of a method refers to the result returned by calling the abstract
operation thisTimeValue with the this value of the method invocation passed as
the argument.
21.4.4.1 DATE.PROTOTYPE.CONSTRUCTOR
The initial value of Date.prototype.constructor is %Date%.
21.4.4.2 DATE.PROTOTYPE.GETDATE ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return DateFromTime(LocalTime(t)).
21.4.4.3 DATE.PROTOTYPE.GETDAY ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return WeekDay(LocalTime(t)).
21.4.4.4 DATE.PROTOTYPE.GETFULLYEAR ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return YearFromTime(LocalTime(t)).
21.4.4.5 DATE.PROTOTYPE.GETHOURS ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return HourFromTime(LocalTime(t)).
21.4.4.6 DATE.PROTOTYPE.GETMILLISECONDS ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return msFromTime(LocalTime(t)).
21.4.4.7 DATE.PROTOTYPE.GETMINUTES ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return MinFromTime(LocalTime(t)).
21.4.4.8 DATE.PROTOTYPE.GETMONTH ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return MonthFromTime(LocalTime(t)).
21.4.4.9 DATE.PROTOTYPE.GETSECONDS ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return SecFromTime(LocalTime(t)).
21.4.4.10 DATE.PROTOTYPE.GETTIME ( )
This method performs the following steps when called:
1. 1. 1. Return ? thisTimeValue(this value).
21.4.4.11 DATE.PROTOTYPE.GETTIMEZONEOFFSET ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return (t - LocalTime(t)) / msPerMinute.
21.4.4.12 DATE.PROTOTYPE.GETUTCDATE ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return DateFromTime(t).
21.4.4.13 DATE.PROTOTYPE.GETUTCDAY ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return WeekDay(t).
21.4.4.14 DATE.PROTOTYPE.GETUTCFULLYEAR ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return YearFromTime(t).
21.4.4.15 DATE.PROTOTYPE.GETUTCHOURS ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return HourFromTime(t).
21.4.4.16 DATE.PROTOTYPE.GETUTCMILLISECONDS ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return msFromTime(t).
21.4.4.17 DATE.PROTOTYPE.GETUTCMINUTES ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return MinFromTime(t).
21.4.4.18 DATE.PROTOTYPE.GETUTCMONTH ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return MonthFromTime(t).
21.4.4.19 DATE.PROTOTYPE.GETUTCSECONDS ( )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return SecFromTime(t).
21.4.4.20 DATE.PROTOTYPE.SETDATE ( DATE )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let dt be ? ToNumber(date).
3. 3. 3. If t is NaN, return NaN.
4. 4. 4. Set t to LocalTime(t).
5. 5. 5. Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t),
dt), TimeWithinDay(t)).
6. 6. 6. Let u be TimeClip(UTC(newDate)).
7. 7. 7. Set the [[DateValue]] internal slot of this Date object to u.
8. 8. 8. Return u.
21.4.4.21 DATE.PROTOTYPE.SETFULLYEAR ( YEAR [ , MONTH [ , DATE ] ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let y be ? ToNumber(year).
3. 3. 3. If t is NaN, set t to +0𝔽; otherwise, set t to LocalTime(t).
4. 4. 4. If month is not present, let m be MonthFromTime(t); otherwise, let m
be ? ToNumber(month).
5. 5. 5. If date is not present, let dt be DateFromTime(t); otherwise, let dt
be ? ToNumber(date).
6. 6. 6. Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
7. 7. 7. Let u be TimeClip(UTC(newDate)).
8. 8. 8. Set the [[DateValue]] internal slot of this Date object to u.
9. 9. 9. Return u.
The "length" property of this method is 3𝔽.
Note
If month is not present, this method behaves as if month was present with the
value getMonth(). If date is not present, it behaves as if date was present with
the value getDate().
21.4.4.22 DATE.PROTOTYPE.SETHOURS ( HOUR [ , MIN [ , SEC [ , MS ] ] ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let h be ? ToNumber(hour).
3. 3. 3. If min is present, let m be ? ToNumber(min).
4. 4. 4. If sec is present, let s be ? ToNumber(sec).
5. 5. 5. If ms is present, let milli be ? ToNumber(ms).
6. 6. 6. If t is NaN, return NaN.
7. 7. 7. Set t to LocalTime(t).
8. 8. 8. If min is not present, let m be MinFromTime(t).
9. 9. 9. If sec is not present, let s be SecFromTime(t).
10. 10. 10. If ms is not present, let milli be msFromTime(t).
11. 11. 11. Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
12. 12. 12. Let u be TimeClip(UTC(date)).
13. 13. 13. Set the [[DateValue]] internal slot of this Date object to u.
14. 14. 14. Return u.
The "length" property of this method is 4𝔽.
Note
If min is not present, this method behaves as if min was present with the value
getMinutes(). If sec is not present, it behaves as if sec was present with the
value getSeconds(). If ms is not present, it behaves as if ms was present with
the value getMilliseconds().
21.4.4.23 DATE.PROTOTYPE.SETMILLISECONDS ( MS )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Set ms to ? ToNumber(ms).
3. 3. 3. If t is NaN, return NaN.
4. 4. 4. Set t to LocalTime(t).
5. 5. 5. Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t),
ms).
6. 6. 6. Let u be TimeClip(UTC(MakeDate(Day(t), time))).
7. 7. 7. Set the [[DateValue]] internal slot of this Date object to u.
8. 8. 8. Return u.
21.4.4.24 DATE.PROTOTYPE.SETMINUTES ( MIN [ , SEC [ , MS ] ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let m be ? ToNumber(min).
3. 3. 3. If sec is present, let s be ? ToNumber(sec).
4. 4. 4. If ms is present, let milli be ? ToNumber(ms).
5. 5. 5. If t is NaN, return NaN.
6. 6. 6. Set t to LocalTime(t).
7. 7. 7. If sec is not present, let s be SecFromTime(t).
8. 8. 8. If ms is not present, let milli be msFromTime(t).
9. 9. 9. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
10. 10. 10. Let u be TimeClip(UTC(date)).
11. 11. 11. Set the [[DateValue]] internal slot of this Date object to u.
12. 12. 12. Return u.
The "length" property of this method is 3𝔽.
Note
If sec is not present, this method behaves as if sec was present with the value
getSeconds(). If ms is not present, this behaves as if ms was present with the
value getMilliseconds().
21.4.4.25 DATE.PROTOTYPE.SETMONTH ( MONTH [ , DATE ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let m be ? ToNumber(month).
3. 3. 3. If date is present, let dt be ? ToNumber(date).
4. 4. 4. If t is NaN, return NaN.
5. 5. 5. Set t to LocalTime(t).
6. 6. 6. If date is not present, let dt be DateFromTime(t).
7. 7. 7. Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt),
TimeWithinDay(t)).
8. 8. 8. Let u be TimeClip(UTC(newDate)).
9. 9. 9. Set the [[DateValue]] internal slot of this Date object to u.
10. 10. 10. Return u.
The "length" property of this method is 2𝔽.
Note
If date is not present, this method behaves as if date was present with the
value getDate().
21.4.4.26 DATE.PROTOTYPE.SETSECONDS ( SEC [ , MS ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let s be ? ToNumber(sec).
3. 3. 3. If ms is present, let milli be ? ToNumber(ms).
4. 4. 4. If t is NaN, return NaN.
5. 5. 5. Set t to LocalTime(t).
6. 6. 6. If ms is not present, let milli be msFromTime(t).
7. 7. 7. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t),
MinFromTime(t), s, milli)).
8. 8. 8. Let u be TimeClip(UTC(date)).
9. 9. 9. Set the [[DateValue]] internal slot of this Date object to u.
10. 10. 10. Return u.
The "length" property of this method is 2𝔽.
Note
If ms is not present, this method behaves as if ms was present with the value
getMilliseconds().
21.4.4.27 DATE.PROTOTYPE.SETTIME ( TIME )
This method performs the following steps when called:
1. 1. 1. Perform ? thisTimeValue(this value).
2. 2. 2. Let t be ? ToNumber(time).
3. 3. 3. Let v be TimeClip(t).
4. 4. 4. Set the [[DateValue]] internal slot of this Date object to v.
5. 5. 5. Return v.
21.4.4.28 DATE.PROTOTYPE.SETUTCDATE ( DATE )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let dt be ? ToNumber(date).
3. 3. 3. If t is NaN, return NaN.
4. 4. 4. Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t),
dt), TimeWithinDay(t)).
5. 5. 5. Let v be TimeClip(newDate).
6. 6. 6. Set the [[DateValue]] internal slot of this Date object to v.
7. 7. 7. Return v.
21.4.4.29 DATE.PROTOTYPE.SETUTCFULLYEAR ( YEAR [ , MONTH [ , DATE ] ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, set t to +0𝔽.
3. 3. 3. Let y be ? ToNumber(year).
4. 4. 4. If month is not present, let m be MonthFromTime(t); otherwise, let m
be ? ToNumber(month).
5. 5. 5. If date is not present, let dt be DateFromTime(t); otherwise, let dt
be ? ToNumber(date).
6. 6. 6. Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
7. 7. 7. Let v be TimeClip(newDate).
8. 8. 8. Set the [[DateValue]] internal slot of this Date object to v.
9. 9. 9. Return v.
The "length" property of this method is 3𝔽.
Note
If month is not present, this method behaves as if month was present with the
value getUTCMonth(). If date is not present, it behaves as if date was present
with the value getUTCDate().
21.4.4.30 DATE.PROTOTYPE.SETUTCHOURS ( HOUR [ , MIN [ , SEC [ , MS ] ] ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let h be ? ToNumber(hour).
3. 3. 3. If min is present, let m be ? ToNumber(min).
4. 4. 4. If sec is present, let s be ? ToNumber(sec).
5. 5. 5. If ms is present, let milli be ? ToNumber(ms).
6. 6. 6. If t is NaN, return NaN.
7. 7. 7. If min is not present, let m be MinFromTime(t).
8. 8. 8. If sec is not present, let s be SecFromTime(t).
9. 9. 9. If ms is not present, let milli be msFromTime(t).
10. 10. 10. Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
11. 11. 11. Let v be TimeClip(date).
12. 12. 12. Set the [[DateValue]] internal slot of this Date object to v.
13. 13. 13. Return v.
The "length" property of this method is 4𝔽.
Note
If min is not present, this method behaves as if min was present with the value
getUTCMinutes(). If sec is not present, it behaves as if sec was present with
the value getUTCSeconds(). If ms is not present, it behaves as if ms was present
with the value getUTCMilliseconds().
21.4.4.31 DATE.PROTOTYPE.SETUTCMILLISECONDS ( MS )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Set ms to ? ToNumber(ms).
3. 3. 3. If t is NaN, return NaN.
4. 4. 4. Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t),
ms).
5. 5. 5. Let v be TimeClip(MakeDate(Day(t), time)).
6. 6. 6. Set the [[DateValue]] internal slot of this Date object to v.
7. 7. 7. Return v.
21.4.4.32 DATE.PROTOTYPE.SETUTCMINUTES ( MIN [ , SEC [ , MS ] ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let m be ? ToNumber(min).
3. 3. 3. If sec is present, let s be ? ToNumber(sec).
4. 4. 4. If ms is present, let milli be ? ToNumber(ms).
5. 5. 5. If t is NaN, return NaN.
6. 6. 6. If sec is not present, let s be SecFromTime(t).
7. 7. 7. If ms is not present, let milli be msFromTime(t).
8. 8. 8. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
9. 9. 9. Let v be TimeClip(date).
10. 10. 10. Set the [[DateValue]] internal slot of this Date object to v.
11. 11. 11. Return v.
The "length" property of this method is 3𝔽.
Note
If sec is not present, this method behaves as if sec was present with the value
getUTCSeconds(). If ms is not present, it behaves as if ms was present with the
value return by getUTCMilliseconds().
21.4.4.33 DATE.PROTOTYPE.SETUTCMONTH ( MONTH [ , DATE ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let m be ? ToNumber(month).
3. 3. 3. If date is present, let dt be ? ToNumber(date).
4. 4. 4. If t is NaN, return NaN.
5. 5. 5. If date is not present, let dt be DateFromTime(t).
6. 6. 6. Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt),
TimeWithinDay(t)).
7. 7. 7. Let v be TimeClip(newDate).
8. 8. 8. Set the [[DateValue]] internal slot of this Date object to v.
9. 9. 9. Return v.
The "length" property of this method is 2𝔽.
Note
If date is not present, this method behaves as if date was present with the
value getUTCDate().
21.4.4.34 DATE.PROTOTYPE.SETUTCSECONDS ( SEC [ , MS ] )
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let s be ? ToNumber(sec).
3. 3. 3. If ms is present, let milli be ? ToNumber(ms).
4. 4. 4. If t is NaN, return NaN.
5. 5. 5. If ms is not present, let milli be msFromTime(t).
6. 6. 6. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t),
s, milli)).
7. 7. 7. Let v be TimeClip(date).
8. 8. 8. Set the [[DateValue]] internal slot of this Date object to v.
9. 9. 9. Return v.
The "length" property of this method is 2𝔽.
Note
If ms is not present, this method behaves as if ms was present with the value
getUTCMilliseconds().
21.4.4.35 DATE.PROTOTYPE.TODATESTRING ( )
This method performs the following steps when called:
1. 1. 1. Let O be this Date object.
2. 2. 2. Let tv be ? thisTimeValue(O).
3. 3. 3. If tv is NaN, return "Invalid Date".
4. 4. 4. Let t be LocalTime(tv).
5. 5. 5. Return DateString(t).
21.4.4.36 DATE.PROTOTYPE.TOISOSTRING ( )
If this time value is not a finite Number or if it corresponds with a year that
cannot be represented in the Date Time String Format, this method throws a
RangeError exception. Otherwise, it returns a String representation of this time
value in that format on the UTC time scale, including all format elements and
the UTC offset representation "Z".
21.4.4.37 DATE.PROTOTYPE.TOJSON ( KEY )
This method provides a String representation of a Date for use by JSON.stringify
(25.5.2).
It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let tv be ? ToPrimitive(O, number).
3. 3. 3. If tv is a Number and tv is not finite, return null.
4. 4. 4. Return ? Invoke(O, "toISOString").
Note 1
The argument is ignored.
Note 2
This method is intentionally generic; it does not require that its this value be
a Date. Therefore, it can be transferred to other kinds of objects for use as a
method. However, it does require that any such object have a toISOString method.
21.4.4.38 DATE.PROTOTYPE.TOLOCALEDATESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method returns a String value. The contents of the String are
implementation-defined, but are intended to represent the “date” portion of the
Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
21.4.4.39 DATE.PROTOTYPE.TOLOCALESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method returns a String value. The contents of the String are
implementation-defined, but are intended to represent the Date in the current
time zone in a convenient, human-readable form that corresponds to the
conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
21.4.4.40 DATE.PROTOTYPE.TOLOCALETIMESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method returns a String value. The contents of the String are
implementation-defined, but are intended to represent the “time” portion of the
Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
21.4.4.41 DATE.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Let tv be ? thisTimeValue(this value).
2. 2. 2. Return ToDateString(tv).
Note 1
For any Date d such that d.[[DateValue]] is evenly divisible by 1000, the result
of Date.parse(d.toString()) = d.valueOf(). See 21.4.3.2.
Note 2
This method is not generic; it throws a TypeError exception if its this value is
not a Date. Therefore, it cannot be transferred to other kinds of objects for
use as a method.
21.4.4.41.1 TIMESTRING ( TV )
The abstract operation TimeString takes argument tv (a Number, but not NaN) and
returns a String. It performs the following steps when called:
1. 1. 1. Let hour be ToZeroPaddedDecimalString(ℝ(HourFromTime(tv)), 2).
2. 2. 2. Let minute be ToZeroPaddedDecimalString(ℝ(MinFromTime(tv)), 2).
3. 3. 3. Let second be ToZeroPaddedDecimalString(ℝ(SecFromTime(tv)), 2).
4. 4. 4. Return the string-concatenation of hour, ":", minute, ":", second, the
code unit 0x0020 (SPACE), and "GMT".
21.4.4.41.2 DATESTRING ( TV )
The abstract operation DateString takes argument tv (a Number, but not NaN) and
returns a String. It performs the following steps when called:
1. 1. 1. Let weekday be the Name of the entry in Table 61 with the Number
WeekDay(tv).
2. 2. 2. Let month be the Name of the entry in Table 62 with the Number
MonthFromTime(tv).
3. 3. 3. Let day be ToZeroPaddedDecimalString(ℝ(DateFromTime(tv)), 2).
4. 4. 4. Let yv be YearFromTime(tv).
5. 5. 5. If yv is +0𝔽 or yv > +0𝔽, let yearSign be the empty String;
otherwise, let yearSign be "-".
6. 6. 6. Let paddedYear be ToZeroPaddedDecimalString(abs(ℝ(yv)), 4).
7. 7. 7. Return the string-concatenation of weekday, the code unit 0x0020
(SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020
(SPACE), yearSign, and paddedYear.
Table 61: Names of days of the week
Number Name +0𝔽 "Sun" 1𝔽 "Mon" 2𝔽 "Tue" 3𝔽 "Wed" 4𝔽 "Thu" 5𝔽 "Fri" 6𝔽
"Sat"
Table 62: Names of months of the year
Number Name +0𝔽 "Jan" 1𝔽 "Feb" 2𝔽 "Mar" 3𝔽 "Apr" 4𝔽 "May" 5𝔽 "Jun" 6𝔽
"Jul" 7𝔽 "Aug" 8𝔽 "Sep" 9𝔽 "Oct" 10𝔽 "Nov" 11𝔽 "Dec"
21.4.4.41.3 TIMEZONESTRING ( TV )
The abstract operation TimeZoneString takes argument tv (an integral Number) and
returns a String. It performs the following steps when called:
1. 1. 1. Let localTimeZone be DefaultTimeZone().
2. 2. 2. If IsTimeZoneOffsetString(localTimeZone) is true, then
1. a. a. Let offsetNs be ParseTimeZoneOffsetString(localTimeZone).
3. 3. 3. Else,
1. a. a. Let offsetNs be GetNamedTimeZoneOffsetNanoseconds(localTimeZone,
ℤ(ℝ(tv) × 106)).
4. 4. 4. Let offset be 𝔽(truncate(offsetNs / 106)).
5. 5. 5. If offset is +0𝔽 or offset > +0𝔽, then
1. a. a. Let offsetSign be "+".
2. b. b. Let absOffset be offset.
6. 6. 6. Else,
1. a. a. Let offsetSign be "-".
2. b. b. Let absOffset be -offset.
7. 7. 7. Let offsetMin be ToZeroPaddedDecimalString(ℝ(MinFromTime(absOffset)),
2).
8. 8. 8. Let offsetHour be
ToZeroPaddedDecimalString(ℝ(HourFromTime(absOffset)), 2).
9. 9. 9. Let tzName be an implementation-defined string that is either the
empty String or the string-concatenation of the code unit 0x0020 (SPACE),
the code unit 0x0028 (LEFT PARENTHESIS), an implementation-defined timezone
name, and the code unit 0x0029 (RIGHT PARENTHESIS).
10. 10. 10. Return the string-concatenation of offsetSign, offsetHour,
offsetMin, and tzName.
21.4.4.41.4 TODATESTRING ( TV )
The abstract operation ToDateString takes argument tv (an integral Number or
NaN) and returns a String. It performs the following steps when called:
1. 1. 1. If tv is NaN, return "Invalid Date".
2. 2. 2. Let t be LocalTime(tv).
3. 3. 3. Return the string-concatenation of DateString(t), the code unit 0x0020
(SPACE), TimeString(t), and TimeZoneString(tv).
21.4.4.42 DATE.PROTOTYPE.TOTIMESTRING ( )
This method performs the following steps when called:
1. 1. 1. Let O be this Date object.
2. 2. 2. Let tv be ? thisTimeValue(O).
3. 3. 3. If tv is NaN, return "Invalid Date".
4. 4. 4. Let t be LocalTime(tv).
5. 5. 5. Return the string-concatenation of TimeString(t) and
TimeZoneString(tv).
21.4.4.43 DATE.PROTOTYPE.TOUTCSTRING ( )
This method returns a String value representing the instance in time
corresponding to this time value. The format of the String is based upon
"HTTP-date" from RFC 7231, generalized to support the full range of times
supported by ECMAScript Dates.
It performs the following steps when called:
1. 1. 1. Let O be this Date object.
2. 2. 2. Let tv be ? thisTimeValue(O).
3. 3. 3. If tv is NaN, return "Invalid Date".
4. 4. 4. Let weekday be the Name of the entry in Table 61 with the Number
WeekDay(tv).
5. 5. 5. Let month be the Name of the entry in Table 62 with the Number
MonthFromTime(tv).
6. 6. 6. Let day be ToZeroPaddedDecimalString(ℝ(DateFromTime(tv)), 2).
7. 7. 7. Let yv be YearFromTime(tv).
8. 8. 8. If yv is +0𝔽 or yv > +0𝔽, let yearSign be the empty String;
otherwise, let yearSign be "-".
9. 9. 9. Let paddedYear be ToZeroPaddedDecimalString(abs(ℝ(yv)), 4).
10. 10. 10. Return the string-concatenation of weekday, ",", the code unit
0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit
0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), and
TimeString(tv).
21.4.4.44 DATE.PROTOTYPE.VALUEOF ( )
This method performs the following steps when called:
1. 1. 1. Return ? thisTimeValue(this value).
21.4.4.45 DATE.PROTOTYPE [ @@TOPRIMITIVE ] ( HINT )
This method is called by ECMAScript language operators to convert a Date to a
primitive value. The allowed values for hint are "default", "number", and
"string". Dates are unique among built-in ECMAScript object in that they treat
"default" as being equivalent to "string", All other built-in ECMAScript objects
treat "default" as being equivalent to "number".
It performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. If O is not an Object, throw a TypeError exception.
3. 3. 3. If hint is either "string" or "default", then
1. a. a. Let tryFirst be string.
4. 4. 4. Else if hint is "number", then
1. a. a. Let tryFirst be number.
5. 5. 5. Else, throw a TypeError exception.
6. 6. 6. Return ? OrdinaryToPrimitive(O, tryFirst).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
The value of the "name" property of this method is "[Symbol.toPrimitive]".
21.4.5 PROPERTIES OF DATE INSTANCES
Date instances are ordinary objects that inherit properties from the Date
prototype object. Date instances also have a [[DateValue]] internal slot. The
[[DateValue]] internal slot is the time value represented by this Date.
22 TEXT PROCESSING
22.1 STRING OBJECTS
22.1.1 THE STRING CONSTRUCTOR
The String constructor:
* is %String%.
* is the initial value of the "String" property of the global object.
* creates and initializes a new String object when called as a constructor.
* performs a type conversion when called as a function rather than as a
constructor.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified String behaviour must
include a super call to the String constructor to create and initialize the
subclass instance with a [[StringData]] internal slot.
22.1.1.1 STRING ( VALUE )
This function performs the following steps when called:
1. 1. 1. If value is not present, let s be the empty String.
2. 2. 2. Else,
1. a. a. If NewTarget is undefined and value is a Symbol, return
SymbolDescriptiveString(value).
2. b. b. Let s be ? ToString(value).
3. 3. 3. If NewTarget is undefined, return s.
4. 4. 4. Return StringCreate(s, ? GetPrototypeFromConstructor(NewTarget,
"%String.prototype%")).
22.1.2 PROPERTIES OF THE STRING CONSTRUCTOR
The String constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
22.1.2.1 STRING.FROMCHARCODE ( ...CODEUNITS )
This function may be called with any number of arguments which form the rest
parameter codeUnits.
It performs the following steps when called:
1. 1. 1. Let result be the empty String.
2. 2. 2. For each element next of codeUnits, do
1. a. a. Let nextCU be the code unit whose numeric value is ℝ(?
ToUint16(next)).
2. b. b. Set result to the string-concatenation of result and nextCU.
3. 3. 3. Return result.
The "length" property of this function is 1𝔽.
22.1.2.2 STRING.FROMCODEPOINT ( ...CODEPOINTS )
This function may be called with any number of arguments which form the rest
parameter codePoints.
It performs the following steps when called:
1. 1. 1. Let result be the empty String.
2. 2. 2. For each element next of codePoints, do
1. a. a. Let nextCP be ? ToNumber(next).
2. b. b. If IsIntegralNumber(nextCP) is false, throw a RangeError exception.
3. c. c. If ℝ(nextCP) < 0 or ℝ(nextCP) > 0x10FFFF, throw a RangeError
exception.
4. d. d. Set result to the string-concatenation of result and
UTF16EncodeCodePoint(ℝ(nextCP)).
3. 3. 3. Assert: If codePoints is empty, then result is the empty String.
4. 4. 4. Return result.
The "length" property of this function is 1𝔽.
22.1.2.3 STRING.PROTOTYPE
The initial value of String.prototype is the String prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
22.1.2.4 STRING.RAW ( TEMPLATE, ...SUBSTITUTIONS )
This function may be called with a variable number of arguments. The first
argument is template and the remainder of the arguments form the List
substitutions.
It performs the following steps when called:
1. 1. 1. Let substitutionCount be the number of elements in substitutions.
2. 2. 2. Let cooked be ? ToObject(template).
3. 3. 3. Let literals be ? ToObject(? Get(cooked, "raw")).
4. 4. 4. Let literalCount be ? LengthOfArrayLike(literals).
5. 5. 5. If literalCount ≤ 0, return the empty String.
6. 6. 6. Let R be the empty String.
7. 7. 7. Let nextIndex be 0.
8. 8. 8. Repeat,
1. a. a. Let nextLiteralVal be ? Get(literals, ! ToString(𝔽(nextIndex))).
2. b. b. Let nextLiteral be ? ToString(nextLiteralVal).
3. c. c. Set R to the string-concatenation of R and nextLiteral.
4. d. d. If nextIndex + 1 = literalCount, return R.
5. e. e. If nextIndex < substitutionCount, then
1. i. i. Let nextSubVal be substitutions[nextIndex].
2. ii. ii. Let nextSub be ? ToString(nextSubVal).
3. iii. iii. Set R to the string-concatenation of R and nextSub.
6. f. f. Set nextIndex to nextIndex + 1.
Note
This function is intended for use as a tag function of a Tagged Template
(13.3.11). When called as such, the first argument will be a well formed
template object and the rest parameter will contain the substitution values.
22.1.3 PROPERTIES OF THE STRING PROTOTYPE OBJECT
The String prototype object:
* is %String.prototype%.
* is a String exotic object and has the internal methods specified for such
objects.
* has a [[StringData]] internal slot whose value is the empty String.
* has a "length" property whose initial value is +0𝔽 and whose attributes are
{ [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
Unless explicitly stated otherwise, the methods of the String prototype object
defined below are not generic and the this value passed to them must be either a
String value or an object that has a [[StringData]] internal slot that has been
initialized to a String value.
The abstract operation thisStringValue takes argument value. It performs the
following steps when called:
1. 1. 1. If value is a String, return value.
2. 2. 2. If value is an Object and value has a [[StringData]] internal slot,
then
1. a. a. Let s be value.[[StringData]].
2. b. b. Assert: s is a String.
3. c. c. Return s.
3. 3. 3. Throw a TypeError exception.
22.1.3.1 STRING.PROTOTYPE.AT ( INDEX )
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let len be the length of S.
4. 4. 4. Let relativeIndex be ? ToIntegerOrInfinity(index).
5. 5. 5. If relativeIndex ≥ 0, then
1. a. a. Let k be relativeIndex.
6. 6. 6. Else,
1. a. a. Let k be len + relativeIndex.
7. 7. 7. If k < 0 or k ≥ len, return undefined.
8. 8. 8. Return the substring of S from k to k + 1.
22.1.3.2 STRING.PROTOTYPE.CHARAT ( POS )
Note 1
This method returns a single element String containing the code unit at index
pos within the String value resulting from converting this object to a String.
If there is no element at that index, the result is the empty String. The result
is a String value, not a String object.
If pos is an integral Number, then the result of x.charAt(pos) is equivalent to
the result of x.substring(pos, pos + 1).
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let position be ? ToIntegerOrInfinity(pos).
4. 4. 4. Let size be the length of S.
5. 5. 5. If position < 0 or position ≥ size, return the empty String.
6. 6. 6. Return the substring of S from position to position + 1.
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.3 STRING.PROTOTYPE.CHARCODEAT ( POS )
Note 1
This method returns a Number (a non-negative integral Number less than 216) that
is the numeric value of the code unit at index pos within the String resulting
from converting this object to a String. If there is no element at that index,
the result is NaN.
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let position be ? ToIntegerOrInfinity(pos).
4. 4. 4. Let size be the length of S.
5. 5. 5. If position < 0 or position ≥ size, return NaN.
6. 6. 6. Return the Number value for the numeric value of the code unit at
index position within the String S.
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore it can be transferred to other kinds of objects for
use as a method.
22.1.3.4 STRING.PROTOTYPE.CODEPOINTAT ( POS )
Note 1
This method returns a non-negative integral Number less than or equal to
0x10FFFF𝔽 that is the numeric value of the UTF-16 encoded code point (6.1.4)
starting at the string element at index pos within the String resulting from
converting this object to a String. If there is no element at that index, the
result is undefined. If a valid UTF-16 surrogate pair does not begin at pos, the
result is the code unit at pos.
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let position be ? ToIntegerOrInfinity(pos).
4. 4. 4. Let size be the length of S.
5. 5. 5. If position < 0 or position ≥ size, return undefined.
6. 6. 6. Let cp be CodePointAt(S, position).
7. 7. 7. Return 𝔽(cp.[[CodePoint]]).
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore it can be transferred to other kinds of objects for
use as a method.
22.1.3.5 STRING.PROTOTYPE.CONCAT ( ...ARGS )
Note 1
When this method is called it returns the String value consisting of the code
units of the this value (converted to a String) followed by the code units of
each of the arguments converted to a String. The result is a String value, not a
String object.
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let R be S.
4. 4. 4. For each element next of args, do
1. a. a. Let nextString be ? ToString(next).
2. b. b. Set R to the string-concatenation of R and nextString.
5. 5. 5. Return R.
The "length" property of this method is 1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore it can be transferred to other kinds of objects for
use as a method.
22.1.3.6 STRING.PROTOTYPE.CONSTRUCTOR
The initial value of String.prototype.constructor is %String%.
22.1.3.7 STRING.PROTOTYPE.ENDSWITH ( SEARCHSTRING [ , ENDPOSITION ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let isRegExp be ? IsRegExp(searchString).
4. 4. 4. If isRegExp is true, throw a TypeError exception.
5. 5. 5. Let searchStr be ? ToString(searchString).
6. 6. 6. Let len be the length of S.
7. 7. 7. If endPosition is undefined, let pos be len; else let pos be
? ToIntegerOrInfinity(endPosition).
8. 8. 8. Let end be the result of clamping pos between 0 and len.
9. 9. 9. Let searchLength be the length of searchStr.
10. 10. 10. If searchLength = 0, return true.
11. 11. 11. Let start be end - searchLength.
12. 12. 12. If start < 0, return false.
13. 13. 13. Let substring be the substring of S from start to end.
14. 14. 14. If substring is searchStr, return true.
15. 15. 15. Return false.
Note 1
This method returns true if the sequence of code units of searchString converted
to a String is the same as the corresponding code units of this object
(converted to a String) starting at endPosition - length(this). Otherwise it
returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to
allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.8 STRING.PROTOTYPE.INCLUDES ( SEARCHSTRING [ , POSITION ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let isRegExp be ? IsRegExp(searchString).
4. 4. 4. If isRegExp is true, throw a TypeError exception.
5. 5. 5. Let searchStr be ? ToString(searchString).
6. 6. 6. Let pos be ? ToIntegerOrInfinity(position).
7. 7. 7. Assert: If position is undefined, then pos is 0.
8. 8. 8. Let len be the length of S.
9. 9. 9. Let start be the result of clamping pos between 0 and len.
10. 10. 10. Let index be StringIndexOf(S, searchStr, start).
11. 11. 11. If index ≠ -1, return true.
12. 12. 12. Return false.
Note 1
If searchString appears as a substring of the result of converting this object
to a String, at one or more indices that are greater than or equal to position,
this function returns true; otherwise, it returns false. If position is
undefined, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to
allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.9 STRING.PROTOTYPE.INDEXOF ( SEARCHSTRING [ , POSITION ] )
Note 1
If searchString appears as a substring of the result of converting this object
to a String, at one or more indices that are greater than or equal to position,
then the smallest such index is returned; otherwise, -1𝔽 is returned. If
position is undefined, +0𝔽 is assumed, so as to search all of the String.
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let searchStr be ? ToString(searchString).
4. 4. 4. Let pos be ? ToIntegerOrInfinity(position).
5. 5. 5. Assert: If position is undefined, then pos is 0.
6. 6. 6. Let len be the length of S.
7. 7. 7. Let start be the result of clamping pos between 0 and len.
8. 8. 8. Return 𝔽(StringIndexOf(S, searchStr, start)).
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.10 STRING.PROTOTYPE.LASTINDEXOF ( SEARCHSTRING [ , POSITION ] )
Note 1
If searchString appears as a substring of the result of converting this object
to a String at one or more indices that are smaller than or equal to position,
then the greatest such index is returned; otherwise, -1𝔽 is returned. If
position is undefined, the length of the String value is assumed, so as to
search all of the String.
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let searchStr be ? ToString(searchString).
4. 4. 4. Let numPos be ? ToNumber(position).
5. 5. 5. Assert: If position is undefined, then numPos is NaN.
6. 6. 6. If numPos is NaN, let pos be +∞; otherwise, let pos be
! ToIntegerOrInfinity(numPos).
7. 7. 7. Let len be the length of S.
8. 8. 8. Let searchLen be the length of searchStr.
9. 9. 9. Let start be the result of clamping pos between 0 and len -
searchLen.
10. 10. 10. If searchStr is the empty String, return 𝔽(start).
11. 11. 11. For each integer i such that 0 ≤ i ≤ start, in descending order, do
1. a. a. Let candidate be the substring of S from i to i + searchLen.
2. b. b. If candidate is searchStr, return 𝔽(i).
12. 12. 12. Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.11 STRING.PROTOTYPE.LOCALECOMPARE ( THAT [ , RESERVED1 [ , RESERVED2 ] ]
)
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method returns a Number other than NaN representing the result of an
implementation-defined locale-sensitive String comparison of the this value
(converted to a String S) with that (converted to a String thatValue). The
result is intended to correspond with a sort order of String values according to
conventions of the host environment's current locale, and will be negative when
S is ordered before thatValue, positive when S is ordered after thatValue, and
zero in all other cases (representing no relative ordering between S and
thatValue).
Before performing the comparisons, this method performs the following steps to
prepare the Strings:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let thatValue be ? ToString(that).
The meaning of the optional second and third parameters to this method are
defined in the ECMA-402 specification; implementations that do not include
ECMA-402 support must not assign any other interpretation to those parameter
positions.
The actual return values are implementation-defined to permit encoding
additional information in them, but this method, when considered as a method of
two arguments, is required to be a consistent comparator defining a total
ordering on the set of all Strings. This method is also required to recognize
and honour canonical equivalence according to the Unicode Standard, including
returning +0𝔽 when comparing distinguishable Strings that are canonically
equivalent.
Note 1
This method itself is not directly suitable as an argument to
Array.prototype.sort because the latter requires a function of two arguments.
Note 2
This method may rely on whatever language- and/or locale-sensitive comparison
functionality is available to the ECMAScript environment from the host
environment, and is intended to compare according to the conventions of the host
environment's current locale. However, regardless of comparison capabilities,
this method must recognize and honour canonical equivalence according to the
Unicode Standard—for example, the following comparisons must all return +0𝔽:
// Å ANGSTROM SIGN vs.
// Å LATIN CAPITAL LETTER A + COMBINING RING ABOVE
"\u212B".localeCompare("A\u030A")
// Ω OHM SIGN vs.
// Ω GREEK CAPITAL LETTER OMEGA
"\u2126".localeCompare("\u03A9")
// ṩ LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE vs.
// ṩ LATIN SMALL LETTER S + COMBINING DOT ABOVE + COMBINING DOT BELOW
"\u1E69".localeCompare("s\u0307\u0323")
// ḍ̇ LATIN SMALL LETTER D WITH DOT ABOVE + COMBINING DOT BELOW vs.
// ḍ̇ LATIN SMALL LETTER D WITH DOT BELOW + COMBINING DOT ABOVE
"\u1E0B\u0323".localeCompare("\u1E0D\u0307")
// 가 HANGUL CHOSEONG KIYEOK + HANGUL JUNGSEONG A vs.
// 가 HANGUL SYLLABLE GA
"\u1100\u1161".localeCompare("\uAC00")
For a definition and discussion of canonical equivalence see the Unicode
Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode
Normalization Forms and Unicode Technical Note #5, Canonical Equivalence in
Applications. Also see Unicode Technical Standard #10, Unicode Collation
Algorithm.
It is recommended that this method should not honour Unicode compatibility
equivalents or compatibility decompositions as defined in the Unicode Standard,
chapter 3, section 3.7.
Note 3
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.12 STRING.PROTOTYPE.MATCH ( REGEXP )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If regexp is neither undefined nor null, then
1. a. a. Let matcher be ? GetMethod(regexp, @@match).
2. b. b. If matcher is not undefined, then
1. i. i. Return ? Call(matcher, regexp, « O »).
3. 3. 3. Let S be ? ToString(O).
4. 4. 4. Let rx be ? RegExpCreate(regexp, undefined).
5. 5. 5. Return ? Invoke(rx, @@match, « S »).
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.13 STRING.PROTOTYPE.MATCHALL ( REGEXP )
This method performs a regular expression match of the String representing the
this value against regexp and returns an iterator. Each iteration result's value
is an Array containing the results of the match, or null if the String did not
match.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If regexp is neither undefined nor null, then
1. a. a. Let isRegExp be ? IsRegExp(regexp).
2. b. b. If isRegExp is true, then
1. i. i. Let flags be ? Get(regexp, "flags").
2. ii. ii. Perform ? RequireObjectCoercible(flags).
3. iii. iii. If ? ToString(flags) does not contain "g", throw a TypeError
exception.
3. c. c. Let matcher be ? GetMethod(regexp, @@matchAll).
4. d. d. If matcher is not undefined, then
1. i. i. Return ? Call(matcher, regexp, « O »).
3. 3. 3. Let S be ? ToString(O).
4. 4. 4. Let rx be ? RegExpCreate(regexp, "g").
5. 5. 5. Return ? Invoke(rx, @@matchAll, « S »).
Note 1
This method is intentionally generic, it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
Note 2
Similarly to String.prototype.split, String.prototype.matchAll is designed to
typically act without mutating its inputs.
22.1.3.14 STRING.PROTOTYPE.NORMALIZE ( [ FORM ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. If form is undefined, let f be "NFC".
4. 4. 4. Else, let f be ? ToString(form).
5. 5. 5. If f is not one of "NFC", "NFD", "NFKC", or "NFKD", throw a RangeError
exception.
6. 6. 6. Let ns be the String value that is the result of normalizing S into
the normalization form named by f as specified in
https://unicode.org/reports/tr15/.
7. 7. 7. Return ns.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore it can be transferred to other kinds of objects for
use as a method.
22.1.3.15 STRING.PROTOTYPE.PADEND ( MAXLENGTH [ , FILLSTRING ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Return ? StringPad(O, maxLength, fillString, end).
22.1.3.16 STRING.PROTOTYPE.PADSTART ( MAXLENGTH [ , FILLSTRING ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Return ? StringPad(O, maxLength, fillString, start).
22.1.3.16.1 STRINGPAD ( O, MAXLENGTH, FILLSTRING, PLACEMENT )
The abstract operation StringPad takes arguments O (an ECMAScript language
value), maxLength (an ECMAScript language value), fillString (an ECMAScript
language value), and placement (start or end) and returns either a normal
completion containing a String or a throw completion. It performs the following
steps when called:
1. 1. 1. Let S be ? ToString(O).
2. 2. 2. Let intMaxLength be ℝ(? ToLength(maxLength)).
3. 3. 3. Let stringLength be the length of S.
4. 4. 4. If intMaxLength ≤ stringLength, return S.
5. 5. 5. If fillString is undefined, let filler be the String value consisting
solely of the code unit 0x0020 (SPACE).
6. 6. 6. Else, let filler be ? ToString(fillString).
7. 7. 7. If filler is the empty String, return S.
8. 8. 8. Let fillLen be intMaxLength - stringLength.
9. 9. 9. Let truncatedStringFiller be the String value consisting of repeated
concatenations of filler truncated to length fillLen.
10. 10. 10. If placement is start, return the string-concatenation of
truncatedStringFiller and S.
11. 11. 11. Else, return the string-concatenation of S and
truncatedStringFiller.
Note 1
The argument maxLength will be clamped such that it can be no smaller than the
length of S.
Note 2
The argument fillString defaults to " " (the String value consisting of the code
unit 0x0020 SPACE).
22.1.3.16.2 TOZEROPADDEDDECIMALSTRING ( N, MINLENGTH )
The abstract operation ToZeroPaddedDecimalString takes arguments n (a
non-negative integer) and minLength (a non-negative integer) and returns a
String. It performs the following steps when called:
1. 1. 1. Let S be the String representation of n, formatted as a decimal
number.
2. 2. 2. Return ! StringPad(S, 𝔽(minLength), "0", start).
22.1.3.17 STRING.PROTOTYPE.REPEAT ( COUNT )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let n be ? ToIntegerOrInfinity(count).
4. 4. 4. If n < 0 or n = +∞, throw a RangeError exception.
5. 5. 5. If n = 0, return the empty String.
6. 6. 6. Return the String value that is made from n copies of S appended
together.
Note 1
This method creates the String value consisting of the code units of the this
value (converted to String) repeated count times.
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.18 STRING.PROTOTYPE.REPLACE ( SEARCHVALUE, REPLACEVALUE )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If searchValue is neither undefined nor null, then
1. a. a. Let replacer be ? GetMethod(searchValue, @@replace).
2. b. b. If replacer is not undefined, then
1. i. i. Return ? Call(replacer, searchValue, « O, replaceValue »).
3. 3. 3. Let string be ? ToString(O).
4. 4. 4. Let searchString be ? ToString(searchValue).
5. 5. 5. Let functionalReplace be IsCallable(replaceValue).
6. 6. 6. If functionalReplace is false, then
1. a. a. Set replaceValue to ? ToString(replaceValue).
7. 7. 7. Let searchLength be the length of searchString.
8. 8. 8. Let position be StringIndexOf(string, searchString, 0).
9. 9. 9. If position = -1, return string.
10. 10. 10. Let preceding be the substring of string from 0 to position.
11. 11. 11. Let following be the substring of string from position +
searchLength.
12. 12. 12. If functionalReplace is true, then
1. a. a. Let replacement be ? ToString(? Call(replaceValue, undefined, «
searchString, 𝔽(position), string »)).
13. 13. 13. Else,
1. a. a. Assert: replaceValue is a String.
2. b. b. Let captures be a new empty List.
3. c. c. Let replacement be ! GetSubstitution(searchString, string,
position, captures, undefined, replaceValue).
14. 14. 14. Return the string-concatenation of preceding, replacement, and
following.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.18.1 GETSUBSTITUTION ( MATCHED, STR, POSITION, CAPTURES, NAMEDCAPTURES,
REPLACEMENTTEMPLATE )
The abstract operation GetSubstitution takes arguments matched (a String), str
(a String), position (a non-negative integer), captures (a possibly empty List,
each of whose elements is a String or undefined), namedCaptures (an Object or
undefined), and replacementTemplate (a String) and returns either a normal
completion containing a String or a throw completion. For the purposes of this
abstract operation, a decimal digit is a code unit in the inclusive interval
from 0x0030 (DIGIT ZERO) to 0x0039 (DIGIT NINE). It performs the following steps
when called:
1. 1. 1. Let stringLength be the length of str.
2. 2. 2. Assert: position ≤ stringLength.
3. 3. 3. Let result be the empty String.
4. 4. 4. Let templateRemainder be replacementTemplate.
5. 5. 5. Repeat, while templateRemainder is not the empty String,
1. a. a. NOTE: The following steps isolate ref (a prefix of
templateRemainder), determine refReplacement (its replacement), and then
append that replacement to result.
2. b. b. If templateRemainder starts with "$$", then
1. i. i. Let ref be "$$".
2. ii. ii. Let refReplacement be "$".
3. c. c. Else if templateRemainder starts with "$`", then
1. i. i. Let ref be "$`".
2. ii. ii. Let refReplacement be the substring of str from 0 to
position.
4. d. d. Else if templateRemainder starts with "$&", then
1. i. i. Let ref be "$&".
2. ii. ii. Let refReplacement be matched.
5. e. e. Else if templateRemainder starts with "$'" (0x0024 (DOLLAR SIGN)
followed by 0x0027 (APOSTROPHE)), then
1. i. i. Let ref be "$'".
2. ii. ii. Let matchLength be the length of matched.
3. iii. iii. Let tailPos be position + matchLength.
4. iv. iv. Let refReplacement be the substring of str from min(tailPos,
stringLength).
5. v. v. NOTE: tailPos can exceed stringLength only if this abstract
operation was invoked by a call to the intrinsic @@replace method of
%RegExp.prototype% on an object whose "exec" property is not the
intrinsic %RegExp.prototype.exec%.
6. f. f. Else if templateRemainder starts with "$" followed by 1 or more
decimal digits, then
1. i. i. If templateRemainder starts with "$" followed by 2 or more
decimal digits, let digitCount be 2. Otherwise, let digitCount be 1.
2. ii. ii. Let ref be the substring of templateRemainder from 0 to 1 +
digitCount.
3. iii. iii. Let digits be the substring of templateRemainder from 1 to
1 + digitCount.
4. iv. iv. Let index be ℝ(StringToNumber(digits)).
5. v. v. Assert: 0 ≤ index ≤ 99.
6. vi. vi. Let captureLen be the number of elements in captures.
7. vii. vii. If 1 ≤ index ≤ captureLen, then
1. 1. 1. Let capture be captures[index - 1].
2. 2. 2. If capture is undefined, then
1. a. a. Let refReplacement be the empty String.
3. 3. 3. Else,
1. a. a. Let refReplacement be capture.
8. viii. viii. Else,
1. 1. 1. Let refReplacement be ref.
7. g. g. Else if templateRemainder starts with "$<", then
1. i. i. Let gtPos be StringIndexOf(templateRemainder, ">", 0).
2. ii. ii. If gtPos = -1 or namedCaptures is undefined, then
1. 1. 1. Let ref be "$<".
2. 2. 2. Let refReplacement be ref.
3. iii. iii. Else,
1. 1. 1. Let ref be the substring of templateRemainder from 0 to
gtPos + 1.
2. 2. 2. Let groupName be the substring of templateRemainder from 2
to gtPos.
3. 3. 3. Assert: namedCaptures is an Object.
4. 4. 4. Let capture be ? Get(namedCaptures, groupName).
5. 5. 5. If capture is undefined, then
1. a. a. Let refReplacement be the empty String.
6. 6. 6. Else,
1. a. a. Let refReplacement be ? ToString(capture).
8. h. h. Else,
1. i. i. Let ref be the substring of templateRemainder from 0 to 1.
2. ii. ii. Let refReplacement be ref.
9. i. i. Let refLength be the length of ref.
10. j. j. Set templateRemainder to the substring of templateRemainder from
refLength.
11. k. k. Set result to the string-concatenation of result and
refReplacement.
6. 6. 6. Return result.
22.1.3.19 STRING.PROTOTYPE.REPLACEALL ( SEARCHVALUE, REPLACEVALUE )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If searchValue is neither undefined nor null, then
1. a. a. Let isRegExp be ? IsRegExp(searchValue).
2. b. b. If isRegExp is true, then
1. i. i. Let flags be ? Get(searchValue, "flags").
2. ii. ii. Perform ? RequireObjectCoercible(flags).
3. iii. iii. If ? ToString(flags) does not contain "g", throw a
TypeError exception.
3. c. c. Let replacer be ? GetMethod(searchValue, @@replace).
4. d. d. If replacer is not undefined, then
1. i. i. Return ? Call(replacer, searchValue, « O, replaceValue »).
3. 3. 3. Let string be ? ToString(O).
4. 4. 4. Let searchString be ? ToString(searchValue).
5. 5. 5. Let functionalReplace be IsCallable(replaceValue).
6. 6. 6. If functionalReplace is false, then
1. a. a. Set replaceValue to ? ToString(replaceValue).
7. 7. 7. Let searchLength be the length of searchString.
8. 8. 8. Let advanceBy be max(1, searchLength).
9. 9. 9. Let matchPositions be a new empty List.
10. 10. 10. Let position be StringIndexOf(string, searchString, 0).
11. 11. 11. Repeat, while position ≠ -1,
1. a. a. Append position to matchPositions.
2. b. b. Set position to StringIndexOf(string, searchString, position +
advanceBy).
12. 12. 12. Let endOfLastMatch be 0.
13. 13. 13. Let result be the empty String.
14. 14. 14. For each element p of matchPositions, do
1. a. a. Let preserved be the substring of string from endOfLastMatch to p.
2. b. b. If functionalReplace is true, then
1. i. i. Let replacement be ? ToString(? Call(replaceValue, undefined, «
searchString, 𝔽(p), string »)).
3. c. c. Else,
1. i. i. Assert: replaceValue is a String.
2. ii. ii. Let captures be a new empty List.
3. iii. iii. Let replacement be ! GetSubstitution(searchString, string,
p, captures, undefined, replaceValue).
4. d. d. Set result to the string-concatenation of result, preserved, and
replacement.
5. e. e. Set endOfLastMatch to p + searchLength.
15. 15. 15. If endOfLastMatch < the length of string, then
1. a. a. Set result to the string-concatenation of result and the substring
of string from endOfLastMatch.
16. 16. 16. Return result.
22.1.3.20 STRING.PROTOTYPE.SEARCH ( REGEXP )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If regexp is neither undefined nor null, then
1. a. a. Let searcher be ? GetMethod(regexp, @@search).
2. b. b. If searcher is not undefined, then
1. i. i. Return ? Call(searcher, regexp, « O »).
3. 3. 3. Let string be ? ToString(O).
4. 4. 4. Let rx be ? RegExpCreate(regexp, undefined).
5. 5. 5. Return ? Invoke(rx, @@search, « string »).
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.21 STRING.PROTOTYPE.SLICE ( START, END )
This method returns a substring of the result of converting this object to a
String, starting from index start and running to, but not including, index end
(or through the end of the String if end is undefined). If start is negative, it
is treated as sourceLength + start where sourceLength is the length of the
String. If end is negative, it is treated as sourceLength + end where
sourceLength is the length of the String. The result is a String value, not a
String object.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let len be the length of S.
4. 4. 4. Let intStart be ? ToIntegerOrInfinity(start).
5. 5. 5. If intStart = -∞, let from be 0.
6. 6. 6. Else if intStart < 0, let from be max(len + intStart, 0).
7. 7. 7. Else, let from be min(intStart, len).
8. 8. 8. If end is undefined, let intEnd be len; else let intEnd be
? ToIntegerOrInfinity(end).
9. 9. 9. If intEnd = -∞, let to be 0.
10. 10. 10. Else if intEnd < 0, let to be max(len + intEnd, 0).
11. 11. 11. Else, let to be min(intEnd, len).
12. 12. 12. If from ≥ to, return the empty String.
13. 13. 13. Return the substring of S from from to to.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore it can be transferred to other kinds of objects for
use as a method.
22.1.3.22 STRING.PROTOTYPE.SPLIT ( SEPARATOR, LIMIT )
This method returns an Array into which substrings of the result of converting
this object to a String have been stored. The substrings are determined by
searching from left to right for occurrences of separator; these occurrences are
not part of any String in the returned array, but serve to divide up the String
value. The value of separator may be a String of any length or it may be an
object, such as a RegExp, that has a @@split method.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. If separator is neither undefined nor null, then
1. a. a. Let splitter be ? GetMethod(separator, @@split).
2. b. b. If splitter is not undefined, then
1. i. i. Return ? Call(splitter, separator, « O, limit »).
3. 3. 3. Let S be ? ToString(O).
4. 4. 4. If limit is undefined, let lim be 232 - 1; else let lim be ℝ(?
ToUint32(limit)).
5. 5. 5. Let R be ? ToString(separator).
6. 6. 6. If lim = 0, then
1. a. a. Return CreateArrayFromList(« »).
7. 7. 7. If separator is undefined, then
1. a. a. Return CreateArrayFromList(« S »).
8. 8. 8. Let separatorLength be the length of R.
9. 9. 9. If separatorLength = 0, then
1. a. a. Let head be the substring of S from 0 to lim.
2. b. b. Let codeUnits be a List consisting of the sequence of code units
that are the elements of head.
3. c. c. Return CreateArrayFromList(codeUnits).
10. 10. 10. If S is the empty String, return CreateArrayFromList(« S »).
11. 11. 11. Let substrings be a new empty List.
12. 12. 12. Let i be 0.
13. 13. 13. Let j be StringIndexOf(S, R, 0).
14. 14. 14. Repeat, while j ≠ -1,
1. a. a. Let T be the substring of S from i to j.
2. b. b. Append T to substrings.
3. c. c. If the number of elements in substrings is lim, return
CreateArrayFromList(substrings).
4. d. d. Set i to j + separatorLength.
5. e. e. Set j to StringIndexOf(S, R, i).
15. 15. 15. Let T be the substring of S from i.
16. 16. 16. Append T to substrings.
17. 17. 17. Return CreateArrayFromList(substrings).
Note 1
The value of separator may be an empty String. In this case, separator does not
match the empty substring at the beginning or end of the input String, nor does
it match the empty substring at the end of the previous separator match. If
separator is the empty String, the String is split up into individual code unit
elements; the length of the result array equals the length of the String, and
each substring contains one code unit.
If the this value is (or converts to) the empty String, the result depends on
whether separator can match the empty String. If it can, the result array
contains no elements. Otherwise, the result array contains one element, which is
the empty String.
If separator is undefined, then the result array contains just one String, which
is the this value (converted to a String). If limit is not undefined, then the
output array is truncated so that it contains no more than limit elements.
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.23 STRING.PROTOTYPE.STARTSWITH ( SEARCHSTRING [ , POSITION ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let isRegExp be ? IsRegExp(searchString).
4. 4. 4. If isRegExp is true, throw a TypeError exception.
5. 5. 5. Let searchStr be ? ToString(searchString).
6. 6. 6. Let len be the length of S.
7. 7. 7. If position is undefined, let pos be 0; else let pos be
? ToIntegerOrInfinity(position).
8. 8. 8. Let start be the result of clamping pos between 0 and len.
9. 9. 9. Let searchLength be the length of searchStr.
10. 10. 10. If searchLength = 0, return true.
11. 11. 11. Let end be start + searchLength.
12. 12. 12. If end > len, return false.
13. 13. 13. Let substring be the substring of S from start to end.
14. 14. 14. If substring is searchStr, return true.
15. 15. 15. Return false.
Note 1
This method returns true if the sequence of code units of searchString converted
to a String is the same as the corresponding code units of this object
(converted to a String) starting at index position. Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to
allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.24 STRING.PROTOTYPE.SUBSTRING ( START, END )
This method returns a substring of the result of converting this object to a
String, starting from index start and running to, but not including, index end
of the String (or through the end of the String if end is undefined). The result
is a String value, not a String object.
If either argument is NaN or negative, it is replaced with zero; if either
argument is strictly greater than the length of the String, it is replaced with
the length of the String.
If start is strictly greater than end, they are swapped.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let len be the length of S.
4. 4. 4. Let intStart be ? ToIntegerOrInfinity(start).
5. 5. 5. If end is undefined, let intEnd be len; else let intEnd be
? ToIntegerOrInfinity(end).
6. 6. 6. Let finalStart be the result of clamping intStart between 0 and len.
7. 7. 7. Let finalEnd be the result of clamping intEnd between 0 and len.
8. 8. 8. Let from be min(finalStart, finalEnd).
9. 9. 9. Let to be max(finalStart, finalEnd).
10. 10. 10. Return the substring of S from from to to.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.25 STRING.PROTOTYPE.TOLOCALELOWERCASE ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It works exactly the same as toLowerCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host environment's
current locale. There will only be a difference in the few cases (such as
Turkish) where the rules for that language conflict with the regular Unicode
case mappings.
The meaning of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.26 STRING.PROTOTYPE.TOLOCALEUPPERCASE ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It works exactly the same as toUpperCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host environment's
current locale. There will only be a difference in the few cases (such as
Turkish) where the rules for that language conflict with the regular Unicode
case mappings.
The meaning of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.27 STRING.PROTOTYPE.TOLOWERCASE ( )
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let sText be StringToCodePoints(S).
4. 4. 4. Let lowerText be the result of toLowercase(sText), according to the
Unicode Default Case Conversion algorithm.
5. 5. 5. Let L be CodePointsToString(lowerText).
6. 6. 6. Return L.
The result must be derived according to the locale-insensitive case mappings in
the Unicode Character Database (this explicitly includes not only the file
UnicodeData.txt, but also all locale-insensitive mappings in the file
SpecialCasing.txt that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this
case the result String may not be the same length as the source String. Because
both toUpperCase and toLowerCase have context-sensitive behaviour, the methods
are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not
necessarily equal to s.toLowerCase().
Note 2
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.28 STRING.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Return ? thisStringValue(this value).
Note
For a String object, this method happens to return the same thing as the valueOf
method.
22.1.3.29 STRING.PROTOTYPE.TOUPPERCASE ( )
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It behaves in exactly the same way as String.prototype.toLowerCase, except that
the String is mapped using the toUppercase algorithm of the Unicode Default Case
Conversion.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.30 STRING.PROTOTYPE.TRIM ( )
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? TrimString(S, start+end).
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.30.1 TRIMSTRING ( STRING, WHERE )
The abstract operation TrimString takes arguments string (an ECMAScript language
value) and where (start, end, or start+end) and returns either a normal
completion containing a String or a throw completion. It interprets string as a
sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the
following steps when called:
1. 1. 1. Let str be ? RequireObjectCoercible(string).
2. 2. 2. Let S be ? ToString(str).
3. 3. 3. If where is start, let T be the String value that is a copy of S with
leading white space removed.
4. 4. 4. Else if where is end, let T be the String value that is a copy of S
with trailing white space removed.
5. 5. 5. Else,
1. a. a. Assert: where is start+end.
2. b. b. Let T be the String value that is a copy of S with both leading and
trailing white space removed.
6. 6. 6. Return T.
The definition of white space is the union of WhiteSpace and LineTerminator.
When determining whether a Unicode code point is in Unicode general category
“Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded
code point sequences as specified in 6.1.4.
22.1.3.31 STRING.PROTOTYPE.TRIMEND ( )
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? TrimString(S, end).
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.32 STRING.PROTOTYPE.TRIMSTART ( )
This method interprets a String value as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? TrimString(S, start).
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
22.1.3.33 STRING.PROTOTYPE.VALUEOF ( )
This method performs the following steps when called:
1. 1. 1. Return ? thisStringValue(this value).
22.1.3.34 STRING.PROTOTYPE [ @@ITERATOR ] ( )
This method returns an Iterator object (27.1.1.2) that iterates over the code
points of a String value, returning each code point as a String value.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let s be ? ToString(O).
3. 3. 3. Let closure be a new Abstract Closure with no parameters that captures
s and performs the following steps when called:
1. a. a. Let len be the length of s.
2. b. b. Let position be 0.
3. c. c. Repeat, while position < len,
1. i. i. Let cp be CodePointAt(s, position).
2. ii. ii. Let nextIndex be position + cp.[[CodeUnitCount]].
3. iii. iii. Let resultString be the substring of s from position to
nextIndex.
4. iv. iv. Set position to nextIndex.
5. v. v. Perform ? GeneratorYield(CreateIterResultObject(resultString,
false)).
4. d. d. Return undefined.
4. 4. 4. Return CreateIteratorFromClosure(closure, "%StringIteratorPrototype%",
%StringIteratorPrototype%).
The value of the "name" property of this method is "[Symbol.iterator]".
22.1.4 PROPERTIES OF STRING INSTANCES
String instances are String exotic objects and have the internal methods
specified for such objects. String instances inherit properties from the String
prototype object. String instances also have a [[StringData]] internal slot.
String instances have a "length" property, and a set of enumerable properties
with integer-indexed names.
22.1.4.1 LENGTH
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false
}.
22.1.5 STRING ITERATOR OBJECTS
A String Iterator is an object, that represents a specific iteration over some
specific String instance object. There is not a named constructor for String
Iterator objects. Instead, String iterator objects are created by calling
certain methods of String instance objects.
22.1.5.1 THE %STRINGITERATORPROTOTYPE% OBJECT
The %StringIteratorPrototype% object:
* has properties that are inherited by all String Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* has the following properties:
22.1.5.1.1 %STRINGITERATORPROTOTYPE%.NEXT ( )
1. 1. 1. Return ? GeneratorResume(this value, empty,
"%StringIteratorPrototype%").
22.1.5.1.2 %STRINGITERATORPROTOTYPE% [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "String
Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
22.2 REGEXP (REGULAR EXPRESSION) OBJECTS
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled after the regular
expression facility in the Perl 5 programming language.
22.2.1 PATTERNS
The RegExp constructor applies the following grammar to the input pattern
String. An error occurs if the grammar cannot interpret the String as an
expansion of Pattern.
SYNTAX
Pattern[UnicodeMode, N] :: Disjunction[?UnicodeMode, ?N]
Disjunction[UnicodeMode, N] :: Alternative[?UnicodeMode, ?N]
Alternative[?UnicodeMode, ?N] | Disjunction[?UnicodeMode, ?N]
Alternative[UnicodeMode, N] :: [empty] Alternative[?UnicodeMode, ?N]
Term[?UnicodeMode, ?N] Term[UnicodeMode, N] :: Assertion[?UnicodeMode, ?N]
Atom[?UnicodeMode, ?N] Atom[?UnicodeMode, ?N] Quantifier Assertion[UnicodeMode,
N] :: ^ $ \b \B (?= Disjunction[?UnicodeMode, ?N] ) (?!
Disjunction[?UnicodeMode, ?N] ) (?<= Disjunction[?UnicodeMode, ?N] ) (?<!
Disjunction[?UnicodeMode, ?N] ) Quantifier :: QuantifierPrefix QuantifierPrefix
? QuantifierPrefix :: * + ? { DecimalDigits[~Sep] } { DecimalDigits[~Sep] ,} {
DecimalDigits[~Sep] , DecimalDigits[~Sep] } Atom[UnicodeMode, N] ::
PatternCharacter . \ AtomEscape[?UnicodeMode, ?N] CharacterClass[?UnicodeMode] (
GroupSpecifier[?UnicodeMode]opt Disjunction[?UnicodeMode, ?N] ) (?:
Disjunction[?UnicodeMode, ?N] ) SyntaxCharacter :: one of ^ $ \ . * + ? ( ) [ ]
{ } | PatternCharacter :: SourceCharacter but not SyntaxCharacter
AtomEscape[UnicodeMode, N] :: DecimalEscape CharacterClassEscape[?UnicodeMode]
CharacterEscape[?UnicodeMode] [+N] k GroupName[?UnicodeMode]
CharacterEscape[UnicodeMode] :: ControlEscape c AsciiLetter 0 [lookahead ∉
DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode]
IdentityEscape[?UnicodeMode] ControlEscape :: one of f n r t v
GroupSpecifier[UnicodeMode] :: ? GroupName[?UnicodeMode] GroupName[UnicodeMode]
:: < RegExpIdentifierName[?UnicodeMode] > RegExpIdentifierName[UnicodeMode] ::
RegExpIdentifierStart[?UnicodeMode] RegExpIdentifierName[?UnicodeMode]
RegExpIdentifierPart[?UnicodeMode] RegExpIdentifierStart[UnicodeMode] ::
IdentifierStartChar \ RegExpUnicodeEscapeSequence[+UnicodeMode] [~UnicodeMode]
UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpIdentifierPart[UnicodeMode] ::
IdentifierPartChar \ RegExpUnicodeEscapeSequence[+UnicodeMode] [~UnicodeMode]
UnicodeLeadSurrogate UnicodeTrailSurrogate
RegExpUnicodeEscapeSequence[UnicodeMode] :: [+UnicodeMode] u HexLeadSurrogate \u
HexTrailSurrogate [+UnicodeMode] u HexLeadSurrogate [+UnicodeMode] u
HexTrailSurrogate [+UnicodeMode] u HexNonSurrogate [~UnicodeMode] u Hex4Digits
[+UnicodeMode] u{ CodePoint } UnicodeLeadSurrogate :: any Unicode code point in
the inclusive interval from U+D800 to U+DBFF UnicodeTrailSurrogate :: any
Unicode code point in the inclusive interval from U+DC00 to U+DFFF
Each \u HexTrailSurrogate for which the choice of associated u HexLeadSurrogate
is ambiguous shall be associated with the nearest possible u HexLeadSurrogate
that would otherwise have no corresponding \u HexTrailSurrogate.
HexLeadSurrogate :: Hex4Digits but only if the MV of Hex4Digits is in the
inclusive interval from 0xD800 to 0xDBFF HexTrailSurrogate :: Hex4Digits but
only if the MV of Hex4Digits is in the inclusive interval from 0xDC00 to 0xDFFF
HexNonSurrogate :: Hex4Digits but only if the MV of Hex4Digits is not in the
inclusive interval from 0xD800 to 0xDFFF IdentityEscape[UnicodeMode] ::
[+UnicodeMode] SyntaxCharacter [+UnicodeMode] / [~UnicodeMode] SourceCharacter
but not UnicodeIDContinue DecimalEscape :: NonZeroDigit DecimalDigits[~Sep]opt
[lookahead ∉ DecimalDigit] CharacterClassEscape[UnicodeMode] :: d D s S w W
[+UnicodeMode] p{ UnicodePropertyValueExpression } [+UnicodeMode] P{
UnicodePropertyValueExpression } UnicodePropertyValueExpression ::
UnicodePropertyName = UnicodePropertyValue LoneUnicodePropertyNameOrValue
UnicodePropertyName :: UnicodePropertyNameCharacters
UnicodePropertyNameCharacters :: UnicodePropertyNameCharacter
UnicodePropertyNameCharactersopt UnicodePropertyValue ::
UnicodePropertyValueCharacters LoneUnicodePropertyNameOrValue ::
UnicodePropertyValueCharacters UnicodePropertyValueCharacters ::
UnicodePropertyValueCharacter UnicodePropertyValueCharactersopt
UnicodePropertyValueCharacter :: UnicodePropertyNameCharacter DecimalDigit
UnicodePropertyNameCharacter :: AsciiLetter _ CharacterClass[UnicodeMode] :: [
[lookahead ≠ ^] ClassRanges[?UnicodeMode] ] [^ ClassRanges[?UnicodeMode] ]
ClassRanges[UnicodeMode] :: [empty] NonemptyClassRanges[?UnicodeMode]
NonemptyClassRanges[UnicodeMode] :: ClassAtom[?UnicodeMode]
ClassAtom[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode]
ClassAtom[?UnicodeMode] - ClassAtom[?UnicodeMode] ClassRanges[?UnicodeMode]
NonemptyClassRangesNoDash[UnicodeMode] :: ClassAtom[?UnicodeMode]
ClassAtomNoDash[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode]
ClassAtomNoDash[?UnicodeMode] - ClassAtom[?UnicodeMode]
ClassRanges[?UnicodeMode] ClassAtom[UnicodeMode] :: -
ClassAtomNoDash[?UnicodeMode] ClassAtomNoDash[UnicodeMode] :: SourceCharacter
but not one of \ or ] or - \ ClassEscape[?UnicodeMode] ClassEscape[UnicodeMode]
:: b [+UnicodeMode] - CharacterClassEscape[?UnicodeMode]
CharacterEscape[?UnicodeMode] Note
A number of productions in this section are given alternative definitions in
section B.1.2.
22.2.1.1 STATIC SEMANTICS: EARLY ERRORS
Note
This section is amended in B.1.2.1.
Pattern :: Disjunction
* It is a Syntax Error if CountLeftCapturingParensWithin(Pattern) ≥ 232 - 1.
* It is a Syntax Error if Pattern contains two or more GroupSpecifiers for
which CapturingGroupName of GroupSpecifier is the same.
QuantifierPrefix :: { DecimalDigits , DecimalDigits }
* It is a Syntax Error if the MV of the first DecimalDigits is strictly greater
than the MV of the second DecimalDigits.
AtomEscape :: k GroupName
* It is a Syntax Error if GroupSpecifiersThatMatch(GroupName) is empty.
AtomEscape :: DecimalEscape
* It is a Syntax Error if the CapturingGroupNumber of DecimalEscape is strictly
greater than CountLeftCapturingParensWithin(the Pattern containing
AtomEscape).
NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
* It is a Syntax Error if IsCharacterClass of the first ClassAtom is true or
IsCharacterClass of the second ClassAtom is true.
* It is a Syntax Error if IsCharacterClass of the first ClassAtom is false,
IsCharacterClass of the second ClassAtom is false, and the CharacterValue of
the first ClassAtom is strictly greater than the CharacterValue of the second
ClassAtom.
NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
* It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is true or
IsCharacterClass of ClassAtom is true.
* It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is false,
IsCharacterClass of ClassAtom is false, and the CharacterValue of
ClassAtomNoDash is strictly greater than the CharacterValue of ClassAtom.
RegExpIdentifierStart :: \ RegExpUnicodeEscapeSequence
* It is a Syntax Error if the CharacterValue of RegExpUnicodeEscapeSequence is
not the numeric value of some code point matched by the IdentifierStartChar
lexical grammar production.
RegExpIdentifierStart :: UnicodeLeadSurrogate UnicodeTrailSurrogate
* It is a Syntax Error if RegExpIdentifierCodePoint of RegExpIdentifierStart is
not matched by the UnicodeIDStart lexical grammar production.
RegExpIdentifierPart :: \ RegExpUnicodeEscapeSequence
* It is a Syntax Error if the CharacterValue of RegExpUnicodeEscapeSequence is
not the numeric value of some code point matched by the IdentifierPartChar
lexical grammar production.
RegExpIdentifierPart :: UnicodeLeadSurrogate UnicodeTrailSurrogate
* It is a Syntax Error if RegExpIdentifierCodePoint of RegExpIdentifierPart is
not matched by the UnicodeIDContinue lexical grammar production.
UnicodePropertyValueExpression :: UnicodePropertyName = UnicodePropertyValue
* It is a Syntax Error if the source text matched by UnicodePropertyName is not
a Unicode property name or property alias listed in the “Property name and
aliases” column of Table 65.
* It is a Syntax Error if the source text matched by UnicodePropertyValue is
not a property value or property value alias for the Unicode property or
property alias given by the source text matched by UnicodePropertyName listed
in PropertyValueAliases.txt.
UnicodePropertyValueExpression :: LoneUnicodePropertyNameOrValue
* It is a Syntax Error if the source text matched by
LoneUnicodePropertyNameOrValue is not a Unicode property value or property
value alias for the General_Category (gc) property listed in
PropertyValueAliases.txt, nor a binary property or binary property alias
listed in the “Property name and aliases” column of Table 66.
22.2.1.2 STATIC SEMANTICS: COUNTLEFTCAPTURINGPARENSWITHIN ( NODE )
The abstract operation CountLeftCapturingParensWithin takes argument node (a
Parse Node) and returns a non-negative integer. It returns the number of
left-capturing parentheses in node. A left-capturing parenthesis is any (
pattern character that is matched by the ( terminal of the Atom :: (
GroupSpecifieropt Disjunction ) production.
Note
This section is amended in B.1.2.2.
It performs the following steps when called:
1. 1. 1. Assert: node is an instance of a production in the RegExp Pattern
grammar.
2. 2. 2. Return the number of Atom :: ( GroupSpecifieropt Disjunction ) Parse
Nodes contained within node.
22.2.1.3 STATIC SEMANTICS: COUNTLEFTCAPTURINGPARENSBEFORE ( NODE )
The abstract operation CountLeftCapturingParensBefore takes argument node (a
Parse Node) and returns a non-negative integer. It returns the number of
left-capturing parentheses within the enclosing pattern that occur to the left
of node.
Note
This section is amended in B.1.2.2.
It performs the following steps when called:
1. 1. 1. Assert: node is an instance of a production in the RegExp Pattern
grammar.
2. 2. 2. Let pattern be the Pattern containing node.
3. 3. 3. Return the number of Atom :: ( GroupSpecifieropt Disjunction ) Parse
Nodes contained within pattern that either occur before node or contain
node.
22.2.1.4 STATIC SEMANTICS: CAPTURINGGROUPNUMBER
The syntax-directed operation CapturingGroupNumber takes no arguments and
returns a positive integer.
Note
This section is amended in B.1.2.1.
It is defined piecewise over the following productions:
DecimalEscape :: NonZeroDigit
1. 1. 1. Return the MV of NonZeroDigit.
DecimalEscape :: NonZeroDigit DecimalDigits
1. 1. 1. Let n be the number of code points in DecimalDigits.
2. 2. 2. Return (the MV of NonZeroDigit × 10n plus the MV of DecimalDigits).
The definitions of “the MV of NonZeroDigit” and “the MV of DecimalDigits” are in
12.9.3.
22.2.1.5 STATIC SEMANTICS: ISCHARACTERCLASS
The syntax-directed operation IsCharacterClass takes no arguments and returns a
Boolean.
Note
This section is amended in B.1.2.3.
It is defined piecewise over the following productions:
ClassAtom :: - ClassAtomNoDash :: SourceCharacter but not one of \ or ] or -
ClassEscape :: b - CharacterEscape
1. 1. 1. Return false.
ClassEscape :: CharacterClassEscape
1. 1. 1. Return true.
22.2.1.6 STATIC SEMANTICS: CHARACTERVALUE
The syntax-directed operation CharacterValue takes no arguments and returns a
non-negative integer.
Note 1
This section is amended in B.1.2.4.
It is defined piecewise over the following productions:
ClassAtom :: -
1. 1. 1. Return the numeric value of U+002D (HYPHEN-MINUS).
ClassAtomNoDash :: SourceCharacter but not one of \ or ] or -
1. 1. 1. Let ch be the code point matched by SourceCharacter.
2. 2. 2. Return the numeric value of ch.
ClassEscape :: b
1. 1. 1. Return the numeric value of U+0008 (BACKSPACE).
ClassEscape :: -
1. 1. 1. Return the numeric value of U+002D (HYPHEN-MINUS).
CharacterEscape :: ControlEscape
1. 1. 1. Return the numeric value according to Table 63.
Table 63: ControlEscape Code Point Values
ControlEscape Numeric Value Code Point Unicode Name Symbol t 9 U+0009 CHARACTER
TABULATION <HT> n 10 U+000A LINE FEED (LF) <LF> v 11 U+000B LINE TABULATION <VT>
f 12 U+000C FORM FEED (FF) <FF> r 13 U+000D CARRIAGE RETURN (CR) <CR>
CharacterEscape :: c AsciiLetter
1. 1. 1. Let ch be the code point matched by AsciiLetter.
2. 2. 2. Let i be the numeric value of ch.
3. 3. 3. Return the remainder of dividing i by 32.
CharacterEscape :: 0 [lookahead ∉ DecimalDigit]
1. 1. 1. Return the numeric value of U+0000 (NULL).
Note 2
\0 represents the <NUL> character and cannot be followed by a decimal digit.
CharacterEscape :: HexEscapeSequence
1. 1. 1. Return the MV of HexEscapeSequence.
RegExpUnicodeEscapeSequence :: u HexLeadSurrogate \u HexTrailSurrogate
1. 1. 1. Let lead be the CharacterValue of HexLeadSurrogate.
2. 2. 2. Let trail be the CharacterValue of HexTrailSurrogate.
3. 3. 3. Let cp be UTF16SurrogatePairToCodePoint(lead, trail).
4. 4. 4. Return the numeric value of cp.
RegExpUnicodeEscapeSequence :: u Hex4Digits
1. 1. 1. Return the MV of Hex4Digits.
RegExpUnicodeEscapeSequence :: u{ CodePoint }
1. 1. 1. Return the MV of CodePoint.
HexLeadSurrogate :: Hex4Digits HexTrailSurrogate :: Hex4Digits HexNonSurrogate
:: Hex4Digits
1. 1. 1. Return the MV of Hex4Digits.
CharacterEscape :: IdentityEscape
1. 1. 1. Let ch be the code point matched by IdentityEscape.
2. 2. 2. Return the numeric value of ch.
22.2.1.7 STATIC SEMANTICS: GROUPSPECIFIERSTHATMATCH ( THISGROUPNAME )
The abstract operation GroupSpecifiersThatMatch takes argument thisGroupName (a
GroupName Parse Node) and returns a List of GroupSpecifier Parse Nodes. It
performs the following steps when called:
1. 1. 1. Let name be the CapturingGroupName of thisGroupName.
2. 2. 2. Let pattern be the Pattern containing thisGroupName.
3. 3. 3. Let result be a new empty List.
4. 4. 4. For each GroupSpecifier gs that pattern contains, do
1. a. a. If the CapturingGroupName of gs is name, then
1. i. i. Append gs to result.
5. 5. 5. Return result.
22.2.1.8 STATIC SEMANTICS: CAPTURINGGROUPNAME
The syntax-directed operation CapturingGroupName takes no arguments and returns
a String. It is defined piecewise over the following productions:
GroupName :: < RegExpIdentifierName >
1. 1. 1. Let idTextUnescaped be RegExpIdentifierCodePoints of
RegExpIdentifierName.
2. 2. 2. Return CodePointsToString(idTextUnescaped).
22.2.1.9 STATIC SEMANTICS: REGEXPIDENTIFIERCODEPOINTS
The syntax-directed operation RegExpIdentifierCodePoints takes no arguments and
returns a List of code points. It is defined piecewise over the following
productions:
RegExpIdentifierName :: RegExpIdentifierStart
1. 1. 1. Let cp be RegExpIdentifierCodePoint of RegExpIdentifierStart.
2. 2. 2. Return « cp ».
RegExpIdentifierName :: RegExpIdentifierName RegExpIdentifierPart
1. 1. 1. Let cps be RegExpIdentifierCodePoints of the derived
RegExpIdentifierName.
2. 2. 2. Let cp be RegExpIdentifierCodePoint of RegExpIdentifierPart.
3. 3. 3. Return the list-concatenation of cps and « cp ».
22.2.1.10 STATIC SEMANTICS: REGEXPIDENTIFIERCODEPOINT
The syntax-directed operation RegExpIdentifierCodePoint takes no arguments and
returns a code point. It is defined piecewise over the following productions:
RegExpIdentifierStart :: IdentifierStartChar
1. 1. 1. Return the code point matched by IdentifierStartChar.
RegExpIdentifierPart :: IdentifierPartChar
1. 1. 1. Return the code point matched by IdentifierPartChar.
RegExpIdentifierStart :: \ RegExpUnicodeEscapeSequence RegExpIdentifierPart :: \
RegExpUnicodeEscapeSequence
1. 1. 1. Return the code point whose numeric value is the CharacterValue of
RegExpUnicodeEscapeSequence.
RegExpIdentifierStart :: UnicodeLeadSurrogate UnicodeTrailSurrogate
RegExpIdentifierPart :: UnicodeLeadSurrogate UnicodeTrailSurrogate
1. 1. 1. Let lead be the code unit whose numeric value is the numeric value of
the code point matched by UnicodeLeadSurrogate.
2. 2. 2. Let trail be the code unit whose numeric value is the numeric value of
the code point matched by UnicodeTrailSurrogate.
3. 3. 3. Return UTF16SurrogatePairToCodePoint(lead, trail).
22.2.2 PATTERN SEMANTICS
A regular expression pattern is converted into an Abstract Closure using the
process described below. An implementation is encouraged to use more efficient
algorithms than the ones listed below, as long as the results are the same. The
Abstract Closure is used as the value of a RegExp object's [[RegExpMatcher]]
internal slot.
A Pattern is either a BMP pattern or a Unicode pattern depending upon whether or
not its associated flags contain a u. A BMP pattern matches against a String
interpreted as consisting of a sequence of 16-bit values that are Unicode code
points in the range of the Basic Multilingual Plane. A Unicode pattern matches
against a String interpreted as consisting of Unicode code points encoded using
UTF-16. In the context of describing the behaviour of a BMP pattern “character”
means a single 16-bit Unicode BMP code point. In the context of describing the
behaviour of a Unicode pattern “character” means a UTF-16 encoded code point
(6.1.4). In either context, “character value” means the numeric value of the
corresponding non-encoded code point.
The syntax and semantics of Pattern is defined as if the source text for the
Pattern was a List of SourceCharacter values where each SourceCharacter
corresponds to a Unicode code point. If a BMP pattern contains a non-BMP
SourceCharacter the entire pattern is encoded using UTF-16 and the individual
code units of that encoding are used as the elements of the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP
character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it
would be a single element (character) List consisting of the single code point
U+1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to
produce a two element List consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in
which non-BMP characters are UTF-16 encoded. For example, the single character
MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of
length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further
translation of the string would be necessary to process it as a BMP pattern
consisting of two pattern characters. However, to process it as a Unicode
pattern UTF16SurrogatePairToCodePoint must be used in producing a List whose
sole element is a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16,
but the semantics of this specification requires that the result of pattern
matching be as if such translations were performed.
22.2.2.1 NOTATION
The descriptions below use the following internal data structures:
* A CharSet is a mathematical set of characters. In the context of a Unicode
pattern, “all characters” means the CharSet containing all code point values;
otherwise “all characters” means the CharSet containing all code unit values.
* A CaptureRange is an ordered pair (startIndex, endIndex) that represents the
range of characters included in a capture, where startIndex is an integer
representing the start index (inclusive) of the range within Input, and
endIndex is an integer representing the end index (exclusive) of the range
within Input. For any CaptureRange, these indices must satisfy the invariant
that startIndex ≤ endIndex.
* A MatchState is an ordered triple (input, endIndex, captures) where input is
a List of characters representing the String being matched, endIndex is an
integer, and captures is a List of values, one for each left-capturing
parenthesis in the pattern. States are used to represent partial match states
in the regular expression matching algorithms. The endIndex is one plus the
index of the last input character matched so far by the pattern, while
captures holds the results of capturing parentheses. The nth element of
captures is either a CaptureRange representing the range of characters
captured by the nth set of capturing parentheses, or undefined if the nth set
of capturing parentheses hasn't been reached yet. Due to backtracking, many
States may be in use at any time during the matching process.
* A MatchResult is either a MatchState or the special token failure that
indicates that the match failed.
* A MatcherContinuation is an Abstract Closure that takes one MatchState
argument and returns a MatchResult result. The MatcherContinuation attempts
to match the remaining portion (specified by the closure's captured values)
of the pattern against Input, starting at the intermediate state given by its
MatchState argument. If the match succeeds, the MatcherContinuation returns
the final MatchState that it reached; if the match fails, the
MatcherContinuation returns failure.
* A Matcher is an Abstract Closure that takes two arguments—a MatchState and a
MatcherContinuation—and returns a MatchResult result. A Matcher attempts to
match a middle subpattern (specified by the closure's captured values) of the
pattern against the MatchState's input, starting at the intermediate state
given by its MatchState argument. The MatcherContinuation argument should be
a closure that matches the rest of the pattern. After matching the subpattern
of a pattern to obtain a new MatchState, the Matcher then calls
MatcherContinuation on that new MatchState to test if the rest of the pattern
can match as well. If it can, the Matcher returns the MatchState returned by
MatcherContinuation; if not, the Matcher may try different choices at its
choice points, repeatedly calling MatcherContinuation until it either
succeeds or all possibilities have been exhausted.
22.2.2.1.1 REGEXP RECORDS
A RegExp Record is a Record value used to store information about a RegExp that
is needed during compilation and possibly during matching.
It has the following fields:
Table 64: RegExp Record Fields
Field Name Value Meaning [[IgnoreCase]] a Boolean indicates whether "i" appears
in the RegExp's flags [[Multiline]] a Boolean indicates whether "m" appears in
the RegExp's flags [[DotAll]] a Boolean indicates whether "s" appears in the
RegExp's flags [[Unicode]] a Boolean indicates whether "u" appears in the
RegExp's flags [[CapturingGroupsCount]] a non-negative integer the number of
left-capturing parentheses in the RegExp's pattern
22.2.2.2 RUNTIME SEMANTICS: COMPILEPATTERN
The syntax-directed operation CompilePattern takes argument rer (a RegExp
Record) and returns an Abstract Closure that takes a List of characters and a
non-negative integer and returns a MatchResult. It is defined piecewise over the
following productions:
Pattern :: Disjunction
1. 1. 1. Let m be CompileSubpattern of Disjunction with arguments rer and
forward.
2. 2. 2. Return a new Abstract Closure with parameters (Input, index) that
captures rer and m and performs the following steps when called:
1. a. a. Assert: Input is a List of characters.
2. b. b. Assert: 0 ≤ index ≤ the number of elements in Input.
3. c. c. Let c be a new MatcherContinuation with parameters (y) that
captures nothing and performs the following steps when called:
1. i. i. Assert: y is a MatchState.
2. ii. ii. Return y.
4. d. d. Let cap be a List of rer.[[CapturingGroupsCount]] undefined values,
indexed 1 through rer.[[CapturingGroupsCount]].
5. e. e. Let x be the MatchState (Input, index, cap).
6. f. f. Return m(x, c).
Note
A Pattern compiles to an Abstract Closure value. RegExpBuiltinExec can then
apply this procedure to a List of characters and an offset within that List to
determine whether the pattern would match starting at exactly that offset within
the List, and, if it does match, what the values of the capturing parentheses
would be. The algorithms in 22.2.2 are designed so that compiling a pattern may
throw a SyntaxError exception; on the other hand, once the pattern is
successfully compiled, applying the resulting Abstract Closure to find a match
in a List of characters cannot throw an exception (except for any
implementation-defined exceptions that can occur anywhere such as
out-of-memory).
22.2.2.3 RUNTIME SEMANTICS: COMPILESUBPATTERN
The syntax-directed operation CompileSubpattern takes arguments rer (a RegExp
Record) and direction (forward or backward) and returns a Matcher.
Note 1
This section is amended in B.1.2.5.
It is defined piecewise over the following productions:
Disjunction :: Alternative | Disjunction
1. 1. 1. Let m1 be CompileSubpattern of Alternative with arguments rer and
direction.
2. 2. 2. Let m2 be CompileSubpattern of Disjunction with arguments rer and
direction.
3. 3. 3. Return a new Matcher with parameters (x, c) that captures m1 and m2
and performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let r be m1(x, c).
4. d. d. If r is not failure, return r.
5. e. e. Return m2(x, c).
Note 2
The | regular expression operator separates two alternatives. The pattern first
tries to match the left Alternative (followed by the sequel of the regular
expression); if it fails, it tries to match the right Disjunction (followed by
the sequel of the regular expression). If the left Alternative, the right
Disjunction, and the sequel all have choice points, all choices in the sequel
are tried before moving on to the next choice in the left Alternative. If
choices in the left Alternative are exhausted, the right Disjunction is tried
instead of the left Alternative. Any capturing parentheses inside a portion of
the pattern skipped by | produce undefined values instead of Strings. Thus, for
example,
/a|ab/.exec("abc")
returns the result "a" and not "ab". Moreover,
/((a)|(ab))((c)|(bc))/.exec("abc")
returns the array
["abc", "a", "a", undefined, "bc", undefined, "bc"]
and not
["abc", "ab", undefined, "ab", "c", "c", undefined]
The order in which the two alternatives are tried is independent of the value of
direction.
Alternative :: [empty]
1. 1. 1. Return a new Matcher with parameters (x, c) that captures nothing and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Return c(x).
Alternative :: Alternative Term
1. 1. 1. Let m1 be CompileSubpattern of Alternative with arguments rer and
direction.
2. 2. 2. Let m2 be CompileSubpattern of Term with arguments rer and direction.
3. 3. 3. If direction is forward, then
1. a. a. Return a new Matcher with parameters (x, c) that captures m1 and m2
and performs the following steps when called:
1. i. i. Assert: x is a MatchState.
2. ii. ii. Assert: c is a MatcherContinuation.
3. iii. iii. Let d be a new MatcherContinuation with parameters (y) that
captures c and m2 and performs the following steps when called:
1. 1. 1. Assert: y is a MatchState.
2. 2. 2. Return m2(y, c).
4. iv. iv. Return m1(x, d).
4. 4. 4. Else,
1. a. a. Assert: direction is backward.
2. b. b. Return a new Matcher with parameters (x, c) that captures m1 and m2
and performs the following steps when called:
1. i. i. Assert: x is a MatchState.
2. ii. ii. Assert: c is a MatcherContinuation.
3. iii. iii. Let d be a new MatcherContinuation with parameters (y) that
captures c and m1 and performs the following steps when called:
1. 1. 1. Assert: y is a MatchState.
2. 2. 2. Return m1(y, c).
4. iv. iv. Return m2(x, d).
Note 3
Consecutive Terms try to simultaneously match consecutive portions of Input.
When direction is forward, if the left Alternative, the right Term, and the
sequel of the regular expression all have choice points, all choices in the
sequel are tried before moving on to the next choice in the right Term, and all
choices in the right Term are tried before moving on to the next choice in the
left Alternative. When direction is backward, the evaluation order of
Alternative and Term are reversed.
Term :: Assertion
1. 1. 1. Return CompileAssertion of Assertion with argument rer.
Note 4
The resulting Matcher is independent of direction.
Term :: Atom
1. 1. 1. Return CompileAtom of Atom with arguments rer and direction.
Term :: Atom Quantifier
1. 1. 1. Let m be CompileAtom of Atom with arguments rer and direction.
2. 2. 2. Let q be CompileQuantifier of Quantifier.
3. 3. 3. Assert: q.[[Min]] ≤ q.[[Max]].
4. 4. 4. Let parenIndex be CountLeftCapturingParensBefore(Term).
5. 5. 5. Let parenCount be CountLeftCapturingParensWithin(Atom).
6. 6. 6. Return a new Matcher with parameters (x, c) that captures m, q,
parenIndex, and parenCount and performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Return RepeatMatcher(m, q.[[Min]], q.[[Max]], q.[[Greedy]], x, c,
parenIndex, parenCount).
22.2.2.3.1 REPEATMATCHER ( M, MIN, MAX, GREEDY, X, C, PARENINDEX, PARENCOUNT )
The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a
non-negative integer), max (a non-negative integer or +∞), greedy (a Boolean), x
(a MatchState), c (a MatcherContinuation), parenIndex (a non-negative integer),
and parenCount (a non-negative integer) and returns a MatchResult. It performs
the following steps when called:
1. 1. 1. If max = 0, return c(x).
2. 2. 2. Let d be a new MatcherContinuation with parameters (y) that captures
m, min, max, greedy, x, c, parenIndex, and parenCount and performs the
following steps when called:
1. a. a. Assert: y is a MatchState.
2. b. b. If min = 0 and y's endIndex = x's endIndex, return failure.
3. c. c. If min = 0, let min2 be 0; otherwise let min2 be min - 1.
4. d. d. If max = +∞, let max2 be +∞; otherwise let max2 be max - 1.
5. e. e. Return RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex,
parenCount).
3. 3. 3. Let cap be a copy of x's captures List.
4. 4. 4. For each integer k in the inclusive interval from parenIndex + 1 to
parenIndex + parenCount, set cap[k] to undefined.
5. 5. 5. Let Input be x's input.
6. 6. 6. Let e be x's endIndex.
7. 7. 7. Let xr be the MatchState (Input, e, cap).
8. 8. 8. If min ≠ 0, return m(xr, d).
9. 9. 9. If greedy is false, then
1. a. a. Let z be c(x).
2. b. b. If z is not failure, return z.
3. c. c. Return m(xr, d).
10. 10. 10. Let z be m(xr, d).
11. 11. 11. If z is not failure, return z.
12. 12. 12. Return c(x).
Note 1
An Atom followed by a Quantifier is repeated the number of times specified by
the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern
is repeated as few times as possible while still matching the sequel, or it can
be greedy, in which case the Atom pattern is repeated as many times as possible
while still matching the sequel. The Atom pattern is repeated rather than the
input character sequence that it matches, so different repetitions of the Atom
can match different input substrings.
Note 2
If the Atom and the sequel of the regular expression all have choice points, the
Atom is first matched as many (or as few, if non-greedy) times as possible. All
choices in the sequel are tried before moving on to the next choice in the last
repetition of Atom. All choices in the last (nth) repetition of Atom are tried
before moving on to the next choice in the next-to-last (n - 1)st repetition of
Atom; at which point it may turn out that more or fewer repetitions of Atom are
now possible; these are exhausted (again, starting with either as few or as many
as possible) before moving on to the next choice in the (n - 1)st repetition of
Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression
that calculates the greatest common divisor of two numbers (represented in unary
notation). The following example calculates the gcd of 10 and 15:
"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/, "$1")
which returns the gcd in unary notation "aaaaa".
Note 3
Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated.
We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained
in the quantified Atom, which in this case includes capture Strings numbered 2,
3, 4, and 5.
Note 4
Step 2.b of the RepeatMatcher states that once the minimum number of repetitions
has been satisfied, any more expansions of Atom that match the empty character
sequence are not considered for further repetitions. This prevents the regular
expression engine from falling into an infinite loop on patterns such as:
/(a*)*/.exec("b")
or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")
which returns the array
["b", ""]
22.2.2.4 RUNTIME SEMANTICS: COMPILEASSERTION
The syntax-directed operation CompileAssertion takes argument rer (a RegExp
Record) and returns a Matcher.
Note 1
This section is amended in B.1.2.6.
It is defined piecewise over the following productions:
Assertion :: ^
1. 1. 1. Return a new Matcher with parameters (x, c) that captures rer and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let Input be x's input.
4. d. d. Let e be x's endIndex.
5. e. e. If e = 0, or if rer.[[Multiline]] is true and the character Input[e
- 1] is matched by LineTerminator, then
1. i. i. Return c(x).
6. f. f. Return failure.
Note 2
Even when the y flag is used with a pattern, ^ always matches only at the
beginning of Input, or (if rer.[[Multiline]] is true) at the beginning of a
line.
Assertion :: $
1. 1. 1. Return a new Matcher with parameters (x, c) that captures rer and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let Input be x's input.
4. d. d. Let e be x's endIndex.
5. e. e. Let InputLength be the number of elements in Input.
6. f. f. If e = InputLength, or if rer.[[Multiline]] is true and the
character Input[e] is matched by LineTerminator, then
1. i. i. Return c(x).
7. g. g. Return failure.
Assertion :: \b
1. 1. 1. Return a new Matcher with parameters (x, c) that captures rer and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let Input be x's input.
4. d. d. Let e be x's endIndex.
5. e. e. Let a be IsWordChar(rer, Input, e - 1).
6. f. f. Let b be IsWordChar(rer, Input, e).
7. g. g. If a is true and b is false, or if a is false and b is true, return
c(x).
8. h. h. Return failure.
Assertion :: \B
1. 1. 1. Return a new Matcher with parameters (x, c) that captures rer and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let Input be x's input.
4. d. d. Let e be x's endIndex.
5. e. e. Let a be IsWordChar(rer, Input, e - 1).
6. f. f. Let b be IsWordChar(rer, Input, e).
7. g. g. If a is true and b is true, or if a is false and b is false, return
c(x).
8. h. h. Return failure.
Assertion :: (?= Disjunction )
1. 1. 1. Let m be CompileSubpattern of Disjunction with arguments rer and
forward.
2. 2. 2. Return a new Matcher with parameters (x, c) that captures m and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let d be a new MatcherContinuation with parameters (y) that
captures nothing and performs the following steps when called:
1. i. i. Assert: y is a MatchState.
2. ii. ii. Return y.
4. d. d. Let r be m(x, d).
5. e. e. If r is failure, return failure.
6. f. f. Let y be r's MatchState.
7. g. g. Let cap be y's captures List.
8. h. h. Let Input be x's input.
9. i. i. Let xe be x's endIndex.
10. j. j. Let z be the MatchState (Input, xe, cap).
11. k. k. Return c(z).
Note 3
The form (?= Disjunction ) specifies a zero-width positive lookahead. In order
for it to succeed, the pattern inside Disjunction must match at the current
position, but the current position is not advanced before matching the sequel.
If Disjunction can match at the current position in several ways, only the first
one is tried. Unlike other regular expression operators, there is no
backtracking into a (?= form (this unusual behaviour is inherited from Perl).
This only matters when the Disjunction contains capturing parentheses and the
sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the
array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
Assertion :: (?! Disjunction )
1. 1. 1. Let m be CompileSubpattern of Disjunction with arguments rer and
forward.
2. 2. 2. Return a new Matcher with parameters (x, c) that captures m and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let d be a new MatcherContinuation with parameters (y) that
captures nothing and performs the following steps when called:
1. i. i. Assert: y is a MatchState.
2. ii. ii. Return y.
4. d. d. Let r be m(x, d).
5. e. e. If r is not failure, return failure.
6. f. f. Return c(x).
Note 4
The form (?! Disjunction ) specifies a zero-width negative lookahead. In order
for it to succeed, the pattern inside Disjunction must fail to match at the
current position. The current position is not advanced before matching the
sequel. Disjunction can contain capturing parentheses, but backreferences to
them only make sense from within Disjunction itself. Backreferences to these
capturing parentheses from elsewhere in the pattern always return undefined
because the negative lookahead must fail for the pattern to succeed. For
example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a b,
another n a's (specified by the first \2) and a c. The second \2 is outside the
negative lookahead, so it matches against undefined and therefore always
succeeds. The whole expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
Assertion :: (?<= Disjunction )
1. 1. 1. Let m be CompileSubpattern of Disjunction with arguments rer and
backward.
2. 2. 2. Return a new Matcher with parameters (x, c) that captures m and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let d be a new MatcherContinuation with parameters (y) that
captures nothing and performs the following steps when called:
1. i. i. Assert: y is a MatchState.
2. ii. ii. Return y.
4. d. d. Let r be m(x, d).
5. e. e. If r is failure, return failure.
6. f. f. Let y be r's MatchState.
7. g. g. Let cap be y's captures List.
8. h. h. Let Input be x's input.
9. i. i. Let xe be x's endIndex.
10. j. j. Let z be the MatchState (Input, xe, cap).
11. k. k. Return c(z).
Assertion :: (?<! Disjunction )
1. 1. 1. Let m be CompileSubpattern of Disjunction with arguments rer and
backward.
2. 2. 2. Return a new Matcher with parameters (x, c) that captures m and
performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let d be a new MatcherContinuation with parameters (y) that
captures nothing and performs the following steps when called:
1. i. i. Assert: y is a MatchState.
2. ii. ii. Return y.
4. d. d. Let r be m(x, d).
5. e. e. If r is not failure, return failure.
6. f. f. Return c(x).
22.2.2.4.1 ISWORDCHAR ( RER, INPUT, E )
The abstract operation IsWordChar takes arguments rer (a RegExp Record), Input
(a List of characters), and e (an integer) and returns a Boolean. It performs
the following steps when called:
1. 1. 1. Let InputLength be the number of elements in Input.
2. 2. 2. If e = -1 or e = InputLength, return false.
3. 3. 3. Let c be the character Input[e].
4. 4. 4. If WordCharacters(rer) contains c, return true.
5. 5. 5. Return false.
22.2.2.5 RUNTIME SEMANTICS: COMPILEQUANTIFIER
The syntax-directed operation CompileQuantifier takes no arguments and returns a
Record with fields [[Min]] (a non-negative integer), [[Max]] (a non-negative
integer or +∞), and [[Greedy]] (a Boolean). It is defined piecewise over the
following productions:
Quantifier :: QuantifierPrefix
1. 1. 1. Let qp be CompileQuantifierPrefix of QuantifierPrefix.
2. 2. 2. Return the Record { [[Min]]: qp.[[Min]], [[Max]]: qp.[[Max]],
[[Greedy]]: true }.
Quantifier :: QuantifierPrefix ?
1. 1. 1. Let qp be CompileQuantifierPrefix of QuantifierPrefix.
2. 2. 2. Return the Record { [[Min]]: qp.[[Min]], [[Max]]: qp.[[Max]],
[[Greedy]]: false }.
22.2.2.6 RUNTIME SEMANTICS: COMPILEQUANTIFIERPREFIX
The syntax-directed operation CompileQuantifierPrefix takes no arguments and
returns a Record with fields [[Min]] (a non-negative integer) and [[Max]] (a
non-negative integer or +∞). It is defined piecewise over the following
productions:
QuantifierPrefix :: *
1. 1. 1. Return the Record { [[Min]]: 0, [[Max]]: +∞ }.
QuantifierPrefix :: +
1. 1. 1. Return the Record { [[Min]]: 1, [[Max]]: +∞ }.
QuantifierPrefix :: ?
1. 1. 1. Return the Record { [[Min]]: 0, [[Max]]: 1 }.
QuantifierPrefix :: { DecimalDigits }
1. 1. 1. Let i be the MV of DecimalDigits (see 12.9.3).
2. 2. 2. Return the Record { [[Min]]: i, [[Max]]: i }.
QuantifierPrefix :: { DecimalDigits ,}
1. 1. 1. Let i be the MV of DecimalDigits.
2. 2. 2. Return the Record { [[Min]]: i, [[Max]]: +∞ }.
QuantifierPrefix :: { DecimalDigits , DecimalDigits }
1. 1. 1. Let i be the MV of the first DecimalDigits.
2. 2. 2. Let j be the MV of the second DecimalDigits.
3. 3. 3. Return the Record { [[Min]]: i, [[Max]]: j }.
22.2.2.7 RUNTIME SEMANTICS: COMPILEATOM
The syntax-directed operation CompileAtom takes arguments rer (a RegExp Record)
and direction (forward or backward) and returns a Matcher.
Note 1
This section is amended in B.1.2.7.
It is defined piecewise over the following productions:
Atom :: PatternCharacter
1. 1. 1. Let ch be the character matched by PatternCharacter.
2. 2. 2. Let A be a one-element CharSet containing the character ch.
3. 3. 3. Return CharacterSetMatcher(rer, A, false, direction).
Atom :: .
1. 1. 1. Let A be the CharSet of all characters.
2. 2. 2. If rer.[[DotAll]] is not true, then
1. a. a. Remove from A all characters corresponding to a code point on the
right-hand side of the LineTerminator production.
3. 3. 3. Return CharacterSetMatcher(rer, A, false, direction).
Atom :: CharacterClass
1. 1. 1. Let cc be CompileCharacterClass of CharacterClass with argument rer.
2. 2. 2. Return CharacterSetMatcher(rer, cc.[[CharSet]], cc.[[Invert]],
direction).
Atom :: ( GroupSpecifieropt Disjunction )
1. 1. 1. Let m be CompileSubpattern of Disjunction with arguments rer and
direction.
2. 2. 2. Let parenIndex be CountLeftCapturingParensBefore(Atom).
3. 3. 3. Return a new Matcher with parameters (x, c) that captures direction,
m, and parenIndex and performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let d be a new MatcherContinuation with parameters (y) that
captures x, c, direction, and parenIndex and performs the following steps
when called:
1. i. i. Assert: y is a MatchState.
2. ii. ii. Let cap be a copy of y's captures List.
3. iii. iii. Let Input be x's input.
4. iv. iv. Let xe be x's endIndex.
5. v. v. Let ye be y's endIndex.
6. vi. vi. If direction is forward, then
1. 1. 1. Assert: xe ≤ ye.
2. 2. 2. Let r be the CaptureRange (xe, ye).
7. vii. vii. Else,
1. 1. 1. Assert: direction is backward.
2. 2. 2. Assert: ye ≤ xe.
3. 3. 3. Let r be the CaptureRange (ye, xe).
8. viii. viii. Set cap[parenIndex + 1] to r.
9. ix. ix. Let z be the MatchState (Input, ye, cap).
10. x. x. Return c(z).
4. d. d. Return m(x, d).
Note 2
Parentheses of the form ( Disjunction ) serve both to group the components of
the Disjunction pattern together and to save the result of the match. The result
can be used either in a backreference (\ followed by a non-zero decimal number),
referenced in a replace String, or returned as part of an array from the regular
expression matching Abstract Closure. To inhibit the capturing behaviour of
parentheses, use the form (?: Disjunction ) instead.
Atom :: (?: Disjunction )
1. 1. 1. Return CompileSubpattern of Disjunction with arguments rer and
direction.
AtomEscape :: DecimalEscape
1. 1. 1. Let n be the CapturingGroupNumber of DecimalEscape.
2. 2. 2. Assert: n ≤ rer.[[CapturingGroupsCount]].
3. 3. 3. Return BackreferenceMatcher(rer, n, direction).
Note 3
An escape sequence of the form \ followed by a non-zero decimal number n matches
the result of the nth set of capturing parentheses (22.2.2.1). It is an error if
the regular expression has fewer than n capturing parentheses. If the regular
expression has n or more capturing parentheses but the nth one is undefined
because it has not captured anything, then the backreference always succeeds.
AtomEscape :: CharacterEscape
1. 1. 1. Let cv be the CharacterValue of CharacterEscape.
2. 2. 2. Let ch be the character whose character value is cv.
3. 3. 3. Let A be a one-element CharSet containing the character ch.
4. 4. 4. Return CharacterSetMatcher(rer, A, false, direction).
AtomEscape :: CharacterClassEscape
1. 1. 1. Let A be CompileToCharSet of CharacterClassEscape with argument rer.
2. 2. 2. Return CharacterSetMatcher(rer, A, false, direction).
AtomEscape :: k GroupName
1. 1. 1. Let matchingGroupSpecifiers be GroupSpecifiersThatMatch(GroupName).
2. 2. 2. Assert: matchingGroupSpecifiers contains a single GroupSpecifier.
3. 3. 3. Let groupSpecifier be the sole element of matchingGroupSpecifiers.
4. 4. 4. Let parenIndex be CountLeftCapturingParensBefore(groupSpecifier).
5. 5. 5. Return BackreferenceMatcher(rer, parenIndex, direction).
22.2.2.7.1 CHARACTERSETMATCHER ( RER, A, INVERT, DIRECTION )
The abstract operation CharacterSetMatcher takes arguments rer (a RegExp
Record), A (a CharSet), invert (a Boolean), and direction (forward or backward)
and returns a Matcher. It performs the following steps when called:
1. 1. 1. Return a new Matcher with parameters (x, c) that captures rer, A,
invert, and direction and performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let Input be x's input.
4. d. d. Let e be x's endIndex.
5. e. e. If direction is forward, let f be e + 1.
6. f. f. Else, let f be e - 1.
7. g. g. Let InputLength be the number of elements in Input.
8. h. h. If f < 0 or f > InputLength, return failure.
9. i. i. Let index be min(e, f).
10. j. j. Let ch be the character Input[index].
11. k. k. Let cc be Canonicalize(rer, ch).
12. l. l. If there exists a member a of A such that Canonicalize(rer, a) is
cc, let found be true. Otherwise, let found be false.
13. m. m. If invert is false and found is false, return failure.
14. n. n. If invert is true and found is true, return failure.
15. o. o. Let cap be x's captures List.
16. p. p. Let y be the MatchState (Input, f, cap).
17. q. q. Return c(y).
22.2.2.7.2 BACKREFERENCEMATCHER ( RER, N, DIRECTION )
The abstract operation BackreferenceMatcher takes arguments rer (a RegExp
Record), n (a positive integer), and direction (forward or backward) and returns
a Matcher. It performs the following steps when called:
1. 1. 1. Assert: n ≥ 1.
2. 2. 2. Return a new Matcher with parameters (x, c) that captures rer, n, and
direction and performs the following steps when called:
1. a. a. Assert: x is a MatchState.
2. b. b. Assert: c is a MatcherContinuation.
3. c. c. Let Input be x's input.
4. d. d. Let cap be x's captures List.
5. e. e. Let r be cap[n].
6. f. f. If r is undefined, return c(x).
7. g. g. Let e be x's endIndex.
8. h. h. Let rs be r's startIndex.
9. i. i. Let re be r's endIndex.
10. j. j. Let len be re - rs.
11. k. k. If direction is forward, let f be e + len.
12. l. l. Else, let f be e - len.
13. m. m. Let InputLength be the number of elements in Input.
14. n. n. If f < 0 or f > InputLength, return failure.
15. o. o. Let g be min(e, f).
16. p. p. If there exists an integer i in the interval from 0 (inclusive) to
len (exclusive) such that Canonicalize(rer, Input[rs + i]) is not
Canonicalize(rer, Input[g + i]), return failure.
17. q. q. Let y be the MatchState (Input, f, cap).
18. r. r. Return c(y).
22.2.2.7.3 CANONICALIZE ( RER, CH )
The abstract operation Canonicalize takes arguments rer (a RegExp Record) and ch
(a character) and returns a character. It performs the following steps when
called:
1. 1. 1. If rer.[[Unicode]] is true and rer.[[IgnoreCase]] is true, then
1. a. a. If the file CaseFolding.txt of the Unicode Character Database
provides a simple or common case folding mapping for ch, return the
result of applying that mapping to ch.
2. b. b. Return ch.
2. 2. 2. If rer.[[IgnoreCase]] is false, return ch.
3. 3. 3. Assert: ch is a UTF-16 code unit.
4. 4. 4. Let cp be the code point whose numeric value is the numeric value of
ch.
5. 5. 5. Let u be the result of toUppercase(« cp »), according to the Unicode
Default Case Conversion algorithm.
6. 6. 6. Let uStr be CodePointsToString(u).
7. 7. 7. If the length of uStr ≠ 1, return ch.
8. 8. 8. Let cu be uStr's single code unit element.
9. 9. 9. If the numeric value of ch ≥ 128 and the numeric value of cu < 128,
return ch.
10. 10. 10. Return cu.
Note
In case-insignificant matches when rer.[[Unicode]] is true, all characters are
implicitly case-folded using the simple mapping provided by the Unicode Standard
immediately before they are compared. The simple mapping always maps to a single
code point, so it does not map, for example, ß (U+00DF LATIN SMALL LETTER SHARP
S) to ss or SS. It may however map code points outside the Basic Latin block to
code points within it—for example, ſ (U+017F LATIN SMALL LETTER LONG S)
case-folds to s (U+0073 LATIN SMALL LETTER S) and K (U+212A KELVIN SIGN)
case-folds to k (U+006B LATIN SMALL LETTER K). Strings containing those code
points are matched by regular expressions such as /[a-z]/ui.
In case-insignificant matches when rer.[[Unicode]] is false, the mapping is
based on Unicode Default Case Conversion algorithm toUppercase rather than
toCasefold, which results in some subtle differences. For example, Ω (U+2126 OHM
SIGN) is mapped by toUppercase to itself but by toCasefold to ω (U+03C9 GREEK
SMALL LETTER OMEGA) along with Ω (U+03A9 GREEK CAPITAL LETTER OMEGA), so
"\u2126" is matched by /[ω]/ui and /[\u03A9]/ui but not by /[ω]/i or
/[\u03A9]/i. Also, no code point outside the Basic Latin block is mapped to a
code point within it, so strings such as "\u017F ſ" and "\u212A K" are not
matched by /[a-z]/i.
22.2.2.8 RUNTIME SEMANTICS: COMPILECHARACTERCLASS
The syntax-directed operation CompileCharacterClass takes argument rer (a RegExp
Record) and returns a Record with fields [[CharSet]] (a CharSet) and [[Invert]]
(a Boolean). It is defined piecewise over the following productions:
CharacterClass :: [ ClassRanges ]
1. 1. 1. Let A be CompileToCharSet of ClassRanges with argument rer.
2. 2. 2. Return the Record { [[CharSet]]: A, [[Invert]]: false }.
CharacterClass :: [^ ClassRanges ]
1. 1. 1. Let A be CompileToCharSet of ClassRanges with argument rer.
2. 2. 2. Return the Record { [[CharSet]]: A, [[Invert]]: true }.
22.2.2.9 RUNTIME SEMANTICS: COMPILETOCHARSET
The syntax-directed operation CompileToCharSet takes argument rer (a RegExp
Record) and returns a CharSet.
Note 1
This section is amended in B.1.2.8.
It is defined piecewise over the following productions:
ClassRanges :: [empty]
1. 1. 1. Return the empty CharSet.
NonemptyClassRanges :: ClassAtom NonemptyClassRangesNoDash
1. 1. 1. Let A be CompileToCharSet of ClassAtom with argument rer.
2. 2. 2. Let B be CompileToCharSet of NonemptyClassRangesNoDash with argument
rer.
3. 3. 3. Return the union of CharSets A and B.
NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
1. 1. 1. Let A be CompileToCharSet of the first ClassAtom with argument rer.
2. 2. 2. Let B be CompileToCharSet of the second ClassAtom with argument rer.
3. 3. 3. Let C be CompileToCharSet of ClassRanges with argument rer.
4. 4. 4. Let D be CharacterRange(A, B).
5. 5. 5. Return the union of D and C.
NonemptyClassRangesNoDash :: ClassAtomNoDash NonemptyClassRangesNoDash
1. 1. 1. Let A be CompileToCharSet of ClassAtomNoDash with argument rer.
2. 2. 2. Let B be CompileToCharSet of NonemptyClassRangesNoDash with argument
rer.
3. 3. 3. Return the union of CharSets A and B.
NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
1. 1. 1. Let A be CompileToCharSet of ClassAtomNoDash with argument rer.
2. 2. 2. Let B be CompileToCharSet of ClassAtom with argument rer.
3. 3. 3. Let C be CompileToCharSet of ClassRanges with argument rer.
4. 4. 4. Let D be CharacterRange(A, B).
5. 5. 5. Return the union of D and C.
Note 2
ClassRanges can expand into a single ClassAtom and/or ranges of two ClassAtom
separated by dashes. In the latter case the ClassRanges includes all characters
between the first ClassAtom and the second ClassAtom, inclusive; an error occurs
if either ClassAtom does not represent a single character (for example, if one
is \w) or if the first ClassAtom's character value is strictly greater than the
second ClassAtom's character value.
Note 3
Even if the pattern ignores case, the case of the two ends of a range is
significant in determining which characters belong to the range. Thus, for
example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the
pattern /[E-f]/i matches all uppercase and lowercase letters in the Unicode
Basic Latin block as well as the symbols [, \, ], ^, _, and `.
Note 4
A - character can be treated literally or it can denote a range. It is treated
literally if it is the first or last character of ClassRanges, the beginning or
end limit of a range specification, or immediately follows a range
specification.
ClassAtom :: -
1. 1. 1. Return the CharSet containing the single character - U+002D
(HYPHEN-MINUS).
ClassAtomNoDash :: SourceCharacter but not one of \ or ] or -
1. 1. 1. Return the CharSet containing the character matched by
SourceCharacter.
ClassEscape :: b - CharacterEscape
1. 1. 1. Let cv be the CharacterValue of this ClassEscape.
2. 2. 2. Let c be the character whose character value is cv.
3. 3. 3. Return the CharSet containing the single character c.
Note 5
A ClassAtom can use any of the escape sequences that are allowed in the rest of
the regular expression except for \b, \B, and backreferences. Inside a
CharacterClass, \b means the backspace character, while \B and backreferences
raise errors. Using a backreference inside a ClassAtom causes an error.
CharacterClassEscape :: d
1. 1. 1. Return the ten-element CharSet containing the characters 0, 1, 2, 3,
4, 5, 6, 7, 8, and 9.
CharacterClassEscape :: D
1. 1. 1. Return the CharSet containing all characters not in the CharSet
returned by CharacterClassEscape :: d .
CharacterClassEscape :: s
1. 1. 1. Return the CharSet containing all characters corresponding to a code
point on the right-hand side of the WhiteSpace or LineTerminator
productions.
CharacterClassEscape :: S
1. 1. 1. Return the CharSet containing all characters not in the CharSet
returned by CharacterClassEscape :: s .
CharacterClassEscape :: w
1. 1. 1. Return WordCharacters(rer).
CharacterClassEscape :: W
1. 1. 1. Return the CharSet containing all characters not in the CharSet
returned by CharacterClassEscape :: w .
CharacterClassEscape :: p{ UnicodePropertyValueExpression }
1. 1. 1. Return the CharSet containing all Unicode code points included in
CompileToCharSet of UnicodePropertyValueExpression with argument rer.
CharacterClassEscape :: P{ UnicodePropertyValueExpression }
1. 1. 1. Return the CharSet containing all Unicode code points not included in
CompileToCharSet of UnicodePropertyValueExpression with argument rer.
UnicodePropertyValueExpression :: UnicodePropertyName = UnicodePropertyValue
1. 1. 1. Let ps be the source text matched by UnicodePropertyName.
2. 2. 2. Let p be UnicodeMatchProperty(ps).
3. 3. 3. Assert: p is a Unicode property name or property alias listed in the
“Property name and aliases” column of Table 65.
4. 4. 4. Let vs be the source text matched by UnicodePropertyValue.
5. 5. 5. Let v be UnicodeMatchPropertyValue(p, vs).
6. 6. 6. Return the CharSet containing all Unicode code points whose character
database definition includes the property p with value v.
UnicodePropertyValueExpression :: LoneUnicodePropertyNameOrValue
1. 1. 1. Let s be the source text matched by LoneUnicodePropertyNameOrValue.
2. 2. 2. If UnicodeMatchPropertyValue(General_Category, s) is a Unicode
property value or property value alias for the General_Category (gc)
property listed in PropertyValueAliases.txt, then
1. a. a. Return the CharSet containing all Unicode code points whose
character database definition includes the property “General_Category”
with value s.
3. 3. 3. Let p be UnicodeMatchProperty(s).
4. 4. 4. Assert: p is a binary Unicode property or binary property alias listed
in the “Property name and aliases” column of Table 66.
5. 5. 5. Return the CharSet containing all Unicode code points whose character
database definition includes the property p with value “True”.
22.2.2.9.1 CHARACTERRANGE ( A, B )
The abstract operation CharacterRange takes arguments A (a CharSet) and B (a
CharSet) and returns a CharSet. It performs the following steps when called:
1. 1. 1. Assert: A and B each contain exactly one character.
2. 2. 2. Let a be the one character in CharSet A.
3. 3. 3. Let b be the one character in CharSet B.
4. 4. 4. Let i be the character value of character a.
5. 5. 5. Let j be the character value of character b.
6. 6. 6. Assert: i ≤ j.
7. 7. 7. Return the CharSet containing all characters with a character value in
the inclusive interval from i to j.
22.2.2.9.2 WORDCHARACTERS ( RER )
The abstract operation WordCharacters takes argument rer (a RegExp Record) and
returns a CharSet. Returns a CharSet containing the characters considered "word
characters" for the purposes of \b, \B, \w, and \W It performs the following
steps when called:
1. 1. 1. Let basicWordChars be the CharSet containing every character in the
ASCII word characters.
2. 2. 2. Let extraWordChars be the CharSet containing all characters c such
that c is not in basicWordChars but Canonicalize(rer, c) is in
basicWordChars.
3. 3. 3. Assert: extraWordChars is empty unless rer.[[Unicode]] is true and
rer.[[IgnoreCase]] is true.
4. 4. 4. Return the union of basicWordChars and extraWordChars.
22.2.2.9.3 UNICODEMATCHPROPERTY ( P )
The abstract operation UnicodeMatchProperty takes argument p (ECMAScript source
text) and returns a Unicode property name. It performs the following steps when
called:
1. 1. 1. Assert: p is a Unicode property name or property alias listed in the
“Property name and aliases” column of Table 65 or Table 66.
2. 2. 2. Let c be the canonical property name of p as given in the “Canonical
property name” column of the corresponding row.
3. 3. 3. Return the List of Unicode code points c.
Implementations must support the Unicode property names and aliases listed in
Table 65 and Table 66. To ensure interoperability, implementations must not
support any other property names or aliases.
Note 1
For example, Script_Extensions (property name) and scx (property alias) are
valid, but script_extensions or Scx aren't.
Note 2
The listed properties form a superset of what UTS18 RL1.2 requires.
Note 3
The spellings of entries in these tables (including casing) match the spellings
used in the file PropertyAliases.txt in the Unicode Character Database. The
precise spellings in that file are guaranteed to be stable.
Table 65: Non-binary Unicode property aliases and their canonical property names
Property name and aliases Canonical property name General_Category
General_Category gc Script Script sc Script_Extensions Script_Extensions scx
Table 66: Binary Unicode property aliases and their canonical property names
Property name and aliases Canonical property name ASCII ASCII ASCII_Hex_Digit
ASCII_Hex_Digit AHex Alphabetic Alphabetic Alpha Any Any Assigned Assigned
Bidi_Control Bidi_Control Bidi_C Bidi_Mirrored Bidi_Mirrored Bidi_M
Case_Ignorable Case_Ignorable CI Cased Cased Changes_When_Casefolded
Changes_When_Casefolded CWCF Changes_When_Casemapped Changes_When_Casemapped
CWCM Changes_When_Lowercased Changes_When_Lowercased CWL
Changes_When_NFKC_Casefolded Changes_When_NFKC_Casefolded CWKCF
Changes_When_Titlecased Changes_When_Titlecased CWT Changes_When_Uppercased
Changes_When_Uppercased CWU Dash Dash Default_Ignorable_Code_Point
Default_Ignorable_Code_Point DI Deprecated Deprecated Dep Diacritic Diacritic
Dia Emoji Emoji Emoji_Component Emoji_Component EComp Emoji_Modifier
Emoji_Modifier EMod Emoji_Modifier_Base Emoji_Modifier_Base EBase
Emoji_Presentation Emoji_Presentation EPres Extended_Pictographic
Extended_Pictographic ExtPict Extender Extender Ext Grapheme_Base Grapheme_Base
Gr_Base Grapheme_Extend Grapheme_Extend Gr_Ext Hex_Digit Hex_Digit Hex
IDS_Binary_Operator IDS_Binary_Operator IDSB IDS_Trinary_Operator
IDS_Trinary_Operator IDST ID_Continue ID_Continue IDC ID_Start ID_Start IDS
Ideographic Ideographic Ideo Join_Control Join_Control Join_C
Logical_Order_Exception Logical_Order_Exception LOE Lowercase Lowercase Lower
Math Math Noncharacter_Code_Point Noncharacter_Code_Point NChar Pattern_Syntax
Pattern_Syntax Pat_Syn Pattern_White_Space Pattern_White_Space Pat_WS
Quotation_Mark Quotation_Mark QMark Radical Radical Regional_Indicator
Regional_Indicator RI Sentence_Terminal Sentence_Terminal STerm Soft_Dotted
Soft_Dotted SD Terminal_Punctuation Terminal_Punctuation Term Unified_Ideograph
Unified_Ideograph UIdeo Uppercase Uppercase Upper Variation_Selector
Variation_Selector VS White_Space White_Space space XID_Continue XID_Continue
XIDC XID_Start XID_Start XIDS
22.2.2.9.4 UNICODEMATCHPROPERTYVALUE ( P, V )
The abstract operation UnicodeMatchPropertyValue takes arguments p (ECMAScript
source text) and v (ECMAScript source text) and returns a Unicode property
value. It performs the following steps when called:
1. 1. 1. Assert: p is a canonical, unaliased Unicode property name listed in
the “Canonical property name” column of Table 65.
2. 2. 2. Assert: v is a property value or property value alias for the Unicode
property p listed in PropertyValueAliases.txt.
3. 3. 3. Let value be the canonical property value of v as given in the
“Canonical property value” column of the corresponding row.
4. 4. 4. Return the List of Unicode code points value.
Implementations must support the Unicode property values and property value
aliases listed in PropertyValueAliases.txt for the properties listed in Table
65. To ensure interoperability, implementations must not support any other
property values or property value aliases.
Note 1
For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo
and Old Persian aren't.
Note 2
This algorithm differs from the matching rules for symbolic values listed in
UAX44: case, white space, U+002D (HYPHEN-MINUS), and U+005F (LOW LINE) are not
ignored, and the Is prefix is not supported.
22.2.3 ABSTRACT OPERATIONS FOR REGEXP CREATION
22.2.3.1 REGEXPCREATE ( P, F )
The abstract operation RegExpCreate takes arguments P (an ECMAScript language
value) and F (a String or undefined) and returns either a normal completion
containing an Object or a throw completion. It performs the following steps when
called:
1. 1. 1. Let obj be ! RegExpAlloc(%RegExp%).
2. 2. 2. Return ? RegExpInitialize(obj, P, F).
22.2.3.2 REGEXPALLOC ( NEWTARGET )
The abstract operation RegExpAlloc takes argument newTarget (a constructor) and
returns either a normal completion containing an Object or a throw completion.
It performs the following steps when called:
1. 1. 1. Let obj be ? OrdinaryCreateFromConstructor(newTarget,
"%RegExp.prototype%", « [[OriginalSource]], [[OriginalFlags]],
[[RegExpRecord]], [[RegExpMatcher]] »).
2. 2. 2. Perform ! DefinePropertyOrThrow(obj, "lastIndex", PropertyDescriptor {
[[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
3. 3. 3. Return obj.
22.2.3.3 REGEXPINITIALIZE ( OBJ, PATTERN, FLAGS )
The abstract operation RegExpInitialize takes arguments obj (an Object), pattern
(an ECMAScript language value), and flags (an ECMAScript language value) and
returns either a normal completion containing an Object or a throw completion.
It performs the following steps when called:
1. 1. 1. If pattern is undefined, let P be the empty String.
2. 2. 2. Else, let P be ? ToString(pattern).
3. 3. 3. If flags is undefined, let F be the empty String.
4. 4. 4. Else, let F be ? ToString(flags).
5. 5. 5. If F contains any code unit other than "d", "g", "i", "m", "s", "u",
or "y", or if F contains any code unit more than once, throw a SyntaxError
exception.
6. 6. 6. If F contains "i", let i be true; else let i be false.
7. 7. 7. If F contains "m", let m be true; else let m be false.
8. 8. 8. If F contains "s", let s be true; else let s be false.
9. 9. 9. If F contains "u", let u be true; else let u be false.
10. 10. 10. If u is true, then
1. a. a. Let patternText be StringToCodePoints(P).
11. 11. 11. Else,
1. a. a. Let patternText be the result of interpreting each of P's 16-bit
elements as a Unicode BMP code point. UTF-16 decoding is not applied to
the elements.
12. 12. 12. Let parseResult be ParsePattern(patternText, u).
13. 13. 13. If parseResult is a non-empty List of SyntaxError objects, throw a
SyntaxError exception.
14. 14. 14. Assert: parseResult is a Pattern Parse Node.
15. 15. 15. Set obj.[[OriginalSource]] to P.
16. 16. 16. Set obj.[[OriginalFlags]] to F.
17. 17. 17. Let capturingGroupsCount be
CountLeftCapturingParensWithin(parseResult).
18. 18. 18. Let rer be the RegExp Record { [[IgnoreCase]]: i, [[Multiline]]: m,
[[DotAll]]: s, [[Unicode]]: u, [[CapturingGroupsCount]]:
capturingGroupsCount }.
19. 19. 19. Set obj.[[RegExpRecord]] to rer.
20. 20. 20. Set obj.[[RegExpMatcher]] to CompilePattern of parseResult with
argument rer.
21. 21. 21. Perform ? Set(obj, "lastIndex", +0𝔽, true).
22. 22. 22. Return obj.
22.2.3.4 STATIC SEMANTICS: PARSEPATTERN ( PATTERNTEXT, U )
The abstract operation ParsePattern takes arguments patternText (a sequence of
Unicode code points) and u (a Boolean) and returns a Parse Node or a non-empty
List of SyntaxError objects.
Note
This section is amended in B.1.2.9.
It performs the following steps when called:
1. 1. 1. If u is true, then
1. a. a. Let parseResult be ParseText(patternText, Pattern[+UnicodeMode,
+N]).
2. 2. 2. Else,
1. a. a. Let parseResult be ParseText(patternText, Pattern[~UnicodeMode,
+N]).
3. 3. 3. Return parseResult.
22.2.4 THE REGEXP CONSTRUCTOR
The RegExp constructor:
* is %RegExp%.
* is the initial value of the "RegExp" property of the global object.
* creates and initializes a new RegExp object when called as a function rather
than as a constructor. Thus the function call RegExp(…) is equivalent to the
object creation expression new RegExp(…) with the same arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified RegExp behaviour must
include a super call to the RegExp constructor to create and initialize
subclass instances with the necessary internal slots.
22.2.4.1 REGEXP ( PATTERN, FLAGS )
This function performs the following steps when called:
1. 1. 1. Let patternIsRegExp be ? IsRegExp(pattern).
2. 2. 2. If NewTarget is undefined, then
1. a. a. Let newTarget be the active function object.
2. b. b. If patternIsRegExp is true and flags is undefined, then
1. i. i. Let patternConstructor be ? Get(pattern, "constructor").
2. ii. ii. If SameValue(newTarget, patternConstructor) is true, return
pattern.
3. 3. 3. Else, let newTarget be NewTarget.
4. 4. 4. If pattern is an Object and pattern has a [[RegExpMatcher]] internal
slot, then
1. a. a. Let P be pattern.[[OriginalSource]].
2. b. b. If flags is undefined, let F be pattern.[[OriginalFlags]].
3. c. c. Else, let F be flags.
5. 5. 5. Else if patternIsRegExp is true, then
1. a. a. Let P be ? Get(pattern, "source").
2. b. b. If flags is undefined, then
1. i. i. Let F be ? Get(pattern, "flags").
3. c. c. Else, let F be flags.
6. 6. 6. Else,
1. a. a. Let P be pattern.
2. b. b. Let F be flags.
7. 7. 7. Let O be ? RegExpAlloc(newTarget).
8. 8. 8. Return ? RegExpInitialize(O, P, F).
Note
If pattern is supplied using a StringLiteral, the usual escape sequence
substitutions are performed before the String is processed by this function. If
pattern must contain an escape sequence to be recognized by this function, any
U+005C (REVERSE SOLIDUS) code points must be escaped within the StringLiteral to
prevent them being removed when the contents of the StringLiteral are formed.
22.2.5 PROPERTIES OF THE REGEXP CONSTRUCTOR
The RegExp constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
22.2.5.1 REGEXP.PROTOTYPE
The initial value of RegExp.prototype is the RegExp prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
22.2.5.2 GET REGEXP [ @@SPECIES ]
RegExp[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
RegExp prototype methods normally use their this value's constructor to create a
derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
22.2.6 PROPERTIES OF THE REGEXP PROTOTYPE OBJECT
The RegExp prototype object:
* is %RegExp.prototype%.
* is an ordinary object.
* is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot
or any of the other internal slots of RegExp instance objects.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
Note
The RegExp prototype object does not have a "valueOf" property of its own;
however, it inherits the "valueOf" property from the Object prototype object.
22.2.6.1 REGEXP.PROTOTYPE.CONSTRUCTOR
The initial value of RegExp.prototype.constructor is %RegExp%.
22.2.6.2 REGEXP.PROTOTYPE.EXEC ( STRING )
This method searches string for an occurrence of the regular expression pattern
and returns an Array containing the results of the match, or null if string did
not match.
It performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
3. 3. 3. Let S be ? ToString(string).
4. 4. 4. Return ? RegExpBuiltinExec(R, S).
22.2.6.3 GET REGEXP.PROTOTYPE.DOTALL
RegExp.prototype.dotAll is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.6.4 GET REGEXP.PROTOTYPE.FLAGS
RegExp.prototype.flags is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. If R is not an Object, throw a TypeError exception.
3. 3. 3. Let codeUnits be a new empty List.
4. 4. 4. Let hasIndices be ToBoolean(? Get(R, "hasIndices")).
5. 5. 5. If hasIndices is true, append the code unit 0x0064 (LATIN SMALL
LETTER D) to codeUnits.
6. 6. 6. Let global be ToBoolean(? Get(R, "global")).
7. 7. 7. If global is true, append the code unit 0x0067 (LATIN SMALL LETTER G)
to codeUnits.
8. 8. 8. Let ignoreCase be ToBoolean(? Get(R, "ignoreCase")).
9. 9. 9. If ignoreCase is true, append the code unit 0x0069 (LATIN SMALL
LETTER I) to codeUnits.
10. 10. 10. Let multiline be ToBoolean(? Get(R, "multiline")).
11. 11. 11. If multiline is true, append the code unit 0x006D (LATIN SMALL
LETTER M) to codeUnits.
12. 12. 12. Let dotAll be ToBoolean(? Get(R, "dotAll")).
13. 13. 13. If dotAll is true, append the code unit 0x0073 (LATIN SMALL LETTER
S) to codeUnits.
14. 14. 14. Let unicode be ToBoolean(? Get(R, "unicode")).
15. 15. 15. If unicode is true, append the code unit 0x0075 (LATIN SMALL LETTER
U) to codeUnits.
16. 16. 16. Let sticky be ToBoolean(? Get(R, "sticky")).
17. 17. 17. If sticky is true, append the code unit 0x0079 (LATIN SMALL LETTER
Y) to codeUnits.
18. 18. 18. Return the String value whose code units are the elements of the
List codeUnits. If codeUnits has no elements, the empty String is returned.
22.2.6.4.1 REGEXPHASFLAG ( R, CODEUNIT )
The abstract operation RegExpHasFlag takes arguments R (an ECMAScript language
value) and codeUnit (a code unit) and returns either a normal completion
containing either a Boolean or undefined, or a throw completion. It performs the
following steps when called:
1. 1. 1. If R is not an Object, throw a TypeError exception.
2. 2. 2. If R does not have an [[OriginalFlags]] internal slot, then
1. a. a. If SameValue(R, %RegExp.prototype%) is true, return undefined.
2. b. b. Otherwise, throw a TypeError exception.
3. 3. 3. Let flags be R.[[OriginalFlags]].
4. 4. 4. If flags contains codeUnit, return true.
5. 5. 5. Return false.
22.2.6.5 GET REGEXP.PROTOTYPE.GLOBAL
RegExp.prototype.global is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.6.6 GET REGEXP.PROTOTYPE.HASINDICES
RegExp.prototype.hasIndices is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x0064 (LATIN SMALL LETTER D).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.6.7 GET REGEXP.PROTOTYPE.IGNORECASE
RegExp.prototype.ignoreCase is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.6.8 REGEXP.PROTOTYPE [ @@MATCH ] ( STRING )
This method performs the following steps when called:
1. 1. 1. Let rx be the this value.
2. 2. 2. If rx is not an Object, throw a TypeError exception.
3. 3. 3. Let S be ? ToString(string).
4. 4. 4. Let flags be ? ToString(? Get(rx, "flags")).
5. 5. 5. If flags does not contain "g", then
1. a. a. Return ? RegExpExec(rx, S).
6. 6. 6. Else,
1. a. a. If flags contains "u", let fullUnicode be true. Otherwise, let
fullUnicode be false.
2. b. b. Perform ? Set(rx, "lastIndex", +0𝔽, true).
3. c. c. Let A be ! ArrayCreate(0).
4. d. d. Let n be 0.
5. e. e. Repeat,
1. i. i. Let result be ? RegExpExec(rx, S).
2. ii. ii. If result is null, then
1. 1. 1. If n = 0, return null.
2. 2. 2. Return A.
3. iii. iii. Else,
1. 1. 1. Let matchStr be ? ToString(? Get(result, "0")).
2. 2. 2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
matchStr).
3. 3. 3. If matchStr is the empty String, then
1. a. a. Let thisIndex be ℝ(? ToLength(? Get(rx, "lastIndex"))).
2. b. b. Let nextIndex be AdvanceStringIndex(S, thisIndex,
fullUnicode).
3. c. c. Perform ? Set(rx, "lastIndex", 𝔽(nextIndex), true).
4. 4. 4. Set n to n + 1.
The value of the "name" property of this method is "[Symbol.match]".
Note
The @@match property is used by the IsRegExp abstract operation to identify
objects that have the basic behaviour of regular expressions. The absence of a
@@match property or the existence of such a property whose value does not
Boolean coerce to true indicates that the object is not intended to be used as a
regular expression object.
22.2.6.9 REGEXP.PROTOTYPE [ @@MATCHALL ] ( STRING )
This method performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. If R is not an Object, throw a TypeError exception.
3. 3. 3. Let S be ? ToString(string).
4. 4. 4. Let C be ? SpeciesConstructor(R, %RegExp%).
5. 5. 5. Let flags be ? ToString(? Get(R, "flags")).
6. 6. 6. Let matcher be ? Construct(C, « R, flags »).
7. 7. 7. Let lastIndex be ? ToLength(? Get(R, "lastIndex")).
8. 8. 8. Perform ? Set(matcher, "lastIndex", lastIndex, true).
9. 9. 9. If flags contains "g", let global be true.
10. 10. 10. Else, let global be false.
11. 11. 11. If flags contains "u", let fullUnicode be true.
12. 12. 12. Else, let fullUnicode be false.
13. 13. 13. Return CreateRegExpStringIterator(matcher, S, global, fullUnicode).
The value of the "name" property of this method is "[Symbol.matchAll]".
22.2.6.10 GET REGEXP.PROTOTYPE.MULTILINE
RegExp.prototype.multiline is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.6.11 REGEXP.PROTOTYPE [ @@REPLACE ] ( STRING, REPLACEVALUE )
This method performs the following steps when called:
1. 1. 1. Let rx be the this value.
2. 2. 2. If rx is not an Object, throw a TypeError exception.
3. 3. 3. Let S be ? ToString(string).
4. 4. 4. Let lengthS be the length of S.
5. 5. 5. Let functionalReplace be IsCallable(replaceValue).
6. 6. 6. If functionalReplace is false, then
1. a. a. Set replaceValue to ? ToString(replaceValue).
7. 7. 7. Let flags be ? ToString(? Get(rx, "flags")).
8. 8. 8. If flags contains "g", let global be true. Otherwise, let global be
false.
9. 9. 9. If global is true, then
1. a. a. If flags contains "u", let fullUnicode be true. Otherwise, let
fullUnicode be false.
2. b. b. Perform ? Set(rx, "lastIndex", +0𝔽, true).
10. 10. 10. Let results be a new empty List.
11. 11. 11. Let done be false.
12. 12. 12. Repeat, while done is false,
1. a. a. Let result be ? RegExpExec(rx, S).
2. b. b. If result is null, set done to true.
3. c. c. Else,
1. i. i. Append result to results.
2. ii. ii. If global is false, set done to true.
3. iii. iii. Else,
1. 1. 1. Let matchStr be ? ToString(? Get(result, "0")).
2. 2. 2. If matchStr is the empty String, then
1. a. a. Let thisIndex be ℝ(? ToLength(? Get(rx, "lastIndex"))).
2. b. b. Let nextIndex be AdvanceStringIndex(S, thisIndex,
fullUnicode).
3. c. c. Perform ? Set(rx, "lastIndex", 𝔽(nextIndex), true).
13. 13. 13. Let accumulatedResult be the empty String.
14. 14. 14. Let nextSourcePosition be 0.
15. 15. 15. For each element result of results, do
1. a. a. Let resultLength be ? LengthOfArrayLike(result).
2. b. b. Let nCaptures be max(resultLength - 1, 0).
3. c. c. Let matched be ? ToString(? Get(result, "0")).
4. d. d. Let matchLength be the length of matched.
5. e. e. Let position be ? ToIntegerOrInfinity(? Get(result, "index")).
6. f. f. Set position to the result of clamping position between 0 and
lengthS.
7. g. g. Let captures be a new empty List.
8. h. h. Let n be 1.
9. i. i. Repeat, while n ≤ nCaptures,
1. i. i. Let capN be ? Get(result, ! ToString(𝔽(n))).
2. ii. ii. If capN is not undefined, then
1. 1. 1. Set capN to ? ToString(capN).
3. iii. iii. Append capN to captures.
4. iv. iv. NOTE: When n = 1, the preceding step puts the first element
into captures (at index 0). More generally, the nth capture (the
characters captured by the nth set of capturing parentheses) is at
captures[n - 1].
5. v. v. Set n to n + 1.
10. j. j. Let namedCaptures be ? Get(result, "groups").
11. k. k. If functionalReplace is true, then
1. i. i. Let replacerArgs be the list-concatenation of « matched »,
captures, and « 𝔽(position), S ».
2. ii. ii. If namedCaptures is not undefined, then
1. 1. 1. Append namedCaptures to replacerArgs.
3. iii. iii. Let replValue be ? Call(replaceValue, undefined,
replacerArgs).
4. iv. iv. Let replacement be ? ToString(replValue).
12. l. l. Else,
1. i. i. If namedCaptures is not undefined, then
1. 1. 1. Set namedCaptures to ? ToObject(namedCaptures).
2. ii. ii. Let replacement be ? GetSubstitution(matched, S, position,
captures, namedCaptures, replaceValue).
13. m. m. If position ≥ nextSourcePosition, then
1. i. i. NOTE: position should not normally move backwards. If it does,
it is an indication of an ill-behaving RegExp subclass or use of an
access triggered side-effect to change the global flag or other
characteristics of rx. In such cases, the corresponding substitution
is ignored.
2. ii. ii. Set accumulatedResult to the string-concatenation of
accumulatedResult, the substring of S from nextSourcePosition to
position, and replacement.
3. iii. iii. Set nextSourcePosition to position + matchLength.
16. 16. 16. If nextSourcePosition ≥ lengthS, return accumulatedResult.
17. 17. 17. Return the string-concatenation of accumulatedResult and the
substring of S from nextSourcePosition.
The value of the "name" property of this method is "[Symbol.replace]".
22.2.6.12 REGEXP.PROTOTYPE [ @@SEARCH ] ( STRING )
This method performs the following steps when called:
1. 1. 1. Let rx be the this value.
2. 2. 2. If rx is not an Object, throw a TypeError exception.
3. 3. 3. Let S be ? ToString(string).
4. 4. 4. Let previousLastIndex be ? Get(rx, "lastIndex").
5. 5. 5. If SameValue(previousLastIndex, +0𝔽) is false, then
1. a. a. Perform ? Set(rx, "lastIndex", +0𝔽, true).
6. 6. 6. Let result be ? RegExpExec(rx, S).
7. 7. 7. Let currentLastIndex be ? Get(rx, "lastIndex").
8. 8. 8. If SameValue(currentLastIndex, previousLastIndex) is false, then
1. a. a. Perform ? Set(rx, "lastIndex", previousLastIndex, true).
9. 9. 9. If result is null, return -1𝔽.
10. 10. 10. Return ? Get(result, "index").
The value of the "name" property of this method is "[Symbol.search]".
Note
The "lastIndex" and "global" properties of this RegExp object are ignored when
performing the search. The "lastIndex" property is left unchanged.
22.2.6.13 GET REGEXP.PROTOTYPE.SOURCE
RegExp.prototype.source is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. If R is not an Object, throw a TypeError exception.
3. 3. 3. If R does not have an [[OriginalSource]] internal slot, then
1. a. a. If SameValue(R, %RegExp.prototype%) is true, return "(?:)".
2. b. b. Otherwise, throw a TypeError exception.
4. 4. 4. Assert: R has an [[OriginalFlags]] internal slot.
5. 5. 5. Let src be R.[[OriginalSource]].
6. 6. 6. Let flags be R.[[OriginalFlags]].
7. 7. 7. Return EscapeRegExpPattern(src, flags).
22.2.6.13.1 ESCAPEREGEXPPATTERN ( P, F )
The abstract operation EscapeRegExpPattern takes arguments P (a String) and F (a
String) and returns a String. It performs the following steps when called:
1. 1. 1. Let S be a String in the form of a Pattern[~UnicodeMode]
(Pattern[+UnicodeMode] if F contains "u") equivalent to P interpreted as
UTF-16 encoded Unicode code points (6.1.4), in which certain code points are
escaped as described below. S may or may not differ from P; however, the
Abstract Closure that would result from evaluating S as a
Pattern[~UnicodeMode] (Pattern[+UnicodeMode] if F contains "u") must behave
identically to the Abstract Closure given by the constructed object's
[[RegExpMatcher]] internal slot. Multiple calls to this abstract operation
using the same values for P and F must produce identical results.
2. 2. 2. The code points / or any LineTerminator occurring in the pattern shall
be escaped in S as necessary to ensure that the string-concatenation of "/",
S, "/", and F can be parsed (in an appropriate lexical context) as a
RegularExpressionLiteral that behaves identically to the constructed regular
expression. For example, if P is "/", then S could be "\/" or "\u002F",
among other possibilities, but not "/", because /// followed by F would be
parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P
is the empty String, this specification can be met by letting S be "(?:)".
3. 3. 3. Return S.
22.2.6.14 REGEXP.PROTOTYPE [ @@SPLIT ] ( STRING, LIMIT )
Note 1
This method returns an Array into which substrings of the result of converting
string to a String have been stored. The substrings are determined by searching
from left to right for matches of the this value regular expression; these
occurrences are not part of any String in the returned array, but serve to
divide up the String value.
The this value may be an empty regular expression or a regular expression that
can match an empty String. In this case, the regular expression does not match
the empty substring at the beginning or end of the input String, nor does it
match the empty substring at the end of the previous separator match. (For
example, if the regular expression matches the empty String, the String is split
up into individual code unit elements; the length of the result array equals the
length of the String, and each substring contains one code unit.) Only the first
match at a given index of the String is considered, even if backtracking could
yield a non-empty substring match at that index. (For example,
/a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while
/a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If string is (or converts to) the empty String, the result depends on whether
the regular expression can match the empty String. If it can, the result array
contains no elements. Otherwise, the result array contains one element, which is
the empty String.
If the regular expression contains capturing parentheses, then each time
separator is matched the results (including any undefined results) of the
capturing parentheses are spliced into the output array. For example,
/<(\/)?([^<>]+)>/[Symbol.split]("A<B>bold</B>and<CODE>coded</CODE>")
evaluates to the array
["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]
If limit is not undefined, then the output array is truncated so that it
contains no more than limit elements.
This method performs the following steps when called:
1. 1. 1. Let rx be the this value.
2. 2. 2. If rx is not an Object, throw a TypeError exception.
3. 3. 3. Let S be ? ToString(string).
4. 4. 4. Let C be ? SpeciesConstructor(rx, %RegExp%).
5. 5. 5. Let flags be ? ToString(? Get(rx, "flags")).
6. 6. 6. If flags contains "u", let unicodeMatching be true.
7. 7. 7. Else, let unicodeMatching be false.
8. 8. 8. If flags contains "y", let newFlags be flags.
9. 9. 9. Else, let newFlags be the string-concatenation of flags and "y".
10. 10. 10. Let splitter be ? Construct(C, « rx, newFlags »).
11. 11. 11. Let A be ! ArrayCreate(0).
12. 12. 12. Let lengthA be 0.
13. 13. 13. If limit is undefined, let lim be 232 - 1; else let lim be ℝ(?
ToUint32(limit)).
14. 14. 14. If lim = 0, return A.
15. 15. 15. If S is the empty String, then
1. a. a. Let z be ? RegExpExec(splitter, S).
2. b. b. If z is not null, return A.
3. c. c. Perform ! CreateDataPropertyOrThrow(A, "0", S).
4. d. d. Return A.
16. 16. 16. Let size be the length of S.
17. 17. 17. Let p be 0.
18. 18. 18. Let q be p.
19. 19. 19. Repeat, while q < size,
1. a. a. Perform ? Set(splitter, "lastIndex", 𝔽(q), true).
2. b. b. Let z be ? RegExpExec(splitter, S).
3. c. c. If z is null, set q to AdvanceStringIndex(S, q, unicodeMatching).
4. d. d. Else,
1. i. i. Let e be ℝ(? ToLength(? Get(splitter, "lastIndex"))).
2. ii. ii. Set e to min(e, size).
3. iii. iii. If e = p, set q to AdvanceStringIndex(S, q,
unicodeMatching).
4. iv. iv. Else,
1. 1. 1. Let T be the substring of S from p to q.
2. 2. 2. Perform ! CreateDataPropertyOrThrow(A,
! ToString(𝔽(lengthA)), T).
3. 3. 3. Set lengthA to lengthA + 1.
4. 4. 4. If lengthA = lim, return A.
5. 5. 5. Set p to e.
6. 6. 6. Let numberOfCaptures be ? LengthOfArrayLike(z).
7. 7. 7. Set numberOfCaptures to max(numberOfCaptures - 1, 0).
8. 8. 8. Let i be 1.
9. 9. 9. Repeat, while i ≤ numberOfCaptures,
1. a. a. Let nextCapture be ? Get(z, ! ToString(𝔽(i))).
2. b. b. Perform ! CreateDataPropertyOrThrow(A,
! ToString(𝔽(lengthA)), nextCapture).
3. c. c. Set i to i + 1.
4. d. d. Set lengthA to lengthA + 1.
5. e. e. If lengthA = lim, return A.
10. 10. 10. Set q to p.
20. 20. 20. Let T be the substring of S from p to size.
21. 21. 21. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
22. 22. 22. Return A.
The value of the "name" property of this method is "[Symbol.split]".
Note 2
This method ignores the value of the "global" and "sticky" properties of this
RegExp object.
22.2.6.15 GET REGEXP.PROTOTYPE.STICKY
RegExp.prototype.sticky is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.6.16 REGEXP.PROTOTYPE.TEST ( S )
This method performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. If R is not an Object, throw a TypeError exception.
3. 3. 3. Let string be ? ToString(S).
4. 4. 4. Let match be ? RegExpExec(R, string).
5. 5. 5. If match is not null, return true; else return false.
22.2.6.17 REGEXP.PROTOTYPE.TOSTRING ( )
1. 1. 1. Let R be the this value.
2. 2. 2. If R is not an Object, throw a TypeError exception.
3. 3. 3. Let pattern be ? ToString(? Get(R, "source")).
4. 4. 4. Let flags be ? ToString(? Get(R, "flags")).
5. 5. 5. Let result be the string-concatenation of "/", pattern, "/", and
flags.
6. 6. 6. Return result.
Note
The returned String has the form of a RegularExpressionLiteral that evaluates to
another RegExp object with the same behaviour as this object.
22.2.6.18 GET REGEXP.PROTOTYPE.UNICODE
RegExp.prototype.unicode is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let R be the this value.
2. 2. 2. Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
3. 3. 3. Return ? RegExpHasFlag(R, cu).
22.2.7 ABSTRACT OPERATIONS FOR REGEXP MATCHING
22.2.7.1 REGEXPEXEC ( R, S )
The abstract operation RegExpExec takes arguments R (an Object) and S (a String)
and returns either a normal completion containing either an Object or null, or a
throw completion. It performs the following steps when called:
1. 1. 1. Let exec be ? Get(R, "exec").
2. 2. 2. If IsCallable(exec) is true, then
1. a. a. Let result be ? Call(exec, R, « S »).
2. b. b. If result is not an Object and result is not null, throw a
TypeError exception.
3. c. c. Return result.
3. 3. 3. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
4. 4. 4. Return ? RegExpBuiltinExec(R, S).
Note
If a callable "exec" property is not found this algorithm falls back to
attempting to use the built-in RegExp matching algorithm. This provides
compatible behaviour for code written for prior editions where most built-in
algorithms that use regular expressions did not perform a dynamic property
lookup of "exec".
22.2.7.2 REGEXPBUILTINEXEC ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R (an initialized
RegExp instance) and S (a String) and returns either a normal completion
containing either an Array exotic object or null, or a throw completion. It
performs the following steps when called:
1. 1. 1. Let length be the length of S.
2. 2. 2. Let lastIndex be ℝ(? ToLength(? Get(R, "lastIndex"))).
3. 3. 3. Let flags be R.[[OriginalFlags]].
4. 4. 4. If flags contains "g", let global be true; else let global be false.
5. 5. 5. If flags contains "y", let sticky be true; else let sticky be false.
6. 6. 6. If flags contains "d", let hasIndices be true; else let hasIndices be
false.
7. 7. 7. If global is false and sticky is false, set lastIndex to 0.
8. 8. 8. Let matcher be R.[[RegExpMatcher]].
9. 9. 9. If flags contains "u", let fullUnicode be true; else let fullUnicode
be false.
10. 10. 10. Let matchSucceeded be false.
11. 11. 11. If fullUnicode is true, let input be StringToCodePoints(S).
Otherwise, let input be a List whose elements are the code units that are
the elements of S.
12. 12. 12. NOTE: Each element of input is considered to be a character.
13. 13. 13. Repeat, while matchSucceeded is false,
1. a. a. If lastIndex > length, then
1. i. i. If global is true or sticky is true, then
1. 1. 1. Perform ? Set(R, "lastIndex", +0𝔽, true).
2. ii. ii. Return null.
2. b. b. Let inputIndex be the index into input of the character that was
obtained from element lastIndex of S.
3. c. c. Let r be matcher(input, inputIndex).
4. d. d. If r is failure, then
1. i. i. If sticky is true, then
1. 1. 1. Perform ? Set(R, "lastIndex", +0𝔽, true).
2. 2. 2. Return null.
2. ii. ii. Set lastIndex to AdvanceStringIndex(S, lastIndex,
fullUnicode).
5. e. e. Else,
1. i. i. Assert: r is a MatchState.
2. ii. ii. Set matchSucceeded to true.
14. 14. 14. Let e be r's endIndex value.
15. 15. 15. If fullUnicode is true, set e to GetStringIndex(S, e).
16. 16. 16. If global is true or sticky is true, then
1. a. a. Perform ? Set(R, "lastIndex", 𝔽(e), true).
17. 17. 17. Let n be the number of elements in r's captures List.
18. 18. 18. Assert: n = R.[[RegExpRecord]].[[CapturingGroupsCount]].
19. 19. 19. Assert: n < 232 - 1.
20. 20. 20. Let A be ! ArrayCreate(n + 1).
21. 21. 21. Assert: The mathematical value of A's "length" property is n + 1.
22. 22. 22. Perform ! CreateDataPropertyOrThrow(A, "index", 𝔽(lastIndex)).
23. 23. 23. Perform ! CreateDataPropertyOrThrow(A, "input", S).
24. 24. 24. Let match be the Match Record { [[StartIndex]]: lastIndex,
[[EndIndex]]: e }.
25. 25. 25. Let indices be a new empty List.
26. 26. 26. Let groupNames be a new empty List.
27. 27. 27. Append match to indices.
28. 28. 28. Let matchedSubstr be GetMatchString(S, match).
29. 29. 29. Perform ! CreateDataPropertyOrThrow(A, "0", matchedSubstr).
30. 30. 30. If R contains any GroupName, then
1. a. a. Let groups be OrdinaryObjectCreate(null).
2. b. b. Let hasGroups be true.
31. 31. 31. Else,
1. a. a. Let groups be undefined.
2. b. b. Let hasGroups be false.
32. 32. 32. Perform ! CreateDataPropertyOrThrow(A, "groups", groups).
33. 33. 33. For each integer i such that 1 ≤ i ≤ n, in ascending order, do
1. a. a. Let captureI be ith element of r's captures List.
2. b. b. If captureI is undefined, then
1. i. i. Let capturedValue be undefined.
2. ii. ii. Append undefined to indices.
3. c. c. Else,
1. i. i. Let captureStart be captureI's startIndex.
2. ii. ii. Let captureEnd be captureI's endIndex.
3. iii. iii. If fullUnicode is true, then
1. 1. 1. Set captureStart to GetStringIndex(S, captureStart).
2. 2. 2. Set captureEnd to GetStringIndex(S, captureEnd).
4. iv. iv. Let capture be the Match Record { [[StartIndex]]:
captureStart, [[EndIndex]]: captureEnd }.
5. v. v. Let capturedValue be GetMatchString(S, capture).
6. vi. vi. Append capture to indices.
4. d. d. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(i)),
capturedValue).
5. e. e. If the ith capture of R was defined with a GroupName, then
1. i. i. Let s be the CapturingGroupName of that GroupName.
2. ii. ii. Perform ! CreateDataPropertyOrThrow(groups, s,
capturedValue).
3. iii. iii. Append s to groupNames.
6. f. f. Else,
1. i. i. Append undefined to groupNames.
34. 34. 34. If hasIndices is true, then
1. a. a. Let indicesArray be MakeMatchIndicesIndexPairArray(S, indices,
groupNames, hasGroups).
2. b. b. Perform ! CreateDataPropertyOrThrow(A, "indices", indicesArray).
35. 35. 35. Return A.
22.2.7.3 ADVANCESTRINGINDEX ( S, INDEX, UNICODE )
The abstract operation AdvanceStringIndex takes arguments S (a String), index (a
non-negative integer), and unicode (a Boolean) and returns an integer. It
performs the following steps when called:
1. 1. 1. Assert: index ≤ 253 - 1.
2. 2. 2. If unicode is false, return index + 1.
3. 3. 3. Let length be the length of S.
4. 4. 4. If index + 1 ≥ length, return index + 1.
5. 5. 5. Let cp be CodePointAt(S, index).
6. 6. 6. Return index + cp.[[CodeUnitCount]].
22.2.7.4 GETSTRINGINDEX ( S, CODEPOINTINDEX )
The abstract operation GetStringIndex takes arguments S (a String) and
codePointIndex (a non-negative integer) and returns a non-negative integer. It
interprets S as a sequence of UTF-16 encoded code points, as described in 6.1.4,
and returns the code unit index corresponding to code point index codePointIndex
when such an index exists. Otherwise, it returns the length of S. It performs
the following steps when called:
1. 1. 1. If S is the empty String, return 0.
2. 2. 2. Let len be the length of S.
3. 3. 3. Let codeUnitCount be 0.
4. 4. 4. Let codePointCount be 0.
5. 5. 5. Repeat, while codeUnitCount < len,
1. a. a. If codePointCount = codePointIndex, return codeUnitCount.
2. b. b. Let cp be CodePointAt(S, codeUnitCount).
3. c. c. Set codeUnitCount to codeUnitCount + cp.[[CodeUnitCount]].
4. d. d. Set codePointCount to codePointCount + 1.
6. 6. 6. Return len.
22.2.7.5 MATCH RECORDS
A Match Record is a Record value used to encapsulate the start and end indices
of a regular expression match or capture.
Match Records have the fields listed in Table 67.
Table 67: Match Record Fields
Field Name Value Meaning [[StartIndex]] a non-negative integer The number of
code units from the start of a string at which the match begins (inclusive).
[[EndIndex]] an integer ≥ [[StartIndex]] The number of code units from the start
of a string at which the match ends (exclusive).
22.2.7.6 GETMATCHSTRING ( S, MATCH )
The abstract operation GetMatchString takes arguments S (a String) and match (a
Match Record) and returns a String. It performs the following steps when called:
1. 1. 1. Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
2. 2. 2. Return the substring of S from match.[[StartIndex]] to
match.[[EndIndex]].
22.2.7.7 GETMATCHINDEXPAIR ( S, MATCH )
The abstract operation GetMatchIndexPair takes arguments S (a String) and match
(a Match Record) and returns an Array. It performs the following steps when
called:
1. 1. 1. Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
2. 2. 2. Return CreateArrayFromList(« 𝔽(match.[[StartIndex]]),
𝔽(match.[[EndIndex]]) »).
22.2.7.8 MAKEMATCHINDICESINDEXPAIRARRAY ( S, INDICES, GROUPNAMES, HASGROUPS )
The abstract operation MakeMatchIndicesIndexPairArray takes arguments S (a
String), indices (a List of either Match Records or undefined), groupNames (a
List of either Strings or undefined), and hasGroups (a Boolean) and returns an
Array. It performs the following steps when called:
1. 1. 1. Let n be the number of elements in indices.
2. 2. 2. Assert: n < 232 - 1.
3. 3. 3. Assert: groupNames has n - 1 elements.
4. 4. 4. NOTE: The groupNames List contains elements aligned with the indices
List starting at indices[1].
5. 5. 5. Let A be ! ArrayCreate(n).
6. 6. 6. If hasGroups is true, then
1. a. a. Let groups be OrdinaryObjectCreate(null).
7. 7. 7. Else,
1. a. a. Let groups be undefined.
8. 8. 8. Perform ! CreateDataPropertyOrThrow(A, "groups", groups).
9. 9. 9. For each integer i such that 0 ≤ i < n, in ascending order, do
1. a. a. Let matchIndices be indices[i].
2. b. b. If matchIndices is not undefined, then
1. i. i. Let matchIndexPair be GetMatchIndexPair(S, matchIndices).
3. c. c. Else,
1. i. i. Let matchIndexPair be undefined.
4. d. d. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(i)),
matchIndexPair).
5. e. e. If i > 0 and groupNames[i - 1] is not undefined, then
1. i. i. Assert: groups is not undefined.
2. ii. ii. Perform ! CreateDataPropertyOrThrow(groups, groupNames[i -
1], matchIndexPair).
10. 10. 10. Return A.
22.2.8 PROPERTIES OF REGEXP INSTANCES
RegExp instances are ordinary objects that inherit properties from the RegExp
prototype object. RegExp instances have internal slots [[OriginalSource]],
[[OriginalFlags]], [[RegExpRecord]], and [[RegExpMatcher]]. The value of the
[[RegExpMatcher]] internal slot is an Abstract Closure representation of the
Pattern of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data
properties "source", "global", "ignoreCase", and "multiline". Those properties
are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
22.2.8.1 LASTINDEX
The value of the "lastIndex" property specifies the String index at which to
start the next match. It is coerced to an integral Number when used (see
22.2.7.2). This property shall have the attributes { [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }.
22.2.9 REGEXP STRING ITERATOR OBJECTS
A RegExp String Iterator is an object, that represents a specific iteration over
some specific String instance object, matching against some specific RegExp
instance object. There is not a named constructor for RegExp String Iterator
objects. Instead, RegExp String Iterator objects are created by calling certain
methods of RegExp instance objects.
22.2.9.1 CREATEREGEXPSTRINGITERATOR ( R, S, GLOBAL, FULLUNICODE )
The abstract operation CreateRegExpStringIterator takes arguments R (an Object),
S (a String), global (a Boolean), and fullUnicode (a Boolean) and returns a
Generator. It performs the following steps when called:
1. 1. 1. Let closure be a new Abstract Closure with no parameters that captures
R, S, global, and fullUnicode and performs the following steps when called:
1. a. a. Repeat,
1. i. i. Let match be ? RegExpExec(R, S).
2. ii. ii. If match is null, return undefined.
3. iii. iii. If global is false, then
1. 1. 1. Perform ? GeneratorYield(CreateIterResultObject(match,
false)).
2. 2. 2. Return undefined.
4. iv. iv. Let matchStr be ? ToString(? Get(match, "0")).
5. v. v. If matchStr is the empty String, then
1. 1. 1. Let thisIndex be ℝ(? ToLength(? Get(R, "lastIndex"))).
2. 2. 2. Let nextIndex be AdvanceStringIndex(S, thisIndex,
fullUnicode).
3. 3. 3. Perform ? Set(R, "lastIndex", 𝔽(nextIndex), true).
6. vi. vi. Perform ? GeneratorYield(CreateIterResultObject(match,
false)).
2. 2. 2. Return CreateIteratorFromClosure(closure,
"%RegExpStringIteratorPrototype%", %RegExpStringIteratorPrototype%).
22.2.9.2 THE %REGEXPSTRINGITERATORPROTOTYPE% OBJECT
The %RegExpStringIteratorPrototype% object:
* has properties that are inherited by all RegExp String Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* has the following properties:
22.2.9.2.1 %REGEXPSTRINGITERATORPROTOTYPE%.NEXT ( )
1. 1. 1. Return ? GeneratorResume(this value, empty,
"%RegExpStringIteratorPrototype%").
22.2.9.2.2 %REGEXPSTRINGITERATORPROTOTYPE% [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "RegExp
String Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
23 INDEXED COLLECTIONS
23.1 ARRAY OBJECTS
Arrays are exotic objects that give special treatment to a certain class of
property names. See 10.4.2 for a definition of this special treatment.
23.1.1 THE ARRAY CONSTRUCTOR
The Array constructor:
* is %Array%.
* is the initial value of the "Array" property of the global object.
* creates and initializes a new Array when called as a constructor.
* also creates and initializes a new Array when called as a function rather
than as a constructor. Thus the function call Array(…) is equivalent to the
object creation expression new Array(…) with the same arguments.
* is a function whose behaviour differs based upon the number and types of its
arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the exotic Array behaviour must include a
super call to the Array constructor to initialize subclass instances that are
Array exotic objects. However, most of the Array.prototype methods are
generic methods that are not dependent upon their this value being an Array
exotic object.
* has a "length" property whose value is 1𝔽.
23.1.1.1 ARRAY ( ...VALUES )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
2. 2. 2. Let proto be ? GetPrototypeFromConstructor(newTarget,
"%Array.prototype%").
3. 3. 3. Let numberOfArgs be the number of elements in values.
4. 4. 4. If numberOfArgs = 0, then
1. a. a. Return ! ArrayCreate(0, proto).
5. 5. 5. Else if numberOfArgs = 1, then
1. a. a. Let len be values[0].
2. b. b. Let array be ! ArrayCreate(0, proto).
3. c. c. If len is not a Number, then
1. i. i. Perform ! CreateDataPropertyOrThrow(array, "0", len).
2. ii. ii. Let intLen be 1𝔽.
4. d. d. Else,
1. i. i. Let intLen be ! ToUint32(len).
2. ii. ii. If SameValueZero(intLen, len) is false, throw a RangeError
exception.
5. e. e. Perform ! Set(array, "length", intLen, true).
6. f. f. Return array.
6. 6. 6. Else,
1. a. a. Assert: numberOfArgs ≥ 2.
2. b. b. Let array be ? ArrayCreate(numberOfArgs, proto).
3. c. c. Let k be 0.
4. d. d. Repeat, while k < numberOfArgs,
1. i. i. Let Pk be ! ToString(𝔽(k)).
2. ii. ii. Let itemK be values[k].
3. iii. iii. Perform ! CreateDataPropertyOrThrow(array, Pk, itemK).
4. iv. iv. Set k to k + 1.
5. e. e. Assert: The mathematical value of array's "length" property is
numberOfArgs.
6. f. f. Return array.
23.1.2 PROPERTIES OF THE ARRAY CONSTRUCTOR
The Array constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
23.1.2.1 ARRAY.FROM ( ITEMS [ , MAPFN [ , THISARG ] ] )
This method performs the following steps when called:
1. 1. 1. Let C be the this value.
2. 2. 2. If mapfn is undefined, let mapping be false.
3. 3. 3. Else,
1. a. a. If IsCallable(mapfn) is false, throw a TypeError exception.
2. b. b. Let mapping be true.
4. 4. 4. Let usingIterator be ? GetMethod(items, @@iterator).
5. 5. 5. If usingIterator is not undefined, then
1. a. a. If IsConstructor(C) is true, then
1. i. i. Let A be ? Construct(C).
2. b. b. Else,
1. i. i. Let A be ! ArrayCreate(0).
3. c. c. Let iteratorRecord be ? GetIteratorFromMethod(items,
usingIterator).
4. d. d. Let k be 0.
5. e. e. Repeat,
1. i. i. If k ≥ 253 - 1, then
1. 1. 1. Let error be ThrowCompletion(a newly created TypeError
object).
2. 2. 2. Return ? IteratorClose(iteratorRecord, error).
2. ii. ii. Let Pk be ! ToString(𝔽(k)).
3. iii. iii. Let next be ? IteratorStep(iteratorRecord).
4. iv. iv. If next is false, then
1. 1. 1. Perform ? Set(A, "length", 𝔽(k), true).
2. 2. 2. Return A.
5. v. v. Let nextValue be ? IteratorValue(next).
6. vi. vi. If mapping is true, then
1. 1. 1. Let mappedValue be Completion(Call(mapfn, thisArg, «
nextValue, 𝔽(k) »)).
2. 2. 2. IfAbruptCloseIterator(mappedValue, iteratorRecord).
7. vii. vii. Else, let mappedValue be nextValue.
8. viii. viii. Let defineStatus be
Completion(CreateDataPropertyOrThrow(A, Pk, mappedValue)).
9. ix. ix. IfAbruptCloseIterator(defineStatus, iteratorRecord).
10. x. x. Set k to k + 1.
6. 6. 6. NOTE: items is not an Iterable so assume it is an array-like object.
7. 7. 7. Let arrayLike be ! ToObject(items).
8. 8. 8. Let len be ? LengthOfArrayLike(arrayLike).
9. 9. 9. If IsConstructor(C) is true, then
1. a. a. Let A be ? Construct(C, « 𝔽(len) »).
10. 10. 10. Else,
1. a. a. Let A be ? ArrayCreate(len).
11. 11. 11. Let k be 0.
12. 12. 12. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ? Get(arrayLike, Pk).
3. c. c. If mapping is true, then
1. i. i. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
4. d. d. Else, let mappedValue be kValue.
5. e. e. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
6. f. f. Set k to k + 1.
13. 13. 13. Perform ? Set(A, "length", 𝔽(len), true).
14. 14. 14. Return A.
Note
This method is an intentionally generic factory method; it does not require that
its this value be the Array constructor. Therefore it can be transferred to or
inherited by any other constructors that may be called with a single numeric
argument.
23.1.2.2 ARRAY.ISARRAY ( ARG )
This function performs the following steps when called:
1. 1. 1. Return ? IsArray(arg).
23.1.2.3 ARRAY.OF ( ...ITEMS )
This method performs the following steps when called:
1. 1. 1. Let len be the number of elements in items.
2. 2. 2. Let lenNumber be 𝔽(len).
3. 3. 3. Let C be the this value.
4. 4. 4. If IsConstructor(C) is true, then
1. a. a. Let A be ? Construct(C, « lenNumber »).
5. 5. 5. Else,
1. a. a. Let A be ? ArrayCreate(len).
6. 6. 6. Let k be 0.
7. 7. 7. Repeat, while k < len,
1. a. a. Let kValue be items[k].
2. b. b. Let Pk be ! ToString(𝔽(k)).
3. c. c. Perform ? CreateDataPropertyOrThrow(A, Pk, kValue).
4. d. d. Set k to k + 1.
8. 8. 8. Perform ? Set(A, "length", lenNumber, true).
9. 9. 9. Return A.
Note
This method is an intentionally generic factory method; it does not require that
its this value be the Array constructor. Therefore it can be transferred to or
inherited by other constructors that may be called with a single numeric
argument.
23.1.2.4 ARRAY.PROTOTYPE
The value of Array.prototype is the Array prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.1.2.5 GET ARRAY [ @@SPECIES ]
Array[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
Array prototype methods normally use their this value's constructor to create a
derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
23.1.3 PROPERTIES OF THE ARRAY PROTOTYPE OBJECT
The Array prototype object:
* is %Array.prototype%.
* is an Array exotic object and has the internal methods specified for such
objects.
* has a "length" property whose initial value is +0𝔽 and whose attributes are
{ [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
Note
The Array prototype object is specified to be an Array exotic object to ensure
compatibility with ECMAScript code that was created prior to the ECMAScript 2015
specification.
23.1.3.1 ARRAY.PROTOTYPE.AT ( INDEX )
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeIndex be ? ToIntegerOrInfinity(index).
4. 4. 4. If relativeIndex ≥ 0, then
1. a. a. Let k be relativeIndex.
5. 5. 5. Else,
1. a. a. Let k be len + relativeIndex.
6. 6. 6. If k < 0 or k ≥ len, return undefined.
7. 7. 7. Return ? Get(O, ! ToString(𝔽(k))).
23.1.3.2 ARRAY.PROTOTYPE.CONCAT ( ...ITEMS )
This method returns an array containing the array elements of the object
followed by the array elements of each argument.
It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let A be ? ArraySpeciesCreate(O, 0).
3. 3. 3. Let n be 0.
4. 4. 4. Prepend O to items.
5. 5. 5. For each element E of items, do
1. a. a. Let spreadable be ? IsConcatSpreadable(E).
2. b. b. If spreadable is true, then
1. i. i. Let len be ? LengthOfArrayLike(E).
2. ii. ii. If n + len > 253 - 1, throw a TypeError exception.
3. iii. iii. Let k be 0.
4. iv. iv. Repeat, while k < len,
1. 1. 1. Let P be ! ToString(𝔽(k)).
2. 2. 2. Let exists be ? HasProperty(E, P).
3. 3. 3. If exists is true, then
1. a. a. Let subElement be ? Get(E, P).
2. b. b. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
subElement).
4. 4. 4. Set n to n + 1.
5. 5. 5. Set k to k + 1.
3. c. c. Else,
1. i. i. NOTE: E is added as a single item rather than spread.
2. ii. ii. If n ≥ 253 - 1, throw a TypeError exception.
3. iii. iii. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
E).
4. iv. iv. Set n to n + 1.
6. 6. 6. Perform ? Set(A, "length", 𝔽(n), true).
7. 7. 7. Return A.
The "length" property of this method is 1𝔽.
Note 1
The explicit setting of the "length" property in step 6 is necessary to ensure
that its value is correct in situations where the trailing elements of the
result Array are not present.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.2.1 ISCONCATSPREADABLE ( O )
The abstract operation IsConcatSpreadable takes argument O (an ECMAScript
language value) and returns either a normal completion containing a Boolean or a
throw completion. It performs the following steps when called:
1. 1. 1. If O is not an Object, return false.
2. 2. 2. Let spreadable be ? Get(O, @@isConcatSpreadable).
3. 3. 3. If spreadable is not undefined, return ToBoolean(spreadable).
4. 4. 4. Return ? IsArray(O).
23.1.3.3 ARRAY.PROTOTYPE.CONSTRUCTOR
The initial value of Array.prototype.constructor is %Array%.
23.1.3.4 ARRAY.PROTOTYPE.COPYWITHIN ( TARGET, START [ , END ] )
Note 1
The end argument is optional. If it is not provided, the length of the this
value is used.
Note 2
If target is negative, it is treated as length + target where length is the
length of the array. If start is negative, it is treated as length + start. If
end is negative, it is treated as length + end.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeTarget be ? ToIntegerOrInfinity(target).
4. 4. 4. If relativeTarget = -∞, let to be 0.
5. 5. 5. Else if relativeTarget < 0, let to be max(len + relativeTarget, 0).
6. 6. 6. Else, let to be min(relativeTarget, len).
7. 7. 7. Let relativeStart be ? ToIntegerOrInfinity(start).
8. 8. 8. If relativeStart = -∞, let from be 0.
9. 9. 9. Else if relativeStart < 0, let from be max(len + relativeStart, 0).
10. 10. 10. Else, let from be min(relativeStart, len).
11. 11. 11. If end is undefined, let relativeEnd be len; else let relativeEnd
be ? ToIntegerOrInfinity(end).
12. 12. 12. If relativeEnd = -∞, let final be 0.
13. 13. 13. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
14. 14. 14. Else, let final be min(relativeEnd, len).
15. 15. 15. Let count be min(final - from, len - to).
16. 16. 16. If from < to and to < from + count, then
1. a. a. Let direction be -1.
2. b. b. Set from to from + count - 1.
3. c. c. Set to to to + count - 1.
17. 17. 17. Else,
1. a. a. Let direction be 1.
18. 18. 18. Repeat, while count > 0,
1. a. a. Let fromKey be ! ToString(𝔽(from)).
2. b. b. Let toKey be ! ToString(𝔽(to)).
3. c. c. Let fromPresent be ? HasProperty(O, fromKey).
4. d. d. If fromPresent is true, then
1. i. i. Let fromVal be ? Get(O, fromKey).
2. ii. ii. Perform ? Set(O, toKey, fromVal, true).
5. e. e. Else,
1. i. i. Assert: fromPresent is false.
2. ii. ii. Perform ? DeletePropertyOrThrow(O, toKey).
6. f. f. Set from to from + direction.
7. g. g. Set to to to + direction.
8. h. h. Set count to count - 1.
19. 19. 19. Return O.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.5 ARRAY.PROTOTYPE.ENTRIES ( )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Return CreateArrayIterator(O, key+value).
23.1.3.6 ARRAY.PROTOTYPE.EVERY ( CALLBACKFN [ , THISARG ] )
Note 1
callbackfn should be a function that accepts three arguments and returns a value
that is coercible to a Boolean value. every calls callbackfn once for each
element present in the array, in ascending order, until it finds one where
callbackfn returns false. If such an element is found, every immediately returns
false. Otherwise, if callbackfn returned true for all elements, every will
return true. callbackfn is called only for elements of the array which actually
exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index
of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn.
The range of elements processed by every is set before the first call to
callbackfn. Elements which are appended to the array after the call to every
begins will not be visited by callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will be the value at the time every
visits them; elements that are deleted after the call to every begins and before
being visited are not visited. every acts like the "for all" quantifier in
mathematics. In particular, for an empty array, it returns true.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let k be 0.
5. 5. 5. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Let testResult be ToBoolean(? Call(callbackfn, thisArg, «
kValue, 𝔽(k), O »)).
3. iii. iii. If testResult is false, return false.
4. d. d. Set k to k + 1.
6. 6. 6. Return true.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.7 ARRAY.PROTOTYPE.FILL ( VALUE [ , START [ , END ] ] )
Note 1
The start argument is optional. If it is not provided, +0𝔽 is used.
The end argument is optional. If it is not provided, the length of the this
value is used.
Note 2
If start is negative, it is treated as length + start where length is the length
of the array. If end is negative, it is treated as length + end.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeStart be ? ToIntegerOrInfinity(start).
4. 4. 4. If relativeStart = -∞, let k be 0.
5. 5. 5. Else if relativeStart < 0, let k be max(len + relativeStart, 0).
6. 6. 6. Else, let k be min(relativeStart, len).
7. 7. 7. If end is undefined, let relativeEnd be len; else let relativeEnd be
? ToIntegerOrInfinity(end).
8. 8. 8. If relativeEnd = -∞, let final be 0.
9. 9. 9. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
10. 10. 10. Else, let final be min(relativeEnd, len).
11. 11. 11. Repeat, while k < final,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Perform ? Set(O, Pk, value, true).
3. c. c. Set k to k + 1.
12. 12. 12. Return O.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.8 ARRAY.PROTOTYPE.FILTER ( CALLBACKFN [ , THISARG ] )
Note 1
callbackfn should be a function that accepts three arguments and returns a value
that is coercible to a Boolean value. filter calls callbackfn once for each
element in the array, in ascending order, and constructs a new array of all the
values for which callbackfn returns true. callbackfn is called only for elements
of the array which actually exist; it is not called for missing elements of the
array.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index
of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn.
The range of elements processed by filter is set before the first call to
callbackfn. Elements which are appended to the array after the call to filter
begins will not be visited by callbackfn. If existing elements of the array are
changed their value as passed to callbackfn will be the value at the time filter
visits them; elements that are deleted after the call to filter begins and
before being visited are not visited.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let A be ? ArraySpeciesCreate(O, 0).
5. 5. 5. Let k be 0.
6. 6. 6. Let to be 0.
7. 7. 7. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Let selected be ToBoolean(? Call(callbackfn, thisArg, «
kValue, 𝔽(k), O »)).
3. iii. iii. If selected is true, then
1. 1. 1. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(to)),
kValue).
2. 2. 2. Set to to to + 1.
4. d. d. Set k to k + 1.
8. 8. 8. Return A.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.9 ARRAY.PROTOTYPE.FIND ( PREDICATE [ , THISARG ] )
Note 1
This method calls predicate once for each element of the array, in ascending
index order, until it finds one where predicate returns a value that coerces to
true. If such an element is found, find immediately returns that element value.
Otherwise, find returns undefined.
See FindViaPredicate for additional information.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let findRec be ? FindViaPredicate(O, len, ascending, predicate,
thisArg).
4. 4. 4. Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.10 ARRAY.PROTOTYPE.FINDINDEX ( PREDICATE [ , THISARG ] )
Note 1
This method calls predicate once for each element of the array, in ascending
index order, until it finds one where predicate returns a value that coerces to
true. If such an element is found, findIndex immediately returns the index of
that element value. Otherwise, findIndex returns -1.
See FindViaPredicate for additional information.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let findRec be ? FindViaPredicate(O, len, ascending, predicate,
thisArg).
4. 4. 4. Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.11 ARRAY.PROTOTYPE.FINDLAST ( PREDICATE [ , THISARG ] )
Note 1
This method calls predicate once for each element of the array, in descending
index order, until it finds one where predicate returns a value that coerces to
true. If such an element is found, findLast immediately returns that element
value. Otherwise, findLast returns undefined.
See FindViaPredicate for additional information.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let findRec be ? FindViaPredicate(O, len, descending, predicate,
thisArg).
4. 4. 4. Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for
use as a method.
23.1.3.12 ARRAY.PROTOTYPE.FINDLASTINDEX ( PREDICATE [ , THISARG ] )
Note 1
This method calls predicate once for each element of the array, in descending
index order, until it finds one where predicate returns a value that coerces to
true. If such an element is found, findLastIndex immediately returns the index
of that element value. Otherwise, findLastIndex returns -1.
See FindViaPredicate for additional information.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let findRec be ? FindViaPredicate(O, len, descending, predicate,
thisArg).
4. 4. 4. Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for
use as a method.
23.1.3.12.1 FINDVIAPREDICATE ( O, LEN, DIRECTION, PREDICATE, THISARG )
The abstract operation FindViaPredicate takes arguments O (an Object), len (a
non-negative integer), direction (ascending or descending), predicate (an
ECMAScript language value), and thisArg (an ECMAScript language value) and
returns either a normal completion containing a Record with fields [[Index]] (an
integral Number) and [[Value]] (an ECMAScript language value) or a throw
completion.
O should be an array-like object or a TypedArray. This operation calls predicate
once for each element of O, in either ascending index order or descending index
order (as indicated by direction), until it finds one where predicate returns a
value that coerces to true. At that point, this operation returns a Record that
gives the index and value of the element found. If no such element is found,
this operation returns a Record that specifies -1𝔽 for the index and undefined
for the value.
predicate should be a function. When called for an element of the array, it is
passed three arguments: the value of the element, the index of the element, and
the object being traversed. Its return value will be coerced to a Boolean value.
thisArg will be used as the this value for each invocation of predicate.
This operation does not directly mutate the object on which it is called, but
the object may be mutated by the calls to predicate.
The range of elements processed is set before the first call to predicate, just
before the traversal begins. Elements that are appended to the array after this
will not be visited by predicate. If existing elements of the array are changed,
their value as passed to predicate will be the value at the time that this
operation visits them. Elements that are deleted after traversal begins and
before being visited are still visited and are either looked up from the
prototype or are undefined.
It performs the following steps when called:
1. 1. 1. If IsCallable(predicate) is false, throw a TypeError exception.
2. 2. 2. If direction is ascending, then
1. a. a. Let indices be a List of the integers in the interval from 0
(inclusive) to len (exclusive), in ascending order.
3. 3. 3. Else,
1. a. a. Let indices be a List of the integers in the interval from 0
(inclusive) to len (exclusive), in descending order.
4. 4. 4. For each integer k of indices, do
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. NOTE: If O is a TypedArray, the following invocation of Get will
return a normal completion.
3. c. c. Let kValue be ? Get(O, Pk).
4. d. d. Let testResult be ? Call(predicate, thisArg, « kValue, 𝔽(k), O »).
5. e. e. If ToBoolean(testResult) is true, return the Record { [[Index]]:
𝔽(k), [[Value]]: kValue }.
5. 5. 5. Return the Record { [[Index]]: -1𝔽, [[Value]]: undefined }.
23.1.3.13 ARRAY.PROTOTYPE.FLAT ( [ DEPTH ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let sourceLen be ? LengthOfArrayLike(O).
3. 3. 3. Let depthNum be 1.
4. 4. 4. If depth is not undefined, then
1. a. a. Set depthNum to ? ToIntegerOrInfinity(depth).
2. b. b. If depthNum < 0, set depthNum to 0.
5. 5. 5. Let A be ? ArraySpeciesCreate(O, 0).
6. 6. 6. Perform ? FlattenIntoArray(A, O, sourceLen, 0, depthNum).
7. 7. 7. Return A.
23.1.3.13.1 FLATTENINTOARRAY ( TARGET, SOURCE, SOURCELEN, START, DEPTH [ ,
MAPPERFUNCTION [ , THISARG ] ] )
The abstract operation FlattenIntoArray takes arguments target (an Object),
source (an Object), sourceLen (a non-negative integer), start (a non-negative
integer), and depth (a non-negative integer or +∞) and optional arguments
mapperFunction (a function object) and thisArg (an ECMAScript language value)
and returns either a normal completion containing a non-negative integer or a
throw completion. It performs the following steps when called:
1. 1. 1. Assert: If mapperFunction is present, then IsCallable(mapperFunction)
is true, thisArg is present, and depth is 1.
2. 2. 2. Let targetIndex be start.
3. 3. 3. Let sourceIndex be +0𝔽.
4. 4. 4. Repeat, while ℝ(sourceIndex) < sourceLen,
1. a. a. Let P be ! ToString(sourceIndex).
2. b. b. Let exists be ? HasProperty(source, P).
3. c. c. If exists is true, then
1. i. i. Let element be ? Get(source, P).
2. ii. ii. If mapperFunction is present, then
1. 1. 1. Set element to ? Call(mapperFunction, thisArg, « element,
sourceIndex, source »).
3. iii. iii. Let shouldFlatten be false.
4. iv. iv. If depth > 0, then
1. 1. 1. Set shouldFlatten to ? IsArray(element).
5. v. v. If shouldFlatten is true, then
1. 1. 1. If depth = +∞, let newDepth be +∞.
2. 2. 2. Else, let newDepth be depth - 1.
3. 3. 3. Let elementLen be ? LengthOfArrayLike(element).
4. 4. 4. Set targetIndex to ? FlattenIntoArray(target, element,
elementLen, targetIndex, newDepth).
6. vi. vi. Else,
1. 1. 1. If targetIndex ≥ 253 - 1, throw a TypeError exception.
2. 2. 2. Perform ? CreateDataPropertyOrThrow(target,
! ToString(𝔽(targetIndex)), element).
3. 3. 3. Set targetIndex to targetIndex + 1.
4. d. d. Set sourceIndex to sourceIndex + 1𝔽.
5. 5. 5. Return targetIndex.
23.1.3.14 ARRAY.PROTOTYPE.FLATMAP ( MAPPERFUNCTION [ , THISARG ] )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let sourceLen be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(mapperFunction) is false, throw a TypeError exception.
4. 4. 4. Let A be ? ArraySpeciesCreate(O, 0).
5. 5. 5. Perform ? FlattenIntoArray(A, O, sourceLen, 0, 1, mapperFunction,
thisArg).
6. 6. 6. Return A.
23.1.3.15 ARRAY.PROTOTYPE.FOREACH ( CALLBACKFN [ , THISARG ] )
Note 1
callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each element present in the array, in ascending order.
callbackfn is called only for elements of the array which actually exist; it is
not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index
of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn.
The range of elements processed by forEach is set before the first call to
callbackfn. Elements which are appended to the array after the call to forEach
begins will not be visited by callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will be the value at the time
forEach visits them; elements that are deleted after the call to forEach begins
and before being visited are not visited.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let k be 0.
5. 5. 5. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Perform ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »).
4. d. d. Set k to k + 1.
6. 6. 6. Return undefined.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.16 ARRAY.PROTOTYPE.INCLUDES ( SEARCHELEMENT [ , FROMINDEX ] )
Note 1
This method compares searchElement to the elements of the array, in ascending
order, using the SameValueZero algorithm, and if found at any position, returns
true; otherwise, it returns false.
The optional second argument fromIndex defaults to +0𝔽 (i.e. the whole array is
searched). If it is greater than or equal to the length of the array, false is
returned, i.e. the array will not be searched. If it is less than -0𝔽, it is
used as the offset from the end of the array to compute fromIndex. If the
computed index is less than or equal to +0𝔽, the whole array will be searched.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If len = 0, return false.
4. 4. 4. Let n be ? ToIntegerOrInfinity(fromIndex).
5. 5. 5. Assert: If fromIndex is undefined, then n is 0.
6. 6. 6. If n = +∞, return false.
7. 7. 7. Else if n = -∞, set n to 0.
8. 8. 8. If n ≥ 0, then
1. a. a. Let k be n.
9. 9. 9. Else,
1. a. a. Let k be len + n.
2. b. b. If k < 0, set k to 0.
10. 10. 10. Repeat, while k < len,
1. a. a. Let elementK be ? Get(O, ! ToString(𝔽(k))).
2. b. b. If SameValueZero(searchElement, elementK) is true, return true.
3. c. c. Set k to k + 1.
11. 11. 11. Return false.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
Note 3
This method intentionally differs from the similar indexOf method in two ways.
First, it uses the SameValueZero algorithm, instead of IsStrictlyEqual, allowing
it to detect NaN array elements. Second, it does not skip missing array
elements, instead treating them as undefined.
23.1.3.17 ARRAY.PROTOTYPE.INDEXOF ( SEARCHELEMENT [ , FROMINDEX ] )
This method compares searchElement to the elements of the array, in ascending
order, using the IsStrictlyEqual algorithm, and if found at one or more indices,
returns the smallest such index; otherwise, it returns -1𝔽.
Note 1
The optional second argument fromIndex defaults to +0𝔽 (i.e. the whole array is
searched). If it is greater than or equal to the length of the array, -1𝔽 is
returned, i.e. the array will not be searched. If it is less than -0𝔽, it is
used as the offset from the end of the array to compute fromIndex. If the
computed index is less than or equal to +0𝔽, the whole array will be searched.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If len = 0, return -1𝔽.
4. 4. 4. Let n be ? ToIntegerOrInfinity(fromIndex).
5. 5. 5. Assert: If fromIndex is undefined, then n is 0.
6. 6. 6. If n = +∞, return -1𝔽.
7. 7. 7. Else if n = -∞, set n to 0.
8. 8. 8. If n ≥ 0, then
1. a. a. Let k be n.
9. 9. 9. Else,
1. a. a. Let k be len + n.
2. b. b. If k < 0, set k to 0.
10. 10. 10. Repeat, while k < len,
1. a. a. Let kPresent be ? HasProperty(O, ! ToString(𝔽(k))).
2. b. b. If kPresent is true, then
1. i. i. Let elementK be ? Get(O, ! ToString(𝔽(k))).
2. ii. ii. If IsStrictlyEqual(searchElement, elementK) is true, return
𝔽(k).
3. c. c. Set k to k + 1.
11. 11. 11. Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.18 ARRAY.PROTOTYPE.JOIN ( SEPARATOR )
This method converts the elements of the array to Strings, and then concatenates
these Strings, separated by occurrences of the separator. If no separator is
provided, a single comma is used as the separator.
It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If separator is undefined, let sep be ",".
4. 4. 4. Else, let sep be ? ToString(separator).
5. 5. 5. Let R be the empty String.
6. 6. 6. Let k be 0.
7. 7. 7. Repeat, while k < len,
1. a. a. If k > 0, set R to the string-concatenation of R and sep.
2. b. b. Let element be ? Get(O, ! ToString(𝔽(k))).
3. c. c. If element is either undefined or null, let next be the empty
String; otherwise, let next be ? ToString(element).
4. d. d. Set R to the string-concatenation of R and next.
5. e. e. Set k to k + 1.
8. 8. 8. Return R.
Note
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as
a method.
23.1.3.19 ARRAY.PROTOTYPE.KEYS ( )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Return CreateArrayIterator(O, key).
23.1.3.20 ARRAY.PROTOTYPE.LASTINDEXOF ( SEARCHELEMENT [ , FROMINDEX ] )
Note 1
This method compares searchElement to the elements of the array in descending
order using the IsStrictlyEqual algorithm, and if found at one or more indices,
returns the largest such index; otherwise, it returns -1𝔽.
The optional second argument fromIndex defaults to the array's length minus one
(i.e. the whole array is searched). If it is greater than or equal to the length
of the array, the whole array will be searched. If it is less than -0𝔽, it is
used as the offset from the end of the array to compute fromIndex. If the
computed index is less than or equal to +0𝔽, -1𝔽 is returned.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If len = 0, return -1𝔽.
4. 4. 4. If fromIndex is present, let n be ? ToIntegerOrInfinity(fromIndex);
else let n be len - 1.
5. 5. 5. If n = -∞, return -1𝔽.
6. 6. 6. If n ≥ 0, then
1. a. a. Let k be min(n, len - 1).
7. 7. 7. Else,
1. a. a. Let k be len + n.
8. 8. 8. Repeat, while k ≥ 0,
1. a. a. Let kPresent be ? HasProperty(O, ! ToString(𝔽(k))).
2. b. b. If kPresent is true, then
1. i. i. Let elementK be ? Get(O, ! ToString(𝔽(k))).
2. ii. ii. If IsStrictlyEqual(searchElement, elementK) is true, return
𝔽(k).
3. c. c. Set k to k - 1.
9. 9. 9. Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.21 ARRAY.PROTOTYPE.MAP ( CALLBACKFN [ , THISARG ] )
Note 1
callbackfn should be a function that accepts three arguments. map calls
callbackfn once for each element in the array, in ascending order, and
constructs a new Array from the results. callbackfn is called only for elements
of the array which actually exist; it is not called for missing elements of the
array.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index
of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may
be mutated by the calls to callbackfn.
The range of elements processed by map is set before the first call to
callbackfn. Elements which are appended to the array after the call to map
begins will not be visited by callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will be the value at the time map
visits them; elements that are deleted after the call to map begins and before
being visited are not visited.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let A be ? ArraySpeciesCreate(O, len).
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Let mappedValue be ? Call(callbackfn, thisArg, « kValue,
𝔽(k), O »).
3. iii. iii. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
4. d. d. Set k to k + 1.
7. 7. 7. Return A.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.22 ARRAY.PROTOTYPE.POP ( )
Note 1
This method removes the last element of the array and returns it.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If len = 0, then
1. a. a. Perform ? Set(O, "length", +0𝔽, true).
2. b. b. Return undefined.
4. 4. 4. Else,
1. a. a. Assert: len > 0.
2. b. b. Let newLen be 𝔽(len - 1).
3. c. c. Let index be ! ToString(newLen).
4. d. d. Let element be ? Get(O, index).
5. e. e. Perform ? DeletePropertyOrThrow(O, index).
6. f. f. Perform ? Set(O, "length", newLen, true).
7. g. g. Return element.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.23 ARRAY.PROTOTYPE.PUSH ( ...ITEMS )
Note 1
This method appends the arguments to the end of the array, in the order in which
they appear. It returns the new length of the array.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let argCount be the number of elements in items.
4. 4. 4. If len + argCount > 253 - 1, throw a TypeError exception.
5. 5. 5. For each element E of items, do
1. a. a. Perform ? Set(O, ! ToString(𝔽(len)), E, true).
2. b. b. Set len to len + 1.
6. 6. 6. Perform ? Set(O, "length", 𝔽(len), true).
7. 7. 7. Return 𝔽(len).
The "length" property of this method is 1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.24 ARRAY.PROTOTYPE.REDUCE ( CALLBACKFN [ , INITIALVALUE ] )
Note 1
callbackfn should be a function that takes four arguments. reduce calls the
callback, as a function, once for each element after the first element present
in the array, in ascending order.
callbackfn is called with four arguments: the previousValue (value from the
previous call to callbackfn), the currentValue (value of the current element),
the currentIndex, and the object being traversed. The first time that callback
is called, the previousValue and currentValue can be one of two values. If an
initialValue was supplied in the call to reduce, then previousValue will be
initialValue and currentValue will be the first value in the array. If no
initialValue was supplied, then previousValue will be the first value in the
array and currentValue will be the second. It is a TypeError if the array
contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn.
The range of elements processed by reduce is set before the first call to
callbackfn. Elements that are appended to the array after the call to reduce
begins will not be visited by callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will be the value at the time
reduce visits them; elements that are deleted after the call to reduce begins
and before being visited are not visited.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. If len = 0 and initialValue is not present, throw a TypeError
exception.
5. 5. 5. Let k be 0.
6. 6. 6. Let accumulator be undefined.
7. 7. 7. If initialValue is present, then
1. a. a. Set accumulator to initialValue.
8. 8. 8. Else,
1. a. a. Let kPresent be false.
2. b. b. Repeat, while kPresent is false and k < len,
1. i. i. Let Pk be ! ToString(𝔽(k)).
2. ii. ii. Set kPresent to ? HasProperty(O, Pk).
3. iii. iii. If kPresent is true, then
1. 1. 1. Set accumulator to ? Get(O, Pk).
4. iv. iv. Set k to k + 1.
3. c. c. If kPresent is false, throw a TypeError exception.
9. 9. 9. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Set accumulator to ? Call(callbackfn, undefined, «
accumulator, kValue, 𝔽(k), O »).
4. d. d. Set k to k + 1.
10. 10. 10. Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.25 ARRAY.PROTOTYPE.REDUCERIGHT ( CALLBACKFN [ , INITIALVALUE ] )
Note 1
callbackfn should be a function that takes four arguments. reduceRight calls the
callback, as a function, once for each element after the first element present
in the array, in descending order.
callbackfn is called with four arguments: the previousValue (value from the
previous call to callbackfn), the currentValue (value of the current element),
the currentIndex, and the object being traversed. The first time the function is
called, the previousValue and currentValue can be one of two values. If an
initialValue was supplied in the call to reduceRight, then previousValue will be
initialValue and currentValue will be the last value in the array. If no
initialValue was supplied, then previousValue will be the last value in the
array and currentValue will be the second-to-last value. It is a TypeError if
the array contains no elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the
object may be mutated by the calls to callbackfn.
The range of elements processed by reduceRight is set before the first call to
callbackfn. Elements that are appended to the array after the call to
reduceRight begins will not be visited by callbackfn. If existing elements of
the array are changed by callbackfn, their value as passed to callbackfn will be
the value at the time reduceRight visits them; elements that are deleted after
the call to reduceRight begins and before being visited are not visited.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. If len = 0 and initialValue is not present, throw a TypeError
exception.
5. 5. 5. Let k be len - 1.
6. 6. 6. Let accumulator be undefined.
7. 7. 7. If initialValue is present, then
1. a. a. Set accumulator to initialValue.
8. 8. 8. Else,
1. a. a. Let kPresent be false.
2. b. b. Repeat, while kPresent is false and k ≥ 0,
1. i. i. Let Pk be ! ToString(𝔽(k)).
2. ii. ii. Set kPresent to ? HasProperty(O, Pk).
3. iii. iii. If kPresent is true, then
1. 1. 1. Set accumulator to ? Get(O, Pk).
4. iv. iv. Set k to k - 1.
3. c. c. If kPresent is false, throw a TypeError exception.
9. 9. 9. Repeat, while k ≥ 0,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Set accumulator to ? Call(callbackfn, undefined, «
accumulator, kValue, 𝔽(k), O »).
4. d. d. Set k to k - 1.
10. 10. 10. Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.26 ARRAY.PROTOTYPE.REVERSE ( )
Note 1
This method rearranges the elements of the array so as to reverse their order.
It returns the object as the result of the call.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let middle be floor(len / 2).
4. 4. 4. Let lower be 0.
5. 5. 5. Repeat, while lower ≠ middle,
1. a. a. Let upper be len - lower - 1.
2. b. b. Let upperP be ! ToString(𝔽(upper)).
3. c. c. Let lowerP be ! ToString(𝔽(lower)).
4. d. d. Let lowerExists be ? HasProperty(O, lowerP).
5. e. e. If lowerExists is true, then
1. i. i. Let lowerValue be ? Get(O, lowerP).
6. f. f. Let upperExists be ? HasProperty(O, upperP).
7. g. g. If upperExists is true, then
1. i. i. Let upperValue be ? Get(O, upperP).
8. h. h. If lowerExists is true and upperExists is true, then
1. i. i. Perform ? Set(O, lowerP, upperValue, true).
2. ii. ii. Perform ? Set(O, upperP, lowerValue, true).
9. i. i. Else if lowerExists is false and upperExists is true, then
1. i. i. Perform ? Set(O, lowerP, upperValue, true).
2. ii. ii. Perform ? DeletePropertyOrThrow(O, upperP).
10. j. j. Else if lowerExists is true and upperExists is false, then
1. i. i. Perform ? DeletePropertyOrThrow(O, lowerP).
2. ii. ii. Perform ? Set(O, upperP, lowerValue, true).
11. k. k. Else,
1. i. i. Assert: lowerExists and upperExists are both false.
2. ii. ii. NOTE: No action is required.
12. l. l. Set lower to lower + 1.
6. 6. 6. Return O.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as
a method.
23.1.3.27 ARRAY.PROTOTYPE.SHIFT ( )
This method removes the first element of the array and returns it.
It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If len = 0, then
1. a. a. Perform ? Set(O, "length", +0𝔽, true).
2. b. b. Return undefined.
4. 4. 4. Let first be ? Get(O, "0").
5. 5. 5. Let k be 1.
6. 6. 6. Repeat, while k < len,
1. a. a. Let from be ! ToString(𝔽(k)).
2. b. b. Let to be ! ToString(𝔽(k - 1)).
3. c. c. Let fromPresent be ? HasProperty(O, from).
4. d. d. If fromPresent is true, then
1. i. i. Let fromVal be ? Get(O, from).
2. ii. ii. Perform ? Set(O, to, fromVal, true).
5. e. e. Else,
1. i. i. Assert: fromPresent is false.
2. ii. ii. Perform ? DeletePropertyOrThrow(O, to).
6. f. f. Set k to k + 1.
7. 7. 7. Perform ? DeletePropertyOrThrow(O, ! ToString(𝔽(len - 1))).
8. 8. 8. Perform ? Set(O, "length", 𝔽(len - 1), true).
9. 9. 9. Return first.
Note
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.28 ARRAY.PROTOTYPE.SLICE ( START, END )
This method returns an array containing the elements of the array from element
start up to, but not including, element end (or through the end of the array if
end is undefined). If start is negative, it is treated as length + start where
length is the length of the array. If end is negative, it is treated as length +
end where length is the length of the array.
It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeStart be ? ToIntegerOrInfinity(start).
4. 4. 4. If relativeStart = -∞, let k be 0.
5. 5. 5. Else if relativeStart < 0, let k be max(len + relativeStart, 0).
6. 6. 6. Else, let k be min(relativeStart, len).
7. 7. 7. If end is undefined, let relativeEnd be len; else let relativeEnd be
? ToIntegerOrInfinity(end).
8. 8. 8. If relativeEnd = -∞, let final be 0.
9. 9. 9. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
10. 10. 10. Else, let final be min(relativeEnd, len).
11. 11. 11. Let count be max(final - k, 0).
12. 12. 12. Let A be ? ArraySpeciesCreate(O, count).
13. 13. 13. Let n be 0.
14. 14. 14. Repeat, while k < final,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)),
kValue).
4. d. d. Set k to k + 1.
5. e. e. Set n to n + 1.
15. 15. 15. Perform ? Set(A, "length", 𝔽(n), true).
16. 16. 16. Return A.
Note 1
The explicit setting of the "length" property of the result Array in step 15 was
necessary in previous editions of ECMAScript to ensure that its length was
correct in situations where the trailing elements of the result Array were not
present. Setting "length" became unnecessary starting in ES2015 when the result
Array was initialized to its proper length rather than an empty Array but is
carried forward to preserve backward compatibility.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.29 ARRAY.PROTOTYPE.SOME ( CALLBACKFN [ , THISARG ] )
Note 1
callbackfn should be a function that accepts three arguments and returns a value
that is coercible to a Boolean value. some calls callbackfn once for each
element present in the array, in ascending order, until it finds one where
callbackfn returns true. If such an element is found, some immediately returns
true. Otherwise, some returns false. callbackfn is called only for elements of
the array which actually exist; it is not called for missing elements of the
array.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index
of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn.
The range of elements processed by some is set before the first call to
callbackfn. Elements that are appended to the array after the call to some
begins will not be visited by callbackfn. If existing elements of the array are
changed, their value as passed to callbackfn will be the value at the time that
some visits them; elements that are deleted after the call to some begins and
before being visited are not visited. some acts like the "exists" quantifier in
mathematics. In particular, for an empty array, it returns false.
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let k be 0.
5. 5. 5. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kPresent be ? HasProperty(O, Pk).
3. c. c. If kPresent is true, then
1. i. i. Let kValue be ? Get(O, Pk).
2. ii. ii. Let testResult be ToBoolean(? Call(callbackfn, thisArg, «
kValue, 𝔽(k), O »)).
3. iii. iii. If testResult is true, return true.
4. d. d. Set k to k + 1.
6. 6. 6. Return false.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.30 ARRAY.PROTOTYPE.SORT ( COMPAREFN )
This method sorts the elements of this array. The sort must be stable (that is,
elements that compare equal must remain in their original order). If comparefn
is not undefined, it should be a function that accepts two arguments x and y and
returns a negative Number if x < y, a positive Number if x > y, or a zero
otherwise.
It performs the following steps when called:
1. 1. 1. If comparefn is not undefined and IsCallable(comparefn) is false,
throw a TypeError exception.
2. 2. 2. Let obj be ? ToObject(this value).
3. 3. 3. Let len be ? LengthOfArrayLike(obj).
4. 4. 4. Let SortCompare be a new Abstract Closure with parameters (x, y) that
captures comparefn and performs the following steps when called:
1. a. a. Return ? CompareArrayElements(x, y, comparefn).
5. 5. 5. Let sortedList be ? SortIndexedProperties(obj, len, SortCompare,
skip-holes).
6. 6. 6. Let itemCount be the number of elements in sortedList.
7. 7. 7. Let j be 0.
8. 8. 8. Repeat, while j < itemCount,
1. a. a. Perform ? Set(obj, ! ToString(𝔽(j)), sortedList[j], true).
2. b. b. Set j to j + 1.
9. 9. 9. NOTE: The call to SortIndexedProperties in step 5 uses skip-holes.
The remaining indices are deleted to preserve the number of holes that were
detected and excluded from the sort.
10. 10. 10. Repeat, while j < len,
1. a. a. Perform ? DeletePropertyOrThrow(obj, ! ToString(𝔽(j))).
2. b. b. Set j to j + 1.
11. 11. 11. Return obj.
Note 1
Because non-existent property values always compare greater than undefined
property values, and undefined always compares greater than any other value (see
CompareArrayElements), undefined property values always sort to the end of the
result, followed by non-existent property values.
Note 2
Method calls performed by the ToString abstract operations in steps 5 and 6 have
the potential to cause SortCompare to not behave as a consistent comparator.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as
a method.
23.1.3.30.1 SORTINDEXEDPROPERTIES ( OBJ, LEN, SORTCOMPARE, HOLES )
The abstract operation SortIndexedProperties takes arguments obj (an Object),
len (a non-negative integer), SortCompare (an Abstract Closure with two
parameters), and holes (skip-holes or read-through-holes) and returns either a
normal completion containing a List of ECMAScript language values or a throw
completion. It performs the following steps when called:
1. 1. 1. Let items be a new empty List.
2. 2. 2. Let k be 0.
3. 3. 3. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. If holes is skip-holes, then
1. i. i. Let kRead be ? HasProperty(obj, Pk).
3. c. c. Else,
1. i. i. Assert: holes is read-through-holes.
2. ii. ii. Let kRead be true.
4. d. d. If kRead is true, then
1. i. i. Let kValue be ? Get(obj, Pk).
2. ii. ii. Append kValue to items.
5. e. e. Set k to k + 1.
4. 4. 4. Sort items using an implementation-defined sequence of calls to
SortCompare. If any such call returns an abrupt completion, stop before
performing any further calls to SortCompare and return that Completion
Record.
5. 5. 5. Return items.
The sort order is the ordering of items after completion of step 4 of the
algorithm above. The sort order is implementation-defined if SortCompare is not
a consistent comparator for the elements of items. When SortIndexedProperties is
invoked by Array.prototype.sort, the sort order is also implementation-defined
if comparefn is undefined, and all applications of ToString, to any specific
value passed as an argument to SortCompare, do not produce the same result.
Unless the sort order is specified to be implementation-defined, it must satisfy
all of the following conditions:
* There must be some mathematical permutation π of the non-negative integers
less than itemCount, such that for every non-negative integer j less than
itemCount, the element old[j] is exactly the same as new[π(j)].
* Then for all non-negative integers j and k, each less than itemCount, if
ℝ(SortCompare(old[j], old[k])) < 0, then π(j) < π(k).
Here the notation old[j] is used to refer to items[j] before step 4 is executed,
and the notation new[j] to refer to items[j] after step 4 has been executed.
An abstract closure or function comparator is a consistent comparator for a set
of values S if all of the requirements below are met for all values a, b, and c
(possibly the same value) in the set S: The notation a <C b means
ℝ(comparator(a, b)) < 0; a =C b means ℝ(comparator(a, b)) = 0; and a >C b means
ℝ(comparator(a, b)) > 0.
* Calling comparator(a, b) always returns the same value v when given a
specific pair of values a and b as its two arguments. Furthermore, v is a
Number, and v is not NaN. Note that this implies that exactly one of a <C b,
a =C b, and a >C b will be true for a given pair of a and b.
* Calling comparator(a, b) does not modify obj or any object on obj's prototype
chain.
* a =C a (reflexivity)
* If a =C b, then b =C a (symmetry)
* If a =C b and b =C c, then a =C c (transitivity of =C)
* If a <C b and b <C c, then a <C c (transitivity of <C)
* If a >C b and b >C c, then a >C c (transitivity of >C)
Note
The above conditions are necessary and sufficient to ensure that comparator
divides the set S into equivalence classes and that these equivalence classes
are totally ordered.
23.1.3.30.2 COMPAREARRAYELEMENTS ( X, Y, COMPAREFN )
The abstract operation CompareArrayElements takes arguments x (an ECMAScript
language value), y (an ECMAScript language value), and comparefn (a function
object or undefined) and returns either a normal completion containing a Number
or an abrupt completion. It performs the following steps when called:
1. 1. 1. If x and y are both undefined, return +0𝔽.
2. 2. 2. If x is undefined, return 1𝔽.
3. 3. 3. If y is undefined, return -1𝔽.
4. 4. 4. If comparefn is not undefined, then
1. a. a. Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
2. b. b. If v is NaN, return +0𝔽.
3. c. c. Return v.
5. 5. 5. Let xString be ? ToString(x).
6. 6. 6. Let yString be ? ToString(y).
7. 7. 7. Let xSmaller be ! IsLessThan(xString, yString, true).
8. 8. 8. If xSmaller is true, return -1𝔽.
9. 9. 9. Let ySmaller be ! IsLessThan(yString, xString, true).
10. 10. 10. If ySmaller is true, return 1𝔽.
11. 11. 11. Return +0𝔽.
23.1.3.31 ARRAY.PROTOTYPE.SPLICE ( START, DELETECOUNT, ...ITEMS )
Note 1
This method deletes the deleteCount elements of the array starting at integer
index start and replaces them with the elements of items. It returns an Array
containing the deleted elements (if any).
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeStart be ? ToIntegerOrInfinity(start).
4. 4. 4. If relativeStart = -∞, let actualStart be 0.
5. 5. 5. Else if relativeStart < 0, let actualStart be max(len +
relativeStart, 0).
6. 6. 6. Else, let actualStart be min(relativeStart, len).
7. 7. 7. Let itemCount be the number of elements in items.
8. 8. 8. If start is not present, then
1. a. a. Let actualDeleteCount be 0.
9. 9. 9. Else if deleteCount is not present, then
1. a. a. Let actualDeleteCount be len - actualStart.
10. 10. 10. Else,
1. a. a. Let dc be ? ToIntegerOrInfinity(deleteCount).
2. b. b. Let actualDeleteCount be the result of clamping dc between 0 and
len - actualStart.
11. 11. 11. If len + itemCount - actualDeleteCount > 253 - 1, throw a TypeError
exception.
12. 12. 12. Let A be ? ArraySpeciesCreate(O, actualDeleteCount).
13. 13. 13. Let k be 0.
14. 14. 14. Repeat, while k < actualDeleteCount,
1. a. a. Let from be ! ToString(𝔽(actualStart + k)).
2. b. b. If ? HasProperty(O, from) is true, then
1. i. i. Let fromValue be ? Get(O, from).
2. ii. ii. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(k)),
fromValue).
3. c. c. Set k to k + 1.
15. 15. 15. Perform ? Set(A, "length", 𝔽(actualDeleteCount), true).
16. 16. 16. If itemCount < actualDeleteCount, then
1. a. a. Set k to actualStart.
2. b. b. Repeat, while k < (len - actualDeleteCount),
1. i. i. Let from be ! ToString(𝔽(k + actualDeleteCount)).
2. ii. ii. Let to be ! ToString(𝔽(k + itemCount)).
3. iii. iii. If ? HasProperty(O, from) is true, then
1. 1. 1. Let fromValue be ? Get(O, from).
2. 2. 2. Perform ? Set(O, to, fromValue, true).
4. iv. iv. Else,
1. 1. 1. Perform ? DeletePropertyOrThrow(O, to).
5. v. v. Set k to k + 1.
3. c. c. Set k to len.
4. d. d. Repeat, while k > (len - actualDeleteCount + itemCount),
1. i. i. Perform ? DeletePropertyOrThrow(O, ! ToString(𝔽(k - 1))).
2. ii. ii. Set k to k - 1.
17. 17. 17. Else if itemCount > actualDeleteCount, then
1. a. a. Set k to (len - actualDeleteCount).
2. b. b. Repeat, while k > actualStart,
1. i. i. Let from be ! ToString(𝔽(k + actualDeleteCount - 1)).
2. ii. ii. Let to be ! ToString(𝔽(k + itemCount - 1)).
3. iii. iii. If ? HasProperty(O, from) is true, then
1. 1. 1. Let fromValue be ? Get(O, from).
2. 2. 2. Perform ? Set(O, to, fromValue, true).
4. iv. iv. Else,
1. 1. 1. Perform ? DeletePropertyOrThrow(O, to).
5. v. v. Set k to k - 1.
18. 18. 18. Set k to actualStart.
19. 19. 19. For each element E of items, do
1. a. a. Perform ? Set(O, ! ToString(𝔽(k)), E, true).
2. b. b. Set k to k + 1.
20. 20. 20. Perform ? Set(O, "length", 𝔽(len - actualDeleteCount + itemCount),
true).
21. 21. 21. Return A.
Note 2
The explicit setting of the "length" property of the result Array in step 20 was
necessary in previous editions of ECMAScript to ensure that its length was
correct in situations where the trailing elements of the result Array were not
present. Setting "length" became unnecessary starting in ES2015 when the result
Array was initialized to its proper length rather than an empty Array but is
carried forward to preserve backward compatibility.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.32 ARRAY.PROTOTYPE.TOLOCALESTRING ( [ RESERVED1 [ , RESERVED2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API
must implement this method as specified in the ECMA-402 specification. If an
ECMAScript implementation does not include the ECMA-402 API the following
specification of this method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for
this method.
The meanings of the optional parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support
must not use those parameter positions for anything else.
This method performs the following steps when called:
1. 1. 1. Let array be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(array).
3. 3. 3. Let separator be the implementation-defined list-separator String
value appropriate for the host environment's current locale (such as ", ").
4. 4. 4. Let R be the empty String.
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. If k > 0, then
1. i. i. Set R to the string-concatenation of R and separator.
2. b. b. Let nextElement be ? Get(array, ! ToString(𝔽(k))).
3. c. c. If nextElement is neither undefined nor null, then
1. i. i. Let S be ? ToString(? Invoke(nextElement, "toLocaleString")).
2. ii. ii. Set R to the string-concatenation of R and S.
4. d. d. Set k to k + 1.
7. 7. 7. Return R.
Note 2
This method converts the elements of the array to Strings using their
toLocaleString methods, and then concatenates these Strings, separated by
occurrences of an implementation-defined locale-sensitive separator String. This
method is analogous to toString except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host environment's
current locale.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.33 ARRAY.PROTOTYPE.TOREVERSED ( )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let A be ? ArrayCreate(len).
4. 4. 4. Let k be 0.
5. 5. 5. Repeat, while k < len,
1. a. a. Let from be ! ToString(𝔽(len - k - 1)).
2. b. b. Let Pk be ! ToString(𝔽(k)).
3. c. c. Let fromValue be ? Get(O, from).
4. d. d. Perform ! CreateDataPropertyOrThrow(A, Pk, fromValue).
5. e. e. Set k to k + 1.
6. 6. 6. Return A.
23.1.3.34 ARRAY.PROTOTYPE.TOSORTED ( COMPAREFN )
This method performs the following steps when called:
1. 1. 1. If comparefn is not undefined and IsCallable(comparefn) is false,
throw a TypeError exception.
2. 2. 2. Let O be ? ToObject(this value).
3. 3. 3. Let len be ? LengthOfArrayLike(O).
4. 4. 4. Let A be ? ArrayCreate(len).
5. 5. 5. Let SortCompare be a new Abstract Closure with parameters (x, y) that
captures comparefn and performs the following steps when called:
1. a. a. Return ? CompareArrayElements(x, y, comparefn).
6. 6. 6. Let sortedList be ? SortIndexedProperties(O, len, SortCompare,
read-through-holes).
7. 7. 7. Let j be 0.
8. 8. 8. Repeat, while j < len,
1. a. a. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(j)),
sortedList[j]).
2. b. b. Set j to j + 1.
9. 9. 9. Return A.
23.1.3.35 ARRAY.PROTOTYPE.TOSPLICED ( START, SKIPCOUNT, ...ITEMS )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeStart be ? ToIntegerOrInfinity(start).
4. 4. 4. If relativeStart is -∞, let actualStart be 0.
5. 5. 5. Else if relativeStart < 0, let actualStart be max(len +
relativeStart, 0).
6. 6. 6. Else, let actualStart be min(relativeStart, len).
7. 7. 7. Let insertCount be the number of elements in items.
8. 8. 8. If start is not present, then
1. a. a. Let actualSkipCount be 0.
9. 9. 9. Else if skipCount is not present, then
1. a. a. Let actualSkipCount be len - actualStart.
10. 10. 10. Else,
1. a. a. Let sc be ? ToIntegerOrInfinity(skipCount).
2. b. b. Let actualSkipCount be the result of clamping sc between 0 and len
- actualStart.
11. 11. 11. Let newLen be len + insertCount - actualSkipCount.
12. 12. 12. If newLen > 253 - 1, throw a TypeError exception.
13. 13. 13. Let A be ? ArrayCreate(newLen).
14. 14. 14. Let i be 0.
15. 15. 15. Let r be actualStart + actualSkipCount.
16. 16. 16. Repeat, while i < actualStart,
1. a. a. Let Pi be ! ToString(𝔽(i)).
2. b. b. Let iValue be ? Get(O, Pi).
3. c. c. Perform ! CreateDataPropertyOrThrow(A, Pi, iValue).
4. d. d. Set i to i + 1.
17. 17. 17. For each element E of items, do
1. a. a. Let Pi be ! ToString(𝔽(i)).
2. b. b. Perform ! CreateDataPropertyOrThrow(A, Pi, E).
3. c. c. Set i to i + 1.
18. 18. 18. Repeat, while i < newLen,
1. a. a. Let Pi be ! ToString(𝔽(i)).
2. b. b. Let from be ! ToString(𝔽(r)).
3. c. c. Let fromValue be ? Get(O, from).
4. d. d. Perform ! CreateDataPropertyOrThrow(A, Pi, fromValue).
5. e. e. Set i to i + 1.
6. f. f. Set r to r + 1.
19. 19. 19. Return A.
23.1.3.36 ARRAY.PROTOTYPE.TOSTRING ( )
This method performs the following steps when called:
1. 1. 1. Let array be ? ToObject(this value).
2. 2. 2. Let func be ? Get(array, "join").
3. 3. 3. If IsCallable(func) is false, set func to the intrinsic function
%Object.prototype.toString%.
4. 4. 4. Return ? Call(func, array).
Note
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.37 ARRAY.PROTOTYPE.UNSHIFT ( ...ITEMS )
This method prepends the arguments to the start of the array, such that their
order within the array is the same as the order in which they appear in the
argument list.
It performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let argCount be the number of elements in items.
4. 4. 4. If argCount > 0, then
1. a. a. If len + argCount > 253 - 1, throw a TypeError exception.
2. b. b. Let k be len.
3. c. c. Repeat, while k > 0,
1. i. i. Let from be ! ToString(𝔽(k - 1)).
2. ii. ii. Let to be ! ToString(𝔽(k + argCount - 1)).
3. iii. iii. Let fromPresent be ? HasProperty(O, from).
4. iv. iv. If fromPresent is true, then
1. 1. 1. Let fromValue be ? Get(O, from).
2. 2. 2. Perform ? Set(O, to, fromValue, true).
5. v. v. Else,
1. 1. 1. Assert: fromPresent is false.
2. 2. 2. Perform ? DeletePropertyOrThrow(O, to).
6. vi. vi. Set k to k - 1.
4. d. d. Let j be +0𝔽.
5. e. e. For each element E of items, do
1. i. i. Perform ? Set(O, ! ToString(j), E, true).
2. ii. ii. Set j to j + 1𝔽.
5. 5. 5. Perform ? Set(O, "length", 𝔽(len + argCount), true).
6. 6. 6. Return 𝔽(len + argCount).
The "length" property of this method is 1𝔽.
Note
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a
method.
23.1.3.38 ARRAY.PROTOTYPE.VALUES ( )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Return CreateArrayIterator(O, value).
23.1.3.39 ARRAY.PROTOTYPE.WITH ( INDEX, VALUE )
This method performs the following steps when called:
1. 1. 1. Let O be ? ToObject(this value).
2. 2. 2. Let len be ? LengthOfArrayLike(O).
3. 3. 3. Let relativeIndex be ? ToIntegerOrInfinity(index).
4. 4. 4. If relativeIndex ≥ 0, let actualIndex be relativeIndex.
5. 5. 5. Else, let actualIndex be len + relativeIndex.
6. 6. 6. If actualIndex ≥ len or actualIndex < 0, throw a RangeError
exception.
7. 7. 7. Let A be ? ArrayCreate(len).
8. 8. 8. Let k be 0.
9. 9. 9. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. If k is actualIndex, let fromValue be value.
3. c. c. Else, let fromValue be ? Get(O, Pk).
4. d. d. Perform ! CreateDataPropertyOrThrow(A, Pk, fromValue).
5. e. e. Set k to k + 1.
10. 10. 10. Return A.
23.1.3.40 ARRAY.PROTOTYPE [ @@ITERATOR ] ( )
The initial value of the @@iterator property is %Array.prototype.values%,
defined in 23.1.3.38.
23.1.3.41 ARRAY.PROTOTYPE [ @@UNSCOPABLES ]
The initial value of the @@unscopables data property is an object created by the
following steps:
1. 1. 1. Let unscopableList be OrdinaryObjectCreate(null).
2. 2. 2. Perform ! CreateDataPropertyOrThrow(unscopableList, "at", true).
3. 3. 3. Perform ! CreateDataPropertyOrThrow(unscopableList, "copyWithin",
true).
4. 4. 4. Perform ! CreateDataPropertyOrThrow(unscopableList, "entries", true).
5. 5. 5. Perform ! CreateDataPropertyOrThrow(unscopableList, "fill", true).
6. 6. 6. Perform ! CreateDataPropertyOrThrow(unscopableList, "find", true).
7. 7. 7. Perform ! CreateDataPropertyOrThrow(unscopableList, "findIndex",
true).
8. 8. 8. Perform ! CreateDataPropertyOrThrow(unscopableList, "findLast",
true).
9. 9. 9. Perform ! CreateDataPropertyOrThrow(unscopableList, "findLastIndex",
true).
10. 10. 10. Perform ! CreateDataPropertyOrThrow(unscopableList, "flat", true).
11. 11. 11. Perform ! CreateDataPropertyOrThrow(unscopableList, "flatMap",
true).
12. 12. 12. Perform ! CreateDataPropertyOrThrow(unscopableList, "includes",
true).
13. 13. 13. Perform ! CreateDataPropertyOrThrow(unscopableList, "keys", true).
14. 14. 14. Perform ! CreateDataPropertyOrThrow(unscopableList, "toReversed",
true).
15. 15. 15. Perform ! CreateDataPropertyOrThrow(unscopableList, "toSorted",
true).
16. 16. 16. Perform ! CreateDataPropertyOrThrow(unscopableList, "toSpliced",
true).
17. 17. 17. Perform ! CreateDataPropertyOrThrow(unscopableList, "values",
true).
18. 18. 18. Return unscopableList.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
Note
The own property names of this object are property names that were not included
as standard properties of Array.prototype prior to the ECMAScript 2015
specification. These names are ignored for with statement binding purposes in
order to preserve the behaviour of existing code that might use one of these
names as a binding in an outer scope that is shadowed by a with statement whose
binding object is an Array.
The reason that "with" is not included in the unscopableList is because it is
already a reserved word.
23.1.4 PROPERTIES OF ARRAY INSTANCES
Array instances are Array exotic objects and have the internal methods specified
for such objects. Array instances inherit properties from the Array prototype
object.
Array instances have a "length" property, and a set of enumerable properties
with array index names.
23.1.4.1 LENGTH
The "length" property of an Array instance is a data property whose value is
always numerically greater than the name of every configurable own property
whose name is an array index.
The "length" property initially has the attributes { [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }.
Note
Reducing the value of the "length" property has the side-effect of deleting own
array elements whose array index is between the old and new length values.
However, non-configurable properties can not be deleted. Attempting to set the
"length" property of an Array to a value that is numerically less than or equal
to the largest numeric own property name of an existing non-configurable
array-indexed property of the array will result in the length being set to a
numeric value that is one greater than that non-configurable numeric own
property name. See 10.4.2.1.
23.1.5 ARRAY ITERATOR OBJECTS
An Array Iterator is an object, that represents a specific iteration over some
specific Array instance object. There is not a named constructor for Array
Iterator objects. Instead, Array iterator objects are created by calling certain
methods of Array instance objects.
23.1.5.1 CREATEARRAYITERATOR ( ARRAY, KIND )
The abstract operation CreateArrayIterator takes arguments array (an Object) and
kind (key+value, key, or value) and returns a Generator. It is used to create
iterator objects for Array methods that return such iterators. It performs the
following steps when called:
1. 1. 1. Let closure be a new Abstract Closure with no parameters that captures
kind and array and performs the following steps when called:
1. a. a. Let index be 0.
2. b. b. Repeat,
1. i. i. If array has a [[TypedArrayName]] internal slot, then
1. 1. 1. If IsDetachedBuffer(array.[[ViewedArrayBuffer]]) is true,
throw a TypeError exception.
2. 2. 2. Let len be array.[[ArrayLength]].
2. ii. ii. Else,
1. 1. 1. Let len be ? LengthOfArrayLike(array).
3. iii. iii. If index ≥ len, return NormalCompletion(undefined).
4. iv. iv. If kind is key, perform
? GeneratorYield(CreateIterResultObject(𝔽(index), false)).
5. v. v. Else,
1. 1. 1. Let elementKey be ! ToString(𝔽(index)).
2. 2. 2. Let elementValue be ? Get(array, elementKey).
3. 3. 3. If kind is value, perform
? GeneratorYield(CreateIterResultObject(elementValue, false)).
4. 4. 4. Else,
1. a. a. Assert: kind is key+value.
2. b. b. Let result be CreateArrayFromList(« 𝔽(index),
elementValue »).
3. c. c. Perform ? GeneratorYield(CreateIterResultObject(result,
false)).
6. vi. vi. Set index to index + 1.
2. 2. 2. Return CreateIteratorFromClosure(closure, "%ArrayIteratorPrototype%",
%ArrayIteratorPrototype%).
23.1.5.2 THE %ARRAYITERATORPROTOTYPE% OBJECT
The %ArrayIteratorPrototype% object:
* has properties that are inherited by all Array Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* has the following properties:
23.1.5.2.1 %ARRAYITERATORPROTOTYPE%.NEXT ( )
1. 1. 1. Return ? GeneratorResume(this value, empty,
"%ArrayIteratorPrototype%").
23.1.5.2.2 %ARRAYITERATORPROTOTYPE% [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Array
Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
23.2 TYPEDARRAY OBJECTS
A TypedArray presents an array-like view of an underlying binary data buffer
(25.1). A TypedArray element type is the underlying binary scalar data type that
all elements of a TypedArray instance have. There is a distinct TypedArray
constructor, listed in Table 68, for each of the supported element types. Each
constructor in Table 68 has a corresponding distinct prototype object.
Table 68: The TypedArray Constructors
Constructor Name and Intrinsic Element Type Element Size Conversion Operation
Description Int8Array
%Int8Array% Int8 1 ToInt8 8-bit two's complement signed integer Uint8Array
%Uint8Array% Uint8 1 ToUint8 8-bit unsigned integer Uint8ClampedArray
%Uint8ClampedArray% Uint8C 1 ToUint8Clamp 8-bit unsigned integer (clamped
conversion) Int16Array
%Int16Array% Int16 2 ToInt16 16-bit two's complement signed integer Uint16Array
%Uint16Array% Uint16 2 ToUint16 16-bit unsigned integer Int32Array
%Int32Array% Int32 4 ToInt32 32-bit two's complement signed integer Uint32Array
%Uint32Array% Uint32 4 ToUint32 32-bit unsigned integer BigInt64Array
%BigInt64Array% BigInt64 8 ToBigInt64 64-bit two's complement signed integer
BigUint64Array
%BigUint64Array% BigUint64 8 ToBigUint64 64-bit unsigned integer Float32Array
%Float32Array% Float32 4 32-bit IEEE floating point Float64Array
%Float64Array% Float64 8 64-bit IEEE floating point
In the definitions below, references to TypedArray should be replaced with the
appropriate constructor name from the above table.
23.2.1 THE %TYPEDARRAY% INTRINSIC OBJECT
The %TypedArray% intrinsic object:
* is a constructor function object that all of the TypedArray constructor
objects inherit from.
* along with its corresponding prototype object, provides common properties
that are inherited by all TypedArray constructors and their instances.
* does not have a global name or appear as a property of the global object.
* acts as the abstract superclass of the various TypedArray constructors.
* will throw an error when invoked, because it is an abstract class
constructor. The TypedArray constructors do not perform a super call to it.
23.2.1.1 %TYPEDARRAY% ( )
This function performs the following steps when called:
1. 1. 1. Throw a TypeError exception.
The "length" property of this function is +0𝔽.
23.2.2 PROPERTIES OF THE %TYPEDARRAY% INTRINSIC OBJECT
The %TypedArray% intrinsic object:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has a "name" property whose value is "TypedArray".
* has the following properties:
23.2.2.1 %TYPEDARRAY%.FROM ( SOURCE [ , MAPFN [ , THISARG ] ] )
This method performs the following steps when called:
1. 1. 1. Let C be the this value.
2. 2. 2. If IsConstructor(C) is false, throw a TypeError exception.
3. 3. 3. If mapfn is undefined, let mapping be false.
4. 4. 4. Else,
1. a. a. If IsCallable(mapfn) is false, throw a TypeError exception.
2. b. b. Let mapping be true.
5. 5. 5. Let usingIterator be ? GetMethod(source, @@iterator).
6. 6. 6. If usingIterator is not undefined, then
1. a. a. Let values be ? IteratorToList(? GetIteratorFromMethod(source,
usingIterator)).
2. b. b. Let len be the number of elements in values.
3. c. c. Let targetObj be ? TypedArrayCreate(C, « 𝔽(len) »).
4. d. d. Let k be 0.
5. e. e. Repeat, while k < len,
1. i. i. Let Pk be ! ToString(𝔽(k)).
2. ii. ii. Let kValue be the first element of values.
3. iii. iii. Remove the first element from values.
4. iv. iv. If mapping is true, then
1. 1. 1. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k)
»).
5. v. v. Else, let mappedValue be kValue.
6. vi. vi. Perform ? Set(targetObj, Pk, mappedValue, true).
7. vii. vii. Set k to k + 1.
6. f. f. Assert: values is now an empty List.
7. g. g. Return targetObj.
7. 7. 7. NOTE: source is not an Iterable so assume it is already an array-like
object.
8. 8. 8. Let arrayLike be ! ToObject(source).
9. 9. 9. Let len be ? LengthOfArrayLike(arrayLike).
10. 10. 10. Let targetObj be ? TypedArrayCreate(C, « 𝔽(len) »).
11. 11. 11. Let k be 0.
12. 12. 12. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ? Get(arrayLike, Pk).
3. c. c. If mapping is true, then
1. i. i. Let mappedValue be ? Call(mapfn, thisArg, « kValue, 𝔽(k) »).
4. d. d. Else, let mappedValue be kValue.
5. e. e. Perform ? Set(targetObj, Pk, mappedValue, true).
6. f. f. Set k to k + 1.
13. 13. 13. Return targetObj.
23.2.2.2 %TYPEDARRAY%.OF ( ...ITEMS )
This method performs the following steps when called:
1. 1. 1. Let len be the number of elements in items.
2. 2. 2. Let C be the this value.
3. 3. 3. If IsConstructor(C) is false, throw a TypeError exception.
4. 4. 4. Let newObj be ? TypedArrayCreate(C, « 𝔽(len) »).
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let kValue be items[k].
2. b. b. Let Pk be ! ToString(𝔽(k)).
3. c. c. Perform ? Set(newObj, Pk, kValue, true).
4. d. d. Set k to k + 1.
7. 7. 7. Return newObj.
23.2.2.3 %TYPEDARRAY%.PROTOTYPE
The initial value of %TypedArray%.prototype is the %TypedArray% prototype
object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.2.2.4 GET %TYPEDARRAY% [ @@SPECIES ]
%TypedArray%[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
%TypedArray.prototype% methods normally use their this value's constructor to
create a derived object. However, a subclass constructor may over-ride that
default behaviour by redefining its @@species property.
23.2.3 PROPERTIES OF THE %TYPEDARRAY% PROTOTYPE OBJECT
The %TypedArray% prototype object:
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is %TypedArray.prototype%.
* is an ordinary object.
* does not have a [[ViewedArrayBuffer]] or any other of the internal slots that
are specific to TypedArray instance objects.
23.2.3.1 %TYPEDARRAY%.PROTOTYPE.AT ( INDEX )
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let relativeIndex be ? ToIntegerOrInfinity(index).
5. 5. 5. If relativeIndex ≥ 0, then
1. a. a. Let k be relativeIndex.
6. 6. 6. Else,
1. a. a. Let k be len + relativeIndex.
7. 7. 7. If k < 0 or k ≥ len, return undefined.
8. 8. 8. Return ! Get(O, ! ToString(𝔽(k))).
23.2.3.2 GET %TYPEDARRAY%.PROTOTYPE.BUFFER
%TypedArray%.prototype.buffer is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. Return buffer.
23.2.3.3 GET %TYPEDARRAY%.PROTOTYPE.BYTELENGTH
%TypedArray%.prototype.byteLength is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. If IsDetachedBuffer(buffer) is true, return +0𝔽.
6. 6. 6. Let size be O.[[ByteLength]].
7. 7. 7. Return 𝔽(size).
23.2.3.4 GET %TYPEDARRAY%.PROTOTYPE.BYTEOFFSET
%TypedArray%.prototype.byteOffset is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. If IsDetachedBuffer(buffer) is true, return +0𝔽.
6. 6. 6. Let offset be O.[[ByteOffset]].
7. 7. 7. Return 𝔽(offset).
23.2.3.5 %TYPEDARRAY%.PROTOTYPE.CONSTRUCTOR
The initial value of %TypedArray%.prototype.constructor is %TypedArray%.
23.2.3.6 %TYPEDARRAY%.PROTOTYPE.COPYWITHIN ( TARGET, START [ , END ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.copyWithin as defined in 23.1.3.4.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let relativeTarget be ? ToIntegerOrInfinity(target).
5. 5. 5. If relativeTarget = -∞, let to be 0.
6. 6. 6. Else if relativeTarget < 0, let to be max(len + relativeTarget, 0).
7. 7. 7. Else, let to be min(relativeTarget, len).
8. 8. 8. Let relativeStart be ? ToIntegerOrInfinity(start).
9. 9. 9. If relativeStart = -∞, let from be 0.
10. 10. 10. Else if relativeStart < 0, let from be max(len + relativeStart, 0).
11. 11. 11. Else, let from be min(relativeStart, len).
12. 12. 12. If end is undefined, let relativeEnd be len; else let relativeEnd
be ? ToIntegerOrInfinity(end).
13. 13. 13. If relativeEnd = -∞, let final be 0.
14. 14. 14. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
15. 15. 15. Else, let final be min(relativeEnd, len).
16. 16. 16. Let count be min(final - from, len - to).
17. 17. 17. If count > 0, then
1. a. a. NOTE: The copying must be performed in a manner that preserves
the bit-level encoding of the source data.
2. b. b. Let buffer be O.[[ViewedArrayBuffer]].
3. c. c. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
4. d. d. Let elementSize be TypedArrayElementSize(O).
5. e. e. Let byteOffset be O.[[ByteOffset]].
6. f. f. Let toByteIndex be to × elementSize + byteOffset.
7. g. g. Let fromByteIndex be from × elementSize + byteOffset.
8. h. h. Let countBytes be count × elementSize.
9. i. i. If fromByteIndex < toByteIndex and toByteIndex < fromByteIndex +
countBytes, then
1. i. i. Let direction be -1.
2. ii. ii. Set fromByteIndex to fromByteIndex + countBytes - 1.
3. iii. iii. Set toByteIndex to toByteIndex + countBytes - 1.
10. j. j. Else,
1. i. i. Let direction be 1.
11. k. k. Repeat, while countBytes > 0,
1. i. i. Let value be GetValueFromBuffer(buffer, fromByteIndex, Uint8,
true, Unordered).
2. ii. ii. Perform SetValueInBuffer(buffer, toByteIndex, Uint8, value,
true, Unordered).
3. iii. iii. Set fromByteIndex to fromByteIndex + direction.
4. iv. iv. Set toByteIndex to toByteIndex + direction.
5. v. v. Set countBytes to countBytes - 1.
18. 18. 18. Return O.
23.2.3.7 %TYPEDARRAY%.PROTOTYPE.ENTRIES ( )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Return CreateArrayIterator(O, key+value).
23.2.3.8 %TYPEDARRAY%.PROTOTYPE.EVERY ( CALLBACKFN [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.every as defined in 23.1.3.6.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Let testResult be ToBoolean(? Call(callbackfn, thisArg, « kValue,
𝔽(k), O »)).
4. d. d. If testResult is false, return false.
5. e. e. Set k to k + 1.
7. 7. 7. Return true.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.9 %TYPEDARRAY%.PROTOTYPE.FILL ( VALUE [ , START [ , END ] ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.fill as defined in 23.1.3.7.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If O.[[ContentType]] is BigInt, set value to ? ToBigInt(value).
5. 5. 5. Otherwise, set value to ? ToNumber(value).
6. 6. 6. Let relativeStart be ? ToIntegerOrInfinity(start).
7. 7. 7. If relativeStart = -∞, let k be 0.
8. 8. 8. Else if relativeStart < 0, let k be max(len + relativeStart, 0).
9. 9. 9. Else, let k be min(relativeStart, len).
10. 10. 10. If end is undefined, let relativeEnd be len; else let relativeEnd
be ? ToIntegerOrInfinity(end).
11. 11. 11. If relativeEnd = -∞, let final be 0.
12. 12. 12. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
13. 13. 13. Else, let final be min(relativeEnd, len).
14. 14. 14. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, throw a
TypeError exception.
15. 15. 15. Repeat, while k < final,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Perform ! Set(O, Pk, value, true).
3. c. c. Set k to k + 1.
16. 16. 16. Return O.
23.2.3.10 %TYPEDARRAY%.PROTOTYPE.FILTER ( CALLBACKFN [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.filter as defined in 23.1.3.8.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. Let kept be a new empty List.
6. 6. 6. Let captured be 0.
7. 7. 7. Let k be 0.
8. 8. 8. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Let selected be ToBoolean(? Call(callbackfn, thisArg, « kValue,
𝔽(k), O »)).
4. d. d. If selected is true, then
1. i. i. Append kValue to kept.
2. ii. ii. Set captured to captured + 1.
5. e. e. Set k to k + 1.
9. 9. 9. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(captured) »).
10. 10. 10. Let n be 0.
11. 11. 11. For each element e of kept, do
1. a. a. Perform ! Set(A, ! ToString(𝔽(n)), e, true).
2. b. b. Set n to n + 1.
12. 12. 12. Return A.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.11 %TYPEDARRAY%.PROTOTYPE.FIND ( PREDICATE [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.find as defined in 23.1.3.9.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let findRec be ? FindViaPredicate(O, len, ascending, predicate,
thisArg).
5. 5. 5. Return findRec.[[Value]].
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.12 %TYPEDARRAY%.PROTOTYPE.FINDINDEX ( PREDICATE [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.findIndex as defined in 23.1.3.10.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let findRec be ? FindViaPredicate(O, len, ascending, predicate,
thisArg).
5. 5. 5. Return findRec.[[Index]].
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.13 %TYPEDARRAY%.PROTOTYPE.FINDLAST ( PREDICATE [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.findLast as defined in 23.1.3.11.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let findRec be ? FindViaPredicate(O, len, descending, predicate,
thisArg).
5. 5. 5. Return findRec.[[Value]].
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.14 %TYPEDARRAY%.PROTOTYPE.FINDLASTINDEX ( PREDICATE [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.findLastIndex as defined in 23.1.3.12.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let findRec be ? FindViaPredicate(O, len, descending, predicate,
thisArg).
5. 5. 5. Return findRec.[[Index]].
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.15 %TYPEDARRAY%.PROTOTYPE.FOREACH ( CALLBACKFN [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.forEach as defined in 23.1.3.15.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Perform ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O »).
4. d. d. Set k to k + 1.
7. 7. 7. Return undefined.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.16 %TYPEDARRAY%.PROTOTYPE.INCLUDES ( SEARCHELEMENT [ , FROMINDEX ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.includes as defined in 23.1.3.16.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If len = 0, return false.
5. 5. 5. Let n be ? ToIntegerOrInfinity(fromIndex).
6. 6. 6. Assert: If fromIndex is undefined, then n is 0.
7. 7. 7. If n = +∞, return false.
8. 8. 8. Else if n = -∞, set n to 0.
9. 9. 9. If n ≥ 0, then
1. a. a. Let k be n.
10. 10. 10. Else,
1. a. a. Let k be len + n.
2. b. b. If k < 0, set k to 0.
11. 11. 11. Repeat, while k < len,
1. a. a. Let elementK be ! Get(O, ! ToString(𝔽(k))).
2. b. b. If SameValueZero(searchElement, elementK) is true, return true.
3. c. c. Set k to k + 1.
12. 12. 12. Return false.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.17 %TYPEDARRAY%.PROTOTYPE.INDEXOF ( SEARCHELEMENT [ , FROMINDEX ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.indexOf as defined in 23.1.3.17.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If len = 0, return -1𝔽.
5. 5. 5. Let n be ? ToIntegerOrInfinity(fromIndex).
6. 6. 6. Assert: If fromIndex is undefined, then n is 0.
7. 7. 7. If n = +∞, return -1𝔽.
8. 8. 8. Else if n = -∞, set n to 0.
9. 9. 9. If n ≥ 0, then
1. a. a. Let k be n.
10. 10. 10. Else,
1. a. a. Let k be len + n.
2. b. b. If k < 0, set k to 0.
11. 11. 11. Repeat, while k < len,
1. a. a. Let kPresent be ! HasProperty(O, ! ToString(𝔽(k))).
2. b. b. If kPresent is true, then
1. i. i. Let elementK be ! Get(O, ! ToString(𝔽(k))).
2. ii. ii. If IsStrictlyEqual(searchElement, elementK) is true, return
𝔽(k).
3. c. c. Set k to k + 1.
12. 12. 12. Return -1𝔽.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.18 %TYPEDARRAY%.PROTOTYPE.JOIN ( SEPARATOR )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.join as defined in 23.1.3.18.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If separator is undefined, let sep be ",".
5. 5. 5. Else, let sep be ? ToString(separator).
6. 6. 6. Let R be the empty String.
7. 7. 7. Let k be 0.
8. 8. 8. Repeat, while k < len,
1. a. a. If k > 0, set R to the string-concatenation of R and sep.
2. b. b. Let element be ! Get(O, ! ToString(𝔽(k))).
3. c. c. If element is undefined, let next be the empty String; otherwise,
let next be ! ToString(element).
4. d. d. Set R to the string-concatenation of R and next.
5. e. e. Set k to k + 1.
9. 9. 9. Return R.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.19 %TYPEDARRAY%.PROTOTYPE.KEYS ( )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Return CreateArrayIterator(O, key).
23.2.3.20 %TYPEDARRAY%.PROTOTYPE.LASTINDEXOF ( SEARCHELEMENT [ , FROMINDEX ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.lastIndexOf as defined in 23.1.3.20.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If len = 0, return -1𝔽.
5. 5. 5. If fromIndex is present, let n be ? ToIntegerOrInfinity(fromIndex);
else let n be len - 1.
6. 6. 6. If n = -∞, return -1𝔽.
7. 7. 7. If n ≥ 0, then
1. a. a. Let k be min(n, len - 1).
8. 8. 8. Else,
1. a. a. Let k be len + n.
9. 9. 9. Repeat, while k ≥ 0,
1. a. a. Let kPresent be ! HasProperty(O, ! ToString(𝔽(k))).
2. b. b. If kPresent is true, then
1. i. i. Let elementK be ! Get(O, ! ToString(𝔽(k))).
2. ii. ii. If IsStrictlyEqual(searchElement, elementK) is true, return
𝔽(k).
3. c. c. Set k to k - 1.
10. 10. 10. Return -1𝔽.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.21 GET %TYPEDARRAY%.PROTOTYPE.LENGTH
%TypedArray%.prototype.length is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
3. 3. 3. Assert: O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal
slots.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. If IsDetachedBuffer(buffer) is true, return +0𝔽.
6. 6. 6. Let length be O.[[ArrayLength]].
7. 7. 7. Return 𝔽(length).
This function is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.22 %TYPEDARRAY%.PROTOTYPE.MAP ( CALLBACKFN [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.map as defined in 23.1.3.21.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(len) »).
6. 6. 6. Let k be 0.
7. 7. 7. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Let mappedValue be ? Call(callbackfn, thisArg, « kValue, 𝔽(k), O
»).
4. d. d. Perform ? Set(A, Pk, mappedValue, true).
5. e. e. Set k to k + 1.
8. 8. 8. Return A.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.23 %TYPEDARRAY%.PROTOTYPE.REDUCE ( CALLBACKFN [ , INITIALVALUE ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.reduce as defined in 23.1.3.24.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. If len = 0 and initialValue is not present, throw a TypeError
exception.
6. 6. 6. Let k be 0.
7. 7. 7. Let accumulator be undefined.
8. 8. 8. If initialValue is present, then
1. a. a. Set accumulator to initialValue.
9. 9. 9. Else,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Set accumulator to ! Get(O, Pk).
3. c. c. Set k to k + 1.
10. 10. 10. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Set accumulator to ? Call(callbackfn, undefined, « accumulator,
kValue, 𝔽(k), O »).
4. d. d. Set k to k + 1.
11. 11. 11. Return accumulator.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.24 %TYPEDARRAY%.PROTOTYPE.REDUCERIGHT ( CALLBACKFN [ , INITIALVALUE ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.reduceRight as defined in 23.1.3.25.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. If len = 0 and initialValue is not present, throw a TypeError
exception.
6. 6. 6. Let k be len - 1.
7. 7. 7. Let accumulator be undefined.
8. 8. 8. If initialValue is present, then
1. a. a. Set accumulator to initialValue.
9. 9. 9. Else,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Set accumulator to ! Get(O, Pk).
3. c. c. Set k to k - 1.
10. 10. 10. Repeat, while k ≥ 0,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Set accumulator to ? Call(callbackfn, undefined, « accumulator,
kValue, 𝔽(k), O »).
4. d. d. Set k to k - 1.
11. 11. 11. Return accumulator.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.25 %TYPEDARRAY%.PROTOTYPE.REVERSE ( )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.reverse as defined in 23.1.3.26.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let middle be floor(len / 2).
5. 5. 5. Let lower be 0.
6. 6. 6. Repeat, while lower ≠ middle,
1. a. a. Let upper be len - lower - 1.
2. b. b. Let upperP be ! ToString(𝔽(upper)).
3. c. c. Let lowerP be ! ToString(𝔽(lower)).
4. d. d. Let lowerValue be ! Get(O, lowerP).
5. e. e. Let upperValue be ! Get(O, upperP).
6. f. f. Perform ! Set(O, lowerP, upperValue, true).
7. g. g. Perform ! Set(O, upperP, lowerValue, true).
8. h. h. Set lower to lower + 1.
7. 7. 7. Return O.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.26 %TYPEDARRAY%.PROTOTYPE.SET ( SOURCE [ , OFFSET ] )
This method sets multiple values in this TypedArray, reading the values from
source. The details differ based upon the type of source. The optional offset
value indicates the first element index in this TypedArray where values are
written. If omitted, it is assumed to be 0.
It performs the following steps when called:
1. 1. 1. Let target be the this value.
2. 2. 2. Perform ? RequireInternalSlot(target, [[TypedArrayName]]).
3. 3. 3. Assert: target has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let targetOffset be ? ToIntegerOrInfinity(offset).
5. 5. 5. If targetOffset < 0, throw a RangeError exception.
6. 6. 6. If source is an Object that has a [[TypedArrayName]] internal slot,
then
1. a. a. Perform ? SetTypedArrayFromTypedArray(target, targetOffset,
source).
7. 7. 7. Else,
1. a. a. Perform ? SetTypedArrayFromArrayLike(target, targetOffset, source).
8. 8. 8. Return undefined.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.26.1 SETTYPEDARRAYFROMTYPEDARRAY ( TARGET, TARGETOFFSET, SOURCE )
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a
TypedArray), targetOffset (a non-negative integer or +∞), and source (a
TypedArray) and returns either a normal completion containing unused or a throw
completion. It sets multiple values in target, starting at index targetOffset,
reading the values from source. It performs the following steps when called:
1. 1. 1. Let targetBuffer be target.[[ViewedArrayBuffer]].
2. 2. 2. If IsDetachedBuffer(targetBuffer) is true, throw a TypeError
exception.
3. 3. 3. Let targetLength be target.[[ArrayLength]].
4. 4. 4. Let srcBuffer be source.[[ViewedArrayBuffer]].
5. 5. 5. If IsDetachedBuffer(srcBuffer) is true, throw a TypeError exception.
6. 6. 6. Let targetType be TypedArrayElementType(target).
7. 7. 7. Let targetElementSize be TypedArrayElementSize(target).
8. 8. 8. Let targetByteOffset be target.[[ByteOffset]].
9. 9. 9. Let srcType be TypedArrayElementType(source).
10. 10. 10. Let srcElementSize be TypedArrayElementSize(source).
11. 11. 11. Let srcLength be source.[[ArrayLength]].
12. 12. 12. Let srcByteOffset be source.[[ByteOffset]].
13. 13. 13. If targetOffset = +∞, throw a RangeError exception.
14. 14. 14. If srcLength + targetOffset > targetLength, throw a RangeError
exception.
15. 15. 15. If target.[[ContentType]] is not source.[[ContentType]], throw a
TypeError exception.
16. 16. 16. If IsSharedArrayBuffer(srcBuffer) is true,
IsSharedArrayBuffer(targetBuffer) is true, and
srcBuffer.[[ArrayBufferData]] is targetBuffer.[[ArrayBufferData]], let
sameSharedArrayBuffer be true; otherwise, let sameSharedArrayBuffer be
false.
17. 17. 17. If SameValue(srcBuffer, targetBuffer) is true or
sameSharedArrayBuffer is true, then
1. a. a. Let srcByteLength be source.[[ByteLength]].
2. b. b. Set srcBuffer to ? CloneArrayBuffer(srcBuffer, srcByteOffset,
srcByteLength).
3. c. c. Let srcByteIndex be 0.
18. 18. 18. Else, let srcByteIndex be srcByteOffset.
19. 19. 19. Let targetByteIndex be targetOffset × targetElementSize +
targetByteOffset.
20. 20. 20. Let limit be targetByteIndex + targetElementSize × srcLength.
21. 21. 21. If srcType is targetType, then
1. a. a. NOTE: The transfer must be performed in a manner that preserves
the bit-level encoding of the source data.
2. b. b. Repeat, while targetByteIndex < limit,
1. i. i. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex, Uint8,
true, Unordered).
2. ii. ii. Perform SetValueInBuffer(targetBuffer, targetByteIndex,
Uint8, value, true, Unordered).
3. iii. iii. Set srcByteIndex to srcByteIndex + 1.
4. iv. iv. Set targetByteIndex to targetByteIndex + 1.
22. 22. 22. Else,
1. a. a. Repeat, while targetByteIndex < limit,
1. i. i. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex,
srcType, true, Unordered).
2. ii. ii. Perform SetValueInBuffer(targetBuffer, targetByteIndex,
targetType, value, true, Unordered).
3. iii. iii. Set srcByteIndex to srcByteIndex + srcElementSize.
4. iv. iv. Set targetByteIndex to targetByteIndex + targetElementSize.
23. 23. 23. Return unused.
23.2.3.26.2 SETTYPEDARRAYFROMARRAYLIKE ( TARGET, TARGETOFFSET, SOURCE )
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a
TypedArray), targetOffset (a non-negative integer or +∞), and source (an
ECMAScript language value, but not a TypedArray) and returns either a normal
completion containing unused or a throw completion. It sets multiple values in
target, starting at index targetOffset, reading the values from source. It
performs the following steps when called:
1. 1. 1. Let targetBuffer be target.[[ViewedArrayBuffer]].
2. 2. 2. If IsDetachedBuffer(targetBuffer) is true, throw a TypeError
exception.
3. 3. 3. Let targetLength be target.[[ArrayLength]].
4. 4. 4. Let src be ? ToObject(source).
5. 5. 5. Let srcLength be ? LengthOfArrayLike(src).
6. 6. 6. If targetOffset = +∞, throw a RangeError exception.
7. 7. 7. If srcLength + targetOffset > targetLength, throw a RangeError
exception.
8. 8. 8. Let k be 0.
9. 9. 9. Repeat, while k < srcLength,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let value be ? Get(src, Pk).
3. c. c. Let targetIndex be 𝔽(targetOffset + k).
4. d. d. Perform ? IntegerIndexedElementSet(target, targetIndex, value).
5. e. e. Set k to k + 1.
10. 10. 10. Return unused.
23.2.3.27 %TYPEDARRAY%.PROTOTYPE.SLICE ( START, END )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.slice as defined in 23.1.3.28.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let relativeStart be ? ToIntegerOrInfinity(start).
5. 5. 5. If relativeStart = -∞, let k be 0.
6. 6. 6. Else if relativeStart < 0, let k be max(len + relativeStart, 0).
7. 7. 7. Else, let k be min(relativeStart, len).
8. 8. 8. If end is undefined, let relativeEnd be len; else let relativeEnd be
? ToIntegerOrInfinity(end).
9. 9. 9. If relativeEnd = -∞, let final be 0.
10. 10. 10. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
11. 11. 11. Else, let final be min(relativeEnd, len).
12. 12. 12. Let count be max(final - k, 0).
13. 13. 13. Let A be ? TypedArraySpeciesCreate(O, « 𝔽(count) »).
14. 14. 14. If count > 0, then
1. a. a. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, throw a
TypeError exception.
2. b. b. Let srcType be TypedArrayElementType(O).
3. c. c. Let targetType be TypedArrayElementType(A).
4. d. d. If srcType is targetType, then
1. i. i. NOTE: The transfer must be performed in a manner that preserves
the bit-level encoding of the source data.
2. ii. ii. Let srcBuffer be O.[[ViewedArrayBuffer]].
3. iii. iii. Let targetBuffer be A.[[ViewedArrayBuffer]].
4. iv. iv. Let elementSize be TypedArrayElementSize(O).
5. v. v. Let srcByteOffset be O.[[ByteOffset]].
6. vi. vi. Let srcByteIndex be (k × elementSize) + srcByteOffset.
7. vii. vii. Let targetByteIndex be A.[[ByteOffset]].
8. viii. viii. Let limit be targetByteIndex + count × elementSize.
9. ix. ix. Repeat, while targetByteIndex < limit,
1. 1. 1. Let value be GetValueFromBuffer(srcBuffer, srcByteIndex,
Uint8, true, Unordered).
2. 2. 2. Perform SetValueInBuffer(targetBuffer, targetByteIndex,
Uint8, value, true, Unordered).
3. 3. 3. Set srcByteIndex to srcByteIndex + 1.
4. 4. 4. Set targetByteIndex to targetByteIndex + 1.
5. e. e. Else,
1. i. i. Let n be 0.
2. ii. ii. Repeat, while k < final,
1. 1. 1. Let Pk be ! ToString(𝔽(k)).
2. 2. 2. Let kValue be ! Get(O, Pk).
3. 3. 3. Perform ! Set(A, ! ToString(𝔽(n)), kValue, true).
4. 4. 4. Set k to k + 1.
5. 5. 5. Set n to n + 1.
15. 15. 15. Return A.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.28 %TYPEDARRAY%.PROTOTYPE.SOME ( CALLBACKFN [ , THISARG ] )
The interpretation and use of the arguments of this method are the same as for
Array.prototype.some as defined in 23.1.3.29.
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ! Get(O, Pk).
3. c. c. Let testResult be ToBoolean(? Call(callbackfn, thisArg, « kValue,
𝔽(k), O »)).
4. d. d. If testResult is true, return true.
5. e. e. Set k to k + 1.
7. 7. 7. Return false.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.29 %TYPEDARRAY%.PROTOTYPE.SORT ( COMPAREFN )
This is a distinct method that, except as described below, implements the same
requirements as those of Array.prototype.sort as defined in 23.1.3.30. The
implementation of this method may be optimized with the knowledge that the this
value is an object that has a fixed length and whose integer-indexed properties
are not sparse.
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
It performs the following steps when called:
1. 1. 1. If comparefn is not undefined and IsCallable(comparefn) is false,
throw a TypeError exception.
2. 2. 2. Let obj be the this value.
3. 3. 3. Perform ? ValidateTypedArray(obj).
4. 4. 4. Let len be obj.[[ArrayLength]].
5. 5. 5. NOTE: The following closure performs a numeric comparison rather than
the string comparison used in 23.1.3.30.
6. 6. 6. Let SortCompare be a new Abstract Closure with parameters (x, y) that
captures comparefn and performs the following steps when called:
1. a. a. Return ? CompareTypedArrayElements(x, y, comparefn).
7. 7. 7. Let sortedList be ? SortIndexedProperties(obj, len, SortCompare,
read-through-holes).
8. 8. 8. Let j be 0.
9. 9. 9. Repeat, while j < len,
1. a. a. Perform ! Set(obj, ! ToString(𝔽(j)), sortedList[j], true).
2. b. b. Set j to j + 1.
10. 10. 10. Return obj.
Note
Because NaN always compares greater than any other value (see
CompareTypedArrayElements), NaN property values always sort to the end of the
result when comparefn is not provided.
23.2.3.30 %TYPEDARRAY%.PROTOTYPE.SUBARRAY ( BEGIN, END )
This method returns a new TypedArray whose element type is the element type of
this TypedArray and whose ArrayBuffer is the ArrayBuffer of this TypedArray,
referencing the elements in the interval from begin (inclusive) to end
(exclusive). If either begin or end is negative, it refers to an index from the
end of the array, as opposed to from the beginning.
It performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. Let srcLength be O.[[ArrayLength]].
6. 6. 6. Let relativeBegin be ? ToIntegerOrInfinity(begin).
7. 7. 7. If relativeBegin = -∞, let beginIndex be 0.
8. 8. 8. Else if relativeBegin < 0, let beginIndex be max(srcLength +
relativeBegin, 0).
9. 9. 9. Else, let beginIndex be min(relativeBegin, srcLength).
10. 10. 10. If end is undefined, let relativeEnd be srcLength; else let
relativeEnd be ? ToIntegerOrInfinity(end).
11. 11. 11. If relativeEnd = -∞, let endIndex be 0.
12. 12. 12. Else if relativeEnd < 0, let endIndex be max(srcLength +
relativeEnd, 0).
13. 13. 13. Else, let endIndex be min(relativeEnd, srcLength).
14. 14. 14. Let newLength be max(endIndex - beginIndex, 0).
15. 15. 15. Let elementSize be TypedArrayElementSize(O).
16. 16. 16. Let srcByteOffset be O.[[ByteOffset]].
17. 17. 17. Let beginByteOffset be srcByteOffset + beginIndex × elementSize.
18. 18. 18. Let argumentsList be « buffer, 𝔽(beginByteOffset), 𝔽(newLength)
».
19. 19. 19. Return ? TypedArraySpeciesCreate(O, argumentsList).
This method is not generic. The this value must be an object with a
[[TypedArrayName]] internal slot.
23.2.3.31 %TYPEDARRAY%.PROTOTYPE.TOLOCALESTRING ( [ RESERVED1 [ , RESERVED2 ] ]
)
This is a distinct method that implements the same algorithm as
Array.prototype.toLocaleString as defined in 23.1.3.32 except that the this
value's [[ArrayLength]] internal slot is accessed in place of performing a
[[Get]] of "length". The implementation of the algorithm may be optimized with
the knowledge that the this value is an object that has a fixed length and whose
integer-indexed properties are not sparse. However, such optimization must not
introduce any observable changes in the specified behaviour of the algorithm.
This method is not generic. ValidateTypedArray is applied to the this value
prior to evaluating the algorithm. If its result is an abrupt completion that
exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API
this method is based upon the algorithm for Array.prototype.toLocaleString that
is in the ECMA-402 specification.
23.2.3.32 %TYPEDARRAY%.PROTOTYPE.TOREVERSED ( )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let length be O.[[ArrayLength]].
4. 4. 4. Let A be ? TypedArrayCreateSameType(O, « 𝔽(length) »).
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < length,
1. a. a. Let from be ! ToString(𝔽(length - k - 1)).
2. b. b. Let Pk be ! ToString(𝔽(k)).
3. c. c. Let fromValue be ! Get(O, from).
4. d. d. Perform ! Set(A, Pk, fromValue, true).
5. e. e. Set k to k + 1.
7. 7. 7. Return A.
23.2.3.33 %TYPEDARRAY%.PROTOTYPE.TOSORTED ( COMPAREFN )
This method performs the following steps when called:
1. 1. 1. If comparefn is not undefined and IsCallable(comparefn) is false,
throw a TypeError exception.
2. 2. 2. Let O be the this value.
3. 3. 3. Perform ? ValidateTypedArray(O).
4. 4. 4. Let len be O.[[ArrayLength]].
5. 5. 5. Let A be ? TypedArrayCreateSameType(O, « 𝔽(len) »).
6. 6. 6. NOTE: The following closure performs a numeric comparison rather than
the string comparison used in 23.1.3.34.
7. 7. 7. Let SortCompare be a new Abstract Closure with parameters (x, y) that
captures comparefn and performs the following steps when called:
1. a. a. Return ? CompareTypedArrayElements(x, y, comparefn).
8. 8. 8. Let sortedList be ? SortIndexedProperties(O, len, SortCompare,
read-through-holes).
9. 9. 9. Let j be 0.
10. 10. 10. Repeat, while j < len,
1. a. a. Perform ! Set(A, ! ToString(𝔽(j)), sortedList[j], true).
2. b. b. Set j to j + 1.
11. 11. 11. Return A.
23.2.3.34 %TYPEDARRAY%.PROTOTYPE.TOSTRING ( )
The initial value of the "toString" property is %Array.prototype.toString%,
defined in 23.1.3.36.
23.2.3.35 %TYPEDARRAY%.PROTOTYPE.VALUES ( )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Return CreateArrayIterator(O, value).
23.2.3.36 %TYPEDARRAY%.PROTOTYPE.WITH ( INDEX, VALUE )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? ValidateTypedArray(O).
3. 3. 3. Let len be O.[[ArrayLength]].
4. 4. 4. Let relativeIndex be ? ToIntegerOrInfinity(index).
5. 5. 5. If relativeIndex ≥ 0, let actualIndex be relativeIndex.
6. 6. 6. Else, let actualIndex be len + relativeIndex.
7. 7. 7. If O.[[ContentType]] is BigInt, let numericValue be
? ToBigInt(value).
8. 8. 8. Else, let numericValue be ? ToNumber(value).
9. 9. 9. If IsValidIntegerIndex(O, 𝔽(actualIndex)) is false, throw a
RangeError exception.
10. 10. 10. Let A be ? TypedArrayCreateSameType(O, « 𝔽(len) »).
11. 11. 11. Let k be 0.
12. 12. 12. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. If k is actualIndex, let fromValue be numericValue.
3. c. c. Else, let fromValue be ! Get(O, Pk).
4. d. d. Perform ! Set(A, Pk, fromValue, true).
5. e. e. Set k to k + 1.
13. 13. 13. Return A.
23.2.3.37 %TYPEDARRAY%.PROTOTYPE [ @@ITERATOR ] ( )
The initial value of the @@iterator property is %TypedArray.prototype.values%,
defined in 23.2.3.35.
23.2.3.38 GET %TYPEDARRAY%.PROTOTYPE [ @@TOSTRINGTAG ]
%TypedArray%.prototype[@@toStringTag] is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. If O is not an Object, return undefined.
3. 3. 3. If O does not have a [[TypedArrayName]] internal slot, return
undefined.
4. 4. 4. Let name be O.[[TypedArrayName]].
5. 5. 5. Assert: name is a String.
6. 6. 6. Return name.
This property has the attributes { [[Enumerable]]: false, [[Configurable]]: true
}.
The initial value of the "name" property of this function is "get
[Symbol.toStringTag]".
23.2.4 ABSTRACT OPERATIONS FOR TYPEDARRAY OBJECTS
23.2.4.1 TYPEDARRAYSPECIESCREATE ( EXEMPLAR, ARGUMENTLIST )
The abstract operation TypedArraySpeciesCreate takes arguments exemplar (a
TypedArray) and argumentList (a List of ECMAScript language values) and returns
either a normal completion containing a TypedArray or a throw completion. It is
used to specify the creation of a new TypedArray using a constructor function
that is derived from exemplar. Unlike ArraySpeciesCreate, which can create
non-Array objects through the use of @@species, this operation enforces that the
constructor function creates an actual TypedArray. It performs the following
steps when called:
1. 1. 1. Let defaultConstructor be the intrinsic object associated with the
constructor name exemplar.[[TypedArrayName]] in Table 68.
2. 2. 2. Let constructor be ? SpeciesConstructor(exemplar, defaultConstructor).
3. 3. 3. Let result be ? TypedArrayCreate(constructor, argumentList).
4. 4. 4. Assert: result has [[TypedArrayName]] and [[ContentType]] internal
slots.
5. 5. 5. If result.[[ContentType]] is not exemplar.[[ContentType]], throw a
TypeError exception.
6. 6. 6. Return result.
23.2.4.2 TYPEDARRAYCREATE ( CONSTRUCTOR, ARGUMENTLIST )
The abstract operation TypedArrayCreate takes arguments constructor (a
constructor) and argumentList (a List of ECMAScript language values) and returns
either a normal completion containing a TypedArray or a throw completion. It is
used to specify the creation of a new TypedArray using a constructor function.
It performs the following steps when called:
1. 1. 1. Let newTypedArray be ? Construct(constructor, argumentList).
2. 2. 2. Perform ? ValidateTypedArray(newTypedArray).
3. 3. 3. If the number of elements in argumentList is 1 and argumentList[0] is
a Number, then
1. a. a. If newTypedArray.[[ArrayLength]] < ℝ(argumentList[0]), throw a
TypeError exception.
4. 4. 4. Return newTypedArray.
23.2.4.3 TYPEDARRAYCREATESAMETYPE ( EXEMPLAR, ARGUMENTLIST )
The abstract operation TypedArrayCreateSameType takes arguments exemplar (a
TypedArray) and argumentList (a List of ECMAScript language values) and returns
either a normal completion containing a TypedArray or a throw completion. It is
used to specify the creation of a new TypedArray using a constructor function
that is derived from exemplar. Unlike TypedArraySpeciesCreate, which can
construct custom TypedArray subclasses through the use of @@species, this
operation always uses one of the built-in TypedArray constructors. It performs
the following steps when called:
1. 1. 1. Let constructor be the intrinsic object associated with the
constructor name exemplar.[[TypedArrayName]] in Table 68.
2. 2. 2. Let result be ? TypedArrayCreate(constructor, argumentList).
3. 3. 3. Assert: result has [[TypedArrayName]] and [[ContentType]] internal
slots.
4. 4. 4. Assert: result.[[ContentType]] is exemplar.[[ContentType]].
5. 5. 5. Return result.
23.2.4.4 VALIDATETYPEDARRAY ( O )
The abstract operation ValidateTypedArray takes argument O (an ECMAScript
language value) and returns either a normal completion containing unused or a
throw completion. It performs the following steps when called:
1. 1. 1. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
2. 2. 2. Assert: O has a [[ViewedArrayBuffer]] internal slot.
3. 3. 3. Let buffer be O.[[ViewedArrayBuffer]].
4. 4. 4. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
5. 5. 5. Return unused.
23.2.4.5 TYPEDARRAYELEMENTSIZE ( O )
The abstract operation TypedArrayElementSize takes argument O (a TypedArray) and
returns a non-negative integer. It performs the following steps when called:
1. 1. 1. Return the Element Size value specified in Table 68 for
O.[[TypedArrayName]].
23.2.4.6 TYPEDARRAYELEMENTTYPE ( O )
The abstract operation TypedArrayElementType takes argument O (a TypedArray) and
returns a TypedArray element type. It performs the following steps when called:
1. 1. 1. Return the Element Type value specified in Table 68 for
O.[[TypedArrayName]].
23.2.4.7 COMPARETYPEDARRAYELEMENTS ( X, Y, COMPAREFN )
The abstract operation CompareTypedArrayElements takes arguments x (a Number or
a BigInt), y (a Number or a BigInt), and comparefn (a function object or
undefined) and returns either a normal completion containing a Number or an
abrupt completion. It performs the following steps when called:
1. 1. 1. Assert: x is a Number and y is a Number, or x is a BigInt and y is a
BigInt.
2. 2. 2. If comparefn is not undefined, then
1. a. a. Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
2. b. b. If v is NaN, return +0𝔽.
3. c. c. Return v.
3. 3. 3. If x and y are both NaN, return +0𝔽.
4. 4. 4. If x is NaN, return 1𝔽.
5. 5. 5. If y is NaN, return -1𝔽.
6. 6. 6. If x < y, return -1𝔽.
7. 7. 7. If x > y, return 1𝔽.
8. 8. 8. If x is -0𝔽 and y is +0𝔽, return -1𝔽.
9. 9. 9. If x is +0𝔽 and y is -0𝔽, return 1𝔽.
10. 10. 10. Return +0𝔽.
Note
This performs a numeric comparison rather than the string comparison used in
23.1.3.30.2.
23.2.5 THE TYPEDARRAY CONSTRUCTORS
Each TypedArray constructor:
* is an intrinsic object that has the structure described below, differing only
in the name used as the constructor name instead of TypedArray, in Table 68.
* is a function whose behaviour differs based upon the number and types of its
arguments. The actual behaviour of a call of TypedArray depends upon the
number and kind of arguments that are passed to it.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified TypedArray behaviour must
include a super call to the TypedArray constructor to create and initialize
the subclass instance with the internal state necessary to support the
%TypedArray%.prototype built-in methods.
* has a "length" property whose value is 3𝔽.
23.2.5.1 TYPEDARRAY ( ...ARGS )
Each TypedArray constructor performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let constructorName be the String value of the Constructor Name value
specified in Table 68 for this TypedArray constructor.
3. 3. 3. Let proto be "%TypedArray.prototype%".
4. 4. 4. Let numberOfArgs be the number of elements in args.
5. 5. 5. If numberOfArgs = 0, then
1. a. a. Return ? AllocateTypedArray(constructorName, NewTarget, proto, 0).
6. 6. 6. Else,
1. a. a. Let firstArgument be args[0].
2. b. b. If firstArgument is an Object, then
1. i. i. Let O be ? AllocateTypedArray(constructorName, NewTarget,
proto).
2. ii. ii. If firstArgument has a [[TypedArrayName]] internal slot, then
1. 1. 1. Perform ? InitializeTypedArrayFromTypedArray(O,
firstArgument).
3. iii. iii. Else if firstArgument has an [[ArrayBufferData]] internal
slot, then
1. 1. 1. If numberOfArgs > 1, let byteOffset be args[1]; else let
byteOffset be undefined.
2. 2. 2. If numberOfArgs > 2, let length be args[2]; else let length
be undefined.
3. 3. 3. Perform ? InitializeTypedArrayFromArrayBuffer(O,
firstArgument, byteOffset, length).
4. iv. iv. Else,
1. 1. 1. Assert: firstArgument is an Object and firstArgument does not
have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal
slot.
2. 2. 2. Let usingIterator be ? GetMethod(firstArgument, @@iterator).
3. 3. 3. If usingIterator is not undefined, then
1. a. a. Let values be ? IteratorToList(?
GetIteratorFromMethod(firstArgument, usingIterator)).
2. b. b. Perform ? InitializeTypedArrayFromList(O, values).
4. 4. 4. Else,
1. a. a. NOTE: firstArgument is not an Iterable so assume it is
already an array-like object.
2. b. b. Perform ? InitializeTypedArrayFromArrayLike(O,
firstArgument).
5. v. v. Return O.
3. c. c. Else,
1. i. i. Assert: firstArgument is not an Object.
2. ii. ii. Let elementLength be ? ToIndex(firstArgument).
3. iii. iii. Return ? AllocateTypedArray(constructorName, NewTarget,
proto, elementLength).
23.2.5.1.1 ALLOCATETYPEDARRAY ( CONSTRUCTORNAME, NEWTARGET, DEFAULTPROTO [ ,
LENGTH ] )
The abstract operation AllocateTypedArray takes arguments constructorName (a
String which is the name of a TypedArray constructor in Table 68), newTarget (a
constructor), and defaultProto (a String) and optional argument length (a
non-negative integer) and returns either a normal completion containing a
TypedArray or a throw completion. It is used to validate and create an instance
of a TypedArray constructor. If the length argument is passed, an ArrayBuffer of
that length is also allocated and associated with the new TypedArray instance.
AllocateTypedArray provides common semantics that is used by TypedArray. It
performs the following steps when called:
1. 1. 1. Let proto be ? GetPrototypeFromConstructor(newTarget, defaultProto).
2. 2. 2. Let obj be IntegerIndexedObjectCreate(proto).
3. 3. 3. Assert: obj.[[ViewedArrayBuffer]] is undefined.
4. 4. 4. Set obj.[[TypedArrayName]] to constructorName.
5. 5. 5. If constructorName is either "BigInt64Array" or "BigUint64Array", set
obj.[[ContentType]] to BigInt.
6. 6. 6. Otherwise, set obj.[[ContentType]] to Number.
7. 7. 7. If length is not present, then
1. a. a. Set obj.[[ByteLength]] to 0.
2. b. b. Set obj.[[ByteOffset]] to 0.
3. c. c. Set obj.[[ArrayLength]] to 0.
8. 8. 8. Else,
1. a. a. Perform ? AllocateTypedArrayBuffer(obj, length).
9. 9. 9. Return obj.
23.2.5.1.2 INITIALIZETYPEDARRAYFROMTYPEDARRAY ( O, SRCARRAY )
The abstract operation InitializeTypedArrayFromTypedArray takes arguments O (a
TypedArray) and srcArray (a TypedArray) and returns either a normal completion
containing unused or a throw completion. It performs the following steps when
called:
1. 1. 1. Let srcData be srcArray.[[ViewedArrayBuffer]].
2. 2. 2. If IsDetachedBuffer(srcData) is true, throw a TypeError exception.
3. 3. 3. Let elementType be TypedArrayElementType(O).
4. 4. 4. Let elementSize be TypedArrayElementSize(O).
5. 5. 5. Let srcType be TypedArrayElementType(srcArray).
6. 6. 6. Let srcElementSize be TypedArrayElementSize(srcArray).
7. 7. 7. Let srcByteOffset be srcArray.[[ByteOffset]].
8. 8. 8. Let elementLength be srcArray.[[ArrayLength]].
9. 9. 9. Let byteLength be elementSize × elementLength.
10. 10. 10. If elementType is srcType, then
1. a. a. Let data be ? CloneArrayBuffer(srcData, srcByteOffset,
byteLength).
11. 11. 11. Else,
1. a. a. Let data be ? AllocateArrayBuffer(%ArrayBuffer%, byteLength).
2. b. b. If srcArray.[[ContentType]] is not O.[[ContentType]], throw a
TypeError exception.
3. c. c. Let srcByteIndex be srcByteOffset.
4. d. d. Let targetByteIndex be 0.
5. e. e. Let count be elementLength.
6. f. f. Repeat, while count > 0,
1. i. i. Let value be GetValueFromBuffer(srcData, srcByteIndex, srcType,
true, Unordered).
2. ii. ii. Perform SetValueInBuffer(data, targetByteIndex, elementType,
value, true, Unordered).
3. iii. iii. Set srcByteIndex to srcByteIndex + srcElementSize.
4. iv. iv. Set targetByteIndex to targetByteIndex + elementSize.
5. v. v. Set count to count - 1.
12. 12. 12. Set O.[[ViewedArrayBuffer]] to data.
13. 13. 13. Set O.[[ByteLength]] to byteLength.
14. 14. 14. Set O.[[ByteOffset]] to 0.
15. 15. 15. Set O.[[ArrayLength]] to elementLength.
16. 16. 16. Return unused.
23.2.5.1.3 INITIALIZETYPEDARRAYFROMARRAYBUFFER ( O, BUFFER, BYTEOFFSET, LENGTH )
The abstract operation InitializeTypedArrayFromArrayBuffer takes arguments O (a
TypedArray), buffer (an ArrayBuffer or a SharedArrayBuffer), byteOffset (an
ECMAScript language value), and length (an ECMAScript language value) and
returns either a normal completion containing unused or a throw completion. It
performs the following steps when called:
1. 1. 1. Let elementSize be TypedArrayElementSize(O).
2. 2. 2. Let offset be ? ToIndex(byteOffset).
3. 3. 3. If offset modulo elementSize ≠ 0, throw a RangeError exception.
4. 4. 4. If length is not undefined, then
1. a. a. Let newLength be ? ToIndex(length).
5. 5. 5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
6. 6. 6. Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
7. 7. 7. If length is undefined, then
1. a. a. If bufferByteLength modulo elementSize ≠ 0, throw a RangeError
exception.
2. b. b. Let newByteLength be bufferByteLength - offset.
3. c. c. If newByteLength < 0, throw a RangeError exception.
8. 8. 8. Else,
1. a. a. Let newByteLength be newLength × elementSize.
2. b. b. If offset + newByteLength > bufferByteLength, throw a RangeError
exception.
9. 9. 9. Set O.[[ViewedArrayBuffer]] to buffer.
10. 10. 10. Set O.[[ByteLength]] to newByteLength.
11. 11. 11. Set O.[[ByteOffset]] to offset.
12. 12. 12. Set O.[[ArrayLength]] to newByteLength / elementSize.
13. 13. 13. Return unused.
23.2.5.1.4 INITIALIZETYPEDARRAYFROMLIST ( O, VALUES )
The abstract operation InitializeTypedArrayFromList takes arguments O (a
TypedArray) and values (a List of ECMAScript language values) and returns either
a normal completion containing unused or a throw completion. It performs the
following steps when called:
1. 1. 1. Let len be the number of elements in values.
2. 2. 2. Perform ? AllocateTypedArrayBuffer(O, len).
3. 3. 3. Let k be 0.
4. 4. 4. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be the first element of values.
3. c. c. Remove the first element from values.
4. d. d. Perform ? Set(O, Pk, kValue, true).
5. e. e. Set k to k + 1.
5. 5. 5. Assert: values is now an empty List.
6. 6. 6. Return unused.
23.2.5.1.5 INITIALIZETYPEDARRAYFROMARRAYLIKE ( O, ARRAYLIKE )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a
TypedArray) and arrayLike (an Object, but not a TypedArray or an ArrayBuffer)
and returns either a normal completion containing unused or a throw completion.
It performs the following steps when called:
1. 1. 1. Let len be ? LengthOfArrayLike(arrayLike).
2. 2. 2. Perform ? AllocateTypedArrayBuffer(O, len).
3. 3. 3. Let k be 0.
4. 4. 4. Repeat, while k < len,
1. a. a. Let Pk be ! ToString(𝔽(k)).
2. b. b. Let kValue be ? Get(arrayLike, Pk).
3. c. c. Perform ? Set(O, Pk, kValue, true).
4. d. d. Set k to k + 1.
5. 5. 5. Return unused.
23.2.5.1.6 ALLOCATETYPEDARRAYBUFFER ( O, LENGTH )
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray)
and length (a non-negative integer) and returns either a normal completion
containing unused or a throw completion. It allocates and associates an
ArrayBuffer with O. It performs the following steps when called:
1. 1. 1. Assert: O.[[ViewedArrayBuffer]] is undefined.
2. 2. 2. Let elementSize be TypedArrayElementSize(O).
3. 3. 3. Let byteLength be elementSize × length.
4. 4. 4. Let data be ? AllocateArrayBuffer(%ArrayBuffer%, byteLength).
5. 5. 5. Set O.[[ViewedArrayBuffer]] to data.
6. 6. 6. Set O.[[ByteLength]] to byteLength.
7. 7. 7. Set O.[[ByteOffset]] to 0.
8. 8. 8. Set O.[[ArrayLength]] to length.
9. 9. 9. Return unused.
23.2.6 PROPERTIES OF THE TYPEDARRAY CONSTRUCTORS
Each TypedArray constructor:
* has a [[Prototype]] internal slot whose value is %TypedArray%.
* has a "name" property whose value is the String value of the constructor name
specified for it in Table 68.
* has the following properties:
23.2.6.1 TYPEDARRAY.BYTES_PER_ELEMENT
The value of TypedArray.BYTES_PER_ELEMENT is the Element Size value specified in
Table 68 for TypedArray.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.2.6.2 TYPEDARRAY.PROTOTYPE
The initial value of TypedArray.prototype is the corresponding TypedArray
prototype intrinsic object (23.2.7).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.2.7 PROPERTIES OF THE TYPEDARRAY PROTOTYPE OBJECTS
Each TypedArray prototype object:
* has a [[Prototype]] internal slot whose value is %TypedArray.prototype%.
* is an ordinary object.
* does not have a [[ViewedArrayBuffer]] or any other of the internal slots that
are specific to TypedArray instance objects.
23.2.7.1 TYPEDARRAY.PROTOTYPE.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value
specified in Table 68 for TypedArray.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.2.7.2 TYPEDARRAY.PROTOTYPE.CONSTRUCTOR
The initial value of a TypedArray.prototype.constructor is the corresponding
%TypedArray% intrinsic object.
23.2.8 PROPERTIES OF TYPEDARRAY INSTANCES
TypedArray instances are Integer-Indexed exotic objects. Each TypedArray
instance inherits properties from the corresponding TypedArray prototype object.
Each TypedArray instance has the following internal slots: [[TypedArrayName]],
[[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
24 KEYED COLLECTIONS
24.1 MAP OBJECTS
Maps are collections of key/value pairs where both the keys and values may be
arbitrary ECMAScript language values. A distinct key value may only occur in one
key/value pair within the Map's collection. Distinct key values are
discriminated using the SameValueZero comparison algorithm.
Maps must be implemented using either hash tables or other mechanisms that, on
average, provide access times that are sublinear on the number of elements in
the collection. The data structure used in this specification is only intended
to describe the required observable semantics of Maps. It is not intended to be
a viable implementation model.
24.1.1 THE MAP CONSTRUCTOR
The Map constructor:
* is %Map%.
* is the initial value of the "Map" property of the global object.
* creates and initializes a new Map when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Map behaviour must include
a super call to the Map constructor to create and initialize the subclass
instance with the internal state necessary to support the Map.prototype
built-in methods.
24.1.1.1 MAP ( [ ITERABLE ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let map be ? OrdinaryCreateFromConstructor(NewTarget,
"%Map.prototype%", « [[MapData]] »).
3. 3. 3. Set map.[[MapData]] to a new empty List.
4. 4. 4. If iterable is either undefined or null, return map.
5. 5. 5. Let adder be ? Get(map, "set").
6. 6. 6. If IsCallable(adder) is false, throw a TypeError exception.
7. 7. 7. Return ? AddEntriesFromIterable(map, iterable, adder).
Note
If the parameter iterable is present, it is expected to be an object that
implements an @@iterator method that returns an iterator object that produces a
two element array-like object whose first element is a value that will be used
as a Map key and whose second element is the value to associate with that key.
24.1.1.2 ADDENTRIESFROMITERABLE ( TARGET, ITERABLE, ADDER )
The abstract operation AddEntriesFromIterable takes arguments target (an
Object), iterable (an ECMAScript language value, but not undefined or null), and
adder (a function object) and returns either a normal completion containing an
ECMAScript language value or a throw completion. adder will be invoked, with
target as the receiver. It performs the following steps when called:
1. 1. 1. Let iteratorRecord be ? GetIterator(iterable, sync).
2. 2. 2. Repeat,
1. a. a. Let next be ? IteratorStep(iteratorRecord).
2. b. b. If next is false, return target.
3. c. c. Let nextItem be ? IteratorValue(next).
4. d. d. If nextItem is not an Object, then
1. i. i. Let error be ThrowCompletion(a newly created TypeError object).
2. ii. ii. Return ? IteratorClose(iteratorRecord, error).
5. e. e. Let k be Completion(Get(nextItem, "0")).
6. f. f. IfAbruptCloseIterator(k, iteratorRecord).
7. g. g. Let v be Completion(Get(nextItem, "1")).
8. h. h. IfAbruptCloseIterator(v, iteratorRecord).
9. i. i. Let status be Completion(Call(adder, target, « k, v »)).
10. j. j. IfAbruptCloseIterator(status, iteratorRecord).
Note
The parameter iterable is expected to be an object that implements an @@iterator
method that returns an iterator object that produces a two element array-like
object whose first element is a value that will be used as a Map key and whose
second element is the value to associate with that key.
24.1.2 PROPERTIES OF THE MAP CONSTRUCTOR
The Map constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
24.1.2.1 MAP.PROTOTYPE
The initial value of Map.prototype is the Map prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
24.1.2.2 GET MAP [ @@SPECIES ]
Map[@@species] is an accessor property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to
determine the constructor to use to create the derived objects. Subclass
constructor may over-ride @@species to change the default constructor
assignment.
24.1.3 PROPERTIES OF THE MAP PROTOTYPE OBJECT
The Map prototype object:
* is %Map.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[MapData]] internal slot.
24.1.3.1 MAP.PROTOTYPE.CLEAR ( )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
1. a. a. Set p.[[Key]] to empty.
2. b. b. Set p.[[Value]] to empty.
4. 4. 4. Return undefined.
Note
The existing [[MapData]] List is preserved because there may be existing Map
Iterator objects that are suspended midway through iterating over that List.
24.1.3.2 MAP.PROTOTYPE.CONSTRUCTOR
The initial value of Map.prototype.constructor is %Map%.
24.1.3.3 MAP.PROTOTYPE.DELETE ( KEY )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
1. a. a. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is
true, then
1. i. i. Set p.[[Key]] to empty.
2. ii. ii. Set p.[[Value]] to empty.
3. iii. iii. Return true.
4. 4. 4. Return false.
Note
The value empty is used as a specification device to indicate that an entry has
been deleted. Actual implementations may take other actions such as physically
removing the entry from internal data structures.
24.1.3.4 MAP.PROTOTYPE.ENTRIES ( )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Return ? CreateMapIterator(M, key+value).
24.1.3.5 MAP.PROTOTYPE.FOREACH ( CALLBACKFN [ , THISARG ] )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let entries be M.[[MapData]].
5. 5. 5. Let numEntries be the number of elements in entries.
6. 6. 6. Let index be 0.
7. 7. 7. Repeat, while index < numEntries,
1. a. a. Let e be entries[index].
2. b. b. Set index to index + 1.
3. c. c. If e.[[Key]] is not empty, then
1. i. i. Perform ? Call(callbackfn, thisArg, « e.[[Value]], e.[[Key]], M
»).
2. ii. ii. NOTE: The number of elements in entries may have increased
during execution of callbackfn.
3. iii. iii. Set numEntries to the number of elements in entries.
8. 8. 8. Return undefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each key/value pair present in the Map, in key insertion
order. callbackfn is called only for keys of the Map which actually exist; it is
not called for keys that have been deleted from the Map.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the item, the key of the
item, and the Map being traversed.
forEach does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is
only visited once. New keys added after the call to forEach begins are visited.
A key will be revisited if it is deleted after it has been visited and then
re-added before the forEach call completes. Keys that are deleted after the call
to forEach begins and before being visited are not visited unless the key is
added again before the forEach call completes.
24.1.3.6 MAP.PROTOTYPE.GET ( KEY )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
1. a. a. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is
true, return p.[[Value]].
4. 4. 4. Return undefined.
24.1.3.7 MAP.PROTOTYPE.HAS ( KEY )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
1. a. a. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is
true, return true.
4. 4. 4. Return false.
24.1.3.8 MAP.PROTOTYPE.KEYS ( )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Return ? CreateMapIterator(M, key).
24.1.3.9 MAP.PROTOTYPE.SET ( KEY, VALUE )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
1. a. a. If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is
true, then
1. i. i. Set p.[[Value]] to value.
2. ii. ii. Return M.
4. 4. 4. If key is -0𝔽, set key to +0𝔽.
5. 5. 5. Let p be the Record { [[Key]]: key, [[Value]]: value }.
6. 6. 6. Append p to M.[[MapData]].
7. 7. 7. Return M.
24.1.3.10 GET MAP.PROTOTYPE.SIZE
Map.prototype.size is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[MapData]]).
3. 3. 3. Let count be 0.
4. 4. 4. For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
1. a. a. If p.[[Key]] is not empty, set count to count + 1.
5. 5. 5. Return 𝔽(count).
24.1.3.11 MAP.PROTOTYPE.VALUES ( )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Return ? CreateMapIterator(M, value).
24.1.3.12 MAP.PROTOTYPE [ @@ITERATOR ] ( )
The initial value of the @@iterator property is %Map.prototype.entries%, defined
in 24.1.3.4.
24.1.3.13 MAP.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Map".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.1.4 PROPERTIES OF MAP INSTANCES
Map instances are ordinary objects that inherit properties from the Map
prototype. Map instances also have a [[MapData]] internal slot.
24.1.5 MAP ITERATOR OBJECTS
A Map Iterator is an object, that represents a specific iteration over some
specific Map instance object. There is not a named constructor for Map Iterator
objects. Instead, map iterator objects are created by calling certain methods of
Map instance objects.
24.1.5.1 CREATEMAPITERATOR ( MAP, KIND )
The abstract operation CreateMapIterator takes arguments map (an ECMAScript
language value) and kind (key+value, key, or value) and returns either a normal
completion containing a Generator or a throw completion. It is used to create
iterator objects for Map methods that return such iterators. It performs the
following steps when called:
1. 1. 1. Perform ? RequireInternalSlot(map, [[MapData]]).
2. 2. 2. Let closure be a new Abstract Closure with no parameters that captures
map and kind and performs the following steps when called:
1. a. a. Let entries be map.[[MapData]].
2. b. b. Let index be 0.
3. c. c. Let numEntries be the number of elements in entries.
4. d. d. Repeat, while index < numEntries,
1. i. i. Let e be entries[index].
2. ii. ii. Set index to index + 1.
3. iii. iii. If e.[[Key]] is not empty, then
1. 1. 1. If kind is key, let result be e.[[Key]].
2. 2. 2. Else if kind is value, let result be e.[[Value]].
3. 3. 3. Else,
1. a. a. Assert: kind is key+value.
2. b. b. Let result be CreateArrayFromList(« e.[[Key]], e.[[Value]]
»).
4. 4. 4. Perform ? GeneratorYield(CreateIterResultObject(result,
false)).
5. 5. 5. NOTE: The number of elements in entries may have increased
while execution of this abstract operation was paused by Yield.
6. 6. 6. Set numEntries to the number of elements in entries.
5. e. e. Return undefined.
3. 3. 3. Return CreateIteratorFromClosure(closure, "%MapIteratorPrototype%",
%MapIteratorPrototype%).
24.1.5.2 THE %MAPITERATORPROTOTYPE% OBJECT
The %MapIteratorPrototype% object:
* has properties that are inherited by all Map Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* has the following properties:
24.1.5.2.1 %MAPITERATORPROTOTYPE%.NEXT ( )
1. 1. 1. Return ? GeneratorResume(this value, empty, "%MapIteratorPrototype%").
24.1.5.2.2 %MAPITERATORPROTOTYPE% [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Map
Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.2 SET OBJECTS
Set objects are collections of ECMAScript language values. A distinct value may
only occur once as an element of a Set's collection. Distinct values are
discriminated using the SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms
that, on average, provide access times that are sublinear on the number of
elements in the collection. The data structure used in this specification is
only intended to describe the required observable semantics of Set objects. It
is not intended to be a viable implementation model.
24.2.1 THE SET CONSTRUCTOR
The Set constructor:
* is %Set%.
* is the initial value of the "Set" property of the global object.
* creates and initializes a new Set object when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Set behaviour must include
a super call to the Set constructor to create and initialize the subclass
instance with the internal state necessary to support the Set.prototype
built-in methods.
24.2.1.1 SET ( [ ITERABLE ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let set be ? OrdinaryCreateFromConstructor(NewTarget,
"%Set.prototype%", « [[SetData]] »).
3. 3. 3. Set set.[[SetData]] to a new empty List.
4. 4. 4. If iterable is either undefined or null, return set.
5. 5. 5. Let adder be ? Get(set, "add").
6. 6. 6. If IsCallable(adder) is false, throw a TypeError exception.
7. 7. 7. Let iteratorRecord be ? GetIterator(iterable, sync).
8. 8. 8. Repeat,
1. a. a. Let next be ? IteratorStep(iteratorRecord).
2. b. b. If next is false, return set.
3. c. c. Let nextValue be ? IteratorValue(next).
4. d. d. Let status be Completion(Call(adder, set, « nextValue »)).
5. e. e. IfAbruptCloseIterator(status, iteratorRecord).
24.2.2 PROPERTIES OF THE SET CONSTRUCTOR
The Set constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
24.2.2.1 SET.PROTOTYPE
The initial value of Set.prototype is the Set prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
24.2.2.2 GET SET [ @@SPECIES ]
Set[@@species] is an accessor property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to
determine the constructor to use to create the derived objects. Subclass
constructor may over-ride @@species to change the default constructor
assignment.
24.2.3 PROPERTIES OF THE SET PROTOTYPE OBJECT
The Set prototype object:
* is %Set.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[SetData]] internal slot.
24.2.3.1 SET.PROTOTYPE.ADD ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[SetData]]).
3. 3. 3. For each element e of S.[[SetData]], do
1. a. a. If e is not empty and SameValueZero(e, value) is true, then
1. i. i. Return S.
4. 4. 4. If value is -0𝔽, set value to +0𝔽.
5. 5. 5. Append value to S.[[SetData]].
6. 6. 6. Return S.
24.2.3.2 SET.PROTOTYPE.CLEAR ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[SetData]]).
3. 3. 3. For each element e of S.[[SetData]], do
1. a. a. Replace the element of S.[[SetData]] whose value is e with an
element whose value is empty.
4. 4. 4. Return undefined.
Note
The existing [[SetData]] List is preserved because there may be existing Set
Iterator objects that are suspended midway through iterating over that List.
24.2.3.3 SET.PROTOTYPE.CONSTRUCTOR
The initial value of Set.prototype.constructor is %Set%.
24.2.3.4 SET.PROTOTYPE.DELETE ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[SetData]]).
3. 3. 3. For each element e of S.[[SetData]], do
1. a. a. If e is not empty and SameValueZero(e, value) is true, then
1. i. i. Replace the element of S.[[SetData]] whose value is e with an
element whose value is empty.
2. ii. ii. Return true.
4. 4. 4. Return false.
Note
The value empty is used as a specification device to indicate that an entry has
been deleted. Actual implementations may take other actions such as physically
removing the entry from internal data structures.
24.2.3.5 SET.PROTOTYPE.ENTRIES ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateSetIterator(S, key+value).
Note
For iteration purposes, a Set appears similar to a Map where each entry has the
same value for its key and value.
24.2.3.6 SET.PROTOTYPE.FOREACH ( CALLBACKFN [ , THISARG ] )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[SetData]]).
3. 3. 3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. 4. 4. Let entries be S.[[SetData]].
5. 5. 5. Let numEntries be the number of elements in entries.
6. 6. 6. Let index be 0.
7. 7. 7. Repeat, while index < numEntries,
1. a. a. Let e be entries[index].
2. b. b. Set index to index + 1.
3. c. c. If e is not empty, then
1. i. i. Perform ? Call(callbackfn, thisArg, « e, e, S »).
2. ii. ii. NOTE: The number of elements in entries may have increased
during execution of callbackfn.
3. iii. iii. Set numEntries to the number of elements in entries.
8. 8. 8. Return undefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each value present in the Set object, in value insertion
order. callbackfn is called only for values of the Set which actually exist; it
is not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the this value for each
invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the first two arguments are a value
contained in the Set. The same value is passed for both arguments. The Set
object being traversed is passed as the third argument.
The callbackfn is called with three arguments to be consistent with the call
back functions used by forEach methods for Map and Array. For Sets, each item
value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object
may be mutated by the calls to callbackfn.
Each value is normally visited only once. However, a value will be revisited if
it is deleted after it has been visited and then re-added before the forEach
call completes. Values that are deleted after the call to forEach begins and
before being visited are not visited unless the value is added again before the
forEach call completes. New values added after the call to forEach begins are
visited.
24.2.3.7 SET.PROTOTYPE.HAS ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[SetData]]).
3. 3. 3. For each element e of S.[[SetData]], do
1. a. a. If e is not empty and SameValueZero(e, value) is true, return true.
4. 4. 4. Return false.
24.2.3.8 SET.PROTOTYPE.KEYS ( )
The initial value of the "keys" property is %Set.prototype.values%, defined in
24.2.3.10.
Note
For iteration purposes, a Set appears similar to a Map where each entry has the
same value for its key and value.
24.2.3.9 GET SET.PROTOTYPE.SIZE
Set.prototype.size is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[SetData]]).
3. 3. 3. Let count be 0.
4. 4. 4. For each element e of S.[[SetData]], do
1. a. a. If e is not empty, set count to count + 1.
5. 5. 5. Return 𝔽(count).
24.2.3.10 SET.PROTOTYPE.VALUES ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateSetIterator(S, value).
24.2.3.11 SET.PROTOTYPE [ @@ITERATOR ] ( )
The initial value of the @@iterator property is %Set.prototype.values%, defined
in 24.2.3.10.
24.2.3.12 SET.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Set".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.2.4 PROPERTIES OF SET INSTANCES
Set instances are ordinary objects that inherit properties from the Set
prototype. Set instances also have a [[SetData]] internal slot.
24.2.5 SET ITERATOR OBJECTS
A Set Iterator is an ordinary object, with the structure defined below, that
represents a specific iteration over some specific Set instance object. There is
not a named constructor for Set Iterator objects. Instead, set iterator objects
are created by calling certain methods of Set instance objects.
24.2.5.1 CREATESETITERATOR ( SET, KIND )
The abstract operation CreateSetIterator takes arguments set (an ECMAScript
language value) and kind (key+value or value) and returns either a normal
completion containing a Generator or a throw completion. It is used to create
iterator objects for Set methods that return such iterators. It performs the
following steps when called:
1. 1. 1. Perform ? RequireInternalSlot(set, [[SetData]]).
2. 2. 2. Let closure be a new Abstract Closure with no parameters that captures
set and kind and performs the following steps when called:
1. a. a. Let index be 0.
2. b. b. Let entries be set.[[SetData]].
3. c. c. Let numEntries be the number of elements in entries.
4. d. d. Repeat, while index < numEntries,
1. i. i. Let e be entries[index].
2. ii. ii. Set index to index + 1.
3. iii. iii. If e is not empty, then
1. 1. 1. If kind is key+value, then
1. a. a. Let result be CreateArrayFromList(« e, e »).
2. b. b. Perform ? GeneratorYield(CreateIterResultObject(result,
false)).
2. 2. 2. Else,
1. a. a. Assert: kind is value.
2. b. b. Perform ? GeneratorYield(CreateIterResultObject(e,
false)).
3. 3. 3. NOTE: The number of elements in entries may have increased
while execution of this abstract operation was paused by Yield.
4. 4. 4. Set numEntries to the number of elements in entries.
5. e. e. Return undefined.
3. 3. 3. Return CreateIteratorFromClosure(closure, "%SetIteratorPrototype%",
%SetIteratorPrototype%).
24.2.5.2 THE %SETITERATORPROTOTYPE% OBJECT
The %SetIteratorPrototype% object:
* has properties that are inherited by all Set Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* has the following properties:
24.2.5.2.1 %SETITERATORPROTOTYPE%.NEXT ( )
1. 1. 1. Return ? GeneratorResume(this value, empty, "%SetIteratorPrototype%").
24.2.5.2.2 %SETITERATORPROTOTYPE% [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Set
Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.3 WEAKMAP OBJECTS
WeakMaps are collections of key/value pairs where the keys are objects and/or
symbols and values may be arbitrary ECMAScript language values. A WeakMap may be
queried to see if it contains a key/value pair with a specific key, but no
mechanism is provided for enumerating the values it holds as keys. In certain
conditions, values which are not live are removed as WeakMap keys, as described
in 9.10.3.
An implementation may impose an arbitrarily determined latency between the time
a key/value pair of a WeakMap becomes inaccessible and the time when the
key/value pair is removed from the WeakMap. If this latency was observable to
ECMAScript program, it would be a source of indeterminacy that could impact
program execution. For that reason, an ECMAScript implementation must not
provide any means to observe a key of a WeakMap that does not require the
observer to present the observed key.
WeakMaps must be implemented using either hash tables or other mechanisms that,
on average, provide access times that are sublinear on the number of key/value
pairs in the collection. The data structure used in this specification is only
intended to describe the required observable semantics of WeakMaps. It is not
intended to be a viable implementation model.
Note
WeakMap and WeakSet are intended to provide mechanisms for dynamically
associating state with an object or symbol in a manner that does not “leak”
memory resources if, in the absence of the WeakMap or WeakSet instance, the
object or symbol otherwise became inaccessible and subject to resource
reclamation by the implementation's garbage collection mechanisms. This
characteristic can be achieved by using an inverted per-object/symbol mapping of
WeakMap or WeakSet instances to keys. Alternatively, each WeakMap or WeakSet
instance may internally store its key and value data, but this approach requires
coordination between the WeakMap or WeakSet implementation and the garbage
collector. The following references describe mechanism that may be useful to
implementations of WeakMap and WeakSet:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of
the 12th ACM SIGPLAN conference on Object-oriented programming, systems,
languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New
York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in Weak Tables.
Journal of Universal Computer Science - J.UCS, vol. 14, no. 21, pp. 3481-3497,
2008, http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak
24.3.1 THE WEAKMAP CONSTRUCTOR
The WeakMap constructor:
* is %WeakMap%.
* is the initial value of the "WeakMap" property of the global object.
* creates and initializes a new WeakMap when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified WeakMap behaviour must
include a super call to the WeakMap constructor to create and initialize the
subclass instance with the internal state necessary to support the
WeakMap.prototype built-in methods.
24.3.1.1 WEAKMAP ( [ ITERABLE ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let map be ? OrdinaryCreateFromConstructor(NewTarget,
"%WeakMap.prototype%", « [[WeakMapData]] »).
3. 3. 3. Set map.[[WeakMapData]] to a new empty List.
4. 4. 4. If iterable is either undefined or null, return map.
5. 5. 5. Let adder be ? Get(map, "set").
6. 6. 6. If IsCallable(adder) is false, throw a TypeError exception.
7. 7. 7. Return ? AddEntriesFromIterable(map, iterable, adder).
Note
If the parameter iterable is present, it is expected to be an object that
implements an @@iterator method that returns an iterator object that produces a
two element array-like object whose first element is a value that will be used
as a WeakMap key and whose second element is the value to associate with that
key.
24.3.2 PROPERTIES OF THE WEAKMAP CONSTRUCTOR
The WeakMap constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
24.3.2.1 WEAKMAP.PROTOTYPE
The initial value of WeakMap.prototype is the WeakMap prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
24.3.3 PROPERTIES OF THE WEAKMAP PROTOTYPE OBJECT
The WeakMap prototype object:
* is %WeakMap.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[WeakMapData]] internal slot.
24.3.3.1 WEAKMAP.PROTOTYPE.CONSTRUCTOR
The initial value of WeakMap.prototype.constructor is %WeakMap%.
24.3.3.2 WEAKMAP.PROTOTYPE.DELETE ( KEY )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
3. 3. 3. If CanBeHeldWeakly(key) is false, return false.
4. 4. 4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
1. a. a. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true,
then
1. i. i. Set p.[[Key]] to empty.
2. ii. ii. Set p.[[Value]] to empty.
3. iii. iii. Return true.
5. 5. 5. Return false.
Note
The value empty is used as a specification device to indicate that an entry has
been deleted. Actual implementations may take other actions such as physically
removing the entry from internal data structures.
24.3.3.3 WEAKMAP.PROTOTYPE.GET ( KEY )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
3. 3. 3. If CanBeHeldWeakly(key) is false, return undefined.
4. 4. 4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
1. a. a. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true,
return p.[[Value]].
5. 5. 5. Return undefined.
24.3.3.4 WEAKMAP.PROTOTYPE.HAS ( KEY )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
3. 3. 3. If CanBeHeldWeakly(key) is false, return false.
4. 4. 4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
1. a. a. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true,
return true.
5. 5. 5. Return false.
24.3.3.5 WEAKMAP.PROTOTYPE.SET ( KEY, VALUE )
This method performs the following steps when called:
1. 1. 1. Let M be the this value.
2. 2. 2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
3. 3. 3. If CanBeHeldWeakly(key) is false, throw a TypeError exception.
4. 4. 4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
1. a. a. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true,
then
1. i. i. Set p.[[Value]] to value.
2. ii. ii. Return M.
5. 5. 5. Let p be the Record { [[Key]]: key, [[Value]]: value }.
6. 6. 6. Append p to M.[[WeakMapData]].
7. 7. 7. Return M.
24.3.3.6 WEAKMAP.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "WeakMap".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.3.4 PROPERTIES OF WEAKMAP INSTANCES
WeakMap instances are ordinary objects that inherit properties from the WeakMap
prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
24.4 WEAKSET OBJECTS
WeakSets are collections of objects and/or symbols. A distinct object or symbol
may only occur once as an element of a WeakSet's collection. A WeakSet may be
queried to see if it contains a specific value, but no mechanism is provided for
enumerating the values it holds. In certain conditions, values which are not
live are removed as WeakSet elements, as described in 9.10.3.
An implementation may impose an arbitrarily determined latency between the time
a value contained in a WeakSet becomes inaccessible and the time when the value
is removed from the WeakSet. If this latency was observable to ECMAScript
program, it would be a source of indeterminacy that could impact program
execution. For that reason, an ECMAScript implementation must not provide any
means to determine if a WeakSet contains a particular value that does not
require the observer to present the observed value.
WeakSets must be implemented using either hash tables or other mechanisms that,
on average, provide access times that are sublinear on the number of elements in
the collection. The data structure used in this specification is only intended
to describe the required observable semantics of WeakSets. It is not intended to
be a viable implementation model.
Note
See the NOTE in 24.3.
24.4.1 THE WEAKSET CONSTRUCTOR
The WeakSet constructor:
* is %WeakSet%.
* is the initial value of the "WeakSet" property of the global object.
* creates and initializes a new WeakSet when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified WeakSet behaviour must
include a super call to the WeakSet constructor to create and initialize the
subclass instance with the internal state necessary to support the
WeakSet.prototype built-in methods.
24.4.1.1 WEAKSET ( [ ITERABLE ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let set be ? OrdinaryCreateFromConstructor(NewTarget,
"%WeakSet.prototype%", « [[WeakSetData]] »).
3. 3. 3. Set set.[[WeakSetData]] to a new empty List.
4. 4. 4. If iterable is either undefined or null, return set.
5. 5. 5. Let adder be ? Get(set, "add").
6. 6. 6. If IsCallable(adder) is false, throw a TypeError exception.
7. 7. 7. Let iteratorRecord be ? GetIterator(iterable, sync).
8. 8. 8. Repeat,
1. a. a. Let next be ? IteratorStep(iteratorRecord).
2. b. b. If next is false, return set.
3. c. c. Let nextValue be ? IteratorValue(next).
4. d. d. Let status be Completion(Call(adder, set, « nextValue »)).
5. e. e. IfAbruptCloseIterator(status, iteratorRecord).
24.4.2 PROPERTIES OF THE WEAKSET CONSTRUCTOR
The WeakSet constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
24.4.2.1 WEAKSET.PROTOTYPE
The initial value of WeakSet.prototype is the WeakSet prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
24.4.3 PROPERTIES OF THE WEAKSET PROTOTYPE OBJECT
The WeakSet prototype object:
* is %WeakSet.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[WeakSetData]] internal slot.
24.4.3.1 WEAKSET.PROTOTYPE.ADD ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
3. 3. 3. If CanBeHeldWeakly(value) is false, throw a TypeError exception.
4. 4. 4. For each element e of S.[[WeakSetData]], do
1. a. a. If e is not empty and SameValue(e, value) is true, then
1. i. i. Return S.
5. 5. 5. Append value to S.[[WeakSetData]].
6. 6. 6. Return S.
24.4.3.2 WEAKSET.PROTOTYPE.CONSTRUCTOR
The initial value of WeakSet.prototype.constructor is %WeakSet%.
24.4.3.3 WEAKSET.PROTOTYPE.DELETE ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
3. 3. 3. If CanBeHeldWeakly(value) is false, return false.
4. 4. 4. For each element e of S.[[WeakSetData]], do
1. a. a. If e is not empty and SameValue(e, value) is true, then
1. i. i. Replace the element of S.[[WeakSetData]] whose value is e with
an element whose value is empty.
2. ii. ii. Return true.
5. 5. 5. Return false.
Note
The value empty is used as a specification device to indicate that an entry has
been deleted. Actual implementations may take other actions such as physically
removing the entry from internal data structures.
24.4.3.4 WEAKSET.PROTOTYPE.HAS ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
3. 3. 3. If CanBeHeldWeakly(value) is false, return false.
4. 4. 4. For each element e of S.[[WeakSetData]], do
1. a. a. If e is not empty and SameValue(e, value) is true, return true.
5. 5. 5. Return false.
24.4.3.5 WEAKSET.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "WeakSet".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.4.4 PROPERTIES OF WEAKSET INSTANCES
WeakSet instances are ordinary objects that inherit properties from the WeakSet
prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
25 STRUCTURED DATA
25.1 ARRAYBUFFER OBJECTS
25.1.1 NOTATION
The descriptions below in this section, 25.4, and 29 use the read-modify-write
modification function internal data structure.
A read-modify-write modification function is a mathematical function that is
notationally represented as an abstract closure that takes two Lists of byte
values as arguments and returns a List of byte values. These abstract closures
satisfy all of the following properties:
* They perform all their algorithm steps atomically.
* Their individual algorithm steps are not observable.
Note
To aid verifying that a read-modify-write modification function's algorithm
steps constitute a pure, mathematical function, the following editorial
conventions are recommended:
* They do not access, directly or transitively via invoked abstract operations
and abstract closures, any language or specification values except their
parameters and captured values.
* They do not return Completion Records.
25.1.2 ABSTRACT OPERATIONS FOR ARRAYBUFFER OBJECTS
25.1.2.1 ALLOCATEARRAYBUFFER ( CONSTRUCTOR, BYTELENGTH )
The abstract operation AllocateArrayBuffer takes arguments constructor (a
constructor) and byteLength (a non-negative integer) and returns either a normal
completion containing an ArrayBuffer or a throw completion. It is used to create
an ArrayBuffer. It performs the following steps when called:
1. 1. 1. Let obj be ? OrdinaryCreateFromConstructor(constructor,
"%ArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]],
[[ArrayBufferDetachKey]] »).
2. 2. 2. Let block be ? CreateByteDataBlock(byteLength).
3. 3. 3. Set obj.[[ArrayBufferData]] to block.
4. 4. 4. Set obj.[[ArrayBufferByteLength]] to byteLength.
5. 5. 5. Return obj.
25.1.2.2 ISDETACHEDBUFFER ( ARRAYBUFFER )
The abstract operation IsDetachedBuffer takes argument arrayBuffer (an
ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the
following steps when called:
1. 1. 1. If arrayBuffer.[[ArrayBufferData]] is null, return true.
2. 2. 2. Return false.
25.1.2.3 DETACHARRAYBUFFER ( ARRAYBUFFER [ , KEY ] )
The abstract operation DetachArrayBuffer takes argument arrayBuffer (an
ArrayBuffer) and optional argument key (anything) and returns either a normal
completion containing unused or a throw completion. It performs the following
steps when called:
1. 1. 1. Assert: IsSharedArrayBuffer(arrayBuffer) is false.
2. 2. 2. If key is not present, set key to undefined.
3. 3. 3. If arrayBuffer.[[ArrayBufferDetachKey]] is not key, throw a TypeError
exception.
4. 4. 4. Set arrayBuffer.[[ArrayBufferData]] to null.
5. 5. 5. Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
6. 6. 6. Return unused.
Note
Detaching an ArrayBuffer instance disassociates the Data Block used as its
backing store from the instance and sets the byte length of the buffer to 0. No
operations defined by this specification use the DetachArrayBuffer abstract
operation. However, an ECMAScript host or implementation may define such
operations.
25.1.2.4 CLONEARRAYBUFFER ( SRCBUFFER, SRCBYTEOFFSET, SRCLENGTH )
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an
ArrayBuffer or a SharedArrayBuffer), srcByteOffset (a non-negative integer), and
srcLength (a non-negative integer) and returns either a normal completion
containing an ArrayBuffer or a throw completion. It creates a new ArrayBuffer
whose data is a copy of srcBuffer's data over the range starting at
srcByteOffset and continuing for srcLength bytes. It performs the following
steps when called:
1. 1. 1. Assert: IsDetachedBuffer(srcBuffer) is false.
2. 2. 2. Let targetBuffer be ? AllocateArrayBuffer(%ArrayBuffer%, srcLength).
3. 3. 3. Let srcBlock be srcBuffer.[[ArrayBufferData]].
4. 4. 4. Let targetBlock be targetBuffer.[[ArrayBufferData]].
5. 5. 5. Perform CopyDataBlockBytes(targetBlock, 0, srcBlock, srcByteOffset,
srcLength).
6. 6. 6. Return targetBuffer.
25.1.2.5 ISUNSIGNEDELEMENTTYPE ( TYPE )
The abstract operation IsUnsignedElementType takes argument type (a TypedArray
element type) and returns a Boolean. It verifies if the argument type is an
unsigned TypedArray element type. It performs the following steps when called:
1. 1. 1. If type is one of Uint8, Uint8C, Uint16, Uint32, or BigUint64, return
true.
2. 2. 2. Return false.
25.1.2.6 ISUNCLAMPEDINTEGERELEMENTTYPE ( TYPE )
The abstract operation IsUnclampedIntegerElementType takes argument type (a
TypedArray element type) and returns a Boolean. It verifies if the argument type
is an Integer TypedArray element type not including Uint8C. It performs the
following steps when called:
1. 1. 1. If type is one of Int8, Uint8, Int16, Uint16, Int32, or Uint32, return
true.
2. 2. 2. Return false.
25.1.2.7 ISBIGINTELEMENTTYPE ( TYPE )
The abstract operation IsBigIntElementType takes argument type (a TypedArray
element type) and returns a Boolean. It verifies if the argument type is a
BigInt TypedArray element type. It performs the following steps when called:
1. 1. 1. If type is either BigUint64 or BigInt64, return true.
2. 2. 2. Return false.
25.1.2.8 ISNOTEARCONFIGURATION ( TYPE, ORDER )
The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray
element type) and order (SeqCst, Unordered, or Init) and returns a Boolean. It
performs the following steps when called:
1. 1. 1. If IsUnclampedIntegerElementType(type) is true, return true.
2. 2. 2. If IsBigIntElementType(type) is true and order is neither Init nor
Unordered, return true.
3. 3. 3. Return false.
25.1.2.9 RAWBYTESTONUMERIC ( TYPE, RAWBYTES, ISLITTLEENDIAN )
The abstract operation RawBytesToNumeric takes arguments type (a TypedArray
element type), rawBytes (a List of byte values), and isLittleEndian (a Boolean)
and returns a Number or a BigInt. It performs the following steps when called:
1. 1. 1. Let elementSize be the Element Size value specified in Table 68 for
Element Type type.
2. 2. 2. If isLittleEndian is false, reverse the order of the elements of
rawBytes.
3. 3. 3. If type is Float32, then
1. a. a. Let value be the byte elements of rawBytes concatenated and
interpreted as a little-endian bit string encoding of an IEEE 754-2019
binary32 value.
2. b. b. If value is an IEEE 754-2019 binary32 NaN value, return the NaN
Number value.
3. c. c. Return the Number value that corresponds to value.
4. 4. 4. If type is Float64, then
1. a. a. Let value be the byte elements of rawBytes concatenated and
interpreted as a little-endian bit string encoding of an IEEE 754-2019
binary64 value.
2. b. b. If value is an IEEE 754-2019 binary64 NaN value, return the NaN
Number value.
3. c. c. Return the Number value that corresponds to value.
5. 5. 5. If IsUnsignedElementType(type) is true, then
1. a. a. Let intValue be the byte elements of rawBytes concatenated and
interpreted as a bit string encoding of an unsigned little-endian binary
number.
6. 6. 6. Else,
1. a. a. Let intValue be the byte elements of rawBytes concatenated and
interpreted as a bit string encoding of a binary little-endian two's
complement number of bit length elementSize × 8.
7. 7. 7. If IsBigIntElementType(type) is true, return the BigInt value that
corresponds to intValue.
8. 8. 8. Otherwise, return the Number value that corresponds to intValue.
25.1.2.10 GETVALUEFROMBUFFER ( ARRAYBUFFER, BYTEINDEX, TYPE, ISTYPEDARRAY, ORDER
[ , ISLITTLEENDIAN ] )
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an
ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a
TypedArray element type), isTypedArray (a Boolean), and order (SeqCst or
Unordered) and optional argument isLittleEndian (a Boolean) and returns a Number
or a BigInt. It performs the following steps when called:
1. 1. 1. Assert: IsDetachedBuffer(arrayBuffer) is false.
2. 2. 2. Assert: There are sufficient bytes in arrayBuffer starting at
byteIndex to represent a value of type.
3. 3. 3. Let block be arrayBuffer.[[ArrayBufferData]].
4. 4. 4. Let elementSize be the Element Size value specified in Table 68 for
Element Type type.
5. 5. 5. If IsSharedArrayBuffer(arrayBuffer) is true, then
1. a. a. Let execution be the [[CandidateExecution]] field of the
surrounding agent's Agent Record.
2. b. b. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
3. c. c. If isTypedArray is true and IsNoTearConfiguration(type, order) is
true, let noTear be true; otherwise let noTear be false.
4. d. d. Let rawValue be a List of length elementSize whose elements are
nondeterministically chosen byte values.
5. e. e. NOTE: In implementations, rawValue is the result of a non-atomic or
atomic read instruction on the underlying hardware. The nondeterminism is
a semantic prescription of the memory model to describe observable
behaviour of hardware with weak consistency.
6. f. f. Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]:
noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]:
elementSize }.
7. g. g. Append readEvent to eventsRecord.[[EventList]].
8. h. h. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]:
rawValue } to execution.[[ChosenValues]].
6. 6. 6. Else, let rawValue be a List whose elements are bytes from block at
indices in the interval from byteIndex (inclusive) to byteIndex +
elementSize (exclusive).
7. 7. 7. Assert: The number of elements in rawValue is elementSize.
8. 8. 8. If isLittleEndian is not present, set isLittleEndian to the value of
the [[LittleEndian]] field of the surrounding agent's Agent Record.
9. 9. 9. Return RawBytesToNumeric(type, rawValue, isLittleEndian).
25.1.2.11 NUMERICTORAWBYTES ( TYPE, VALUE, ISLITTLEENDIAN )
The abstract operation NumericToRawBytes takes arguments type (a TypedArray
element type), value (a Number or a BigInt), and isLittleEndian (a Boolean) and
returns a List of byte values. It performs the following steps when called:
1. 1. 1. If type is Float32, then
1. a. a. Let rawBytes be a List whose elements are the 4 bytes that are the
result of converting value to IEEE 754-2019 binary32 format using
roundTiesToEven mode. The bytes are arranged in little endian order. If
value is NaN, rawBytes may be set to any implementation chosen IEEE
754-2019 binary32 format Not-a-Number encoding. An implementation must
always choose the same encoding for each implementation distinguishable
NaN value.
2. 2. 2. Else if type is Float64, then
1. a. a. Let rawBytes be a List whose elements are the 8 bytes that are the
IEEE 754-2019 binary64 format encoding of value. The bytes are arranged
in little endian order. If value is NaN, rawBytes may be set to any
implementation chosen IEEE 754-2019 binary64 format Not-a-Number
encoding. An implementation must always choose the same encoding for each
implementation distinguishable NaN value.
3. 3. 3. Else,
1. a. a. Let n be the Element Size value specified in Table 68 for Element
Type type.
2. b. b. Let convOp be the abstract operation named in the Conversion
Operation column in Table 68 for Element Type type.
3. c. c. Let intValue be ℝ(convOp(value)).
4. d. d. If intValue ≥ 0, then
1. i. i. Let rawBytes be a List whose elements are the n-byte binary
encoding of intValue. The bytes are ordered in little endian order.
5. e. e. Else,
1. i. i. Let rawBytes be a List whose elements are the n-byte binary
two's complement encoding of intValue. The bytes are ordered in little
endian order.
4. 4. 4. If isLittleEndian is false, reverse the order of the elements of
rawBytes.
5. 5. 5. Return rawBytes.
25.1.2.12 SETVALUEINBUFFER ( ARRAYBUFFER, BYTEINDEX, TYPE, VALUE, ISTYPEDARRAY,
ORDER [ , ISLITTLEENDIAN ] )
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an
ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a
TypedArray element type), value (a Number or a BigInt), isTypedArray (a
Boolean), and order (SeqCst, Unordered, or Init) and optional argument
isLittleEndian (a Boolean) and returns unused. It performs the following steps
when called:
1. 1. 1. Assert: IsDetachedBuffer(arrayBuffer) is false.
2. 2. 2. Assert: There are sufficient bytes in arrayBuffer starting at
byteIndex to represent a value of type.
3. 3. 3. Assert: value is a BigInt if IsBigIntElementType(type) is true;
otherwise, value is a Number.
4. 4. 4. Let block be arrayBuffer.[[ArrayBufferData]].
5. 5. 5. Let elementSize be the Element Size value specified in Table 68 for
Element Type type.
6. 6. 6. If isLittleEndian is not present, set isLittleEndian to the value of
the [[LittleEndian]] field of the surrounding agent's Agent Record.
7. 7. 7. Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
8. 8. 8. If IsSharedArrayBuffer(arrayBuffer) is true, then
1. a. a. Let execution be the [[CandidateExecution]] field of the
surrounding agent's Agent Record.
2. b. b. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier().
3. c. c. If isTypedArray is true and IsNoTearConfiguration(type, order) is
true, let noTear be true; otherwise let noTear be false.
4. d. d. Append WriteSharedMemory { [[Order]]: order, [[NoTear]]: noTear,
[[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]:
elementSize, [[Payload]]: rawBytes } to eventsRecord.[[EventList]].
9. 9. 9. Else, store the individual bytes of rawBytes into block, starting at
block[byteIndex].
10. 10. 10. Return unused.
25.1.2.13 GETMODIFYSETVALUEINBUFFER ( ARRAYBUFFER, BYTEINDEX, TYPE, VALUE, OP [
, ISLITTLEENDIAN ] )
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an
ArrayBuffer or a SharedArrayBuffer), byteIndex (a non-negative integer), type (a
TypedArray element type), value (a Number or a BigInt), and op (a
read-modify-write modification function) and optional argument isLittleEndian (a
Boolean) and returns a Number or a BigInt. It performs the following steps when
called:
1. 1. 1. Assert: IsDetachedBuffer(arrayBuffer) is false.
2. 2. 2. Assert: There are sufficient bytes in arrayBuffer starting at
byteIndex to represent a value of type.
3. 3. 3. Assert: value is a BigInt if IsBigIntElementType(type) is true;
otherwise, value is a Number.
4. 4. 4. Let block be arrayBuffer.[[ArrayBufferData]].
5. 5. 5. Let elementSize be the Element Size value specified in Table 68 for
Element Type type.
6. 6. 6. If isLittleEndian is not present, set isLittleEndian to the value of
the [[LittleEndian]] field of the surrounding agent's Agent Record.
7. 7. 7. Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
8. 8. 8. If IsSharedArrayBuffer(arrayBuffer) is true, then
1. a. a. Let execution be the [[CandidateExecution]] field of the
surrounding agent's Agent Record.
2. b. b. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier().
3. c. c. Let rawBytesRead be a List of length elementSize whose elements
are nondeterministically chosen byte values.
4. d. d. NOTE: In implementations, rawBytesRead is the result of a
load-link, of a load-exclusive, or of an operand of a read-modify-write
instruction on the underlying hardware. The nondeterminism is a semantic
prescription of the memory model to describe observable behaviour of
hardware with weak consistency.
5. e. e. Let rmwEvent be ReadModifyWriteSharedMemory { [[Order]]: SeqCst,
[[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndex,
[[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
6. f. f. Append rmwEvent to eventsRecord.[[EventList]].
7. g. g. Append Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]:
rawBytesRead } to execution.[[ChosenValues]].
9. 9. 9. Else,
1. a. a. Let rawBytesRead be a List of length elementSize whose elements
are the sequence of elementSize bytes starting with block[byteIndex].
2. b. b. Let rawBytesModified be op(rawBytesRead, rawBytes).
3. c. c. Store the individual bytes of rawBytesModified into block,
starting at block[byteIndex].
10. 10. 10. Return RawBytesToNumeric(type, rawBytesRead, isLittleEndian).
25.1.3 THE ARRAYBUFFER CONSTRUCTOR
The ArrayBuffer constructor:
* is %ArrayBuffer%.
* is the initial value of the "ArrayBuffer" property of the global object.
* creates and initializes a new ArrayBuffer when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified ArrayBuffer behaviour must
include a super call to the ArrayBuffer constructor to create and initialize
subclass instances with the internal state necessary to support the
ArrayBuffer.prototype built-in methods.
25.1.3.1 ARRAYBUFFER ( LENGTH )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let byteLength be ? ToIndex(length).
3. 3. 3. Return ? AllocateArrayBuffer(NewTarget, byteLength).
25.1.4 PROPERTIES OF THE ARRAYBUFFER CONSTRUCTOR
The ArrayBuffer constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
25.1.4.1 ARRAYBUFFER.ISVIEW ( ARG )
This function performs the following steps when called:
1. 1. 1. If arg is not an Object, return false.
2. 2. 2. If arg has a [[ViewedArrayBuffer]] internal slot, return true.
3. 3. 3. Return false.
25.1.4.2 ARRAYBUFFER.PROTOTYPE
The initial value of ArrayBuffer.prototype is the ArrayBuffer prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
25.1.4.3 GET ARRAYBUFFER [ @@SPECIES ]
ArrayBuffer[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
ArrayBuffer prototype methods normally use their this value's constructor to
create a derived object. However, a subclass constructor may over-ride that
default behaviour by redefining its @@species property.
25.1.5 PROPERTIES OF THE ARRAYBUFFER PROTOTYPE OBJECT
The ArrayBuffer prototype object:
* is %ArrayBuffer.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal
slot.
25.1.5.1 GET ARRAYBUFFER.PROTOTYPE.BYTELENGTH
ArrayBuffer.prototype.byteLength is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
3. 3. 3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
4. 4. 4. If IsDetachedBuffer(O) is true, return +0𝔽.
5. 5. 5. Let length be O.[[ArrayBufferByteLength]].
6. 6. 6. Return 𝔽(length).
25.1.5.2 ARRAYBUFFER.PROTOTYPE.CONSTRUCTOR
The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.
25.1.5.3 ARRAYBUFFER.PROTOTYPE.SLICE ( START, END )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
3. 3. 3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
4. 4. 4. If IsDetachedBuffer(O) is true, throw a TypeError exception.
5. 5. 5. Let len be O.[[ArrayBufferByteLength]].
6. 6. 6. Let relativeStart be ? ToIntegerOrInfinity(start).
7. 7. 7. If relativeStart = -∞, let first be 0.
8. 8. 8. Else if relativeStart < 0, let first be max(len + relativeStart, 0).
9. 9. 9. Else, let first be min(relativeStart, len).
10. 10. 10. If end is undefined, let relativeEnd be len; else let relativeEnd
be ? ToIntegerOrInfinity(end).
11. 11. 11. If relativeEnd = -∞, let final be 0.
12. 12. 12. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
13. 13. 13. Else, let final be min(relativeEnd, len).
14. 14. 14. Let newLen be max(final - first, 0).
15. 15. 15. Let ctor be ? SpeciesConstructor(O, %ArrayBuffer%).
16. 16. 16. Let new be ? Construct(ctor, « 𝔽(newLen) »).
17. 17. 17. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
18. 18. 18. If IsSharedArrayBuffer(new) is true, throw a TypeError exception.
19. 19. 19. If IsDetachedBuffer(new) is true, throw a TypeError exception.
20. 20. 20. If SameValue(new, O) is true, throw a TypeError exception.
21. 21. 21. If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError
exception.
22. 22. 22. NOTE: Side-effects of the above steps may have detached O.
23. 23. 23. If IsDetachedBuffer(O) is true, throw a TypeError exception.
24. 24. 24. Let fromBuf be O.[[ArrayBufferData]].
25. 25. 25. Let toBuf be new.[[ArrayBufferData]].
26. 26. 26. Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
27. 27. 27. Return new.
25.1.5.4 ARRAYBUFFER.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"ArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.1.6 PROPERTIES OF ARRAYBUFFER INSTANCES
ArrayBuffer instances inherit properties from the ArrayBuffer prototype object.
ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an
[[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]]
internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is null are considered to be
detached and all operators to access or modify data contained in the ArrayBuffer
instance will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other
than undefined need to have all DetachArrayBuffer calls passing that same
"detach key" as an argument, otherwise a TypeError will result. This internal
slot is only ever set by certain embedding environments, not by algorithms in
this specification.
25.2 SHAREDARRAYBUFFER OBJECTS
25.2.1 ABSTRACT OPERATIONS FOR SHAREDARRAYBUFFER OBJECTS
25.2.1.1 ALLOCATESHAREDARRAYBUFFER ( CONSTRUCTOR, BYTELENGTH )
The abstract operation AllocateSharedArrayBuffer takes arguments constructor (a
constructor) and byteLength (a non-negative integer) and returns either a normal
completion containing a SharedArrayBuffer or a throw completion. It is used to
create a SharedArrayBuffer. It performs the following steps when called:
1. 1. 1. Let obj be ? OrdinaryCreateFromConstructor(constructor,
"%SharedArrayBuffer.prototype%", « [[ArrayBufferData]],
[[ArrayBufferByteLength]] »).
2. 2. 2. Let block be ? CreateSharedByteDataBlock(byteLength).
3. 3. 3. Set obj.[[ArrayBufferData]] to block.
4. 4. 4. Set obj.[[ArrayBufferByteLength]] to byteLength.
5. 5. 5. Return obj.
25.2.1.2 ISSHAREDARRAYBUFFER ( OBJ )
The abstract operation IsSharedArrayBuffer takes argument obj (an ArrayBuffer or
a SharedArrayBuffer) and returns a Boolean. It tests whether an object is an
ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the
following steps when called:
1. 1. 1. Let bufferData be obj.[[ArrayBufferData]].
2. 2. 2. If bufferData is null, return false.
3. 3. 3. If bufferData is a Data Block, return false.
4. 4. 4. Assert: bufferData is a Shared Data Block.
5. 5. 5. Return true.
25.2.2 THE SHAREDARRAYBUFFER CONSTRUCTOR
The SharedArrayBuffer constructor:
* is %SharedArrayBuffer%.
* is the initial value of the "SharedArrayBuffer" property of the global
object, if that property is present (see below).
* creates and initializes a new SharedArrayBuffer when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified SharedArrayBuffer behaviour
must include a super call to the SharedArrayBuffer constructor to create and
initialize subclass instances with the internal state necessary to support
the SharedArrayBuffer.prototype built-in methods.
Whenever a host does not provide concurrent access to SharedArrayBuffers it may
omit the "SharedArrayBuffer" property of the global object.
Note
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its
internal [[ArrayBufferData]] slot is never null.
25.2.2.1 SHAREDARRAYBUFFER ( LENGTH )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Let byteLength be ? ToIndex(length).
3. 3. 3. Return ? AllocateSharedArrayBuffer(NewTarget, byteLength).
25.2.3 PROPERTIES OF THE SHAREDARRAYBUFFER CONSTRUCTOR
The SharedArrayBuffer constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
25.2.3.1 SHAREDARRAYBUFFER.PROTOTYPE
The initial value of SharedArrayBuffer.prototype is the SharedArrayBuffer
prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
25.2.3.2 GET SHAREDARRAYBUFFER [ @@SPECIES ]
SharedArrayBuffer[@@species] is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
25.2.4 PROPERTIES OF THE SHAREDARRAYBUFFER PROTOTYPE OBJECT
The SharedArrayBuffer prototype object:
* is %SharedArrayBuffer.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal
slot.
25.2.4.1 GET SHAREDARRAYBUFFER.PROTOTYPE.BYTELENGTH
SharedArrayBuffer.prototype.byteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following
steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
3. 3. 3. If IsSharedArrayBuffer(O) is false, throw a TypeError exception.
4. 4. 4. Let length be O.[[ArrayBufferByteLength]].
5. 5. 5. Return 𝔽(length).
25.2.4.2 SHAREDARRAYBUFFER.PROTOTYPE.CONSTRUCTOR
The initial value of SharedArrayBuffer.prototype.constructor is
%SharedArrayBuffer%.
25.2.4.3 SHAREDARRAYBUFFER.PROTOTYPE.SLICE ( START, END )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
3. 3. 3. If IsSharedArrayBuffer(O) is false, throw a TypeError exception.
4. 4. 4. Let len be O.[[ArrayBufferByteLength]].
5. 5. 5. Let relativeStart be ? ToIntegerOrInfinity(start).
6. 6. 6. If relativeStart = -∞, let first be 0.
7. 7. 7. Else if relativeStart < 0, let first be max(len + relativeStart, 0).
8. 8. 8. Else, let first be min(relativeStart, len).
9. 9. 9. If end is undefined, let relativeEnd be len; else let relativeEnd be
? ToIntegerOrInfinity(end).
10. 10. 10. If relativeEnd = -∞, let final be 0.
11. 11. 11. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
12. 12. 12. Else, let final be min(relativeEnd, len).
13. 13. 13. Let newLen be max(final - first, 0).
14. 14. 14. Let ctor be ? SpeciesConstructor(O, %SharedArrayBuffer%).
15. 15. 15. Let new be ? Construct(ctor, « 𝔽(newLen) »).
16. 16. 16. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
17. 17. 17. If IsSharedArrayBuffer(new) is false, throw a TypeError exception.
18. 18. 18. If new.[[ArrayBufferData]] is O.[[ArrayBufferData]], throw a
TypeError exception.
19. 19. 19. If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError
exception.
20. 20. 20. Let fromBuf be O.[[ArrayBufferData]].
21. 21. 21. Let toBuf be new.[[ArrayBufferData]].
22. 22. 22. Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
23. 23. 23. Return new.
25.2.4.4 SHAREDARRAYBUFFER.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"SharedArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.2.5 PROPERTIES OF SHAREDARRAYBUFFER INSTANCES
SharedArrayBuffer instances inherit properties from the SharedArrayBuffer
prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]]
internal slot and an [[ArrayBufferByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
25.3 DATAVIEW OBJECTS
25.3.1 ABSTRACT OPERATIONS FOR DATAVIEW OBJECTS
25.3.1.1 GETVIEWVALUE ( VIEW, REQUESTINDEX, ISLITTLEENDIAN, TYPE )
The abstract operation GetViewValue takes arguments view (an ECMAScript language
value), requestIndex (an ECMAScript language value), isLittleEndian (an
ECMAScript language value), and type (a TypedArray element type) and returns
either a normal completion containing either a Number or a BigInt, or a throw
completion. It is used by functions on DataView instances to retrieve values
from the view's buffer. It performs the following steps when called:
1. 1. 1. Perform ? RequireInternalSlot(view, [[DataView]]).
2. 2. 2. Assert: view has a [[ViewedArrayBuffer]] internal slot.
3. 3. 3. Let getIndex be ? ToIndex(requestIndex).
4. 4. 4. Set isLittleEndian to ToBoolean(isLittleEndian).
5. 5. 5. Let buffer be view.[[ViewedArrayBuffer]].
6. 6. 6. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
7. 7. 7. Let viewOffset be view.[[ByteOffset]].
8. 8. 8. Let viewSize be view.[[ByteLength]].
9. 9. 9. Let elementSize be the Element Size value specified in Table 68 for
Element Type type.
10. 10. 10. If getIndex + elementSize > viewSize, throw a RangeError exception.
11. 11. 11. Let bufferIndex be getIndex + viewOffset.
12. 12. 12. Return GetValueFromBuffer(buffer, bufferIndex, type, false,
Unordered, isLittleEndian).
25.3.1.2 SETVIEWVALUE ( VIEW, REQUESTINDEX, ISLITTLEENDIAN, TYPE, VALUE )
The abstract operation SetViewValue takes arguments view (an ECMAScript language
value), requestIndex (an ECMAScript language value), isLittleEndian (an
ECMAScript language value), type (a TypedArray element type), and value (an
ECMAScript language value) and returns either a normal completion containing
undefined or a throw completion. It is used by functions on DataView instances
to store values into the view's buffer. It performs the following steps when
called:
1. 1. 1. Perform ? RequireInternalSlot(view, [[DataView]]).
2. 2. 2. Assert: view has a [[ViewedArrayBuffer]] internal slot.
3. 3. 3. Let getIndex be ? ToIndex(requestIndex).
4. 4. 4. If IsBigIntElementType(type) is true, let numberValue be
? ToBigInt(value).
5. 5. 5. Otherwise, let numberValue be ? ToNumber(value).
6. 6. 6. Set isLittleEndian to ToBoolean(isLittleEndian).
7. 7. 7. Let buffer be view.[[ViewedArrayBuffer]].
8. 8. 8. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
9. 9. 9. Let viewOffset be view.[[ByteOffset]].
10. 10. 10. Let viewSize be view.[[ByteLength]].
11. 11. 11. Let elementSize be the Element Size value specified in Table 68 for
Element Type type.
12. 12. 12. If getIndex + elementSize > viewSize, throw a RangeError exception.
13. 13. 13. Let bufferIndex be getIndex + viewOffset.
14. 14. 14. Perform SetValueInBuffer(buffer, bufferIndex, type, numberValue,
false, Unordered, isLittleEndian).
15. 15. 15. Return undefined.
25.3.2 THE DATAVIEW CONSTRUCTOR
The DataView constructor:
* is %DataView%.
* is the initial value of the "DataView" property of the global object.
* creates and initializes a new DataView when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified DataView behaviour must
include a super call to the DataView constructor to create and initialize
subclass instances with the internal state necessary to support the
DataView.prototype built-in methods.
25.3.2.1 DATAVIEW ( BUFFER [ , BYTEOFFSET [ , BYTELENGTH ] ] )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Perform ? RequireInternalSlot(buffer, [[ArrayBufferData]]).
3. 3. 3. Let offset be ? ToIndex(byteOffset).
4. 4. 4. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
5. 5. 5. Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
6. 6. 6. If offset > bufferByteLength, throw a RangeError exception.
7. 7. 7. If byteLength is undefined, then
1. a. a. Let viewByteLength be bufferByteLength - offset.
8. 8. 8. Else,
1. a. a. Let viewByteLength be ? ToIndex(byteLength).
2. b. b. If offset + viewByteLength > bufferByteLength, throw a RangeError
exception.
9. 9. 9. Let O be ? OrdinaryCreateFromConstructor(NewTarget,
"%DataView.prototype%", « [[DataView]], [[ViewedArrayBuffer]],
[[ByteLength]], [[ByteOffset]] »).
10. 10. 10. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
11. 11. 11. Set O.[[ViewedArrayBuffer]] to buffer.
12. 12. 12. Set O.[[ByteLength]] to viewByteLength.
13. 13. 13. Set O.[[ByteOffset]] to offset.
14. 14. 14. Return O.
25.3.3 PROPERTIES OF THE DATAVIEW CONSTRUCTOR
The DataView constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
25.3.3.1 DATAVIEW.PROTOTYPE
The initial value of DataView.prototype is the DataView prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
25.3.4 PROPERTIES OF THE DATAVIEW PROTOTYPE OBJECT
The DataView prototype object:
* is %DataView.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or
[[ByteOffset]] internal slot.
25.3.4.1 GET DATAVIEW.PROTOTYPE.BUFFER
DataView.prototype.buffer is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[DataView]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. Return buffer.
25.3.4.2 GET DATAVIEW.PROTOTYPE.BYTELENGTH
DataView.prototype.byteLength is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[DataView]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
6. 6. 6. Let size be O.[[ByteLength]].
7. 7. 7. Return 𝔽(size).
25.3.4.3 GET DATAVIEW.PROTOTYPE.BYTEOFFSET
DataView.prototype.byteOffset is an accessor property whose set accessor
function is undefined. Its get accessor function performs the following steps
when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[DataView]]).
3. 3. 3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. 4. 4. Let buffer be O.[[ViewedArrayBuffer]].
5. 5. 5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
6. 6. 6. Let offset be O.[[ByteOffset]].
7. 7. 7. Return 𝔽(offset).
25.3.4.4 DATAVIEW.PROTOTYPE.CONSTRUCTOR
The initial value of DataView.prototype.constructor is %DataView%.
25.3.4.5 DATAVIEW.PROTOTYPE.GETBIGINT64 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? GetViewValue(v, byteOffset, littleEndian, BigInt64).
25.3.4.6 DATAVIEW.PROTOTYPE.GETBIGUINT64 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? GetViewValue(v, byteOffset, littleEndian, BigUint64).
25.3.4.7 DATAVIEW.PROTOTYPE.GETFLOAT32 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? GetViewValue(v, byteOffset, littleEndian, Float32).
25.3.4.8 DATAVIEW.PROTOTYPE.GETFLOAT64 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? GetViewValue(v, byteOffset, littleEndian, Float64).
25.3.4.9 DATAVIEW.PROTOTYPE.GETINT8 ( BYTEOFFSET )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? GetViewValue(v, byteOffset, true, Int8).
25.3.4.10 DATAVIEW.PROTOTYPE.GETINT16 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? GetViewValue(v, byteOffset, littleEndian, Int16).
25.3.4.11 DATAVIEW.PROTOTYPE.GETINT32 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? GetViewValue(v, byteOffset, littleEndian, Int32).
25.3.4.12 DATAVIEW.PROTOTYPE.GETUINT8 ( BYTEOFFSET )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? GetViewValue(v, byteOffset, true, Uint8).
25.3.4.13 DATAVIEW.PROTOTYPE.GETUINT16 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? GetViewValue(v, byteOffset, littleEndian, Uint16).
25.3.4.14 DATAVIEW.PROTOTYPE.GETUINT32 ( BYTEOFFSET [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? GetViewValue(v, byteOffset, littleEndian, Uint32).
25.3.4.15 DATAVIEW.PROTOTYPE.SETBIGINT64 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ]
)
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? SetViewValue(v, byteOffset, littleEndian, BigInt64, value).
25.3.4.16 DATAVIEW.PROTOTYPE.SETBIGUINT64 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ]
)
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? SetViewValue(v, byteOffset, littleEndian, BigUint64, value).
25.3.4.17 DATAVIEW.PROTOTYPE.SETFLOAT32 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? SetViewValue(v, byteOffset, littleEndian, Float32, value).
25.3.4.18 DATAVIEW.PROTOTYPE.SETFLOAT64 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? SetViewValue(v, byteOffset, littleEndian, Float64, value).
25.3.4.19 DATAVIEW.PROTOTYPE.SETINT8 ( BYTEOFFSET, VALUE )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? SetViewValue(v, byteOffset, true, Int8, value).
25.3.4.20 DATAVIEW.PROTOTYPE.SETINT16 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? SetViewValue(v, byteOffset, littleEndian, Int16, value).
25.3.4.21 DATAVIEW.PROTOTYPE.SETINT32 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? SetViewValue(v, byteOffset, littleEndian, Int32, value).
25.3.4.22 DATAVIEW.PROTOTYPE.SETUINT8 ( BYTEOFFSET, VALUE )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. Return ? SetViewValue(v, byteOffset, true, Uint8, value).
25.3.4.23 DATAVIEW.PROTOTYPE.SETUINT16 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? SetViewValue(v, byteOffset, littleEndian, Uint16, value).
25.3.4.24 DATAVIEW.PROTOTYPE.SETUINT32 ( BYTEOFFSET, VALUE [ , LITTLEENDIAN ] )
This method performs the following steps when called:
1. 1. 1. Let v be the this value.
2. 2. 2. If littleEndian is not present, set littleEndian to false.
3. 3. 3. Return ? SetViewValue(v, byteOffset, littleEndian, Uint32, value).
25.3.4.25 DATAVIEW.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "DataView".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.3.5 PROPERTIES OF DATAVIEW INSTANCES
DataView instances are ordinary objects that inherit properties from the
DataView prototype object. DataView instances each have [[DataView]],
[[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this
specification. The simple presence of that internal slot is used within the
specification to identify objects created using the DataView constructor.
25.4 THE ATOMICS OBJECT
The Atomics object:
* is %Atomics%.
* is the initial value of the "Atomics" property of the global object.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* does not have a [[Construct]] internal method; it cannot be used as a
constructor with the new operator.
* does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly (atomically) on
shared memory array cells as well as functions that let agents wait for and
dispatch primitive events. When used with discipline, the Atomics functions
allow multi-agent programs that communicate through shared memory to execute in
a well-understood order even on parallel CPUs. The rules that govern
shared-memory communication are provided by the memory model, defined below.
Note
For informative guidelines for programming and implementing shared memory in
ECMAScript, please see the notes at the end of the memory model section.
25.4.1 WAITERLIST OBJECTS
A WaiterList is a semantic object that contains an ordered list of agent
signifiers for those agents that are waiting on a location (block, i) in shared
memory; block is a Shared Data Block and i a byte offset into the memory of
block. A WaiterList object also optionally contains a Synchronize event denoting
the previous leaving of its critical section.
Initially a WaiterList object has an empty list and no Synchronize event.
The agent cluster has a store of WaiterList objects; the store is indexed by
(block, i). WaiterLists are agent-independent: a lookup in the store of
WaiterLists by (block, i) will result in the same WaiterList object in any agent
in the agent cluster.
Each WaiterList has a critical section that controls exclusive access to that
WaiterList during evaluation. Only a single agent may enter a WaiterList's
critical section at one time. Entering and leaving a WaiterList's critical
section is controlled by the abstract operations EnterCriticalSection and
LeaveCriticalSection. Operations on a WaiterList—adding and removing waiting
agents, traversing the list of agents, suspending and notifying agents on the
list, setting and retrieving the Synchronize event—may only be performed by
agents that have entered the WaiterList's critical section.
25.4.2 ABSTRACT OPERATIONS FOR ATOMICS
25.4.2.1 VALIDATEINTEGERTYPEDARRAY ( TYPEDARRAY [ , WAITABLE ] )
The abstract operation ValidateIntegerTypedArray takes argument typedArray (an
ECMAScript language value) and optional argument waitable (a Boolean) and
returns either a normal completion containing either an ArrayBuffer or a
SharedArrayBuffer, or a throw completion. It performs the following steps when
called:
1. 1. 1. If waitable is not present, set waitable to false.
2. 2. 2. Perform ? ValidateTypedArray(typedArray).
3. 3. 3. Let buffer be typedArray.[[ViewedArrayBuffer]].
4. 4. 4. If waitable is true, then
1. a. a. If typedArray.[[TypedArrayName]] is neither "Int32Array" nor
"BigInt64Array", throw a TypeError exception.
5. 5. 5. Else,
1. a. a. Let type be TypedArrayElementType(typedArray).
2. b. b. If IsUnclampedIntegerElementType(type) is false and
IsBigIntElementType(type) is false, throw a TypeError exception.
6. 6. 6. Return buffer.
25.4.2.2 VALIDATEATOMICACCESS ( TYPEDARRAY, REQUESTINDEX )
The abstract operation ValidateAtomicAccess takes arguments typedArray (a
TypedArray) and requestIndex (an ECMAScript language value) and returns either a
normal completion containing an integer or a throw completion. It performs the
following steps when called:
1. 1. 1. Let length be typedArray.[[ArrayLength]].
2. 2. 2. Let accessIndex be ? ToIndex(requestIndex).
3. 3. 3. Assert: accessIndex ≥ 0.
4. 4. 4. If accessIndex ≥ length, throw a RangeError exception.
5. 5. 5. Let elementSize be TypedArrayElementSize(typedArray).
6. 6. 6. Let offset be typedArray.[[ByteOffset]].
7. 7. 7. Return (accessIndex × elementSize) + offset.
25.4.2.3 GETWAITERLIST ( BLOCK, I )
The abstract operation GetWaiterList takes arguments block (a Shared Data Block)
and i (a non-negative integer that is evenly divisible by 4) and returns a
WaiterList. It performs the following steps when called:
1. 1. 1. Assert: i and i + 3 are valid byte offsets within the memory of block.
2. 2. 2. Return the WaiterList that is referenced by the pair (block, i).
25.4.2.4 ENTERCRITICALSECTION ( WL )
The abstract operation EnterCriticalSection takes argument WL (a WaiterList) and
returns unused. It performs the following steps when called:
1. 1. 1. Assert: The surrounding agent is not in the critical section for any
WaiterList.
2. 2. 2. Wait until no agent is in the critical section for WL, then enter the
critical section for WL (without allowing any other agent to enter).
3. 3. 3. If WL has a Synchronize event, then
1. a. a. NOTE: A WL whose critical section has been entered at least once
has a Synchronize event set by LeaveCriticalSection.
2. b. b. Let execution be the [[CandidateExecution]] field of the
surrounding agent's Agent Record.
3. c. c. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
4. d. d. Let enterEvent be a new Synchronize event.
5. e. e. Append enterEvent to eventsRecord.[[EventList]].
6. f. f. Let leaveEvent be the Synchronize event in WL.
7. g. g. Append (leaveEvent, enterEvent) to
eventsRecord.[[AgentSynchronizesWith]].
4. 4. 4. Return unused.
EnterCriticalSection has contention when an agent attempting to enter the
critical section must wait for another agent to leave it. When there is no
contention, FIFO order of EnterCriticalSection calls is observable. When there
is contention, an implementation may choose an arbitrary order but may not cause
an agent to wait indefinitely.
25.4.2.5 LEAVECRITICALSECTION ( WL )
The abstract operation LeaveCriticalSection takes argument WL (a WaiterList) and
returns unused. It performs the following steps when called:
1. 1. 1. Assert: The surrounding agent is in the critical section for WL.
2. 2. 2. Let execution be the [[CandidateExecution]] field of the surrounding
agent's Agent Record.
3. 3. 3. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
4. 4. 4. Let leaveEvent be a new Synchronize event.
5. 5. 5. Append leaveEvent to eventsRecord.[[EventList]].
6. 6. 6. Set the Synchronize event in WL to leaveEvent.
7. 7. 7. Leave the critical section for WL.
8. 8. 8. Return unused.
25.4.2.6 ADDWAITER ( WL, W )
The abstract operation AddWaiter takes arguments WL (a WaiterList) and W (an
agent signifier) and returns unused. It performs the following steps when
called:
1. 1. 1. Assert: The surrounding agent is in the critical section for WL.
2. 2. 2. Assert: W is not on the list of waiters in any WaiterList.
3. 3. 3. Append W to WL.
4. 4. 4. Return unused.
25.4.2.7 REMOVEWAITER ( WL, W )
The abstract operation RemoveWaiter takes arguments WL (a WaiterList) and W (an
agent signifier) and returns unused. It performs the following steps when
called:
1. 1. 1. Assert: The surrounding agent is in the critical section for WL.
2. 2. 2. Assert: W is on the list of waiters in WL.
3. 3. 3. Remove W from the list of waiters in WL.
4. 4. 4. Return unused.
25.4.2.8 REMOVEWAITERS ( WL, C )
The abstract operation RemoveWaiters takes arguments WL (a WaiterList) and c (a
non-negative integer or +∞) and returns a List of agent signifiers. It performs
the following steps when called:
1. 1. 1. Assert: The surrounding agent is in the critical section for WL.
2. 2. 2. Let S be a reference to the list of waiters in WL.
3. 3. 3. Let len be the number of elements in S.
4. 4. 4. Let n be min(c, len).
5. 5. 5. Let L be a List whose elements are the first n elements of S.
6. 6. 6. Remove the first n elements of S.
7. 7. 7. Return L.
25.4.2.9 SUSPENDAGENT ( WL, W, MINIMUMTIMEOUT )
The abstract operation SuspendAgent takes arguments WL (a WaiterList), W (an
agent signifier), and minimumTimeout (a non-negative extended mathematical
value) and returns a Boolean. It performs the following steps when called:
1. 1. 1. Assert: The surrounding agent is in the critical section for WL.
2. 2. 2. Assert: W is AgentSignifier().
3. 3. 3. Assert: W is on the list of waiters in WL.
4. 4. 4. Assert: AgentCanSuspend() is true.
5. 5. 5. Let additionalTimeout be an implementation-defined non-negative
mathematical value.
6. 6. 6. Let timeout be minimumTimeout + additionalTimeout.
7. 7. 7. NOTE: When minimumTimeout is +∞, timeout is also +∞ and the following
step can terminate only by another agent calling NotifyWaiter.
8. 8. 8. Perform LeaveCriticalSection(WL) and suspend W for up to timeout
milliseconds, performing the combined operation in such a way that a
notification that arrives after the critical section is exited but before
the suspension takes effect is not lost. W can wake from suspension either
because the timeout expired or because it was notified explicitly by
another agent calling NotifyWaiter with arguments WL and W, and not for any
other reasons at all.
9. 9. 9. Perform EnterCriticalSection(WL).
10. 10. 10. If W was notified explicitly by another agent calling NotifyWaiter
with arguments WL and W, return true.
11. 11. 11. Return false.
Note
additionalTimeout allows implementations to pad timeouts as necessary, such as
for reducing power consumption or coarsening timer resolution to mitigate timing
attacks. This value may differ from call to call of SuspendAgent.
25.4.2.10 NOTIFYWAITER ( WL, W )
The abstract operation NotifyWaiter takes arguments WL (a WaiterList) and W (an
agent signifier) and returns unused. It performs the following steps when
called:
1. 1. 1. Assert: The surrounding agent is in the critical section for WL.
2. 2. 2. Notify the agent W.
3. 3. 3. Return unused.
Note
The embedding may delay notifying W, e.g. for resource management reasons, but W
must eventually be notified in order to guarantee forward progress.
25.4.2.11 ATOMICREADMODIFYWRITE ( TYPEDARRAY, INDEX, VALUE, OP )
The abstract operation AtomicReadModifyWrite takes arguments typedArray (an
ECMAScript language value), index (an ECMAScript language value), value (an
ECMAScript language value), and op (a read-modify-write modification function)
and returns either a normal completion containing either a Number or a BigInt,
or a throw completion. op takes two List of byte values arguments and returns a
List of byte values. This operation atomically loads a value, combines it with
another value, and stores the result of the combination. It returns the loaded
value. It performs the following steps when called:
1. 1. 1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
2. 2. 2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
3. 3. 3. If typedArray.[[ContentType]] is BigInt, let v be ? ToBigInt(value).
4. 4. 4. Otherwise, let v be 𝔽(? ToIntegerOrInfinity(value)).
5. 5. 5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
6. 6. 6. NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call to ToBigInt or
ToIntegerOrInfinity on the preceding lines can have arbitrary side effects,
which could cause the buffer to become detached.
7. 7. 7. Let elementType be TypedArrayElementType(typedArray).
8. 8. 8. Return GetModifySetValueInBuffer(buffer, indexedPosition, elementType,
v, op).
25.4.2.12 BYTELISTBITWISEOP ( OP, XBYTES, YBYTES )
The abstract operation ByteListBitwiseOp takes arguments op (&, ^, or |), xBytes
(a List of byte values), and yBytes (a List of byte values) and returns a List
of byte values. The operation atomically performs a bitwise operation on all
byte values of the arguments and returns a List of byte values. It performs the
following steps when called:
1. 1. 1. Assert: xBytes and yBytes have the same number of elements.
2. 2. 2. Let result be a new empty List.
3. 3. 3. Let i be 0.
4. 4. 4. For each element xByte of xBytes, do
1. a. a. Let yByte be yBytes[i].
2. b. b. If op is &, let resultByte be the result of applying the bitwise
AND operation to xByte and yByte.
3. c. c. Else if op is ^, let resultByte be the result of applying the
bitwise exclusive OR (XOR) operation to xByte and yByte.
4. d. d. Else, op is |. Let resultByte be the result of applying the bitwise
inclusive OR operation to xByte and yByte.
5. e. e. Set i to i + 1.
6. f. f. Append resultByte to result.
5. 5. 5. Return result.
25.4.2.13 BYTELISTEQUAL ( XBYTES, YBYTES )
The abstract operation ByteListEqual takes arguments xBytes (a List of byte
values) and yBytes (a List of byte values) and returns a Boolean. It performs
the following steps when called:
1. 1. 1. If xBytes and yBytes do not have the same number of elements, return
false.
2. 2. 2. Let i be 0.
3. 3. 3. For each element xByte of xBytes, do
1. a. a. Let yByte be yBytes[i].
2. b. b. If xByte ≠ yByte, return false.
3. c. c. Set i to i + 1.
4. 4. 4. Return true.
25.4.3 ATOMICS.ADD ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let type be TypedArrayElementType(typedArray).
2. 2. 2. Let isLittleEndian be the value of the [[LittleEndian]] field of the
surrounding agent's Agent Record.
3. 3. 3. Let add be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures type and isLittleEndian and
performs the following steps atomically when called:
1. a. a. Let x be RawBytesToNumeric(type, xBytes, isLittleEndian).
2. b. b. Let y be RawBytesToNumeric(type, yBytes, isLittleEndian).
3. c. c. If x is a Number, then
1. i. i. Let sum be Number::add(x, y).
4. d. d. Else,
1. i. i. Assert: x is a BigInt.
2. ii. ii. Let sum be BigInt::add(x, y).
5. e. e. Let sumBytes be NumericToRawBytes(type, sum, isLittleEndian).
6. f. f. Assert: sumBytes, xBytes, and yBytes have the same number of
elements.
7. g. g. Return sumBytes.
4. 4. 4. Return ? AtomicReadModifyWrite(typedArray, index, value, add).
25.4.4 ATOMICS.AND ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let and be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures nothing and performs the following
steps atomically when called:
1. a. a. Return ByteListBitwiseOp(&, xBytes, yBytes).
2. 2. 2. Return ? AtomicReadModifyWrite(typedArray, index, value, and).
25.4.5 ATOMICS.COMPAREEXCHANGE ( TYPEDARRAY, INDEX, EXPECTEDVALUE,
REPLACEMENTVALUE )
This function performs the following steps when called:
1. 1. 1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
2. 2. 2. Let block be buffer.[[ArrayBufferData]].
3. 3. 3. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
4. 4. 4. If typedArray.[[ContentType]] is BigInt, then
1. a. a. Let expected be ? ToBigInt(expectedValue).
2. b. b. Let replacement be ? ToBigInt(replacementValue).
5. 5. 5. Else,
1. a. a. Let expected be 𝔽(? ToIntegerOrInfinity(expectedValue)).
2. b. b. Let replacement be 𝔽(? ToIntegerOrInfinity(replacementValue)).
6. 6. 6. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
7. 7. 7. NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call to ToBigInt or
ToIntegerOrInfinity on the preceding lines can have arbitrary side effects,
which could cause the buffer to become detached.
8. 8. 8. Let elementType be TypedArrayElementType(typedArray).
9. 9. 9. Let elementSize be TypedArrayElementSize(typedArray).
10. 10. 10. Let isLittleEndian be the value of the [[LittleEndian]] field of
the surrounding agent's Agent Record.
11. 11. 11. Let expectedBytes be NumericToRawBytes(elementType, expected,
isLittleEndian).
12. 12. 12. Let replacementBytes be NumericToRawBytes(elementType, replacement,
isLittleEndian).
13. 13. 13. If IsSharedArrayBuffer(buffer) is true, then
1. a. a. Let execution be the [[CandidateExecution]] field of the
surrounding agent's Agent Record.
2. b. b. Let eventsRecord be the Agent Events Record of
execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier().
3. c. c. Let rawBytesRead be a List of length elementSize whose elements
are nondeterministically chosen byte values.
4. d. d. NOTE: In implementations, rawBytesRead is the result of a
load-link, of a load-exclusive, or of an operand of a read-modify-write
instruction on the underlying hardware. The nondeterminism is a semantic
prescription of the memory model to describe observable behaviour of
hardware with weak consistency.
5. e. e. NOTE: The comparison of the expected value and the read value is
performed outside of the read-modify-write modification function to
avoid needlessly strong synchronization when the expected value is not
equal to the read value.
6. f. f. If ByteListEqual(rawBytesRead, expectedBytes) is true, then
1. i. i. Let second be a new read-modify-write modification function
with parameters (oldBytes, newBytes) that captures nothing and
performs the following steps atomically when called:
1. 1. 1. Return newBytes.
2. ii. ii. Let event be ReadModifyWriteSharedMemory { [[Order]]: SeqCst,
[[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: indexedPosition,
[[ElementSize]]: elementSize, [[Payload]]: replacementBytes,
[[ModifyOp]]: second }.
7. g. g. Else,
1. i. i. Let event be ReadSharedMemory { [[Order]]: SeqCst, [[NoTear]]:
true, [[Block]]: block, [[ByteIndex]]: indexedPosition,
[[ElementSize]]: elementSize }.
8. h. h. Append event to eventsRecord.[[EventList]].
9. i. i. Append Chosen Value Record { [[Event]]: event, [[ChosenValue]]:
rawBytesRead } to execution.[[ChosenValues]].
14. 14. 14. Else,
1. a. a. Let rawBytesRead be a List of length elementSize whose elements
are the sequence of elementSize bytes starting with
block[indexedPosition].
2. b. b. If ByteListEqual(rawBytesRead, expectedBytes) is true, then
1. i. i. Store the individual bytes of replacementBytes into block,
starting at block[indexedPosition].
15. 15. 15. Return RawBytesToNumeric(elementType, rawBytesRead,
isLittleEndian).
25.4.6 ATOMICS.EXCHANGE ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let second be a new read-modify-write modification function with
parameters (oldBytes, newBytes) that captures nothing and performs the
following steps atomically when called:
1. a. a. Return newBytes.
2. 2. 2. Return ? AtomicReadModifyWrite(typedArray, index, value, second).
25.4.7 ATOMICS.ISLOCKFREE ( SIZE )
This function performs the following steps when called:
1. 1. 1. Let n be ? ToIntegerOrInfinity(size).
2. 2. 2. Let AR be the Agent Record of the surrounding agent.
3. 3. 3. If n = 1, return AR.[[IsLockFree1]].
4. 4. 4. If n = 2, return AR.[[IsLockFree2]].
5. 5. 5. If n = 4, return true.
6. 6. 6. If n = 8, return AR.[[IsLockFree8]].
7. 7. 7. Return false.
Note
This function is an optimization primitive. The intuition is that if the atomic
step of an atomic primitive (compareExchange, load, store, add, sub, and, or,
xor, or exchange) on a datum of size n bytes will be performed without the
surrounding agent acquiring a lock outside the n bytes comprising the datum,
then Atomics.isLockFree(n) will return true. High-performance algorithms will
use this function to determine whether to use locks or atomic operations in
critical sections. If an atomic primitive is not lock-free then it is often more
efficient for an algorithm to provide its own locking.
Atomics.isLockFree(4) always returns true as that can be supported on all known
relevant hardware. Being able to assume this will generally simplify programs.
Regardless of the value returned by this function, all atomic operations are
guaranteed to be atomic. For example, they will never have a visible operation
take place in the middle of the operation (e.g., "tearing").
25.4.8 ATOMICS.LOAD ( TYPEDARRAY, INDEX )
This function performs the following steps when called:
1. 1. 1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
2. 2. 2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
3. 3. 3. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
4. 4. 4. NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call to ValidateAtomicAccess on the
preceding line can have arbitrary side effects, which could cause the buffer
to become detached.
5. 5. 5. Let elementType be TypedArrayElementType(typedArray).
6. 6. 6. Return GetValueFromBuffer(buffer, indexedPosition, elementType, true,
SeqCst).
25.4.9 ATOMICS.OR ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let or be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures nothing and performs the following
steps atomically when called:
1. a. a. Return ByteListBitwiseOp(|, xBytes, yBytes).
2. 2. 2. Return ? AtomicReadModifyWrite(typedArray, index, value, or).
25.4.10 ATOMICS.STORE ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
2. 2. 2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
3. 3. 3. If typedArray.[[ContentType]] is BigInt, let v be ? ToBigInt(value).
4. 4. 4. Otherwise, let v be 𝔽(? ToIntegerOrInfinity(value)).
5. 5. 5. If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
6. 6. 6. NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call to ToBigInt or
ToIntegerOrInfinity on the preceding lines can have arbitrary side effects,
which could cause the buffer to become detached.
7. 7. 7. Let elementType be TypedArrayElementType(typedArray).
8. 8. 8. Perform SetValueInBuffer(buffer, indexedPosition, elementType, v,
true, SeqCst).
9. 9. 9. Return v.
25.4.11 ATOMICS.SUB ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let type be TypedArrayElementType(typedArray).
2. 2. 2. Let isLittleEndian be the value of the [[LittleEndian]] field of the
surrounding agent's Agent Record.
3. 3. 3. Let subtract be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures type and isLittleEndian and
performs the following steps atomically when called:
1. a. a. Let x be RawBytesToNumeric(type, xBytes, isLittleEndian).
2. b. b. Let y be RawBytesToNumeric(type, yBytes, isLittleEndian).
3. c. c. If x is a Number, then
1. i. i. Let difference be Number::subtract(x, y).
4. d. d. Else,
1. i. i. Assert: x is a BigInt.
2. ii. ii. Let difference be BigInt::subtract(x, y).
5. e. e. Let differenceBytes be NumericToRawBytes(type, difference,
isLittleEndian).
6. f. f. Assert: differenceBytes, xBytes, and yBytes have the same number of
elements.
7. g. g. Return differenceBytes.
4. 4. 4. Return ? AtomicReadModifyWrite(typedArray, index, value, subtract).
25.4.12 ATOMICS.WAIT ( TYPEDARRAY, INDEX, VALUE, TIMEOUT )
This function puts the surrounding agent in a wait queue and suspends it until
notified or until the wait times out, returning a String differentiating those
cases.
It performs the following steps when called:
1. 1. 1. Let buffer be ? ValidateIntegerTypedArray(typedArray, true).
2. 2. 2. If IsSharedArrayBuffer(buffer) is false, throw a TypeError exception.
3. 3. 3. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
4. 4. 4. If typedArray.[[TypedArrayName]] is "BigInt64Array", let v be
? ToBigInt64(value).
5. 5. 5. Otherwise, let v be ? ToInt32(value).
6. 6. 6. Let q be ? ToNumber(timeout).
7. 7. 7. If q is either NaN or +∞𝔽, let t be +∞; else if q is -∞𝔽, let t be
0; else let t be max(ℝ(q), 0).
8. 8. 8. Let B be AgentCanSuspend().
9. 9. 9. If B is false, throw a TypeError exception.
10. 10. 10. Let block be buffer.[[ArrayBufferData]].
11. 11. 11. Let WL be GetWaiterList(block, indexedPosition).
12. 12. 12. Perform EnterCriticalSection(WL).
13. 13. 13. Let elementType be TypedArrayElementType(typedArray).
14. 14. 14. Let w be GetValueFromBuffer(buffer, indexedPosition, elementType,
true, SeqCst).
15. 15. 15. If v ≠ w, then
1. a. a. Perform LeaveCriticalSection(WL).
2. b. b. Return "not-equal".
16. 16. 16. Let W be AgentSignifier().
17. 17. 17. Perform AddWaiter(WL, W).
18. 18. 18. Let notified be SuspendAgent(WL, W, t).
19. 19. 19. If notified is true, then
1. a. a. Assert: W is not on the list of waiters in WL.
20. 20. 20. Else,
1. a. a. Perform RemoveWaiter(WL, W).
21. 21. 21. Perform LeaveCriticalSection(WL).
22. 22. 22. If notified is true, return "ok".
23. 23. 23. Return "timed-out".
25.4.13 ATOMICS.NOTIFY ( TYPEDARRAY, INDEX, COUNT )
This function notifies some agents that are sleeping in the wait queue.
It performs the following steps when called:
1. 1. 1. Let buffer be ? ValidateIntegerTypedArray(typedArray, true).
2. 2. 2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
3. 3. 3. If count is undefined, let c be +∞.
4. 4. 4. Else,
1. a. a. Let intCount be ? ToIntegerOrInfinity(count).
2. b. b. Let c be max(intCount, 0).
5. 5. 5. Let block be buffer.[[ArrayBufferData]].
6. 6. 6. If IsSharedArrayBuffer(buffer) is false, return +0𝔽.
7. 7. 7. Let WL be GetWaiterList(block, indexedPosition).
8. 8. 8. Perform EnterCriticalSection(WL).
9. 9. 9. Let S be RemoveWaiters(WL, c).
10. 10. 10. For each element W of S, do
1. a. a. Perform NotifyWaiter(WL, W).
11. 11. 11. Perform LeaveCriticalSection(WL).
12. 12. 12. Let n be the number of elements in S.
13. 13. 13. Return 𝔽(n).
25.4.14 ATOMICS.XOR ( TYPEDARRAY, INDEX, VALUE )
This function performs the following steps when called:
1. 1. 1. Let xor be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures nothing and performs the following
steps atomically when called:
1. a. a. Return ByteListBitwiseOp(^, xBytes, yBytes).
2. 2. 2. Return ? AtomicReadModifyWrite(typedArray, index, value, xor).
25.4.15 ATOMICS [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Atomics".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.5 THE JSON OBJECT
The JSON object:
* is %JSON%.
* is the initial value of the "JSON" property of the global object.
* is an ordinary object.
* contains two functions, parse and stringify, that are used to parse and
construct JSON texts.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* does not have a [[Construct]] internal method; it cannot be used as a
constructor with the new operator.
* does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange
format used in this specification is exactly that described by ECMA-404.
Conforming implementations of JSON.parse and JSON.stringify must support the
exact interchange format described in the ECMA-404 specification without any
deletions or extensions to the format.
25.5.1 JSON.PARSE ( TEXT [ , REVIVER ] )
This function parses a JSON text (a JSON-formatted String) and produces an
ECMAScript language value. The JSON format represents literals, arrays, and
objects with a syntax similar to the syntax for ECMAScript literals, Array
Initializers, and Object Initializers. After parsing, JSON objects are realized
as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances.
JSON strings, numbers, booleans, and null are realized as ECMAScript Strings,
Numbers, Booleans, and null.
The optional reviver parameter is a function that takes two parameters, key and
value. It can filter and transform the results. It is called with each of the
key/value pairs produced by the parse, and its return value is used instead of
the original value. If it returns what it received, the structure is not
modified. If it returns undefined then the property is deleted from the result.
1. 1. 1. Let jsonString be ? ToString(text).
2. 2. 2. Parse StringToCodePoints(jsonString) as a JSON text as specified in
ECMA-404. Throw a SyntaxError exception if it is not a valid JSON text as
defined in that specification.
3. 3. 3. Let scriptString be the string-concatenation of "(", jsonString, and
");".
4. 4. 4. Let script be ParseText(StringToCodePoints(scriptString), Script).
5. 5. 5. NOTE: The early error rules defined in 13.2.5.1 have special handling
for the above invocation of ParseText.
6. 6. 6. Assert: script is a Parse Node.
7. 7. 7. Let completion be Completion(Evaluation of script).
8. 8. 8. NOTE: The PropertyDefinitionEvaluation semantics defined in 13.2.5.5
have special handling for the above evaluation.
9. 9. 9. Let unfiltered be completion.[[Value]].
10. 10. 10. Assert: unfiltered is either a String, a Number, a Boolean, an
Object that is defined by either an ArrayLiteral or an ObjectLiteral, or
null.
11. 11. 11. If IsCallable(reviver) is true, then
1. a. a. Let root be OrdinaryObjectCreate(%Object.prototype%).
2. b. b. Let rootName be the empty String.
3. c. c. Perform ! CreateDataPropertyOrThrow(root, rootName, unfiltered).
4. d. d. Return ? InternalizeJSONProperty(root, rootName, reviver).
12. 12. 12. Else,
1. a. a. Return unfiltered.
The "length" property of this function is 2𝔽.
Note
Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Step 2
verifies that jsonString conforms to that subset, and step 10 asserts that that
parsing and evaluation returns a value of an appropriate type.
However, because 13.2.5.5 behaves differently during JSON.parse, the same source
text can produce different results when evaluated as a PrimaryExpression rather
than as JSON. Furthermore, the Early Error for duplicate "__proto__" properties
in object literals, which likewise does not apply during JSON.parse, means that
not all texts accepted by JSON.parse are valid as a PrimaryExpression, despite
matching the grammar.
25.5.1.1 INTERNALIZEJSONPROPERTY ( HOLDER, NAME, REVIVER )
The abstract operation InternalizeJSONProperty takes arguments holder (an
Object), name (a String), and reviver (a function object) and returns either a
normal completion containing an ECMAScript language value or a throw completion.
Note 1
This algorithm intentionally does not throw an exception if either [[Delete]] or
CreateDataProperty return false.
It performs the following steps when called:
1. 1. 1. Let val be ? Get(holder, name).
2. 2. 2. If val is an Object, then
1. a. a. Let isArray be ? IsArray(val).
2. b. b. If isArray is true, then
1. i. i. Let len be ? LengthOfArrayLike(val).
2. ii. ii. Let I be 0.
3. iii. iii. Repeat, while I < len,
1. 1. 1. Let prop be ! ToString(𝔽(I)).
2. 2. 2. Let newElement be ? InternalizeJSONProperty(val, prop,
reviver).
3. 3. 3. If newElement is undefined, then
1. a. a. Perform ? val.[[Delete]](prop).
4. 4. 4. Else,
1. a. a. Perform ? CreateDataProperty(val, prop, newElement).
5. 5. 5. Set I to I + 1.
3. c. c. Else,
1. i. i. Let keys be ? EnumerableOwnProperties(val, key).
2. ii. ii. For each String P of keys, do
1. 1. 1. Let newElement be ? InternalizeJSONProperty(val, P, reviver).
2. 2. 2. If newElement is undefined, then
1. a. a. Perform ? val.[[Delete]](P).
3. 3. 3. Else,
1. a. a. Perform ? CreateDataProperty(val, P, newElement).
3. 3. 3. Return ? Call(reviver, holder, « name, val »).
It is not permitted for a conforming implementation of JSON.parse to extend the
JSON grammars. If an implementation wishes to support a modified or extended
JSON interchange format it must do so by defining a different parse function.
Note 2
In the case where there are duplicate name Strings within an object, lexically
preceding values for the same key shall be overwritten.
25.5.2 JSON.STRINGIFY ( VALUE [ , REPLACER [ , SPACE ] ] )
This function returns a String in UTF-16 encoded JSON format representing an
ECMAScript language value, or undefined. It can take three parameters. The value
parameter is an ECMAScript language value, which is usually an object or array,
although it can also be a String, Boolean, Number or null. The optional replacer
parameter is either a function that alters the way objects and arrays are
stringified, or an array of Strings and Numbers that acts as an inclusion list
for selecting the object properties that will be stringified. The optional space
parameter is a String or Number that allows the result to have white space
injected into it to improve human readability.
It performs the following steps when called:
1. 1. 1. Let stack be a new empty List.
2. 2. 2. Let indent be the empty String.
3. 3. 3. Let PropertyList be undefined.
4. 4. 4. Let ReplacerFunction be undefined.
5. 5. 5. If replacer is an Object, then
1. a. a. If IsCallable(replacer) is true, then
1. i. i. Set ReplacerFunction to replacer.
2. b. b. Else,
1. i. i. Let isArray be ? IsArray(replacer).
2. ii. ii. If isArray is true, then
1. 1. 1. Set PropertyList to a new empty List.
2. 2. 2. Let len be ? LengthOfArrayLike(replacer).
3. 3. 3. Let k be 0.
4. 4. 4. Repeat, while k < len,
1. a. a. Let prop be ! ToString(𝔽(k)).
2. b. b. Let v be ? Get(replacer, prop).
3. c. c. Let item be undefined.
4. d. d. If v is a String, set item to v.
5. e. e. Else if v is a Number, set item to ! ToString(v).
6. f. f. Else if v is an Object, then
1. i. i. If v has a [[StringData]] or [[NumberData]] internal
slot, set item to ? ToString(v).
7. g. g. If item is not undefined and PropertyList does not
contain item, then
1. i. i. Append item to PropertyList.
8. h. h. Set k to k + 1.
6. 6. 6. If space is an Object, then
1. a. a. If space has a [[NumberData]] internal slot, then
1. i. i. Set space to ? ToNumber(space).
2. b. b. Else if space has a [[StringData]] internal slot, then
1. i. i. Set space to ? ToString(space).
7. 7. 7. If space is a Number, then
1. a. a. Let spaceMV be ! ToIntegerOrInfinity(space).
2. b. b. Set spaceMV to min(10, spaceMV).
3. c. c. If spaceMV < 1, let gap be the empty String; otherwise let gap be
the String value containing spaceMV occurrences of the code unit 0x0020
(SPACE).
8. 8. 8. Else if space is a String, then
1. a. a. If the length of space ≤ 10, let gap be space; otherwise let gap
be the substring of space from 0 to 10.
9. 9. 9. Else,
1. a. a. Let gap be the empty String.
10. 10. 10. Let wrapper be OrdinaryObjectCreate(%Object.prototype%).
11. 11. 11. Perform ! CreateDataPropertyOrThrow(wrapper, the empty String,
value).
12. 12. 12. Let state be the JSON Serialization Record { [[ReplacerFunction]]:
ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap,
[[PropertyList]]: PropertyList }.
13. 13. 13. Return ? SerializeJSONProperty(state, the empty String, wrapper).
The "length" property of this function is 3𝔽.
Note 1
JSON structures are allowed to be nested to any depth, but they must be acyclic.
If value is or contains a cyclic structure, then this function must throw a
TypeError exception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
* The null value is rendered in JSON text as the String value "null".
* The undefined value is not rendered.
* The true value is rendered in JSON text as the String value "true".
* The false value is rendered in JSON text as the String value "false".
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units " and
\ are escaped with \ prefixes. Control characters code units are replaced with
escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM
FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by calling ToString(number). NaN and
Infinity regardless of sign are represented as the String value "null".
Note 5
Values that do not have a JSON representation (such as undefined and functions)
do not produce a String. Instead they produce the undefined value. In arrays
these values are represented as the String value "null". In objects an
unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more
properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY
BRACKET). A property is a quoted String representing the property name, a U+003A
(COLON), and then the stringified property value. An array is rendered as an
opening U+005B (LEFT SQUARE BRACKET) followed by zero or more values, separated
with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
25.5.2.1 JSON SERIALIZATION RECORD
A JSON Serialization Record is a Record value used to enable serialization to
the JSON format.
JSON Serialization Records have the fields listed in Table 69.
Table 69: JSON Serialization Record Fields
Field Name Value Meaning [[ReplacerFunction]] a function object or undefined A
function that can supply replacement values for object properties (from
JSON.stringify's replacer parameter). [[PropertyList]] either a List of Strings
or undefined The names of properties to include when serializing a non-array
object (from JSON.stringify's replacer parameter). [[Gap]] a String The unit of
indentation (from JSON.stringify's space parameter). [[Stack]] a List of Objects
The set of nested objects that are in the process of being serialized. Used to
detect cyclic structures. [[Indent]] a String The current indentation.
25.5.2.2 SERIALIZEJSONPROPERTY ( STATE, KEY, HOLDER )
The abstract operation SerializeJSONProperty takes arguments state (a JSON
Serialization Record), key (a String), and holder (an Object) and returns either
a normal completion containing either a String or undefined, or a throw
completion. It performs the following steps when called:
1. 1. 1. Let value be ? Get(holder, key).
2. 2. 2. If value is an Object or value is a BigInt, then
1. a. a. Let toJSON be ? GetV(value, "toJSON").
2. b. b. If IsCallable(toJSON) is true, then
1. i. i. Set value to ? Call(toJSON, value, « key »).
3. 3. 3. If state.[[ReplacerFunction]] is not undefined, then
1. a. a. Set value to ? Call(state.[[ReplacerFunction]], holder, « key,
value »).
4. 4. 4. If value is an Object, then
1. a. a. If value has a [[NumberData]] internal slot, then
1. i. i. Set value to ? ToNumber(value).
2. b. b. Else if value has a [[StringData]] internal slot, then
1. i. i. Set value to ? ToString(value).
3. c. c. Else if value has a [[BooleanData]] internal slot, then
1. i. i. Set value to value.[[BooleanData]].
4. d. d. Else if value has a [[BigIntData]] internal slot, then
1. i. i. Set value to value.[[BigIntData]].
5. 5. 5. If value is null, return "null".
6. 6. 6. If value is true, return "true".
7. 7. 7. If value is false, return "false".
8. 8. 8. If value is a String, return QuoteJSONString(value).
9. 9. 9. If value is a Number, then
1. a. a. If value is finite, return ! ToString(value).
2. b. b. Return "null".
10. 10. 10. If value is a BigInt, throw a TypeError exception.
11. 11. 11. If value is an Object and IsCallable(value) is false, then
1. a. a. Let isArray be ? IsArray(value).
2. b. b. If isArray is true, return ? SerializeJSONArray(state, value).
3. c. c. Return ? SerializeJSONObject(state, value).
12. 12. 12. Return undefined.
25.5.2.3 QUOTEJSONSTRING ( VALUE )
The abstract operation QuoteJSONString takes argument value (a String) and
returns a String. It wraps value in 0x0022 (QUOTATION MARK) code units and
escapes certain other code units within it. This operation interprets value as a
sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the
following steps when called:
1. 1. 1. Let product be the String value consisting solely of the code unit
0x0022 (QUOTATION MARK).
2. 2. 2. For each code point C of StringToCodePoints(value), do
1. a. a. If C is listed in the “Code Point” column of Table 70, then
1. i. i. Set product to the string-concatenation of product and the
escape sequence for C as specified in the “Escape Sequence” column of
the corresponding row.
2. b. b. Else if C has a numeric value less than 0x0020 (SPACE) or C has the
same numeric value as a leading surrogate or trailing surrogate, then
1. i. i. Let unit be the code unit whose numeric value is the numeric
value of C.
2. ii. ii. Set product to the string-concatenation of product and
UnicodeEscape(unit).
3. c. c. Else,
1. i. i. Set product to the string-concatenation of product and
UTF16EncodeCodePoint(C).
3. 3. 3. Set product to the string-concatenation of product and the code unit
0x0022 (QUOTATION MARK).
4. 4. 4. Return product.
Table 70: JSON Single Character Escape Sequences
Code Point Unicode Character Name Escape Sequence U+0008 BACKSPACE \b U+0009
CHARACTER TABULATION \t U+000A LINE FEED (LF) \n U+000C FORM FEED (FF) \f U+000D
CARRIAGE RETURN (CR) \r U+0022 QUOTATION MARK \" U+005C REVERSE SOLIDUS \\
25.5.2.4 UNICODEESCAPE ( C )
The abstract operation UnicodeEscape takes argument C (a code unit) and returns
a String. It represents C as a Unicode escape sequence. It performs the
following steps when called:
1. 1. 1. Let n be the numeric value of C.
2. 2. 2. Assert: n ≤ 0xFFFF.
3. 3. 3. Let hex be the String representation of n, formatted as a lowercase
hexadecimal number.
4. 4. 4. Return the string-concatenation of the code unit 0x005C (REVERSE
SOLIDUS), "u", and ! StringPad(hex, 4𝔽, "0", start).
25.5.2.5 SERIALIZEJSONOBJECT ( STATE, VALUE )
The abstract operation SerializeJSONObject takes arguments state (a JSON
Serialization Record) and value (an Object) and returns either a normal
completion containing a String or a throw completion. It serializes an object.
It performs the following steps when called:
1. 1. 1. If state.[[Stack]] contains value, throw a TypeError exception
because the structure is cyclical.
2. 2. 2. Append value to state.[[Stack]].
3. 3. 3. Let stepback be state.[[Indent]].
4. 4. 4. Set state.[[Indent]] to the string-concatenation of state.[[Indent]]
and state.[[Gap]].
5. 5. 5. If state.[[PropertyList]] is not undefined, then
1. a. a. Let K be state.[[PropertyList]].
6. 6. 6. Else,
1. a. a. Let K be ? EnumerableOwnProperties(value, key).
7. 7. 7. Let partial be a new empty List.
8. 8. 8. For each element P of K, do
1. a. a. Let strP be ? SerializeJSONProperty(state, P, value).
2. b. b. If strP is not undefined, then
1. i. i. Let member be QuoteJSONString(P).
2. ii. ii. Set member to the string-concatenation of member and ":".
3. iii. iii. If state.[[Gap]] is not the empty String, then
1. 1. 1. Set member to the string-concatenation of member and the
code unit 0x0020 (SPACE).
4. iv. iv. Set member to the string-concatenation of member and strP.
5. v. v. Append member to partial.
9. 9. 9. If partial is empty, then
1. a. a. Let final be "{}".
10. 10. 10. Else,
1. a. a. If state.[[Gap]] is the empty String, then
1. i. i. Let properties be the String value formed by concatenating all
the element Strings of partial with each adjacent pair of Strings
separated with the code unit 0x002C (COMMA). A comma is not inserted
either before the first String or after the last String.
2. ii. ii. Let final be the string-concatenation of "{", properties, and
"}".
2. b. b. Else,
1. i. i. Let separator be the string-concatenation of the code unit
0x002C (COMMA), the code unit 0x000A (LINE FEED), and
state.[[Indent]].
2. ii. ii. Let properties be the String value formed by concatenating
all the element Strings of partial with each adjacent pair of Strings
separated with separator. The separator String is not inserted either
before the first String or after the last String.
3. iii. iii. Let final be the string-concatenation of "{", the code unit
0x000A (LINE FEED), state.[[Indent]], properties, the code unit
0x000A (LINE FEED), stepback, and "}".
11. 11. 11. Remove the last element of state.[[Stack]].
12. 12. 12. Set state.[[Indent]] to stepback.
13. 13. 13. Return final.
25.5.2.6 SERIALIZEJSONARRAY ( STATE, VALUE )
The abstract operation SerializeJSONArray takes arguments state (a JSON
Serialization Record) and value (an ECMAScript language value) and returns
either a normal completion containing a String or a throw completion. It
serializes an array. It performs the following steps when called:
1. 1. 1. If state.[[Stack]] contains value, throw a TypeError exception
because the structure is cyclical.
2. 2. 2. Append value to state.[[Stack]].
3. 3. 3. Let stepback be state.[[Indent]].
4. 4. 4. Set state.[[Indent]] to the string-concatenation of state.[[Indent]]
and state.[[Gap]].
5. 5. 5. Let partial be a new empty List.
6. 6. 6. Let len be ? LengthOfArrayLike(value).
7. 7. 7. Let index be 0.
8. 8. 8. Repeat, while index < len,
1. a. a. Let strP be ? SerializeJSONProperty(state, ! ToString(𝔽(index)),
value).
2. b. b. If strP is undefined, then
1. i. i. Append "null" to partial.
3. c. c. Else,
1. i. i. Append strP to partial.
4. d. d. Set index to index + 1.
9. 9. 9. If partial is empty, then
1. a. a. Let final be "[]".
10. 10. 10. Else,
1. a. a. If state.[[Gap]] is the empty String, then
1. i. i. Let properties be the String value formed by concatenating all
the element Strings of partial with each adjacent pair of Strings
separated with the code unit 0x002C (COMMA). A comma is not inserted
either before the first String or after the last String.
2. ii. ii. Let final be the string-concatenation of "[", properties, and
"]".
2. b. b. Else,
1. i. i. Let separator be the string-concatenation of the code unit
0x002C (COMMA), the code unit 0x000A (LINE FEED), and
state.[[Indent]].
2. ii. ii. Let properties be the String value formed by concatenating
all the element Strings of partial with each adjacent pair of Strings
separated with separator. The separator String is not inserted either
before the first String or after the last String.
3. iii. iii. Let final be the string-concatenation of "[", the code unit
0x000A (LINE FEED), state.[[Indent]], properties, the code unit
0x000A (LINE FEED), stepback, and "]".
11. 11. 11. Remove the last element of state.[[Stack]].
12. 12. 12. Set state.[[Indent]] to stepback.
13. 13. 13. Return final.
Note
The representation of arrays includes only the elements in the interval from
+0𝔽 (inclusive) to array.length (exclusive). Properties whose keys are not
array indices are excluded from the stringification. An array is stringified as
an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT
SQUARE BRACKET.
25.5.3 JSON [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "JSON".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
26 MANAGING MEMORY
26.1 WEAKREF OBJECTS
A WeakRef is an object that is used to refer to a target object or symbol
without preserving it from garbage collection. WeakRefs can be dereferenced to
allow access to the target value, if the target hasn't been reclaimed by garbage
collection.
26.1.1 THE WEAKREF CONSTRUCTOR
The WeakRef constructor:
* is %WeakRef%.
* is the initial value of the "WeakRef" property of the global object.
* creates and initializes a new WeakRef when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified WeakRef behaviour must
include a super call to the WeakRef constructor to create and initialize the
subclass instance with the internal state necessary to support the
WeakRef.prototype built-in methods.
26.1.1.1 WEAKREF ( TARGET )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
3. 3. 3. Let weakRef be ? OrdinaryCreateFromConstructor(NewTarget,
"%WeakRef.prototype%", « [[WeakRefTarget]] »).
4. 4. 4. Perform AddToKeptObjects(target).
5. 5. 5. Set weakRef.[[WeakRefTarget]] to target.
6. 6. 6. Return weakRef.
26.1.2 PROPERTIES OF THE WEAKREF CONSTRUCTOR
The WeakRef constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
26.1.2.1 WEAKREF.PROTOTYPE
The initial value of WeakRef.prototype is the WeakRef prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
26.1.3 PROPERTIES OF THE WEAKREF PROTOTYPE OBJECT
The WeakRef prototype object:
* is %WeakRef.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[WeakRefTarget]] internal slot.
Normative Optional
26.1.3.1 WEAKREF.PROTOTYPE.CONSTRUCTOR
The initial value of WeakRef.prototype.constructor is %WeakRef%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
26.1.3.2 WEAKREF.PROTOTYPE.DEREF ( )
This method performs the following steps when called:
1. 1. 1. Let weakRef be the this value.
2. 2. 2. Perform ? RequireInternalSlot(weakRef, [[WeakRefTarget]]).
3. 3. 3. Return WeakRefDeref(weakRef).
Note
If the WeakRef returns a target value that is not undefined, then this target
value should not be garbage collected until the current execution of ECMAScript
code has completed. The AddToKeptObjects operation makes sure read consistency
is maintained.
let target = { foo() {} };
let weakRef = new WeakRef(target);
// ... later ...
if (weakRef.deref()) {
weakRef.deref().foo();
}
In the above example, if the first deref does not evaluate to undefined then the
second deref cannot either.
26.1.3.3 WEAKREF.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "WeakRef".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
26.1.4 WEAKREF ABSTRACT OPERATIONS
26.1.4.1 WEAKREFDEREF ( WEAKREF )
The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and
returns an ECMAScript language value. It performs the following steps when
called:
1. 1. 1. Let target be weakRef.[[WeakRefTarget]].
2. 2. 2. If target is not empty, then
1. a. a. Perform AddToKeptObjects(target).
2. b. b. Return target.
3. 3. 3. Return undefined.
Note
This abstract operation is defined separately from WeakRef.prototype.deref
strictly to make it possible to succinctly define liveness.
26.1.5 PROPERTIES OF WEAKREF INSTANCES
WeakRef instances are ordinary objects that inherit properties from the WeakRef
prototype. WeakRef instances also have a [[WeakRefTarget]] internal slot.
26.2 FINALIZATIONREGISTRY OBJECTS
A FinalizationRegistry is an object that manages registration and unregistration
of cleanup operations that are performed when target objects and symbols are
garbage collected.
26.2.1 THE FINALIZATIONREGISTRY CONSTRUCTOR
The FinalizationRegistry constructor:
* is %FinalizationRegistry%.
* is the initial value of the "FinalizationRegistry" property of the global
object.
* creates and initializes a new FinalizationRegistry when called as a
constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified FinalizationRegistry
behaviour must include a super call to the FinalizationRegistry constructor
to create and initialize the subclass instance with the internal state
necessary to support the FinalizationRegistry.prototype built-in methods.
26.2.1.1 FINALIZATIONREGISTRY ( CLEANUPCALLBACK )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. If IsCallable(cleanupCallback) is false, throw a TypeError exception.
3. 3. 3. Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget,
"%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]],
[[Cells]] »).
4. 4. 4. Let fn be the active function object.
5. 5. 5. Set finalizationRegistry.[[Realm]] to fn.[[Realm]].
6. 6. 6. Set finalizationRegistry.[[CleanupCallback]] to
HostMakeJobCallback(cleanupCallback).
7. 7. 7. Set finalizationRegistry.[[Cells]] to a new empty List.
8. 8. 8. Return finalizationRegistry.
26.2.2 PROPERTIES OF THE FINALIZATIONREGISTRY CONSTRUCTOR
The FinalizationRegistry constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
26.2.2.1 FINALIZATIONREGISTRY.PROTOTYPE
The initial value of FinalizationRegistry.prototype is the FinalizationRegistry
prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
26.2.3 PROPERTIES OF THE FINALIZATIONREGISTRY PROTOTYPE OBJECT
The FinalizationRegistry prototype object:
* is %FinalizationRegistry.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have [[Cells]] and [[CleanupCallback]] internal slots.
26.2.3.1 FINALIZATIONREGISTRY.PROTOTYPE.CONSTRUCTOR
The initial value of FinalizationRegistry.prototype.constructor is
%FinalizationRegistry%.
26.2.3.2 FINALIZATIONREGISTRY.PROTOTYPE.REGISTER ( TARGET, HELDVALUE [ ,
UNREGISTERTOKEN ] )
This method performs the following steps when called:
1. 1. 1. Let finalizationRegistry be the this value.
2. 2. 2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
3. 3. 3. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
4. 4. 4. If SameValue(target, heldValue) is true, throw a TypeError exception.
5. 5. 5. If CanBeHeldWeakly(unregisterToken) is false, then
1. a. a. If unregisterToken is not undefined, throw a TypeError exception.
2. b. b. Set unregisterToken to empty.
6. 6. 6. Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]:
heldValue, [[UnregisterToken]]: unregisterToken }.
7. 7. 7. Append cell to finalizationRegistry.[[Cells]].
8. 8. 8. Return undefined.
Note
Based on the algorithms and definitions in this specification,
cell.[[HeldValue]] is live when finalizationRegistry.[[Cells]] contains cell;
however, this does not necessarily mean that cell.[[UnregisterToken]] or
cell.[[Target]] are live. For example, registering an object with itself as its
unregister token would not keep the object alive forever.
26.2.3.3 FINALIZATIONREGISTRY.PROTOTYPE.UNREGISTER ( UNREGISTERTOKEN )
This method performs the following steps when called:
1. 1. 1. Let finalizationRegistry be the this value.
2. 2. 2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
3. 3. 3. If CanBeHeldWeakly(unregisterToken) is false, throw a TypeError
exception.
4. 4. 4. Let removed be false.
5. 5. 5. For each Record { [[WeakRefTarget]], [[HeldValue]],
[[UnregisterToken]] } cell of finalizationRegistry.[[Cells]], do
1. a. a. If cell.[[UnregisterToken]] is not empty and
SameValue(cell.[[UnregisterToken]], unregisterToken) is true, then
1. i. i. Remove cell from finalizationRegistry.[[Cells]].
2. ii. ii. Set removed to true.
6. 6. 6. Return removed.
26.2.3.4 FINALIZATIONREGISTRY.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"FinalizationRegistry".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
26.2.4 PROPERTIES OF FINALIZATIONREGISTRY INSTANCES
FinalizationRegistry instances are ordinary objects that inherit properties from
the FinalizationRegistry prototype. FinalizationRegistry instances also have
[[Cells]] and [[CleanupCallback]] internal slots.
27 CONTROL ABSTRACTION OBJECTS
27.1 ITERATION
27.1.1 COMMON ITERATION INTERFACES
An interface is a set of property keys whose associated values match a specific
specification. Any object that provides all the properties as described by an
interface's specification conforms to that interface. An interface is not
represented by a distinct object. There may be many separately implemented
objects that conform to any interface. An individual object may conform to
multiple interfaces.
27.1.1.1 THE ITERABLE INTERFACE
The Iterable interface includes the property described in Table 71:
Table 71: Iterable Interface Required Properties
Property Value Requirements @@iterator a function that returns an Iterator
object The returned object must conform to the Iterator interface.
27.1.1.2 THE ITERATOR INTERFACE
An object that implements the Iterator interface must include the property in
Table 72. Such objects may also implement the properties in Table 73.
Table 72: Iterator Interface Required Properties
Property Value Requirements "next" a function that returns an IteratorResult
object The returned object must conform to the IteratorResult interface. If a
previous call to the next method of an Iterator has returned an IteratorResult
object whose "done" property is true, then all subsequent calls to the next
method of that object should also return an IteratorResult object whose "done"
property is true. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and
validity is dependent upon the target Iterator. The for-of statement and other
common users of Iterators do not pass any arguments, so Iterator objects that
expect to be used in such a manner must be prepared to deal with being called
with no arguments.
Table 73: Iterator Interface Optional Properties
Property Value Requirements "return" a function that returns an IteratorResult
object The returned object must conform to the IteratorResult interface.
Invoking this method notifies the Iterator object that the caller does not
intend to make any more next method calls to the Iterator. The returned
IteratorResult object will typically have a "done" property whose value is true,
and a "value" property with the value passed as the argument of the return
method. However, this requirement is not enforced. "throw" a function that
returns an IteratorResult object The returned object must conform to the
IteratorResult interface. Invoking this method notifies the Iterator object that
the caller has detected an error condition. The argument may be used to identify
the error condition and typically will be an exception object. A typical
response is to throw the value passed as the argument. If the method does not
throw, the returned IteratorResult object will typically have a "done" property
whose value is true.
Note 2
Typically callers of these methods should check for their existence before
invoking them. Certain ECMAScript language features including for-of, yield*,
and array destructuring call these methods after performing an existence check.
Most ECMAScript library functions that accept Iterable objects as arguments also
conditionally call them.
27.1.1.3 THE ASYNCITERABLE INTERFACE
The AsyncIterable interface includes the properties described in Table 74:
Table 74: AsyncIterable Interface Required Properties
Property Value Requirements @@asyncIterator a function that returns an
AsyncIterator object The returned object must conform to the AsyncIterator
interface.
27.1.1.4 THE ASYNCITERATOR INTERFACE
An object that implements the AsyncIterator interface must include the
properties in Table 75. Such objects may also implement the properties in Table
76.
Table 75: AsyncIterator Interface Required Properties
Property Value Requirements "next" a function that returns a promise for an
IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms
to the IteratorResult interface. If a previous call to the next method of an
AsyncIterator has returned a promise for an IteratorResult object whose "done"
property is true, then all subsequent calls to the next method of that object
should also return a promise for an IteratorResult object whose "done" property
is true. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value
should have a "value" property whose value is not a promise (or "thenable").
However, this requirement is also not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and
validity is dependent upon the target AsyncIterator. The for-await-of statement
and other common users of AsyncIterators do not pass any arguments, so
AsyncIterator objects that expect to be used in such a manner must be prepared
to deal with being called with no arguments.
Table 76: AsyncIterator Interface Optional Properties
Property Value Requirements "return" a function that returns a promise for an
IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms
to the IteratorResult interface. Invoking this method notifies the AsyncIterator
object that the caller does not intend to make any more next method calls to the
AsyncIterator. The returned promise will fulfill with an IteratorResult object
which will typically have a "done" property whose value is true, and a "value"
property with the value passed as the argument of the return method. However,
this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value
should have a "value" property whose value is not a promise (or "thenable"). If
the argument value is used in the typical manner, then if it is a rejected
promise, a promise rejected with the same reason should be returned; if it is a
fulfilled promise, then its fulfillment value should be used as the "value"
property of the returned promise's IteratorResult object fulfillment value.
However, these requirements are also not enforced.
"throw" a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms
to the IteratorResult interface. Invoking this method notifies the AsyncIterator
object that the caller has detected an error condition. The argument may be used
to identify the error condition and typically will be an exception object. A
typical response is to return a rejected promise which rejects with the value
passed as the argument.
If the returned promise is fulfilled, the IteratorResult fulfillment value will
typically have a "done" property whose value is true. Additionally, it should
have a "value" property whose value is not a promise (or "thenable"), but this
requirement is not enforced.
Note 2
Typically callers of these methods should check for their existence before
invoking them. Certain ECMAScript language features including for-await-of and
yield* call these methods after performing an existence check.
27.1.1.5 THE ITERATORRESULT INTERFACE
The IteratorResult interface includes the properties listed in Table 77:
Table 77: IteratorResult Interface Properties
Property Value Requirements "done" a Boolean This is the result status of an
iterator next method call. If the end of the iterator was reached "done" is
true. If the end was not reached "done" is false and a value is available. If a
"done" property (either own or inherited) does not exist, it is considered to
have the value false. "value" an ECMAScript language value If done is false,
this is the current iteration element value. If done is true, this is the return
value of the iterator, if it supplied one. If the iterator does not have a
return value, "value" is undefined. In that case, the "value" property may be
absent from the conforming object if it does not inherit an explicit "value"
property.
27.1.2 THE %ITERATORPROTOTYPE% OBJECT
The %IteratorPrototype% object:
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
Note
All objects defined in this specification that implement the Iterator interface
also inherit from %IteratorPrototype%. ECMAScript code may also define objects
that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a
place where additional methods that are applicable to all iterator objects may
be added.
The following expression is one way that ECMAScript code can access the
%IteratorPrototype% object:
Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))
27.1.2.1 %ITERATORPROTOTYPE% [ @@ITERATOR ] ( )
This function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "[Symbol.iterator]".
27.1.3 THE %ASYNCITERATORPROTOTYPE% OBJECT
The %AsyncIteratorPrototype% object:
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
Note
All objects defined in this specification that implement the AsyncIterator
interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also
define objects that inherit from %AsyncIteratorPrototype%. The
%AsyncIteratorPrototype% object provides a place where additional methods that
are applicable to all async iterator objects may be added.
27.1.3.1 %ASYNCITERATORPROTOTYPE% [ @@ASYNCITERATOR ] ( )
This function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "[Symbol.asyncIterator]".
27.1.4 ASYNC-FROM-SYNC ITERATOR OBJECTS
An Async-from-Sync Iterator object is an async iterator that adapts a specific
synchronous iterator. There is not a named constructor for Async-from-Sync
Iterator objects. Instead, Async-from-Sync iterator objects are created by the
CreateAsyncFromSyncIterator abstract operation as needed.
27.1.4.1 CREATEASYNCFROMSYNCITERATOR ( SYNCITERATORRECORD )
The abstract operation CreateAsyncFromSyncIterator takes argument
syncIteratorRecord (an Iterator Record) and returns an Iterator Record. It is
used to create an async Iterator Record from a synchronous Iterator Record. It
performs the following steps when called:
1. 1. 1. Let asyncIterator be
OrdinaryObjectCreate(%AsyncFromSyncIteratorPrototype%, «
[[SyncIteratorRecord]] »).
2. 2. 2. Set asyncIterator.[[SyncIteratorRecord]] to syncIteratorRecord.
3. 3. 3. Let nextMethod be ! Get(asyncIterator, "next").
4. 4. 4. Let iteratorRecord be the Iterator Record { [[Iterator]]:
asyncIterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
5. 5. 5. Return iteratorRecord.
27.1.4.2 THE %ASYNCFROMSYNCITERATORPROTOTYPE% OBJECT
The %AsyncFromSyncIteratorPrototype% object:
* has properties that are inherited by all Async-from-Sync Iterator Objects.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %AsyncIteratorPrototype%.
* has the following properties:
27.1.4.2.1 %ASYNCFROMSYNCITERATORPROTOTYPE%.NEXT ( [ VALUE ] )
1. 1. 1. Let O be the this value.
2. 2. 2. Assert: O is an Object that has a [[SyncIteratorRecord]] internal
slot.
3. 3. 3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
4. 4. 4. Let syncIteratorRecord be O.[[SyncIteratorRecord]].
5. 5. 5. If value is present, then
1. a. a. Let result be Completion(IteratorNext(syncIteratorRecord, value)).
6. 6. 6. Else,
1. a. a. Let result be Completion(IteratorNext(syncIteratorRecord)).
7. 7. 7. IfAbruptRejectPromise(result, promiseCapability).
8. 8. 8. Return AsyncFromSyncIteratorContinuation(result, promiseCapability).
27.1.4.2.2 %ASYNCFROMSYNCITERATORPROTOTYPE%.RETURN ( [ VALUE ] )
1. 1. 1. Let O be the this value.
2. 2. 2. Assert: O is an Object that has a [[SyncIteratorRecord]] internal
slot.
3. 3. 3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
4. 4. 4. Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
5. 5. 5. Let return be Completion(GetMethod(syncIterator, "return")).
6. 6. 6. IfAbruptRejectPromise(return, promiseCapability).
7. 7. 7. If return is undefined, then
1. a. a. Let iterResult be CreateIterResultObject(value, true).
2. b. b. Perform ! Call(promiseCapability.[[Resolve]], undefined, «
iterResult »).
3. c. c. Return promiseCapability.[[Promise]].
8. 8. 8. If value is present, then
1. a. a. Let result be Completion(Call(return, syncIterator, « value »)).
9. 9. 9. Else,
1. a. a. Let result be Completion(Call(return, syncIterator)).
10. 10. 10. IfAbruptRejectPromise(result, promiseCapability).
11. 11. 11. If result is not an Object, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly
created TypeError object »).
2. b. b. Return promiseCapability.[[Promise]].
12. 12. 12. Return AsyncFromSyncIteratorContinuation(result,
promiseCapability).
27.1.4.2.3 %ASYNCFROMSYNCITERATORPROTOTYPE%.THROW ( [ VALUE ] )
Note
In this specification, value is always provided, but is left optional for
consistency with %AsyncFromSyncIteratorPrototype%.return ( [ value ] ).
1. 1. 1. Let O be the this value.
2. 2. 2. Assert: O is an Object that has a [[SyncIteratorRecord]] internal
slot.
3. 3. 3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
4. 4. 4. Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
5. 5. 5. Let throw be Completion(GetMethod(syncIterator, "throw")).
6. 6. 6. IfAbruptRejectPromise(throw, promiseCapability).
7. 7. 7. If throw is undefined, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, « value
»).
2. b. b. Return promiseCapability.[[Promise]].
8. 8. 8. If value is present, then
1. a. a. Let result be Completion(Call(throw, syncIterator, « value »)).
9. 9. 9. Else,
1. a. a. Let result be Completion(Call(throw, syncIterator)).
10. 10. 10. IfAbruptRejectPromise(result, promiseCapability).
11. 11. 11. If result is not an Object, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly
created TypeError object »).
2. b. b. Return promiseCapability.[[Promise]].
12. 12. 12. Return AsyncFromSyncIteratorContinuation(result,
promiseCapability).
27.1.4.3 PROPERTIES OF ASYNC-FROM-SYNC ITERATOR INSTANCES
Async-from-Sync Iterator instances are ordinary objects that inherit properties
from the %AsyncFromSyncIteratorPrototype% intrinsic object. Async-from-Sync
Iterator instances are initially created with the internal slots listed in Table
78. Async-from-Sync Iterator instances are not directly observable from
ECMAScript code.
Table 78: Internal Slots of Async-from-Sync Iterator Instances
Internal Slot Type Description [[SyncIteratorRecord]] an Iterator Record
Represents the original synchronous iterator which is being adapted.
27.1.4.4 ASYNCFROMSYNCITERATORCONTINUATION ( RESULT, PROMISECAPABILITY )
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result
(an Object) and promiseCapability (a PromiseCapability Record for an intrinsic
%Promise%) and returns a Promise. It performs the following steps when called:
1. 1. 1. NOTE: Because promiseCapability is derived from the intrinsic
%Promise%, the calls to promiseCapability.[[Reject]] entailed by the use
IfAbruptRejectPromise below are guaranteed not to throw.
2. 2. 2. Let done be Completion(IteratorComplete(result)).
3. 3. 3. IfAbruptRejectPromise(done, promiseCapability).
4. 4. 4. Let value be Completion(IteratorValue(result)).
5. 5. 5. IfAbruptRejectPromise(value, promiseCapability).
6. 6. 6. Let valueWrapper be Completion(PromiseResolve(%Promise%, value)).
7. 7. 7. IfAbruptRejectPromise(valueWrapper, promiseCapability).
8. 8. 8. Let unwrap be a new Abstract Closure with parameters (v) that
captures done and performs the following steps when called:
1. a. a. Return CreateIterResultObject(v, done).
9. 9. 9. Let onFulfilled be CreateBuiltinFunction(unwrap, 1, "", « »).
10. 10. 10. NOTE: onFulfilled is used when processing the "value" property of
an IteratorResult object in order to wait for its value if it is a promise
and re-package the result in a new "unwrapped" IteratorResult object.
11. 11. 11. Perform PerformPromiseThen(valueWrapper, onFulfilled, undefined,
promiseCapability).
12. 12. 12. Return promiseCapability.[[Promise]].
27.2 PROMISE OBJECTS
A Promise is an object that is used as a placeholder for the eventual results of
a deferred (and possibly asynchronous) computation.
Any Promise is in one of three mutually exclusive states: fulfilled, rejected,
and pending:
* A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to
call the function f.
* A promise p is rejected if p.then(f, r) will immediately enqueue a Job to
call the function r.
* A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either
fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match
the state of another promise. Attempting to resolve or reject a resolved promise
has no effect. A promise is unresolved if it is not resolved. An unresolved
promise is always in the pending state. A resolved promise may be pending,
fulfilled or rejected.
27.2.1 PROMISE ABSTRACT OPERATIONS
27.2.1.1 PROMISECAPABILITY RECORDS
A PromiseCapability Record is a Record value used to encapsulate a Promise or
promise-like object along with the functions that are capable of resolving or
rejecting that promise. PromiseCapability Records are produced by the
NewPromiseCapability abstract operation.
PromiseCapability Records have the fields listed in Table 79.
Table 79: PromiseCapability Record Fields
Field Name Value Meaning [[Promise]] an Object An object that is usable as a
promise. [[Resolve]] a function object The function that is used to resolve the
given promise. [[Reject]] a function object The function that is used to reject
the given promise.
27.2.1.1.1 IFABRUPTREJECTPROMISE ( VALUE, CAPABILITY )
IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use
a PromiseCapability Record. An algorithm step of the form:
1. 1. 1. IfAbruptRejectPromise(value, capability).
means the same thing as:
1. 1. 1. Assert: value is a Completion Record.
2. 2. 2. If value is an abrupt completion, then
1. a. a. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]]
»).
2. b. b. Return capability.[[Promise]].
3. 3. 3. Else, set value to value.[[Value]].
27.2.1.2 PROMISEREACTION RECORDS
The PromiseReaction is a Record value used to store information about how a
promise should react when it becomes resolved or rejected with a given value.
PromiseReaction records are created by the PerformPromiseThen abstract
operation, and are used by the Abstract Closure returned by
NewPromiseReactionJob.
PromiseReaction records have the fields listed in Table 80.
Table 80: PromiseReaction Record Fields
Field Name Value Meaning [[Capability]] a PromiseCapability Record or undefined
The capabilities of the promise for which this record provides a reaction
handler. [[Type]] Fulfill or Reject The [[Type]] is used when [[Handler]] is
empty to allow for behaviour specific to the settlement type. [[Handler]] a
JobCallback Record or empty The function that should be applied to the incoming
value, and whose return value will govern what happens to the derived promise.
If [[Handler]] is empty, a function that depends on the value of [[Type]] will
be used instead.
27.2.1.3 CREATERESOLVINGFUNCTIONS ( PROMISE )
The abstract operation CreateResolvingFunctions takes argument promise (a
Promise) and returns a Record with fields [[Resolve]] (a function object) and
[[Reject]] (a function object). It performs the following steps when called:
1. 1. 1. Let alreadyResolved be the Record { [[Value]]: false }.
2. 2. 2. Let stepsResolve be the algorithm steps defined in Promise Resolve
Functions.
3. 3. 3. Let lengthResolve be the number of non-optional parameters of the
function definition in Promise Resolve Functions.
4. 4. 4. Let resolve be CreateBuiltinFunction(stepsResolve, lengthResolve, "",
« [[Promise]], [[AlreadyResolved]] »).
5. 5. 5. Set resolve.[[Promise]] to promise.
6. 6. 6. Set resolve.[[AlreadyResolved]] to alreadyResolved.
7. 7. 7. Let stepsReject be the algorithm steps defined in Promise Reject
Functions.
8. 8. 8. Let lengthReject be the number of non-optional parameters of the
function definition in Promise Reject Functions.
9. 9. 9. Let reject be CreateBuiltinFunction(stepsReject, lengthReject, "", «
[[Promise]], [[AlreadyResolved]] »).
10. 10. 10. Set reject.[[Promise]] to promise.
11. 11. 11. Set reject.[[AlreadyResolved]] to alreadyResolved.
12. 12. 12. Return the Record { [[Resolve]]: resolve, [[Reject]]: reject }.
27.2.1.3.1 PROMISE REJECT FUNCTIONS
A promise reject function is an anonymous built-in function that has [[Promise]]
and [[AlreadyResolved]] internal slots.
When a promise reject function is called with argument reason, the following
steps are taken:
1. 1. 1. Let F be the active function object.
2. 2. 2. Assert: F has a [[Promise]] internal slot whose value is an Object.
3. 3. 3. Let promise be F.[[Promise]].
4. 4. 4. Let alreadyResolved be F.[[AlreadyResolved]].
5. 5. 5. If alreadyResolved.[[Value]] is true, return undefined.
6. 6. 6. Set alreadyResolved.[[Value]] to true.
7. 7. 7. Perform RejectPromise(promise, reason).
8. 8. 8. Return undefined.
The "length" property of a promise reject function is 1𝔽.
27.2.1.3.2 PROMISE RESOLVE FUNCTIONS
A promise resolve function is an anonymous built-in function that has
[[Promise]] and [[AlreadyResolved]] internal slots.
When a promise resolve function is called with argument resolution, the
following steps are taken:
1. 1. 1. Let F be the active function object.
2. 2. 2. Assert: F has a [[Promise]] internal slot whose value is an Object.
3. 3. 3. Let promise be F.[[Promise]].
4. 4. 4. Let alreadyResolved be F.[[AlreadyResolved]].
5. 5. 5. If alreadyResolved.[[Value]] is true, return undefined.
6. 6. 6. Set alreadyResolved.[[Value]] to true.
7. 7. 7. If SameValue(resolution, promise) is true, then
1. a. a. Let selfResolutionError be a newly created TypeError object.
2. b. b. Perform RejectPromise(promise, selfResolutionError).
3. c. c. Return undefined.
8. 8. 8. If resolution is not an Object, then
1. a. a. Perform FulfillPromise(promise, resolution).
2. b. b. Return undefined.
9. 9. 9. Let then be Completion(Get(resolution, "then")).
10. 10. 10. If then is an abrupt completion, then
1. a. a. Perform RejectPromise(promise, then.[[Value]]).
2. b. b. Return undefined.
11. 11. 11. Let thenAction be then.[[Value]].
12. 12. 12. If IsCallable(thenAction) is false, then
1. a. a. Perform FulfillPromise(promise, resolution).
2. b. b. Return undefined.
13. 13. 13. Let thenJobCallback be HostMakeJobCallback(thenAction).
14. 14. 14. Let job be NewPromiseResolveThenableJob(promise, resolution,
thenJobCallback).
15. 15. 15. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
16. 16. 16. Return undefined.
The "length" property of a promise resolve function is 1𝔽.
27.2.1.4 FULFILLPROMISE ( PROMISE, VALUE )
The abstract operation FulfillPromise takes arguments promise (a Promise) and
value (an ECMAScript language value) and returns unused. It performs the
following steps when called:
1. 1. 1. Assert: The value of promise.[[PromiseState]] is pending.
2. 2. 2. Let reactions be promise.[[PromiseFulfillReactions]].
3. 3. 3. Set promise.[[PromiseResult]] to value.
4. 4. 4. Set promise.[[PromiseFulfillReactions]] to undefined.
5. 5. 5. Set promise.[[PromiseRejectReactions]] to undefined.
6. 6. 6. Set promise.[[PromiseState]] to fulfilled.
7. 7. 7. Perform TriggerPromiseReactions(reactions, value).
8. 8. 8. Return unused.
27.2.1.5 NEWPROMISECAPABILITY ( C )
The abstract operation NewPromiseCapability takes argument C (an ECMAScript
language value) and returns either a normal completion containing a
PromiseCapability Record or a throw completion. It attempts to use C as a
constructor in the fashion of the built-in Promise constructor to create a
promise and extract its resolve and reject functions. The promise plus the
resolve and reject functions are used to initialize a new PromiseCapability
Record. It performs the following steps when called:
1. 1. 1. If IsConstructor(C) is false, throw a TypeError exception.
2. 2. 2. NOTE: C is assumed to be a constructor function that supports the
parameter conventions of the Promise constructor (see 27.2.3.1).
3. 3. 3. Let resolvingFunctions be the Record { [[Resolve]]: undefined,
[[Reject]]: undefined }.
4. 4. 4. Let executorClosure be a new Abstract Closure with parameters
(resolve, reject) that captures resolvingFunctions and performs the
following steps when called:
1. a. a. If resolvingFunctions.[[Resolve]] is not undefined, throw a
TypeError exception.
2. b. b. If resolvingFunctions.[[Reject]] is not undefined, throw a
TypeError exception.
3. c. c. Set resolvingFunctions.[[Resolve]] to resolve.
4. d. d. Set resolvingFunctions.[[Reject]] to reject.
5. e. e. Return undefined.
5. 5. 5. Let executor be CreateBuiltinFunction(executorClosure, 2, "", « »).
6. 6. 6. Let promise be ? Construct(C, « executor »).
7. 7. 7. If IsCallable(resolvingFunctions.[[Resolve]]) is false, throw a
TypeError exception.
8. 8. 8. If IsCallable(resolvingFunctions.[[Reject]]) is false, throw a
TypeError exception.
9. 9. 9. Return the PromiseCapability Record { [[Promise]]: promise,
[[Resolve]]: resolvingFunctions.[[Resolve]], [[Reject]]:
resolvingFunctions.[[Reject]] }.
Note
This abstract operation supports Promise subclassing, as it is generic on any
constructor that calls a passed executor function argument in the same way as
the Promise constructor. It is used to generalize static methods of the Promise
constructor to any subclass.
27.2.1.6 ISPROMISE ( X )
The abstract operation IsPromise takes argument x (an ECMAScript language value)
and returns a Boolean. It checks for the promise brand on an object. It performs
the following steps when called:
1. 1. 1. If x is not an Object, return false.
2. 2. 2. If x does not have a [[PromiseState]] internal slot, return false.
3. 3. 3. Return true.
27.2.1.7 REJECTPROMISE ( PROMISE, REASON )
The abstract operation RejectPromise takes arguments promise (a Promise) and
reason (an ECMAScript language value) and returns unused. It performs the
following steps when called:
1. 1. 1. Assert: The value of promise.[[PromiseState]] is pending.
2. 2. 2. Let reactions be promise.[[PromiseRejectReactions]].
3. 3. 3. Set promise.[[PromiseResult]] to reason.
4. 4. 4. Set promise.[[PromiseFulfillReactions]] to undefined.
5. 5. 5. Set promise.[[PromiseRejectReactions]] to undefined.
6. 6. 6. Set promise.[[PromiseState]] to rejected.
7. 7. 7. If promise.[[PromiseIsHandled]] is false, perform
HostPromiseRejectionTracker(promise, "reject").
8. 8. 8. Perform TriggerPromiseReactions(reactions, reason).
9. 9. 9. Return unused.
27.2.1.8 TRIGGERPROMISEREACTIONS ( REACTIONS, ARGUMENT )
The abstract operation TriggerPromiseReactions takes arguments reactions (a List
of PromiseReaction Records) and argument (an ECMAScript language value) and
returns unused. It enqueues a new Job for each record in reactions. Each such
Job processes the [[Type]] and [[Handler]] of the PromiseReaction Record, and if
the [[Handler]] is not empty, calls it passing the given argument. If the
[[Handler]] is empty, the behaviour is determined by the [[Type]]. It performs
the following steps when called:
1. 1. 1. For each element reaction of reactions, do
1. a. a. Let job be NewPromiseReactionJob(reaction, argument).
2. b. b. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
2. 2. 2. Return unused.
27.2.1.9 HOSTPROMISEREJECTIONTRACKER ( PROMISE, OPERATION )
The host-defined abstract operation HostPromiseRejectionTracker takes arguments
promise (a Promise) and operation ("reject" or "handle") and returns unused. It
allows host environments to track promise rejections.
An implementation of HostPromiseRejectionTracker must conform to the following
requirements:
* It must complete normally (i.e. not return an abrupt completion).
The default implementation of HostPromiseRejectionTracker is to return unused.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
* When a promise is rejected without any handlers, it is called with its
operation argument set to "reject".
* When a handler is added to a rejected promise for the first time, it is
called with its operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify
developers of unhandled rejections, while also being careful to notify them if
such previous notifications are later invalidated by new handlers being
attached.
Note 2
If operation is "handle", an implementation should not hold a reference to
promise in a way that would interfere with garbage collection. An implementation
may hold a reference to promise if operation is "reject", since it is expected
that rejections will be rare and not on hot code paths.
27.2.2 PROMISE JOBS
27.2.2.1 NEWPROMISEREACTIONJOB ( REACTION, ARGUMENT )
The abstract operation NewPromiseReactionJob takes arguments reaction (a
PromiseReaction Record) and argument (an ECMAScript language value) and returns
a Record with fields [[Job]] (a Job Abstract Closure) and [[Realm]] (a Realm
Record or null). It returns a new Job Abstract Closure that applies the
appropriate handler to the incoming value, and uses the handler's return value
to resolve or reject the derived promise associated with that handler. It
performs the following steps when called:
1. 1. 1. Let job be a new Job Abstract Closure with no parameters that captures
reaction and argument and performs the following steps when called:
1. a. a. Let promiseCapability be reaction.[[Capability]].
2. b. b. Let type be reaction.[[Type]].
3. c. c. Let handler be reaction.[[Handler]].
4. d. d. If handler is empty, then
1. i. i. If type is Fulfill, let handlerResult be
NormalCompletion(argument).
2. ii. ii. Else,
1. 1. 1. Assert: type is Reject.
2. 2. 2. Let handlerResult be ThrowCompletion(argument).
5. e. e. Else, let handlerResult be Completion(HostCallJobCallback(handler,
undefined, « argument »)).
6. f. f. If promiseCapability is undefined, then
1. i. i. Assert: handlerResult is not an abrupt completion.
2. ii. ii. Return empty.
7. g. g. Assert: promiseCapability is a PromiseCapability Record.
8. h. h. If handlerResult is an abrupt completion, then
1. i. i. Return ? Call(promiseCapability.[[Reject]], undefined, «
handlerResult.[[Value]] »).
9. i. i. Else,
1. i. i. Return ? Call(promiseCapability.[[Resolve]], undefined, «
handlerResult.[[Value]] »).
2. 2. 2. Let handlerRealm be null.
3. 3. 3. If reaction.[[Handler]] is not empty, then
1. a. a. Let getHandlerRealmResult be
Completion(GetFunctionRealm(reaction.[[Handler]].[[Callback]])).
2. b. b. If getHandlerRealmResult is a normal completion, set handlerRealm
to getHandlerRealmResult.[[Value]].
3. c. c. Else, set handlerRealm to the current Realm Record.
4. d. d. NOTE: handlerRealm is never null unless the handler is undefined.
When the handler is a revoked Proxy and no ECMAScript code runs,
handlerRealm is used to create error objects.
4. 4. 4. Return the Record { [[Job]]: job, [[Realm]]: handlerRealm }.
27.2.2.2 NEWPROMISERESOLVETHENABLEJOB ( PROMISETORESOLVE, THENABLE, THEN )
The abstract operation NewPromiseResolveThenableJob takes arguments
promiseToResolve (a Promise), thenable (an Object), and then (a JobCallback
Record) and returns a Record with fields [[Job]] (a Job Abstract Closure) and
[[Realm]] (a Realm Record). It performs the following steps when called:
1. 1. 1. Let job be a new Job Abstract Closure with no parameters that captures
promiseToResolve, thenable, and then and performs the following steps when
called:
1. a. a. Let resolvingFunctions be
CreateResolvingFunctions(promiseToResolve).
2. b. b. Let thenCallResult be Completion(HostCallJobCallback(then,
thenable, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]]
»)).
3. c. c. If thenCallResult is an abrupt completion, then
1. i. i. Return ? Call(resolvingFunctions.[[Reject]], undefined, «
thenCallResult.[[Value]] »).
4. d. d. Return ? thenCallResult.
2. 2. 2. Let getThenRealmResult be
Completion(GetFunctionRealm(then.[[Callback]])).
3. 3. 3. If getThenRealmResult is a normal completion, let thenRealm be
getThenRealmResult.[[Value]].
4. 4. 4. Else, let thenRealm be the current Realm Record.
5. 5. 5. NOTE: thenRealm is never null. When then.[[Callback]] is a revoked
Proxy and no code runs, thenRealm is used to create error objects.
6. 6. 6. Return the Record { [[Job]]: job, [[Realm]]: thenRealm }.
Note
This Job uses the supplied thenable and its then method to resolve the given
promise. This process must take place as a Job to ensure that the evaluation of
the then method occurs after evaluation of any surrounding code has completed.
27.2.3 THE PROMISE CONSTRUCTOR
The Promise constructor:
* is %Promise%.
* is the initial value of the "Promise" property of the global object.
* creates and initializes a new Promise when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
* may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified Promise behaviour must
include a super call to the Promise constructor to create and initialize the
subclass instance with the internal state necessary to support the Promise
and Promise.prototype built-in methods.
27.2.3.1 PROMISE ( EXECUTOR )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. If IsCallable(executor) is false, throw a TypeError exception.
3. 3. 3. Let promise be ? OrdinaryCreateFromConstructor(NewTarget,
"%Promise.prototype%", « [[PromiseState]], [[PromiseResult]],
[[PromiseFulfillReactions]], [[PromiseRejectReactions]],
[[PromiseIsHandled]] »).
4. 4. 4. Set promise.[[PromiseState]] to pending.
5. 5. 5. Set promise.[[PromiseFulfillReactions]] to a new empty List.
6. 6. 6. Set promise.[[PromiseRejectReactions]] to a new empty List.
7. 7. 7. Set promise.[[PromiseIsHandled]] to false.
8. 8. 8. Let resolvingFunctions be CreateResolvingFunctions(promise).
9. 9. 9. Let completion be Completion(Call(executor, undefined, «
resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »)).
10. 10. 10. If completion is an abrupt completion, then
1. a. a. Perform ? Call(resolvingFunctions.[[Reject]], undefined, «
completion.[[Value]] »).
11. 11. 11. Return promise.
Note
The executor argument must be a function object. It is called for initiating and
reporting completion of the possibly deferred action represented by this
Promise. The executor is called with two arguments: resolve and reject. These
are functions that may be used by the executor function to report eventual
completion or failure of the deferred computation. Returning from the executor
function does not mean that the deferred action has been completed but only that
the request to eventually perform the deferred action has been accepted.
The resolve function that is passed to an executor function accepts a single
argument. The executor code may eventually call the resolve function to indicate
that it wishes to resolve the associated Promise. The argument passed to the
resolve function represents the eventual value of the deferred action and can be
either the actual fulfillment value or another promise which will provide the
value if it is fulfilled.
The reject function that is passed to an executor function accepts a single
argument. The executor code may eventually call the reject function to indicate
that the associated Promise is rejected and will never be fulfilled. The
argument passed to the reject function is used as the rejection value of the
promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the Promise
constructor have the capability to actually resolve and reject the associated
promise. Subclasses may have different constructor behaviour that passes in
customized values for resolve and reject.
27.2.4 PROPERTIES OF THE PROMISE CONSTRUCTOR
The Promise constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* has the following properties:
27.2.4.1 PROMISE.ALL ( ITERABLE )
This function returns a new promise which is fulfilled with an array of
fulfillment values for the passed promises, or rejects with the reason of the
first passed promise that rejects. It resolves all elements of the passed
iterable to promises as it runs this algorithm.
1. 1. 1. Let C be the this value.
2. 2. 2. Let promiseCapability be ? NewPromiseCapability(C).
3. 3. 3. Let promiseResolve be Completion(GetPromiseResolve(C)).
4. 4. 4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
5. 5. 5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
6. 6. 6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
7. 7. 7. Let result be Completion(PerformPromiseAll(iteratorRecord, C,
promiseCapability, promiseResolve)).
8. 8. 8. If result is an abrupt completion, then
1. a. a. If iteratorRecord.[[Done]] is false, set result to
Completion(IteratorClose(iteratorRecord, result)).
2. b. b. IfAbruptRejectPromise(result, promiseCapability).
9. 9. 9. Return ? result.
Note
This function requires its this value to be a constructor function that supports
the parameter conventions of the Promise constructor.
27.2.4.1.1 GETPROMISERESOLVE ( PROMISECONSTRUCTOR )
The abstract operation GetPromiseResolve takes argument promiseConstructor (a
constructor) and returns either a normal completion containing a function object
or a throw completion. It performs the following steps when called:
1. 1. 1. Let promiseResolve be ? Get(promiseConstructor, "resolve").
2. 2. 2. If IsCallable(promiseResolve) is false, throw a TypeError exception.
3. 3. 3. Return promiseResolve.
27.2.4.1.2 PERFORMPROMISEALL ( ITERATORRECORD, CONSTRUCTOR, RESULTCAPABILITY,
PROMISERESOLVE )
The abstract operation PerformPromiseAll takes arguments iteratorRecord (an
Iterator Record), constructor (a constructor), resultCapability (a
PromiseCapability Record), and promiseResolve (a function object) and returns
either a normal completion containing an ECMAScript language value or a throw
completion. It performs the following steps when called:
1. 1. 1. Let values be a new empty List.
2. 2. 2. Let remainingElementsCount be the Record { [[Value]]: 1 }.
3. 3. 3. Let index be 0.
4. 4. 4. Repeat,
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, then
1. i. i. Set iteratorRecord.[[Done]] to true.
2. ii. ii. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
3. iii. iii. If remainingElementsCount.[[Value]] = 0, then
1. 1. 1. Let valuesArray be CreateArrayFromList(values).
2. 2. 2. Perform ? Call(resultCapability.[[Resolve]], undefined, «
valuesArray »).
4. iv. iv. Return resultCapability.[[Promise]].
5. e. e. Let nextValue be Completion(IteratorValue(next)).
6. f. f. If nextValue is an abrupt completion, set iteratorRecord.[[Done]]
to true.
7. g. g. ReturnIfAbrupt(nextValue).
8. h. h. Append undefined to values.
9. i. i. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue
»).
10. j. j. Let steps be the algorithm steps defined in Promise.all Resolve
Element Functions.
11. k. k. Let length be the number of non-optional parameters of the
function definition in Promise.all Resolve Element Functions.
12. l. l. Let onFulfilled be CreateBuiltinFunction(steps, length, "", «
[[AlreadyCalled]], [[Index]], [[Values]], [[Capability]],
[[RemainingElements]] »).
13. m. m. Set onFulfilled.[[AlreadyCalled]] to false.
14. n. n. Set onFulfilled.[[Index]] to index.
15. o. o. Set onFulfilled.[[Values]] to values.
16. p. p. Set onFulfilled.[[Capability]] to resultCapability.
17. q. q. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
18. r. r. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] + 1.
19. s. s. Perform ? Invoke(nextPromise, "then", « onFulfilled,
resultCapability.[[Reject]] »).
20. t. t. Set index to index + 1.
27.2.4.1.3 PROMISE.ALL RESOLVE ELEMENT FUNCTIONS
A Promise.all resolve element function is an anonymous built-in function that is
used to resolve a specific Promise.all element. Each Promise.all resolve element
function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and
[[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument x, the
following steps are taken:
1. 1. 1. Let F be the active function object.
2. 2. 2. If F.[[AlreadyCalled]] is true, return undefined.
3. 3. 3. Set F.[[AlreadyCalled]] to true.
4. 4. 4. Let index be F.[[Index]].
5. 5. 5. Let values be F.[[Values]].
6. 6. 6. Let promiseCapability be F.[[Capability]].
7. 7. 7. Let remainingElementsCount be F.[[RemainingElements]].
8. 8. 8. Set values[index] to x.
9. 9. 9. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
10. 10. 10. If remainingElementsCount.[[Value]] = 0, then
1. a. a. Let valuesArray be CreateArrayFromList(values).
2. b. b. Return ? Call(promiseCapability.[[Resolve]], undefined, «
valuesArray »).
11. 11. 11. Return undefined.
The "length" property of a Promise.all resolve element function is 1𝔽.
27.2.4.2 PROMISE.ALLSETTLED ( ITERABLE )
This function returns a promise that is fulfilled with an array of promise state
snapshots, but only after all the original promises have settled, i.e. become
either fulfilled or rejected. It resolves all elements of the passed iterable to
promises as it runs this algorithm.
1. 1. 1. Let C be the this value.
2. 2. 2. Let promiseCapability be ? NewPromiseCapability(C).
3. 3. 3. Let promiseResolve be Completion(GetPromiseResolve(C)).
4. 4. 4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
5. 5. 5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
6. 6. 6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
7. 7. 7. Let result be Completion(PerformPromiseAllSettled(iteratorRecord, C,
promiseCapability, promiseResolve)).
8. 8. 8. If result is an abrupt completion, then
1. a. a. If iteratorRecord.[[Done]] is false, set result to
Completion(IteratorClose(iteratorRecord, result)).
2. b. b. IfAbruptRejectPromise(result, promiseCapability).
9. 9. 9. Return ? result.
Note
This function requires its this value to be a constructor function that supports
the parameter conventions of the Promise constructor.
27.2.4.2.1 PERFORMPROMISEALLSETTLED ( ITERATORRECORD, CONSTRUCTOR,
RESULTCAPABILITY, PROMISERESOLVE )
The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord
(an Iterator Record), constructor (a constructor), resultCapability (a
PromiseCapability Record), and promiseResolve (a function object) and returns
either a normal completion containing an ECMAScript language value or a throw
completion. It performs the following steps when called:
1. 1. 1. Let values be a new empty List.
2. 2. 2. Let remainingElementsCount be the Record { [[Value]]: 1 }.
3. 3. 3. Let index be 0.
4. 4. 4. Repeat,
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, then
1. i. i. Set iteratorRecord.[[Done]] to true.
2. ii. ii. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
3. iii. iii. If remainingElementsCount.[[Value]] = 0, then
1. 1. 1. Let valuesArray be CreateArrayFromList(values).
2. 2. 2. Perform ? Call(resultCapability.[[Resolve]], undefined, «
valuesArray »).
4. iv. iv. Return resultCapability.[[Promise]].
5. e. e. Let nextValue be Completion(IteratorValue(next)).
6. f. f. If nextValue is an abrupt completion, set iteratorRecord.[[Done]]
to true.
7. g. g. ReturnIfAbrupt(nextValue).
8. h. h. Append undefined to values.
9. i. i. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue
»).
10. j. j. Let stepsFulfilled be the algorithm steps defined in
Promise.allSettled Resolve Element Functions.
11. k. k. Let lengthFulfilled be the number of non-optional parameters of
the function definition in Promise.allSettled Resolve Element Functions.
12. l. l. Let onFulfilled be CreateBuiltinFunction(stepsFulfilled,
lengthFulfilled, "", « [[AlreadyCalled]], [[Index]], [[Values]],
[[Capability]], [[RemainingElements]] »).
13. m. m. Let alreadyCalled be the Record { [[Value]]: false }.
14. n. n. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
15. o. o. Set onFulfilled.[[Index]] to index.
16. p. p. Set onFulfilled.[[Values]] to values.
17. q. q. Set onFulfilled.[[Capability]] to resultCapability.
18. r. r. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
19. s. s. Let stepsRejected be the algorithm steps defined in
Promise.allSettled Reject Element Functions.
20. t. t. Let lengthRejected be the number of non-optional parameters of the
function definition in Promise.allSettled Reject Element Functions.
21. u. u. Let onRejected be CreateBuiltinFunction(stepsRejected,
lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Values]],
[[Capability]], [[RemainingElements]] »).
22. v. v. Set onRejected.[[AlreadyCalled]] to alreadyCalled.
23. w. w. Set onRejected.[[Index]] to index.
24. x. x. Set onRejected.[[Values]] to values.
25. y. y. Set onRejected.[[Capability]] to resultCapability.
26. z. z. Set onRejected.[[RemainingElements]] to remainingElementsCount.
27. aa. aa. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] + 1.
28. ab. ab. Perform ? Invoke(nextPromise, "then", « onFulfilled, onRejected
»).
29. ac. ac. Set index to index + 1.
27.2.4.2.2 PROMISE.ALLSETTLED RESOLVE ELEMENT FUNCTIONS
A Promise.allSettled resolve element function is an anonymous built-in function
that is used to resolve a specific Promise.allSettled element. Each
Promise.allSettled resolve element function has [[Index]], [[Values]],
[[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument x,
the following steps are taken:
1. 1. 1. Let F be the active function object.
2. 2. 2. Let alreadyCalled be F.[[AlreadyCalled]].
3. 3. 3. If alreadyCalled.[[Value]] is true, return undefined.
4. 4. 4. Set alreadyCalled.[[Value]] to true.
5. 5. 5. Let index be F.[[Index]].
6. 6. 6. Let values be F.[[Values]].
7. 7. 7. Let promiseCapability be F.[[Capability]].
8. 8. 8. Let remainingElementsCount be F.[[RemainingElements]].
9. 9. 9. Let obj be OrdinaryObjectCreate(%Object.prototype%).
10. 10. 10. Perform ! CreateDataPropertyOrThrow(obj, "status", "fulfilled").
11. 11. 11. Perform ! CreateDataPropertyOrThrow(obj, "value", x).
12. 12. 12. Set values[index] to obj.
13. 13. 13. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
14. 14. 14. If remainingElementsCount.[[Value]] = 0, then
1. a. a. Let valuesArray be CreateArrayFromList(values).
2. b. b. Return ? Call(promiseCapability.[[Resolve]], undefined, «
valuesArray »).
15. 15. 15. Return undefined.
The "length" property of a Promise.allSettled resolve element function is 1𝔽.
27.2.4.2.3 PROMISE.ALLSETTLED REJECT ELEMENT FUNCTIONS
A Promise.allSettled reject element function is an anonymous built-in function
that is used to reject a specific Promise.allSettled element. Each
Promise.allSettled reject element function has [[Index]], [[Values]],
[[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument x, the
following steps are taken:
1. 1. 1. Let F be the active function object.
2. 2. 2. Let alreadyCalled be F.[[AlreadyCalled]].
3. 3. 3. If alreadyCalled.[[Value]] is true, return undefined.
4. 4. 4. Set alreadyCalled.[[Value]] to true.
5. 5. 5. Let index be F.[[Index]].
6. 6. 6. Let values be F.[[Values]].
7. 7. 7. Let promiseCapability be F.[[Capability]].
8. 8. 8. Let remainingElementsCount be F.[[RemainingElements]].
9. 9. 9. Let obj be OrdinaryObjectCreate(%Object.prototype%).
10. 10. 10. Perform ! CreateDataPropertyOrThrow(obj, "status", "rejected").
11. 11. 11. Perform ! CreateDataPropertyOrThrow(obj, "reason", x).
12. 12. 12. Set values[index] to obj.
13. 13. 13. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
14. 14. 14. If remainingElementsCount.[[Value]] = 0, then
1. a. a. Let valuesArray be CreateArrayFromList(values).
2. b. b. Return ? Call(promiseCapability.[[Resolve]], undefined, «
valuesArray »).
15. 15. 15. Return undefined.
The "length" property of a Promise.allSettled reject element function is 1𝔽.
27.2.4.3 PROMISE.ANY ( ITERABLE )
This function returns a promise that is fulfilled by the first given promise to
be fulfilled, or rejected with an AggregateError holding the rejection reasons
if all of the given promises are rejected. It resolves all elements of the
passed iterable to promises as it runs this algorithm.
1. 1. 1. Let C be the this value.
2. 2. 2. Let promiseCapability be ? NewPromiseCapability(C).
3. 3. 3. Let promiseResolve be Completion(GetPromiseResolve(C)).
4. 4. 4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
5. 5. 5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
6. 6. 6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
7. 7. 7. Let result be Completion(PerformPromiseAny(iteratorRecord, C,
promiseCapability, promiseResolve)).
8. 8. 8. If result is an abrupt completion, then
1. a. a. If iteratorRecord.[[Done]] is false, set result to
Completion(IteratorClose(iteratorRecord, result)).
2. b. b. IfAbruptRejectPromise(result, promiseCapability).
9. 9. 9. Return ? result.
Note
This function requires its this value to be a constructor function that supports
the parameter conventions of the Promise constructor.
27.2.4.3.1 PERFORMPROMISEANY ( ITERATORRECORD, CONSTRUCTOR, RESULTCAPABILITY,
PROMISERESOLVE )
The abstract operation PerformPromiseAny takes arguments iteratorRecord (an
Iterator Record), constructor (a constructor), resultCapability (a
PromiseCapability Record), and promiseResolve (a function object) and returns
either a normal completion containing an ECMAScript language value or a throw
completion. It performs the following steps when called:
1. 1. 1. Let errors be a new empty List.
2. 2. 2. Let remainingElementsCount be the Record { [[Value]]: 1 }.
3. 3. 3. Let index be 0.
4. 4. 4. Repeat,
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, then
1. i. i. Set iteratorRecord.[[Done]] to true.
2. ii. ii. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
3. iii. iii. If remainingElementsCount.[[Value]] = 0, then
1. 1. 1. Let error be a newly created AggregateError object.
2. 2. 2. Perform ! DefinePropertyOrThrow(error, "errors",
PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]:
false, [[Writable]]: true, [[Value]]: CreateArrayFromList(errors)
}).
3. 3. 3. Return ThrowCompletion(error).
4. iv. iv. Return resultCapability.[[Promise]].
5. e. e. Let nextValue be Completion(IteratorValue(next)).
6. f. f. If nextValue is an abrupt completion, set iteratorRecord.[[Done]]
to true.
7. g. g. ReturnIfAbrupt(nextValue).
8. h. h. Append undefined to errors.
9. i. i. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue
»).
10. j. j. Let stepsRejected be the algorithm steps defined in Promise.any
Reject Element Functions.
11. k. k. Let lengthRejected be the number of non-optional parameters of the
function definition in Promise.any Reject Element Functions.
12. l. l. Let onRejected be CreateBuiltinFunction(stepsRejected,
lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Errors]],
[[Capability]], [[RemainingElements]] »).
13. m. m. Set onRejected.[[AlreadyCalled]] to false.
14. n. n. Set onRejected.[[Index]] to index.
15. o. o. Set onRejected.[[Errors]] to errors.
16. p. p. Set onRejected.[[Capability]] to resultCapability.
17. q. q. Set onRejected.[[RemainingElements]] to remainingElementsCount.
18. r. r. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] + 1.
19. s. s. Perform ? Invoke(nextPromise, "then", «
resultCapability.[[Resolve]], onRejected »).
20. t. t. Set index to index + 1.
27.2.4.3.2 PROMISE.ANY REJECT ELEMENT FUNCTIONS
A Promise.any reject element function is an anonymous built-in function that is
used to reject a specific Promise.any element. Each Promise.any reject element
function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and
[[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument x, the
following steps are taken:
1. 1. 1. Let F be the active function object.
2. 2. 2. If F.[[AlreadyCalled]] is true, return undefined.
3. 3. 3. Set F.[[AlreadyCalled]] to true.
4. 4. 4. Let index be F.[[Index]].
5. 5. 5. Let errors be F.[[Errors]].
6. 6. 6. Let promiseCapability be F.[[Capability]].
7. 7. 7. Let remainingElementsCount be F.[[RemainingElements]].
8. 8. 8. Set errors[index] to x.
9. 9. 9. Set remainingElementsCount.[[Value]] to
remainingElementsCount.[[Value]] - 1.
10. 10. 10. If remainingElementsCount.[[Value]] = 0, then
1. a. a. Let error be a newly created AggregateError object.
2. b. b. Perform ! DefinePropertyOrThrow(error, "errors",
PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]: false,
[[Writable]]: true, [[Value]]: CreateArrayFromList(errors) }).
3. c. c. Return ? Call(promiseCapability.[[Reject]], undefined, « error »).
11. 11. 11. Return undefined.
The "length" property of a Promise.any reject element function is 1𝔽.
27.2.4.4 PROMISE.PROTOTYPE
The initial value of Promise.prototype is the Promise prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
27.2.4.5 PROMISE.RACE ( ITERABLE )
This function returns a new promise which is settled in the same way as the
first passed promise to settle. It resolves all elements of the passed iterable
to promises as it runs this algorithm.
1. 1. 1. Let C be the this value.
2. 2. 2. Let promiseCapability be ? NewPromiseCapability(C).
3. 3. 3. Let promiseResolve be Completion(GetPromiseResolve(C)).
4. 4. 4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
5. 5. 5. Let iteratorRecord be Completion(GetIterator(iterable, sync)).
6. 6. 6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
7. 7. 7. Let result be Completion(PerformPromiseRace(iteratorRecord, C,
promiseCapability, promiseResolve)).
8. 8. 8. If result is an abrupt completion, then
1. a. a. If iteratorRecord.[[Done]] is false, set result to
Completion(IteratorClose(iteratorRecord, result)).
2. b. b. IfAbruptRejectPromise(result, promiseCapability).
9. 9. 9. Return ? result.
Note 1
If the iterable argument yields no values or if none of the promises yielded by
iterable ever settle, then the pending promise returned by this method will
never be settled.
Note 2
This function expects its this value to be a constructor function that supports
the parameter conventions of the Promise constructor. It also expects that its
this value provides a resolve method.
27.2.4.5.1 PERFORMPROMISERACE ( ITERATORRECORD, CONSTRUCTOR, RESULTCAPABILITY,
PROMISERESOLVE )
The abstract operation PerformPromiseRace takes arguments iteratorRecord (an
Iterator Record), constructor (a constructor), resultCapability (a
PromiseCapability Record), and promiseResolve (a function object) and returns
either a normal completion containing an ECMAScript language value or a throw
completion. It performs the following steps when called:
1. 1. 1. Repeat,
1. a. a. Let next be Completion(IteratorStep(iteratorRecord)).
2. b. b. If next is an abrupt completion, set iteratorRecord.[[Done]] to
true.
3. c. c. ReturnIfAbrupt(next).
4. d. d. If next is false, then
1. i. i. Set iteratorRecord.[[Done]] to true.
2. ii. ii. Return resultCapability.[[Promise]].
5. e. e. Let nextValue be Completion(IteratorValue(next)).
6. f. f. If nextValue is an abrupt completion, set iteratorRecord.[[Done]]
to true.
7. g. g. ReturnIfAbrupt(nextValue).
8. h. h. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue
»).
9. i. i. Perform ? Invoke(nextPromise, "then", «
resultCapability.[[Resolve]], resultCapability.[[Reject]] »).
27.2.4.6 PROMISE.REJECT ( R )
This function returns a new promise rejected with the passed argument.
1. 1. 1. Let C be the this value.
2. 2. 2. Let promiseCapability be ? NewPromiseCapability(C).
3. 3. 3. Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
4. 4. 4. Return promiseCapability.[[Promise]].
Note
This function expects its this value to be a constructor function that supports
the parameter conventions of the Promise constructor.
27.2.4.7 PROMISE.RESOLVE ( X )
This function returns either a new promise resolved with the passed argument, or
the argument itself if the argument is a promise produced by this constructor.
1. 1. 1. Let C be the this value.
2. 2. 2. If C is not an Object, throw a TypeError exception.
3. 3. 3. Return ? PromiseResolve(C, x).
Note
This function expects its this value to be a constructor function that supports
the parameter conventions of the Promise constructor.
27.2.4.7.1 PROMISERESOLVE ( C, X )
The abstract operation PromiseResolve takes arguments C (a constructor) and x
(an ECMAScript language value) and returns either a normal completion containing
an ECMAScript language value or a throw completion. It returns a new promise
resolved with x. It performs the following steps when called:
1. 1. 1. If IsPromise(x) is true, then
1. a. a. Let xConstructor be ? Get(x, "constructor").
2. b. b. If SameValue(xConstructor, C) is true, return x.
2. 2. 2. Let promiseCapability be ? NewPromiseCapability(C).
3. 3. 3. Perform ? Call(promiseCapability.[[Resolve]], undefined, « x »).
4. 4. 4. Return promiseCapability.[[Promise]].
27.2.4.8 GET PROMISE [ @@SPECIES ]
Promise[@@species] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
1. 1. 1. Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
Promise prototype methods normally use their this value's constructor to create
a derived object. However, a subclass constructor may over-ride that default
behaviour by redefining its @@species property.
27.2.5 PROPERTIES OF THE PROMISE PROTOTYPE OBJECT
The Promise prototype object:
* is %Promise.prototype%.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is an ordinary object.
* does not have a [[PromiseState]] internal slot or any of the other internal
slots of Promise instances.
27.2.5.1 PROMISE.PROTOTYPE.CATCH ( ONREJECTED )
This method performs the following steps when called:
1. 1. 1. Let promise be the this value.
2. 2. 2. Return ? Invoke(promise, "then", « undefined, onRejected »).
27.2.5.2 PROMISE.PROTOTYPE.CONSTRUCTOR
The initial value of Promise.prototype.constructor is %Promise%.
27.2.5.3 PROMISE.PROTOTYPE.FINALLY ( ONFINALLY )
This method performs the following steps when called:
1. 1. 1. Let promise be the this value.
2. 2. 2. If promise is not an Object, throw a TypeError exception.
3. 3. 3. Let C be ? SpeciesConstructor(promise, %Promise%).
4. 4. 4. Assert: IsConstructor(C) is true.
5. 5. 5. If IsCallable(onFinally) is false, then
1. a. a. Let thenFinally be onFinally.
2. b. b. Let catchFinally be onFinally.
6. 6. 6. Else,
1. a. a. Let thenFinallyClosure be a new Abstract Closure with parameters
(value) that captures onFinally and C and performs the following steps
when called:
1. i. i. Let result be ? Call(onFinally, undefined).
2. ii. ii. Let p be ? PromiseResolve(C, result).
3. iii. iii. Let returnValue be a new Abstract Closure with no parameters
that captures value and performs the following steps when called:
1. 1. 1. Return value.
4. iv. iv. Let valueThunk be CreateBuiltinFunction(returnValue, 0, "", «
»).
5. v. v. Return ? Invoke(p, "then", « valueThunk »).
2. b. b. Let thenFinally be CreateBuiltinFunction(thenFinallyClosure, 1, "",
« »).
3. c. c. Let catchFinallyClosure be a new Abstract Closure with parameters
(reason) that captures onFinally and C and performs the following steps
when called:
1. i. i. Let result be ? Call(onFinally, undefined).
2. ii. ii. Let p be ? PromiseResolve(C, result).
3. iii. iii. Let throwReason be a new Abstract Closure with no parameters
that captures reason and performs the following steps when called:
1. 1. 1. Return ThrowCompletion(reason).
4. iv. iv. Let thrower be CreateBuiltinFunction(throwReason, 0, "", « »).
5. v. v. Return ? Invoke(p, "then", « thrower »).
4. d. d. Let catchFinally be CreateBuiltinFunction(catchFinallyClosure, 1,
"", « »).
7. 7. 7. Return ? Invoke(promise, "then", « thenFinally, catchFinally »).
27.2.5.4 PROMISE.PROTOTYPE.THEN ( ONFULFILLED, ONREJECTED )
This method performs the following steps when called:
1. 1. 1. Let promise be the this value.
2. 2. 2. If IsPromise(promise) is false, throw a TypeError exception.
3. 3. 3. Let C be ? SpeciesConstructor(promise, %Promise%).
4. 4. 4. Let resultCapability be ? NewPromiseCapability(C).
5. 5. 5. Return PerformPromiseThen(promise, onFulfilled, onRejected,
resultCapability).
27.2.5.4.1 PERFORMPROMISETHEN ( PROMISE, ONFULFILLED, ONREJECTED [ ,
RESULTCAPABILITY ] )
The abstract operation PerformPromiseThen takes arguments promise (a Promise),
onFulfilled (an ECMAScript language value), and onRejected (an ECMAScript
language value) and optional argument resultCapability (a PromiseCapability
Record) and returns an ECMAScript language value. It performs the “then”
operation on promise using onFulfilled and onRejected as its settlement actions.
If resultCapability is passed, the result is stored by updating
resultCapability's promise. If it is not passed, then PerformPromiseThen is
being called by a specification-internal operation where the result does not
matter. It performs the following steps when called:
1. 1. 1. Assert: IsPromise(promise) is true.
2. 2. 2. If resultCapability is not present, then
1. a. a. Set resultCapability to undefined.
3. 3. 3. If IsCallable(onFulfilled) is false, then
1. a. a. Let onFulfilledJobCallback be empty.
4. 4. 4. Else,
1. a. a. Let onFulfilledJobCallback be HostMakeJobCallback(onFulfilled).
5. 5. 5. If IsCallable(onRejected) is false, then
1. a. a. Let onRejectedJobCallback be empty.
6. 6. 6. Else,
1. a. a. Let onRejectedJobCallback be HostMakeJobCallback(onRejected).
7. 7. 7. Let fulfillReaction be the PromiseReaction { [[Capability]]:
resultCapability, [[Type]]: Fulfill, [[Handler]]: onFulfilledJobCallback }.
8. 8. 8. Let rejectReaction be the PromiseReaction { [[Capability]]:
resultCapability, [[Type]]: Reject, [[Handler]]: onRejectedJobCallback }.
9. 9. 9. If promise.[[PromiseState]] is pending, then
1. a. a. Append fulfillReaction to promise.[[PromiseFulfillReactions]].
2. b. b. Append rejectReaction to promise.[[PromiseRejectReactions]].
10. 10. 10. Else if promise.[[PromiseState]] is fulfilled, then
1. a. a. Let value be promise.[[PromiseResult]].
2. b. b. Let fulfillJob be NewPromiseReactionJob(fulfillReaction, value).
3. c. c. Perform HostEnqueuePromiseJob(fulfillJob.[[Job]],
fulfillJob.[[Realm]]).
11. 11. 11. Else,
1. a. a. Assert: The value of promise.[[PromiseState]] is rejected.
2. b. b. Let reason be promise.[[PromiseResult]].
3. c. c. If promise.[[PromiseIsHandled]] is false, perform
HostPromiseRejectionTracker(promise, "handle").
4. d. d. Let rejectJob be NewPromiseReactionJob(rejectReaction, reason).
5. e. e. Perform HostEnqueuePromiseJob(rejectJob.[[Job]],
rejectJob.[[Realm]]).
12. 12. 12. Set promise.[[PromiseIsHandled]] to true.
13. 13. 13. If resultCapability is undefined, then
1. a. a. Return undefined.
14. 14. 14. Else,
1. a. a. Return resultCapability.[[Promise]].
27.2.5.5 PROMISE.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Promise".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.2.6 PROPERTIES OF PROMISE INSTANCES
Promise instances are ordinary objects that inherit properties from the Promise
prototype object (the intrinsic, %Promise.prototype%). Promise instances are
initially created with the internal slots described in Table 81.
Table 81: Internal Slots of Promise Instances
Internal Slot Type Description [[PromiseState]] pending, fulfilled, or rejected
Governs how a promise will react to incoming calls to its then method.
[[PromiseResult]] an ECMAScript language value The value with which the promise
has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is
not pending. [[PromiseFulfillReactions]] a List of PromiseReaction Records
Records to be processed when/if the promise transitions from the pending state
to the fulfilled state. [[PromiseRejectReactions]] a List of PromiseReaction
Records Records to be processed when/if the promise transitions from the pending
state to the rejected state. [[PromiseIsHandled]] a Boolean Indicates whether
the promise has ever had a fulfillment or rejection handler; used in unhandled
rejection tracking.
27.3 GENERATORFUNCTION OBJECTS
GeneratorFunctions are functions that are usually created by evaluating
GeneratorDeclarations, GeneratorExpressions, and GeneratorMethods. They may also
be created by calling the %GeneratorFunction% intrinsic.
Figure 6 (Informative): Generator Objects Relationships
27.3.1 THE GENERATORFUNCTION CONSTRUCTOR
The GeneratorFunction constructor:
* is %GeneratorFunction%.
* is a subclass of Function.
* creates and initializes a new GeneratorFunction when called as a function
rather than as a constructor. Thus the function call GeneratorFunction (…) is
equivalent to the object creation expression new GeneratorFunction (…) with
the same arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified GeneratorFunction behaviour
must include a super call to the GeneratorFunction constructor to create and
initialize subclass instances with the internal slots necessary for built-in
GeneratorFunction behaviour. All ECMAScript syntactic forms for defining
generator function objects create direct instances of GeneratorFunction.
There is no syntactic means to create instances of GeneratorFunction
subclasses.
27.3.1.1 GENERATORFUNCTION ( ...PARAMETERARGS, BODYARG )
The last argument (if any) specifies the body (executable code) of a generator
function; any preceding arguments specify formal parameters.
This function performs the following steps when called:
1. 1. 1. Let C be the active function object.
2. 2. 2. If bodyArg is not present, set bodyArg to the empty String.
3. 3. 3. Return ? CreateDynamicFunction(C, NewTarget, generator, parameterArgs,
bodyArg).
Note
See NOTE for 20.2.1.1.
27.3.2 PROPERTIES OF THE GENERATORFUNCTION CONSTRUCTOR
The GeneratorFunction constructor:
* is a standard built-in function object that inherits from the Function
constructor.
* has a [[Prototype]] internal slot whose value is %Function%.
* has a "name" property whose value is "GeneratorFunction".
* has the following properties:
27.3.2.1 GENERATORFUNCTION.LENGTH
This is a data property with a value of 1. This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.3.2.2 GENERATORFUNCTION.PROTOTYPE
The initial value of GeneratorFunction.prototype is the GeneratorFunction
prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
27.3.3 PROPERTIES OF THE GENERATORFUNCTION PROTOTYPE OBJECT
The GeneratorFunction prototype object:
* is %GeneratorFunction.prototype% (see Figure 6).
* is an ordinary object.
* is not a function object and does not have an [[ECMAScriptCode]] internal
slot or any other of the internal slots listed in Table 30 or Table 82.
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
27.3.3.1 GENERATORFUNCTION.PROTOTYPE.CONSTRUCTOR
The initial value of GeneratorFunction.prototype.constructor is
%GeneratorFunction%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.3.3.2 GENERATORFUNCTION.PROTOTYPE.PROTOTYPE
The initial value of GeneratorFunction.prototype.prototype is the Generator
prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.3.3.3 GENERATORFUNCTION.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"GeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.3.4 GENERATORFUNCTION INSTANCES
Every GeneratorFunction instance is an ECMAScript function object and has the
internal slots listed in Table 30. The value of the [[IsClassConstructor]]
internal slot for all such instances is false.
Each GeneratorFunction instance has the following own properties:
27.3.4.1 LENGTH
The specification for the "length" property of Function instances given in
20.2.4.1 also applies to GeneratorFunction instances.
27.3.4.2 NAME
The specification for the "name" property of Function instances given in
20.2.4.2 also applies to GeneratorFunction instances.
27.3.4.3 PROTOTYPE
Whenever a GeneratorFunction instance is created another ordinary object is also
created and is the initial value of the generator function's "prototype"
property. The value of the prototype property is used to initialize the
[[Prototype]] internal slot of a newly created Generator when the generator
function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Unlike Function instances, the object that is the value of a GeneratorFunction's
"prototype" property does not have a "constructor" property whose value is the
GeneratorFunction instance.
27.4 ASYNCGENERATORFUNCTION OBJECTS
AsyncGeneratorFunctions are functions that are usually created by evaluating
AsyncGeneratorDeclaration, AsyncGeneratorExpression, and AsyncGeneratorMethod
syntactic productions. They may also be created by calling the
%AsyncGeneratorFunction% intrinsic.
27.4.1 THE ASYNCGENERATORFUNCTION CONSTRUCTOR
The AsyncGeneratorFunction constructor:
* is %AsyncGeneratorFunction%.
* is a subclass of Function.
* creates and initializes a new AsyncGeneratorFunction when called as a
function rather than as a constructor. Thus the function call
AsyncGeneratorFunction (...) is equivalent to the object creation expression
new AsyncGeneratorFunction (...) with the same arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified AsyncGeneratorFunction
behaviour must include a super call to the AsyncGeneratorFunction constructor
to create and initialize subclass instances with the internal slots necessary
for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms
for defining async generator function objects create direct instances of
AsyncGeneratorFunction. There is no syntactic means to create instances of
AsyncGeneratorFunction subclasses.
27.4.1.1 ASYNCGENERATORFUNCTION ( ...PARAMETERARGS, BODYARG )
The last argument (if any) specifies the body (executable code) of an async
generator function; any preceding arguments specify formal parameters.
This function performs the following steps when called:
1. 1. 1. Let C be the active function object.
2. 2. 2. If bodyArg is not present, set bodyArg to the empty String.
3. 3. 3. Return ? CreateDynamicFunction(C, NewTarget, asyncGenerator,
parameterArgs, bodyArg).
Note
See NOTE for 20.2.1.1.
27.4.2 PROPERTIES OF THE ASYNCGENERATORFUNCTION CONSTRUCTOR
The AsyncGeneratorFunction constructor:
* is a standard built-in function object that inherits from the Function
constructor.
* has a [[Prototype]] internal slot whose value is %Function%.
* has a "name" property whose value is "AsyncGeneratorFunction".
* has the following properties:
27.4.2.1 ASYNCGENERATORFUNCTION.LENGTH
This is a data property with a value of 1. This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.4.2.2 ASYNCGENERATORFUNCTION.PROTOTYPE
The initial value of AsyncGeneratorFunction.prototype is the
AsyncGeneratorFunction prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
27.4.3 PROPERTIES OF THE ASYNCGENERATORFUNCTION PROTOTYPE OBJECT
The AsyncGeneratorFunction prototype object:
* is %AsyncGeneratorFunction.prototype%.
* is an ordinary object.
* is not a function object and does not have an [[ECMAScriptCode]] internal
slot or any other of the internal slots listed in Table 30 or Table 83.
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
27.4.3.1 ASYNCGENERATORFUNCTION.PROTOTYPE.CONSTRUCTOR
The initial value of AsyncGeneratorFunction.prototype.constructor is
%AsyncGeneratorFunction%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.4.3.2 ASYNCGENERATORFUNCTION.PROTOTYPE.PROTOTYPE
The initial value of AsyncGeneratorFunction.prototype.prototype is the
AsyncGenerator prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.4.3.3 ASYNCGENERATORFUNCTION.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"AsyncGeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.4.4 ASYNCGENERATORFUNCTION INSTANCES
Every AsyncGeneratorFunction instance is an ECMAScript function object and has
the internal slots listed in Table 30. The value of the [[IsClassConstructor]]
internal slot for all such instances is false.
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 LENGTH
The value of the "length" property is an integral Number that indicates the
typical number of arguments expected by the AsyncGeneratorFunction. However, the
language permits the function to be invoked with some other number of arguments.
The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments
other than the number specified by its "length" property depends on the
function.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.4.4.2 NAME
The specification for the "name" property of Function instances given in
20.2.4.2 also applies to AsyncGeneratorFunction instances.
27.4.4.3 PROTOTYPE
Whenever an AsyncGeneratorFunction instance is created, another ordinary object
is also created and is the initial value of the async generator function's
"prototype" property. The value of the prototype property is used to initialize
the [[Prototype]] internal slot of a newly created AsyncGenerator when the
generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Unlike function instances, the object that is the value of an
AsyncGeneratorFunction's "prototype" property does not have a "constructor"
property whose value is the AsyncGeneratorFunction instance.
27.5 GENERATOR OBJECTS
A Generator is an instance of a generator function and conforms to both the
Iterator and Iterable interfaces.
Generator instances directly inherit properties from the object that is the
initial value of the "prototype" property of the Generator function that created
the instance. Generator instances indirectly inherit properties from the
Generator Prototype intrinsic, %GeneratorFunction.prototype.prototype%.
27.5.1 PROPERTIES OF THE GENERATOR PROTOTYPE OBJECT
The Generator prototype object:
* is %GeneratorFunction.prototype.prototype%.
* is an ordinary object.
* is not a Generator instance and does not have a [[GeneratorState]] internal
slot.
* has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
* has properties that are indirectly inherited by all Generator instances.
27.5.1.1 GENERATOR.PROTOTYPE.CONSTRUCTOR
The initial value of Generator.prototype.constructor is
%GeneratorFunction.prototype%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.5.1.2 GENERATOR.PROTOTYPE.NEXT ( VALUE )
1. 1. 1. Return ? GeneratorResume(this value, value, empty).
27.5.1.3 GENERATOR.PROTOTYPE.RETURN ( VALUE )
This method performs the following steps when called:
1. 1. 1. Let g be the this value.
2. 2. 2. Let C be Completion Record { [[Type]]: return, [[Value]]: value,
[[Target]]: empty }.
3. 3. 3. Return ? GeneratorResumeAbrupt(g, C, empty).
27.5.1.4 GENERATOR.PROTOTYPE.THROW ( EXCEPTION )
This method performs the following steps when called:
1. 1. 1. Let g be the this value.
2. 2. 2. Let C be ThrowCompletion(exception).
3. 3. 3. Return ? GeneratorResumeAbrupt(g, C, empty).
27.5.1.5 GENERATOR.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Generator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.5.2 PROPERTIES OF GENERATOR INSTANCES
Generator instances are initially created with the internal slots described in
Table 82.
Table 82: Internal Slots of Generator Instances
Internal Slot Type Description [[GeneratorState]] undefined, suspendedStart,
suspendedYield, executing, or completed The current execution state of the
generator. [[GeneratorContext]] an execution context The execution context that
is used when executing the code of this generator. [[GeneratorBrand]] a String
or empty A brand used to distinguish different kinds of generators. The
[[GeneratorBrand]] of generators declared by ECMAScript source text is always
empty.
27.5.3 GENERATOR ABSTRACT OPERATIONS
27.5.3.1 GENERATORSTART ( GENERATOR, GENERATORBODY )
The abstract operation GeneratorStart takes arguments generator (a Generator)
and generatorBody (a FunctionBody Parse Node or an Abstract Closure with no
parameters) and returns unused. It performs the following steps when called:
1. 1. 1. Assert: The value of generator.[[GeneratorState]] is undefined.
2. 2. 2. Let genContext be the running execution context.
3. 3. 3. Set the Generator component of genContext to generator.
4. 4. 4. Let closure be a new Abstract Closure with no parameters that captures
generatorBody and performs the following steps when called:
1. a. a. Let acGenContext be the running execution context.
2. b. b. Let acGenerator be the Generator component of acGenContext.
3. c. c. If generatorBody is a Parse Node, then
1. i. i. Let result be Completion(Evaluation of generatorBody).
4. d. d. Else,
1. i. i. Assert: generatorBody is an Abstract Closure with no
parameters.
2. ii. ii. Let result be generatorBody().
5. e. e. Assert: If we return here, the generator either threw an exception
or performed either an implicit or explicit return.
6. f. f. Remove acGenContext from the execution context stack and restore
the execution context that is at the top of the execution context stack
as the running execution context.
7. g. g. Set acGenerator.[[GeneratorState]] to completed.
8. h. h. NOTE: Once a generator enters the completed state it never leaves
it and its associated execution context is never resumed. Any execution
state associated with acGenerator can be discarded at this point.
9. i. i. If result.[[Type]] is normal, let resultValue be undefined.
10. j. j. Else if result.[[Type]] is return, let resultValue be
result.[[Value]].
11. k. k. Else,
1. i. i. Assert: result.[[Type]] is throw.
2. ii. ii. Return ? result.
12. l. l. Return CreateIterResultObject(resultValue, true).
5. 5. 5. Set the code evaluation state of genContext such that when evaluation
is resumed for that execution context, closure will be called with no
arguments.
6. 6. 6. Set generator.[[GeneratorContext]] to genContext.
7. 7. 7. Set generator.[[GeneratorState]] to suspendedStart.
8. 8. 8. Return unused.
27.5.3.2 GENERATORVALIDATE ( GENERATOR, GENERATORBRAND )
The abstract operation GeneratorValidate takes arguments generator (an
ECMAScript language value) and generatorBrand (a String or empty) and returns
either a normal completion containing one of suspendedStart, suspendedYield, or
completed, or a throw completion. It performs the following steps when called:
1. 1. 1. Perform ? RequireInternalSlot(generator, [[GeneratorState]]).
2. 2. 2. Perform ? RequireInternalSlot(generator, [[GeneratorBrand]]).
3. 3. 3. If generator.[[GeneratorBrand]] is not generatorBrand, throw a
TypeError exception.
4. 4. 4. Assert: generator also has a [[GeneratorContext]] internal slot.
5. 5. 5. Let state be generator.[[GeneratorState]].
6. 6. 6. If state is executing, throw a TypeError exception.
7. 7. 7. Return state.
27.5.3.3 GENERATORRESUME ( GENERATOR, VALUE, GENERATORBRAND )
The abstract operation GeneratorResume takes arguments generator (an ECMAScript
language value), value (an ECMAScript language value or empty), and
generatorBrand (a String or empty) and returns either a normal completion
containing an ECMAScript language value or a throw completion. It performs the
following steps when called:
1. 1. 1. Let state be ? GeneratorValidate(generator, generatorBrand).
2. 2. 2. If state is completed, return CreateIterResultObject(undefined,
true).
3. 3. 3. Assert: state is either suspendedStart or suspendedYield.
4. 4. 4. Let genContext be generator.[[GeneratorContext]].
5. 5. 5. Let methodContext be the running execution context.
6. 6. 6. Suspend methodContext.
7. 7. 7. Set generator.[[GeneratorState]] to executing.
8. 8. 8. Push genContext onto the execution context stack; genContext is now
the running execution context.
9. 9. 9. Resume the suspended evaluation of genContext using
NormalCompletion(value) as the result of the operation that suspended it.
Let result be the value returned by the resumed computation.
10. 10. 10. Assert: When we return here, genContext has already been removed
from the execution context stack and methodContext is the currently running
execution context.
11. 11. 11. Return ? result.
27.5.3.4 GENERATORRESUMEABRUPT ( GENERATOR, ABRUPTCOMPLETION, GENERATORBRAND )
The abstract operation GeneratorResumeAbrupt takes arguments generator (an
ECMAScript language value), abruptCompletion (a return completion or a throw
completion), and generatorBrand (a String or empty) and returns either a normal
completion containing an ECMAScript language value or a throw completion. It
performs the following steps when called:
1. 1. 1. Let state be ? GeneratorValidate(generator, generatorBrand).
2. 2. 2. If state is suspendedStart, then
1. a. a. Set generator.[[GeneratorState]] to completed.
2. b. b. NOTE: Once a generator enters the completed state it never leaves
it and its associated execution context is never resumed. Any execution
state associated with generator can be discarded at this point.
3. c. c. Set state to completed.
3. 3. 3. If state is completed, then
1. a. a. If abruptCompletion.[[Type]] is return, then
1. i. i. Return CreateIterResultObject(abruptCompletion.[[Value]],
true).
2. b. b. Return ? abruptCompletion.
4. 4. 4. Assert: state is suspendedYield.
5. 5. 5. Let genContext be generator.[[GeneratorContext]].
6. 6. 6. Let methodContext be the running execution context.
7. 7. 7. Suspend methodContext.
8. 8. 8. Set generator.[[GeneratorState]] to executing.
9. 9. 9. Push genContext onto the execution context stack; genContext is now
the running execution context.
10. 10. 10. Resume the suspended evaluation of genContext using
abruptCompletion as the result of the operation that suspended it. Let
result be the Completion Record returned by the resumed computation.
11. 11. 11. Assert: When we return here, genContext has already been removed
from the execution context stack and methodContext is the currently running
execution context.
12. 12. 12. Return ? result.
27.5.3.5 GETGENERATORKIND ( )
The abstract operation GetGeneratorKind takes no arguments and returns
non-generator, sync, or async. It performs the following steps when called:
1. 1. 1. Let genContext be the running execution context.
2. 2. 2. If genContext does not have a Generator component, return
non-generator.
3. 3. 3. Let generator be the Generator component of genContext.
4. 4. 4. If generator has an [[AsyncGeneratorState]] internal slot, return
async.
5. 5. 5. Else, return sync.
27.5.3.6 GENERATORYIELD ( ITERNEXTOBJ )
The abstract operation GeneratorYield takes argument iterNextObj (an Object that
conforms to the IteratorResult interface) and returns either a normal completion
containing an ECMAScript language value or an abrupt completion. It performs the
following steps when called:
1. 1. 1. Let genContext be the running execution context.
2. 2. 2. Assert: genContext is the execution context of a generator.
3. 3. 3. Let generator be the value of the Generator component of genContext.
4. 4. 4. Assert: GetGeneratorKind() is sync.
5. 5. 5. Set generator.[[GeneratorState]] to suspendedYield.
6. 6. 6. Remove genContext from the execution context stack and restore the
execution context that is at the top of the execution context stack as the
running execution context.
7. 7. 7. Let callerContext be the running execution context.
8. 8. 8. Resume callerContext passing NormalCompletion(iterNextObj). If
genContext is ever resumed again, let resumptionValue be the Completion
Record with which it is resumed.
9. 9. 9. Assert: If control reaches here, then genContext is the running
execution context again.
10. 10. 10. Return resumptionValue.
27.5.3.7 YIELD ( VALUE )
The abstract operation Yield takes argument value (an ECMAScript language value)
and returns either a normal completion containing an ECMAScript language value
or an abrupt completion. It performs the following steps when called:
1. 1. 1. Let generatorKind be GetGeneratorKind().
2. 2. 2. If generatorKind is async, return ? AsyncGeneratorYield(?
Await(value)).
3. 3. 3. Otherwise, return ? GeneratorYield(CreateIterResultObject(value,
false)).
27.5.3.8 CREATEITERATORFROMCLOSURE ( CLOSURE, GENERATORBRAND, GENERATORPROTOTYPE
)
The abstract operation CreateIteratorFromClosure takes arguments closure (an
Abstract Closure with no parameters), generatorBrand (a String or empty), and
generatorPrototype (an Object) and returns a Generator. It performs the
following steps when called:
1. 1. 1. NOTE: closure can contain uses of the Yield operation to yield an
IteratorResult object.
2. 2. 2. Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]],
[[GeneratorBrand]] ».
3. 3. 3. Let generator be OrdinaryObjectCreate(generatorPrototype,
internalSlotsList).
4. 4. 4. Set generator.[[GeneratorBrand]] to generatorBrand.
5. 5. 5. Set generator.[[GeneratorState]] to undefined.
6. 6. 6. Let callerContext be the running execution context.
7. 7. 7. Let calleeContext be a new execution context.
8. 8. 8. Set the Function of calleeContext to null.
9. 9. 9. Set the Realm of calleeContext to the current Realm Record.
10. 10. 10. Set the ScriptOrModule of calleeContext to callerContext's
ScriptOrModule.
11. 11. 11. If callerContext is not already suspended, suspend callerContext.
12. 12. 12. Push calleeContext onto the execution context stack; calleeContext
is now the running execution context.
13. 13. 13. Perform GeneratorStart(generator, closure).
14. 14. 14. Remove calleeContext from the execution context stack and restore
callerContext as the running execution context.
15. 15. 15. Return generator.
27.6 ASYNCGENERATOR OBJECTS
An AsyncGenerator is an instance of an async generator function and conforms to
both the AsyncIterator and AsyncIterable interfaces.
AsyncGenerator instances directly inherit properties from the object that is the
initial value of the "prototype" property of the AsyncGenerator function that
created the instance. AsyncGenerator instances indirectly inherit properties
from the AsyncGenerator Prototype intrinsic,
%AsyncGeneratorFunction.prototype.prototype%.
27.6.1 PROPERTIES OF THE ASYNCGENERATOR PROTOTYPE OBJECT
The AsyncGenerator prototype object:
* is %AsyncGeneratorFunction.prototype.prototype%.
* is an ordinary object.
* is not an AsyncGenerator instance and does not have an
[[AsyncGeneratorState]] internal slot.
* has a [[Prototype]] internal slot whose value is %AsyncIteratorPrototype%.
* has properties that are indirectly inherited by all AsyncGenerator instances.
27.6.1.1 ASYNCGENERATOR.PROTOTYPE.CONSTRUCTOR
The initial value of AsyncGenerator.prototype.constructor is
%AsyncGeneratorFunction.prototype%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.6.1.2 ASYNCGENERATOR.PROTOTYPE.NEXT ( VALUE )
1. 1. 1. Let generator be the this value.
2. 2. 2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
3. 3. 3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
4. 4. 4. IfAbruptRejectPromise(result, promiseCapability).
5. 5. 5. Let state be generator.[[AsyncGeneratorState]].
6. 6. 6. If state is completed, then
1. a. a. Let iteratorResult be CreateIterResultObject(undefined, true).
2. b. b. Perform ! Call(promiseCapability.[[Resolve]], undefined, «
iteratorResult »).
3. c. c. Return promiseCapability.[[Promise]].
7. 7. 7. Let completion be NormalCompletion(value).
8. 8. 8. Perform AsyncGeneratorEnqueue(generator, completion,
promiseCapability).
9. 9. 9. If state is either suspendedStart or suspendedYield, then
1. a. a. Perform AsyncGeneratorResume(generator, completion).
10. 10. 10. Else,
1. a. a. Assert: state is either executing or awaiting-return.
11. 11. 11. Return promiseCapability.[[Promise]].
27.6.1.3 ASYNCGENERATOR.PROTOTYPE.RETURN ( VALUE )
1. 1. 1. Let generator be the this value.
2. 2. 2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
3. 3. 3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
4. 4. 4. IfAbruptRejectPromise(result, promiseCapability).
5. 5. 5. Let completion be Completion Record { [[Type]]: return, [[Value]]:
value, [[Target]]: empty }.
6. 6. 6. Perform AsyncGeneratorEnqueue(generator, completion,
promiseCapability).
7. 7. 7. Let state be generator.[[AsyncGeneratorState]].
8. 8. 8. If state is either suspendedStart or completed, then
1. a. a. Set generator.[[AsyncGeneratorState]] to awaiting-return.
2. b. b. Perform ! AsyncGeneratorAwaitReturn(generator).
9. 9. 9. Else if state is suspendedYield, then
1. a. a. Perform AsyncGeneratorResume(generator, completion).
10. 10. 10. Else,
1. a. a. Assert: state is either executing or awaiting-return.
11. 11. 11. Return promiseCapability.[[Promise]].
27.6.1.4 ASYNCGENERATOR.PROTOTYPE.THROW ( EXCEPTION )
1. 1. 1. Let generator be the this value.
2. 2. 2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
3. 3. 3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
4. 4. 4. IfAbruptRejectPromise(result, promiseCapability).
5. 5. 5. Let state be generator.[[AsyncGeneratorState]].
6. 6. 6. If state is suspendedStart, then
1. a. a. Set generator.[[AsyncGeneratorState]] to completed.
2. b. b. Set state to completed.
7. 7. 7. If state is completed, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, «
exception »).
2. b. b. Return promiseCapability.[[Promise]].
8. 8. 8. Let completion be ThrowCompletion(exception).
9. 9. 9. Perform AsyncGeneratorEnqueue(generator, completion,
promiseCapability).
10. 10. 10. If state is suspendedYield, then
1. a. a. Perform AsyncGeneratorResume(generator, completion).
11. 11. 11. Else,
1. a. a. Assert: state is either executing or awaiting-return.
12. 12. 12. Return promiseCapability.[[Promise]].
27.6.1.5 ASYNCGENERATOR.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"AsyncGenerator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.6.2 PROPERTIES OF ASYNCGENERATOR INSTANCES
AsyncGenerator instances are initially created with the internal slots described
below:
Table 83: Internal Slots of AsyncGenerator Instances
Internal Slot Type Description [[AsyncGeneratorState]] undefined,
suspendedStart, suspendedYield, executing, awaiting-return, or completed The
current execution state of the async generator. [[AsyncGeneratorContext]] an
execution context The execution context that is used when executing the code of
this async generator. [[AsyncGeneratorQueue]] a List of AsyncGeneratorRequest
Records Records which represent requests to resume the async generator. Except
during state transitions, it is non-empty if and only if [[AsyncGeneratorState]]
is either executing or awaiting-return. [[GeneratorBrand]] a String or empty A
brand used to distinguish different kinds of async generators. The
[[GeneratorBrand]] of async generators declared by ECMAScript source text is
always empty.
27.6.3 ASYNCGENERATOR ABSTRACT OPERATIONS
27.6.3.1 ASYNCGENERATORREQUEST RECORDS
An AsyncGeneratorRequest is a Record value used to store information about how
an async generator should be resumed and contains capabilities for fulfilling or
rejecting the corresponding promise.
They have the following fields:
Table 84: AsyncGeneratorRequest Record Fields
Field Name Value Meaning [[Completion]] a Completion Record The Completion
Record which should be used to resume the async generator. [[Capability]] a
PromiseCapability Record The promise capabilities associated with this request.
27.6.3.2 ASYNCGENERATORSTART ( GENERATOR, GENERATORBODY )
The abstract operation AsyncGeneratorStart takes arguments generator (an
AsyncGenerator) and generatorBody (a FunctionBody Parse Node or an Abstract
Closure with no parameters) and returns unused. It performs the following steps
when called:
1. 1. 1. Assert: generator.[[AsyncGeneratorState]] is undefined.
2. 2. 2. Let genContext be the running execution context.
3. 3. 3. Set the Generator component of genContext to generator.
4. 4. 4. Let closure be a new Abstract Closure with no parameters that captures
generatorBody and performs the following steps when called:
1. a. a. Let acGenContext be the running execution context.
2. b. b. Let acGenerator be the Generator component of acGenContext.
3. c. c. If generatorBody is a Parse Node, then
1. i. i. Let result be Completion(Evaluation of generatorBody).
4. d. d. Else,
1. i. i. Assert: generatorBody is an Abstract Closure with no
parameters.
2. ii. ii. Let result be Completion(generatorBody()).
5. e. e. Assert: If we return here, the async generator either threw an
exception or performed either an implicit or explicit return.
6. f. f. Remove acGenContext from the execution context stack and restore
the execution context that is at the top of the execution context stack
as the running execution context.
7. g. g. Set acGenerator.[[AsyncGeneratorState]] to completed.
8. h. h. If result.[[Type]] is normal, set result to
NormalCompletion(undefined).
9. i. i. If result.[[Type]] is return, set result to
NormalCompletion(result.[[Value]]).
10. j. j. Perform AsyncGeneratorCompleteStep(acGenerator, result, true).
11. k. k. Perform AsyncGeneratorDrainQueue(acGenerator).
12. l. l. Return undefined.
5. 5. 5. Set the code evaluation state of genContext such that when evaluation
is resumed for that execution context, closure will be called with no
arguments.
6. 6. 6. Set generator.[[AsyncGeneratorContext]] to genContext.
7. 7. 7. Set generator.[[AsyncGeneratorState]] to suspendedStart.
8. 8. 8. Set generator.[[AsyncGeneratorQueue]] to a new empty List.
9. 9. 9. Return unused.
27.6.3.3 ASYNCGENERATORVALIDATE ( GENERATOR, GENERATORBRAND )
The abstract operation AsyncGeneratorValidate takes arguments generator (an
ECMAScript language value) and generatorBrand (a String or empty) and returns
either a normal completion containing unused or a throw completion. It performs
the following steps when called:
1. 1. 1. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorContext]]).
2. 2. 2. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorState]]).
3. 3. 3. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorQueue]]).
4. 4. 4. If generator.[[GeneratorBrand]] is not generatorBrand, throw a
TypeError exception.
5. 5. 5. Return unused.
27.6.3.4 ASYNCGENERATORENQUEUE ( GENERATOR, COMPLETION, PROMISECAPABILITY )
The abstract operation AsyncGeneratorEnqueue takes arguments generator (an
AsyncGenerator), completion (a Completion Record), and promiseCapability (a
PromiseCapability Record) and returns unused. It performs the following steps
when called:
1. 1. 1. Let request be AsyncGeneratorRequest { [[Completion]]: completion,
[[Capability]]: promiseCapability }.
2. 2. 2. Append request to generator.[[AsyncGeneratorQueue]].
3. 3. 3. Return unused.
27.6.3.5 ASYNCGENERATORCOMPLETESTEP ( GENERATOR, COMPLETION, DONE [ , REALM ] )
The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an
AsyncGenerator), completion (a Completion Record), and done (a Boolean) and
optional argument realm (a Realm Record) and returns unused. It performs the
following steps when called:
1. 1. 1. Assert: generator.[[AsyncGeneratorQueue]] is not empty.
2. 2. 2. Let next be the first element of generator.[[AsyncGeneratorQueue]].
3. 3. 3. Remove the first element from generator.[[AsyncGeneratorQueue]].
4. 4. 4. Let promiseCapability be next.[[Capability]].
5. 5. 5. Let value be completion.[[Value]].
6. 6. 6. If completion.[[Type]] is throw, then
1. a. a. Perform ! Call(promiseCapability.[[Reject]], undefined, « value »).
7. 7. 7. Else,
1. a. a. Assert: completion.[[Type]] is normal.
2. b. b. If realm is present, then
1. i. i. Let oldRealm be the running execution context's Realm.
2. ii. ii. Set the running execution context's Realm to realm.
3. iii. iii. Let iteratorResult be CreateIterResultObject(value, done).
4. iv. iv. Set the running execution context's Realm to oldRealm.
3. c. c. Else,
1. i. i. Let iteratorResult be CreateIterResultObject(value, done).
4. d. d. Perform ! Call(promiseCapability.[[Resolve]], undefined, «
iteratorResult »).
8. 8. 8. Return unused.
27.6.3.6 ASYNCGENERATORRESUME ( GENERATOR, COMPLETION )
The abstract operation AsyncGeneratorResume takes arguments generator (an
AsyncGenerator) and completion (a Completion Record) and returns unused. It
performs the following steps when called:
1. 1. 1. Assert: generator.[[AsyncGeneratorState]] is either suspendedStart or
suspendedYield.
2. 2. 2. Let genContext be generator.[[AsyncGeneratorContext]].
3. 3. 3. Let callerContext be the running execution context.
4. 4. 4. Suspend callerContext.
5. 5. 5. Set generator.[[AsyncGeneratorState]] to executing.
6. 6. 6. Push genContext onto the execution context stack; genContext is now
the running execution context.
7. 7. 7. Resume the suspended evaluation of genContext using completion as the
result of the operation that suspended it. Let result be the Completion
Record returned by the resumed computation.
8. 8. 8. Assert: result is never an abrupt completion.
9. 9. 9. Assert: When we return here, genContext has already been removed from
the execution context stack and callerContext is the currently running
execution context.
10. 10. 10. Return unused.
27.6.3.7 ASYNCGENERATORUNWRAPYIELDRESUMPTION ( RESUMPTIONVALUE )
The abstract operation AsyncGeneratorUnwrapYieldResumption takes argument
resumptionValue (a Completion Record) and returns either a normal completion
containing an ECMAScript language value or an abrupt completion. It performs the
following steps when called:
1. 1. 1. If resumptionValue.[[Type]] is not return, return ? resumptionValue.
2. 2. 2. Let awaited be Completion(Await(resumptionValue.[[Value]])).
3. 3. 3. If awaited.[[Type]] is throw, return ? awaited.
4. 4. 4. Assert: awaited.[[Type]] is normal.
5. 5. 5. Return Completion Record { [[Type]]: return, [[Value]]:
awaited.[[Value]], [[Target]]: empty }.
27.6.3.8 ASYNCGENERATORYIELD ( VALUE )
The abstract operation AsyncGeneratorYield takes argument value (an ECMAScript
language value) and returns either a normal completion containing an ECMAScript
language value or an abrupt completion. It performs the following steps when
called:
1. 1. 1. Let genContext be the running execution context.
2. 2. 2. Assert: genContext is the execution context of a generator.
3. 3. 3. Let generator be the value of the Generator component of genContext.
4. 4. 4. Assert: GetGeneratorKind() is async.
5. 5. 5. Let completion be NormalCompletion(value).
6. 6. 6. Assert: The execution context stack has at least two elements.
7. 7. 7. Let previousContext be the second to top element of the execution
context stack.
8. 8. 8. Let previousRealm be previousContext's Realm.
9. 9. 9. Perform AsyncGeneratorCompleteStep(generator, completion, false,
previousRealm).
10. 10. 10. Let queue be generator.[[AsyncGeneratorQueue]].
11. 11. 11. If queue is not empty, then
1. a. a. NOTE: Execution continues without suspending the generator.
2. b. b. Let toYield be the first element of queue.
3. c. c. Let resumptionValue be Completion(toYield.[[Completion]]).
4. d. d. Return ? AsyncGeneratorUnwrapYieldResumption(resumptionValue).
12. 12. 12. Else,
1. a. a. Set generator.[[AsyncGeneratorState]] to suspendedYield.
2. b. b. Remove genContext from the execution context stack and restore the
execution context that is at the top of the execution context stack as
the running execution context.
3. c. c. Let callerContext be the running execution context.
4. d. d. Resume callerContext passing undefined. If genContext is ever
resumed again, let resumptionValue be the Completion Record with which
it is resumed.
5. e. e. Assert: If control reaches here, then genContext is the running
execution context again.
6. f. f. Return ? AsyncGeneratorUnwrapYieldResumption(resumptionValue).
27.6.3.9 ASYNCGENERATORAWAITRETURN ( GENERATOR )
The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an
AsyncGenerator) and returns either a normal completion containing unused or a
throw completion. It performs the following steps when called:
1. 1. 1. Let queue be generator.[[AsyncGeneratorQueue]].
2. 2. 2. Assert: queue is not empty.
3. 3. 3. Let next be the first element of queue.
4. 4. 4. Let completion be Completion(next.[[Completion]]).
5. 5. 5. Assert: completion.[[Type]] is return.
6. 6. 6. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
7. 7. 7. Let fulfilledClosure be a new Abstract Closure with parameters
(value) that captures generator and performs the following steps when
called:
1. a. a. Set generator.[[AsyncGeneratorState]] to completed.
2. b. b. Let result be NormalCompletion(value).
3. c. c. Perform AsyncGeneratorCompleteStep(generator, result, true).
4. d. d. Perform AsyncGeneratorDrainQueue(generator).
5. e. e. Return undefined.
8. 8. 8. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", «
»).
9. 9. 9. Let rejectedClosure be a new Abstract Closure with parameters
(reason) that captures generator and performs the following steps when
called:
1. a. a. Set generator.[[AsyncGeneratorState]] to completed.
2. b. b. Let result be ThrowCompletion(reason).
3. c. c. Perform AsyncGeneratorCompleteStep(generator, result, true).
4. d. d. Perform AsyncGeneratorDrainQueue(generator).
5. e. e. Return undefined.
10. 10. 10. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", «
»).
11. 11. 11. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
12. 12. 12. Return unused.
27.6.3.10 ASYNCGENERATORDRAINQUEUE ( GENERATOR )
The abstract operation AsyncGeneratorDrainQueue takes argument generator (an
AsyncGenerator) and returns unused. It drains the generator's
AsyncGeneratorQueue until it encounters an AsyncGeneratorRequest which holds a
return completion. It performs the following steps when called:
1. 1. 1. Assert: generator.[[AsyncGeneratorState]] is completed.
2. 2. 2. Let queue be generator.[[AsyncGeneratorQueue]].
3. 3. 3. If queue is empty, return unused.
4. 4. 4. Let done be false.
5. 5. 5. Repeat, while done is false,
1. a. a. Let next be the first element of queue.
2. b. b. Let completion be Completion(next.[[Completion]]).
3. c. c. If completion.[[Type]] is return, then
1. i. i. Set generator.[[AsyncGeneratorState]] to awaiting-return.
2. ii. ii. Perform ! AsyncGeneratorAwaitReturn(generator).
3. iii. iii. Set done to true.
4. d. d. Else,
1. i. i. If completion.[[Type]] is normal, then
1. 1. 1. Set completion to NormalCompletion(undefined).
2. ii. ii. Perform AsyncGeneratorCompleteStep(generator, completion,
true).
3. iii. iii. If queue is empty, set done to true.
6. 6. 6. Return unused.
27.6.3.11 CREATEASYNCITERATORFROMCLOSURE ( CLOSURE, GENERATORBRAND,
GENERATORPROTOTYPE )
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure
(an Abstract Closure with no parameters), generatorBrand (a String or empty),
and generatorPrototype (an Object) and returns an AsyncGenerator. It performs
the following steps when called:
1. 1. 1. NOTE: closure can contain uses of the Await operation and uses of the
Yield operation to yield an IteratorResult object.
2. 2. 2. Let internalSlotsList be « [[AsyncGeneratorState]],
[[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
3. 3. 3. Let generator be OrdinaryObjectCreate(generatorPrototype,
internalSlotsList).
4. 4. 4. Set generator.[[GeneratorBrand]] to generatorBrand.
5. 5. 5. Set generator.[[AsyncGeneratorState]] to undefined.
6. 6. 6. Let callerContext be the running execution context.
7. 7. 7. Let calleeContext be a new execution context.
8. 8. 8. Set the Function of calleeContext to null.
9. 9. 9. Set the Realm of calleeContext to the current Realm Record.
10. 10. 10. Set the ScriptOrModule of calleeContext to callerContext's
ScriptOrModule.
11. 11. 11. If callerContext is not already suspended, suspend callerContext.
12. 12. 12. Push calleeContext onto the execution context stack; calleeContext
is now the running execution context.
13. 13. 13. Perform AsyncGeneratorStart(generator, closure).
14. 14. 14. Remove calleeContext from the execution context stack and restore
callerContext as the running execution context.
15. 15. 15. Return generator.
27.7 ASYNCFUNCTION OBJECTS
AsyncFunctions are functions that are usually created by evaluating
AsyncFunctionDeclarations, AsyncFunctionExpressions, AsyncMethods, and
AsyncArrowFunctions. They may also be created by calling the %AsyncFunction%
intrinsic.
27.7.1 THE ASYNCFUNCTION CONSTRUCTOR
The AsyncFunction constructor:
* is %AsyncFunction%.
* is a subclass of Function.
* creates and initializes a new AsyncFunction when called as a function rather
than as a constructor. Thus the function call AsyncFunction(…) is equivalent
to the object creation expression new AsyncFunction(…) with the same
arguments.
* may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified AsyncFunction behaviour
must include a super call to the AsyncFunction constructor to create and
initialize a subclass instance with the internal slots necessary for built-in
async function behaviour. All ECMAScript syntactic forms for defining async
function objects create direct instances of AsyncFunction. There is no
syntactic means to create instances of AsyncFunction subclasses.
27.7.1.1 ASYNCFUNCTION ( ...PARAMETERARGS, BODYARG )
The last argument (if any) specifies the body (executable code) of an async
function. Any preceding arguments specify formal parameters.
This function performs the following steps when called:
1. 1. 1. Let C be the active function object.
2. 2. 2. If bodyArg is not present, set bodyArg to the empty String.
3. 3. 3. Return ? CreateDynamicFunction(C, NewTarget, async, parameterArgs,
bodyArg).
Note
See NOTE for 20.2.1.1.
27.7.2 PROPERTIES OF THE ASYNCFUNCTION CONSTRUCTOR
The AsyncFunction constructor:
* is a standard built-in function object that inherits from the Function
constructor.
* has a [[Prototype]] internal slot whose value is %Function%.
* has a "name" property whose value is "AsyncFunction".
* has the following properties:
27.7.2.1 ASYNCFUNCTION.LENGTH
This is a data property with a value of 1. This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.7.2.2 ASYNCFUNCTION.PROTOTYPE
The initial value of AsyncFunction.prototype is the AsyncFunction prototype
object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
27.7.3 PROPERTIES OF THE ASYNCFUNCTION PROTOTYPE OBJECT
The AsyncFunction prototype object:
* is %AsyncFunction.prototype%.
* is an ordinary object.
* is not a function object and does not have an [[ECMAScriptCode]] internal
slot or any other of the internal slots listed in Table 30.
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
27.7.3.1 ASYNCFUNCTION.PROTOTYPE.CONSTRUCTOR
The initial value of AsyncFunction.prototype.constructor is %AsyncFunction%
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.7.3.2 ASYNCFUNCTION.PROTOTYPE [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value
"AsyncFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.7.4 ASYNCFUNCTION INSTANCES
Every AsyncFunction instance is an ECMAScript function object and has the
internal slots listed in Table 30. The value of the [[IsClassConstructor]]
internal slot for all such instances is false. AsyncFunction instances are not
constructors and do not have a [[Construct]] internal method. AsyncFunction
instances do not have a prototype property as they are not constructible.
Each AsyncFunction instance has the following own properties:
27.7.4.1 LENGTH
The specification for the "length" property of Function instances given in
20.2.4.1 also applies to AsyncFunction instances.
27.7.4.2 NAME
The specification for the "name" property of Function instances given in
20.2.4.2 also applies to AsyncFunction instances.
27.7.5 ASYNC FUNCTIONS ABSTRACT OPERATIONS
27.7.5.1 ASYNCFUNCTIONSTART ( PROMISECAPABILITY, ASYNCFUNCTIONBODY )
The abstract operation AsyncFunctionStart takes arguments promiseCapability (a
PromiseCapability Record) and asyncFunctionBody (a FunctionBody Parse Node or an
ExpressionBody Parse Node) and returns unused. It performs the following steps
when called:
1. 1. 1. Let runningContext be the running execution context.
2. 2. 2. Let asyncContext be a copy of runningContext.
3. 3. 3. NOTE: Copying the execution state is required for AsyncBlockStart to
resume its execution. It is ill-defined to resume a currently executing
context.
4. 4. 4. Perform AsyncBlockStart(promiseCapability, asyncFunctionBody,
asyncContext).
5. 5. 5. Return unused.
27.7.5.2 ASYNCBLOCKSTART ( PROMISECAPABILITY, ASYNCBODY, ASYNCCONTEXT )
The abstract operation AsyncBlockStart takes arguments promiseCapability (a
PromiseCapability Record), asyncBody (a Parse Node), and asyncContext (an
execution context) and returns unused. It performs the following steps when
called:
1. 1. 1. Assert: promiseCapability is a PromiseCapability Record.
2. 2. 2. Let runningContext be the running execution context.
3. 3. 3. Let closure be a new Abstract Closure with no parameters that captures
promiseCapability and asyncBody and performs the following steps when
called:
1. a. a. Let acAsyncContext be the running execution context.
2. b. b. Let result be Completion(Evaluation of asyncBody).
3. c. c. Assert: If we return here, the async function either threw an
exception or performed an implicit or explicit return; all awaiting is
done.
4. d. d. Remove acAsyncContext from the execution context stack and restore
the execution context that is at the top of the execution context stack
as the running execution context.
5. e. e. If result.[[Type]] is normal, then
1. i. i. Perform ! Call(promiseCapability.[[Resolve]], undefined, «
undefined »).
6. f. f. Else if result.[[Type]] is return, then
1. i. i. Perform ! Call(promiseCapability.[[Resolve]], undefined, «
result.[[Value]] »).
7. g. g. Else,
1. i. i. Assert: result.[[Type]] is throw.
2. ii. ii. Perform ! Call(promiseCapability.[[Reject]], undefined, «
result.[[Value]] »).
8. h. h. Return unused.
4. 4. 4. Set the code evaluation state of asyncContext such that when
evaluation is resumed for that execution context, closure will be called
with no arguments.
5. 5. 5. Push asyncContext onto the execution context stack; asyncContext is
now the running execution context.
6. 6. 6. Resume the suspended evaluation of asyncContext. Let result be the
value returned by the resumed computation.
7. 7. 7. Assert: When we return here, asyncContext has already been removed
from the execution context stack and runningContext is the currently running
execution context.
8. 8. 8. Assert: result is a normal completion with a value of unused. The
possible sources of this value are Await or, if the async function doesn't
await anything, step 3.h above.
9. 9. 9. Return unused.
27.7.5.3 AWAIT ( VALUE )
The abstract operation Await takes argument value (an ECMAScript language value)
and returns either a normal completion containing either an ECMAScript language
value or empty, or a throw completion. It performs the following steps when
called:
1. 1. 1. Let asyncContext be the running execution context.
2. 2. 2. Let promise be ? PromiseResolve(%Promise%, value).
3. 3. 3. Let fulfilledClosure be a new Abstract Closure with parameters (v)
that captures asyncContext and performs the following steps when called:
1. a. a. Let prevContext be the running execution context.
2. b. b. Suspend prevContext.
3. c. c. Push asyncContext onto the execution context stack; asyncContext
is now the running execution context.
4. d. d. Resume the suspended evaluation of asyncContext using
NormalCompletion(v) as the result of the operation that suspended it.
5. e. e. Assert: When we reach this step, asyncContext has already been
removed from the execution context stack and prevContext is the
currently running execution context.
6. f. f. Return undefined.
4. 4. 4. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", «
»).
5. 5. 5. Let rejectedClosure be a new Abstract Closure with parameters
(reason) that captures asyncContext and performs the following steps when
called:
1. a. a. Let prevContext be the running execution context.
2. b. b. Suspend prevContext.
3. c. c. Push asyncContext onto the execution context stack; asyncContext
is now the running execution context.
4. d. d. Resume the suspended evaluation of asyncContext using
ThrowCompletion(reason) as the result of the operation that suspended
it.
5. e. e. Assert: When we reach this step, asyncContext has already been
removed from the execution context stack and prevContext is the
currently running execution context.
6. f. f. Return undefined.
6. 6. 6. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
7. 7. 7. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
8. 8. 8. Remove asyncContext from the execution context stack and restore the
execution context that is at the top of the execution context stack as the
running execution context.
9. 9. 9. Let callerContext be the running execution context.
10. 10. 10. Resume callerContext passing empty. If asyncContext is ever resumed
again, let completion be the Completion Record with which it is resumed.
11. 11. 11. Assert: If control reaches here, then asyncContext is the running
execution context again.
12. 12. 12. Return completion.
28 REFLECTION
28.1 THE REFLECT OBJECT
The Reflect object:
* is %Reflect%.
* is the initial value of the "Reflect" property of the global object.
* is an ordinary object.
* has a [[Prototype]] internal slot whose value is %Object.prototype%.
* is not a function object.
* does not have a [[Construct]] internal method; it cannot be used as a
constructor with the new operator.
* does not have a [[Call]] internal method; it cannot be invoked as a function.
28.1.1 REFLECT.APPLY ( TARGET, THISARGUMENT, ARGUMENTSLIST )
This function performs the following steps when called:
1. 1. 1. If IsCallable(target) is false, throw a TypeError exception.
2. 2. 2. Let args be ? CreateListFromArrayLike(argumentsList).
3. 3. 3. Perform PrepareForTailCall().
4. 4. 4. Return ? Call(target, thisArgument, args).
28.1.2 REFLECT.CONSTRUCT ( TARGET, ARGUMENTSLIST [ , NEWTARGET ] )
This function performs the following steps when called:
1. 1. 1. If IsConstructor(target) is false, throw a TypeError exception.
2. 2. 2. If newTarget is not present, set newTarget to target.
3. 3. 3. Else if IsConstructor(newTarget) is false, throw a TypeError
exception.
4. 4. 4. Let args be ? CreateListFromArrayLike(argumentsList).
5. 5. 5. Return ? Construct(target, args, newTarget).
28.1.3 REFLECT.DEFINEPROPERTY ( TARGET, PROPERTYKEY, ATTRIBUTES )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(propertyKey).
3. 3. 3. Let desc be ? ToPropertyDescriptor(attributes).
4. 4. 4. Return ? target.[[DefineOwnProperty]](key, desc).
28.1.4 REFLECT.DELETEPROPERTY ( TARGET, PROPERTYKEY )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(propertyKey).
3. 3. 3. Return ? target.[[Delete]](key).
28.1.5 REFLECT.GET ( TARGET, PROPERTYKEY [ , RECEIVER ] )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(propertyKey).
3. 3. 3. If receiver is not present, then
1. a. a. Set receiver to target.
4. 4. 4. Return ? target.[[Get]](key, receiver).
28.1.6 REFLECT.GETOWNPROPERTYDESCRIPTOR ( TARGET, PROPERTYKEY )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(propertyKey).
3. 3. 3. Let desc be ? target.[[GetOwnProperty]](key).
4. 4. 4. Return FromPropertyDescriptor(desc).
28.1.7 REFLECT.GETPROTOTYPEOF ( TARGET )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Return ? target.[[GetPrototypeOf]]().
28.1.8 REFLECT.HAS ( TARGET, PROPERTYKEY )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(propertyKey).
3. 3. 3. Return ? target.[[HasProperty]](key).
28.1.9 REFLECT.ISEXTENSIBLE ( TARGET )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Return ? target.[[IsExtensible]]().
28.1.10 REFLECT.OWNKEYS ( TARGET )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let keys be ? target.[[OwnPropertyKeys]]().
3. 3. 3. Return CreateArrayFromList(keys).
28.1.11 REFLECT.PREVENTEXTENSIONS ( TARGET )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Return ? target.[[PreventExtensions]]().
28.1.12 REFLECT.SET ( TARGET, PROPERTYKEY, V [ , RECEIVER ] )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. Let key be ? ToPropertyKey(propertyKey).
3. 3. 3. If receiver is not present, then
1. a. a. Set receiver to target.
4. 4. 4. Return ? target.[[Set]](key, V, receiver).
28.1.13 REFLECT.SETPROTOTYPEOF ( TARGET, PROTO )
This function performs the following steps when called:
1. 1. 1. If target is not an Object, throw a TypeError exception.
2. 2. 2. If proto is not an Object and proto is not null, throw a TypeError
exception.
3. 3. 3. Return ? target.[[SetPrototypeOf]](proto).
28.1.14 REFLECT [ @@TOSTRINGTAG ]
The initial value of the @@toStringTag property is the String value "Reflect".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: true }.
28.2 PROXY OBJECTS
28.2.1 THE PROXY CONSTRUCTOR
The Proxy constructor:
* is %Proxy%.
* is the initial value of the "Proxy" property of the global object.
* creates and initializes a new Proxy object when called as a constructor.
* is not intended to be called as a function and will throw an exception when
called in that manner.
28.2.1.1 PROXY ( TARGET, HANDLER )
This function performs the following steps when called:
1. 1. 1. If NewTarget is undefined, throw a TypeError exception.
2. 2. 2. Return ? ProxyCreate(target, handler).
28.2.2 PROPERTIES OF THE PROXY CONSTRUCTOR
The Proxy constructor:
* has a [[Prototype]] internal slot whose value is %Function.prototype%.
* does not have a "prototype" property because Proxy objects do not have a
[[Prototype]] internal slot that requires initialization.
* has the following properties:
28.2.2.1 PROXY.REVOCABLE ( TARGET, HANDLER )
This function creates a revocable Proxy object.
It performs the following steps when called:
1. 1. 1. Let proxy be ? ProxyCreate(target, handler).
2. 2. 2. Let revokerClosure be a new Abstract Closure with no parameters that
captures nothing and performs the following steps when called:
1. a. a. Let F be the active function object.
2. b. b. Let p be F.[[RevocableProxy]].
3. c. c. If p is null, return undefined.
4. d. d. Set F.[[RevocableProxy]] to null.
5. e. e. Assert: p is a Proxy object.
6. f. f. Set p.[[ProxyTarget]] to null.
7. g. g. Set p.[[ProxyHandler]] to null.
8. h. h. Return undefined.
3. 3. 3. Let revoker be CreateBuiltinFunction(revokerClosure, 0, "", «
[[RevocableProxy]] »).
4. 4. 4. Set revoker.[[RevocableProxy]] to proxy.
5. 5. 5. Let result be OrdinaryObjectCreate(%Object.prototype%).
6. 6. 6. Perform ! CreateDataPropertyOrThrow(result, "proxy", proxy).
7. 7. 7. Perform ! CreateDataPropertyOrThrow(result, "revoke", revoker).
8. 8. 8. Return result.
28.3 MODULE NAMESPACE OBJECTS
A Module Namespace Object is a module namespace exotic object that provides
runtime property-based access to a module's exported bindings. There is no
constructor function for Module Namespace Objects. Instead, such an object is
created for each module that is imported by an ImportDeclaration that contains a
NameSpaceImport.
In addition to the properties specified in 10.4.6 each Module Namespace Object
has the following own property:
28.3.1 @@TOSTRINGTAG
The initial value of the @@toStringTag property is the String value "Module".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }.
29 MEMORY MODEL
The memory consistency model, or memory model, specifies the possible orderings
of Shared Data Block events, arising via accessing TypedArray instances backed
by a SharedArrayBuffer and via methods on the Atomics object. When the program
has no data races (defined below), the ordering of events appears as
sequentially consistent, i.e., as an interleaving of actions from each agent.
When the program has data races, shared memory operations may appear
sequentially inconsistent. For example, programs may exhibit causality-violating
behaviour and other astonishments. These astonishments arise from compiler
transforms and the design of CPUs (e.g., out-of-order execution and
speculation). The memory model defines both the precise conditions under which a
program exhibits sequentially consistent behaviour as well as the possible
values read from data races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced by
abstract operations on SharedArrayBuffer or by methods on the Atomics object
during an evaluation.
Note
This section provides an axiomatic model on events introduced by the abstract
operations on SharedArrayBuffers. It bears stressing that the model is not
expressible algorithmically, unlike the rest of this specification. The
nondeterministic introduction of events by abstract operations is the interface
between the operational semantics of ECMAScript evaluation and the axiomatic
semantics of the memory model. The semantics of these events is defined by
considering graphs of all events in an evaluation. These are neither Static
Semantics nor Runtime Semantics. There is no demonstrated algorithmic
implementation, but instead a set of constraints that determine if a particular
event graph is allowed or disallowed.
29.1 MEMORY MODEL FUNDAMENTALS
Shared memory accesses (reads and writes) are divided into two groups, atomic
accesses and data accesses, defined below. Atomic accesses are sequentially
consistent, i.e., there is a strict total ordering of events agreed upon by all
agents in an agent cluster. Non-atomic accesses do not have a strict total
ordering agreed upon by all agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered,
such as release-acquire, are supported.
A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or
ReadModifyWriteSharedMemory Record.
Table 85: ReadSharedMemory Event Fields
Field Name Value Meaning [[Order]] SeqCst or Unordered The weakest ordering
guaranteed by the memory model for the event. [[NoTear]] a Boolean Whether this
event is allowed to read from multiple write events with equal range as this
event. [[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the read in [[Block]].
[[ElementSize]] a non-negative integer The size of the read.
Table 86: WriteSharedMemory Event Fields
Field Name Value Meaning [[Order]] SeqCst, Unordered, or Init The weakest
ordering guaranteed by the memory model for the event. [[NoTear]] a Boolean
Whether this event is allowed to be read from multiple read events with equal
range as this event. [[Block]] a Shared Data Block The block the event operates
on. [[ByteIndex]] a non-negative integer The byte address of the write in
[[Block]]. [[ElementSize]] a non-negative integer The size of the write.
[[Payload]] a List of byte values The List of byte values to be read by other
events.
Table 87: ReadModifyWriteSharedMemory Event Fields
Field Name Value Meaning [[Order]] SeqCst Read-modify-write events are always
sequentially consistent. [[NoTear]] true Read-modify-write events cannot tear.
[[Block]] a Shared Data Block The block the event operates on. [[ByteIndex]] a
non-negative integer The byte address of the read-modify-write in [[Block]].
[[ElementSize]] a non-negative integer The size of the read-modify-write.
[[Payload]] a List of byte values The List of byte values to be passed to
[[ModifyOp]]. [[ModifyOp]] a read-modify-write modification function An abstract
closure that returns a modified List of byte values from a read List of byte
values and [[Payload]].
These events are introduced by abstract operations or by methods on the Atomics
object.
Some operations may also introduce Synchronize events. A Synchronize event has
no fields, and exists purely to directly constrain the permitted orderings of
other events.
In addition to Shared Data Block and Synchronize events, there are host-specific
events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or
ReadModifyWriteSharedMemory event be the Set of contiguous integers from its
[[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are
equal when the events have the same [[Block]], and the ranges are element-wise
equal. Two events' ranges are overlapping when the events have the same
[[Block]], the ranges are not equal and their intersection is non-empty. Two
events' ranges are disjoint when the events do not have the same [[Block]] or
their ranges are neither equal nor overlapping.
Note 2
Examples of host-specific synchronizing events that should be accounted for are:
sending a SharedArrayBuffer from one agent to another (e.g., by postMessage in a
browser), starting and stopping agents, and communicating within the agent
cluster via channels other than shared memory. It is assumed those events are
appended to agent-order during evaluation like the other SharedArrayBuffer
events.
Events are ordered within candidate executions by the relations defined below.
29.2 AGENT EVENTS RECORDS
An Agent Events Record is a Record with the following fields.
Table 88: Agent Events Record Fields
Field Name Value Meaning [[AgentSignifier]] an agent signifier The agent whose
evaluation resulted in this ordering. [[EventList]] a List of events Events are
appended to the list during evaluation. [[AgentSynchronizesWith]] a List of
pairs of Synchronize events Synchronize relationships introduced by the
operational semantics.
29.3 CHOSEN VALUE RECORDS
A Chosen Value Record is a Record with the following fields.
Table 89: Chosen Value Record Fields
Field Name Value Meaning [[Event]] a Shared Data Block event The
ReadSharedMemory or ReadModifyWriteSharedMemory event that was introduced for
this chosen value. [[ChosenValue]] a List of byte values The bytes that were
nondeterministically chosen during evaluation.
29.4 CANDIDATE EXECUTIONS
A candidate execution of the evaluation of an agent cluster is a Record with the
following fields.
Table 90: Candidate Execution Record Fields
Field Name Value Meaning [[EventsRecords]] a List of Agent Events Records Maps
an agent to Lists of events appended during the evaluation. [[ChosenValues]] a
List of Chosen Value Records Maps ReadSharedMemory or
ReadModifyWriteSharedMemory events to the List of byte values chosen during the
evaluation. [[AgentOrder]] an agent-order Relation Defined below.
[[ReadsBytesFrom]] a reads-bytes-from mathematical function Defined below.
[[ReadsFrom]] a reads-from Relation Defined below. [[HostSynchronizesWith]] a
host-synchronizes-with Relation Defined below. [[SynchronizesWith]] a
synchronizes-with Relation Defined below. [[HappensBefore]] a happens-before
Relation Defined below.
An empty candidate execution is a candidate execution Record whose fields are
empty Lists and Relations.
29.5 ABSTRACT OPERATIONS FOR THE MEMORY MODEL
29.5.1 EVENTSET ( EXECUTION )
The abstract operation EventSet takes argument execution (a candidate execution)
and returns a Set of events. It performs the following steps when called:
1. 1. 1. Let events be an empty Set.
2. 2. 2. For each Agent Events Record aer of execution.[[EventsRecords]], do
1. a. a. For each event E of aer.[[EventList]], do
1. i. i. Add E to events.
3. 3. 3. Return events.
29.5.2 SHAREDDATABLOCKEVENTSET ( EXECUTION )
The abstract operation SharedDataBlockEventSet takes argument execution (a
candidate execution) and returns a Set of events. It performs the following
steps when called:
1. 1. 1. Let events be an empty Set.
2. 2. 2. For each event E of EventSet(execution), do
1. a. a. If E is a ReadSharedMemory, WriteSharedMemory, or
ReadModifyWriteSharedMemory event, add E to events.
3. 3. 3. Return events.
29.5.3 HOSTEVENTSET ( EXECUTION )
The abstract operation HostEventSet takes argument execution (a candidate
execution) and returns a Set of events. It performs the following steps when
called:
1. 1. 1. Let events be an empty Set.
2. 2. 2. For each event E of EventSet(execution), do
1. a. a. If E is not in SharedDataBlockEventSet(execution), add E to events.
3. 3. 3. Return events.
29.5.4 COMPOSEWRITEEVENTBYTES ( EXECUTION, BYTEINDEX, WS )
The abstract operation ComposeWriteEventBytes takes arguments execution (a
candidate execution), byteIndex (a non-negative integer), and Ws (a List of
either WriteSharedMemory or ReadModifyWriteSharedMemory events) and returns a
List of byte values. It performs the following steps when called:
1. 1. 1. Let byteLocation be byteIndex.
2. 2. 2. Let bytesRead be a new empty List.
3. 3. 3. For each element W of Ws, do
1. a. a. Assert: W has byteLocation in its range.
2. b. b. Let payloadIndex be byteLocation - W.[[ByteIndex]].
3. c. c. If W is a WriteSharedMemory event, then
1. i. i. Let byte be W.[[Payload]][payloadIndex].
4. d. d. Else,
1. i. i. Assert: W is a ReadModifyWriteSharedMemory event.
2. ii. ii. Let bytes be ValueOfReadEvent(execution, W).
3. iii. iii. Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
4. iv. iv. Let byte be bytesModified[payloadIndex].
5. e. e. Append byte to bytesRead.
6. f. f. Set byteLocation to byteLocation + 1.
4. 4. 4. Return bytesRead.
Note 1
The read-modify-write modification [[ModifyOp]] is given by the function
properties on the Atomics object that introduce ReadModifyWriteSharedMemory
events.
Note 2
This abstract operation composes a List of write events into a List of byte
values. It is used in the event semantics of ReadSharedMemory and
ReadModifyWriteSharedMemory events.
29.5.5 VALUEOFREADEVENT ( EXECUTION, R )
The abstract operation ValueOfReadEvent takes arguments execution (a candidate
execution) and R (a ReadSharedMemory or ReadModifyWriteSharedMemory event) and
returns a List of byte values. It performs the following steps when called:
1. 1. 1. Let Ws be execution.[[ReadsBytesFrom]](R).
2. 2. 2. Assert: Ws is a List of WriteSharedMemory or
ReadModifyWriteSharedMemory events with length equal to R.[[ElementSize]].
3. 3. 3. Return ComposeWriteEventBytes(execution, R.[[ByteIndex]], Ws).
29.6 RELATIONS OF CANDIDATE EXECUTIONS
29.6.1 AGENT-ORDER
For a candidate execution execution, execution.[[AgentOrder]] is a Relation on
events that satisfies the following.
* For each pair (E, D) in EventSet(execution), execution.[[AgentOrder]]
contains (E, D) if there is some Agent Events Record aer in
execution.[[EventsRecords]] such that E and D are in aer.[[EventList]] and E
is before D in List order of aer.[[EventList]].
Note
Each agent introduces events in a per-agent strict total order during the
evaluation. This is the union of those strict total orders.
29.6.2 READS-BYTES-FROM
For a candidate execution execution, execution.[[ReadsBytesFrom]] is a
mathematical function mapping events in SharedDataBlockEventSet(execution) to
Lists of events in SharedDataBlockEventSet(execution) that satisfies the
following conditions.
* For each ReadSharedMemory or ReadModifyWriteSharedMemory event R in
SharedDataBlockEventSet(execution), execution.[[ReadsBytesFrom]](R) is a List
of length R.[[ElementSize]] whose elements are WriteSharedMemory or
ReadModifyWriteSharedMemory events Ws such that all of the following are
true.
* Each event W with index i in Ws has R.[[ByteIndex]] + i in its range.
* R is not in Ws.
29.6.3 READS-FROM
For a candidate execution execution, execution.[[ReadsFrom]] is the least
Relation on events that satisfies the following.
* For each pair (R, W) in SharedDataBlockEventSet(execution),
execution.[[ReadsFrom]] contains (R, W) if execution.[[ReadsBytesFrom]](R)
contains W.
29.6.4 HOST-SYNCHRONIZES-WITH
For a candidate execution execution, execution.[[HostSynchronizesWith]] is a
host-provided strict partial order on host-specific events that satisfies at
least the following.
* If execution.[[HostSynchronizesWith]] contains (E, D), E and D are in
HostEventSet(execution).
* There is no cycle in the union of execution.[[HostSynchronizesWith]] and
execution.[[AgentOrder]].
Note 1
For two host-specific events E and D, E host-synchronizes-with D implies E
happens-before D.
Note 2
The host-synchronizes-with relation allows the host to provide additional
synchronization mechanisms, such as postMessage between HTML workers.
29.6.5 SYNCHRONIZES-WITH
For a candidate execution execution, execution.[[SynchronizesWith]] is the least
Relation on events that satisfies the following.
* For each pair (R, W) in execution.[[ReadsFrom]],
execution.[[SynchronizesWith]] contains (W, R) if R.[[Order]] is SeqCst,
W.[[Order]] is SeqCst, and R and W have equal ranges.
* For each element eventsRecord of execution.[[EventsRecords]], the following
is true.
* For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]],
execution.[[SynchronizesWith]] contains (S, Sw).
* For each pair (E, D) in execution.[[HostSynchronizesWith]],
execution.[[SynchronizesWith]] contains (E, D).
Note 1
Owing to convention, write events synchronizes-with read events, instead of read
events synchronizes-with write events.
Note 2
Init events do not participate in synchronizes-with, and are instead constrained
directly by happens-before.
Note 3
Not all SeqCst events related by reads-from are related by synchronizes-with.
Only events that also have equal ranges are related by synchronizes-with.
Note 4
For Shared Data Block events R and W such that W synchronizes-with R, R may
reads-from other writes than W.
29.6.6 HAPPENS-BEFORE
For a candidate execution execution, execution.[[HappensBefore]] is the least
Relation on events that satisfies the following.
* For each pair (E, D) in execution.[[AgentOrder]], execution.[[HappensBefore]]
contains (E, D).
* For each pair (E, D) in execution.[[SynchronizesWith]],
execution.[[HappensBefore]] contains (E, D).
* For each pair (E, D) in SharedDataBlockEventSet(execution),
execution.[[HappensBefore]] contains (E, D) if E.[[Order]] is Init and E and
D have overlapping ranges.
* For each pair (E, D) in EventSet(execution), execution.[[HappensBefore]]
contains (E, D) if there is an event F such that the pairs (E, F) and (F, D)
are in execution.[[HappensBefore]].
Note
Because happens-before is a superset of agent-order, candidate executions are
consistent with the single-thread evaluation semantics of ECMAScript.
29.7 PROPERTIES OF VALID EXECUTIONS
29.7.1 VALID CHOSEN READS
A candidate execution execution has valid chosen reads if the following
algorithm returns true.
1. 1. 1. For each ReadSharedMemory or ReadModifyWriteSharedMemory event R of
SharedDataBlockEventSet(execution), do
1. a. a. Let chosenValueRecord be the element of execution.[[ChosenValues]]
whose [[Event]] field is R.
2. b. b. Let chosenValue be chosenValueRecord.[[ChosenValue]].
3. c. c. Let readValue be ValueOfReadEvent(execution, R).
4. d. d. Let chosenLen be the number of elements in chosenValue.
5. e. e. Let readLen be the number of elements in readValue.
6. f. f. If chosenLen ≠ readLen, then
1. i. i. Return false.
7. g. g. If chosenValue[i] ≠ readValue[i] for some integer i in the interval
from 0 (inclusive) to chosenLen (exclusive), then
1. i. i. Return false.
2. 2. 2. Return true.
29.7.2 COHERENT READS
A candidate execution execution has coherent reads if the following algorithm
returns true.
1. 1. 1. For each ReadSharedMemory or ReadModifyWriteSharedMemory event R of
SharedDataBlockEventSet(execution), do
1. a. a. Let Ws be execution.[[ReadsBytesFrom]](R).
2. b. b. Let byteLocation be R.[[ByteIndex]].
3. c. c. For each element W of Ws, do
1. i. i. If execution.[[HappensBefore]] contains (R, W), then
1. 1. 1. Return false.
2. ii. ii. If there exists a WriteSharedMemory or
ReadModifyWriteSharedMemory event V that has byteLocation in its range
such that the pairs (W, V) and (V, R) are in
execution.[[HappensBefore]], then
1. 1. 1. Return false.
3. iii. iii. Set byteLocation to byteLocation + 1.
2. 2. 2. Return true.
29.7.3 TEAR FREE READS
A candidate execution execution has tear free reads if the following algorithm
returns true.
1. 1. 1. For each ReadSharedMemory or ReadModifyWriteSharedMemory event R of
SharedDataBlockEventSet(execution), do
1. a. a. If R.[[NoTear]] is true, then
1. i. i. Assert: The remainder of dividing R.[[ByteIndex]] by
R.[[ElementSize]] is 0.
2. ii. ii. For each event W such that execution.[[ReadsFrom]] contains
(R, W) and W.[[NoTear]] is true, do
1. 1. 1. If R and W have equal ranges and there exists an event V such
that V and W have equal ranges, V.[[NoTear]] is true, W is not V,
and execution.[[ReadsFrom]] contains (R, V), then
1. a. a. Return false.
2. 2. 2. Return true.
Note
An event's [[NoTear]] field is true when that event was introduced via accessing
an integer TypedArray, and false when introduced via accessing a floating point
TypedArray or DataView.
Intuitively, this requirement says when a memory range is accessed in an aligned
fashion via an integer TypedArray, a single write event on that range must "win"
when in a data race with other write events with equal ranges. More precisely,
this requirement says an aligned read event cannot read a value composed of
bytes from multiple, different write events all with equal ranges. It is
possible, however, for an aligned read event to read from multiple write events
with overlapping ranges.
29.7.4 SEQUENTIALLY CONSISTENT ATOMICS
For a candidate execution execution, memory-order is a strict total order of all
events in EventSet(execution) that satisfies the following.
* For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in
memory-order.
* For each pair (R, W) in execution.[[ReadsFrom]], there is no
WriteSharedMemory or ReadModifyWriteSharedMemory event V in
SharedDataBlockEventSet(execution) such that V.[[Order]] is SeqCst, the pairs
(W, V) and (V, R) are in memory-order, and any of the following conditions
are true.
* execution.[[SynchronizesWith]] contains the pair (W, R), and V and R have
equal ranges.
* The pairs (W, R) and (V, R) are in execution.[[HappensBefore]], W.[[Order]]
is SeqCst, and W and V have equal ranges.
* The pairs (W, R) and (W, V) are in execution.[[HappensBefore]], R.[[Order]]
is SeqCst, and V and R have equal ranges.
Note 1
This clause additionally constrains SeqCst events on equal ranges.
* For each WriteSharedMemory or ReadModifyWriteSharedMemory event W in
SharedDataBlockEventSet(execution), if W.[[Order]] is SeqCst, then it is not
the case that there is an infinite number of ReadSharedMemory or
ReadModifyWriteSharedMemory events in SharedDataBlockEventSet(execution) with
equal range that is memory-order before W.
Note 2
This clause together with the forward progress guarantee on agents ensure the
liveness condition that SeqCst writes become visible to SeqCst reads with
equal range in finite time.
A candidate execution has sequentially consistent atomics if a memory-order
exists.
Note 3
While memory-order includes all events in EventSet(execution), those that are
not constrained by happens-before or synchronizes-with are allowed to occur
anywhere in the order.
29.7.5 VALID EXECUTIONS
A candidate execution execution is a valid execution (or simply an execution) if
all of the following are true.
* The host provides a host-synchronizes-with Relation for
execution.[[HostSynchronizesWith]].
* execution.[[HappensBefore]] is a strict partial order.
* execution has valid chosen reads.
* execution has coherent reads.
* execution has tear free reads.
* execution has sequentially consistent atomics.
All programs have at least one valid execution.
29.8 RACES
For an execution execution, two events E and D in
SharedDataBlockEventSet(execution) are in a race if the following algorithm
returns true.
1. 1. 1. If E is not D, then
1. a. a. If the pairs (E, D) and (D, E) are not in
execution.[[HappensBefore]], then
1. i. i. If E and D are both WriteSharedMemory or
ReadModifyWriteSharedMemory events and E and D do not have disjoint
ranges, then
1. 1. 1. Return true.
2. ii. ii. If execution.[[ReadsFrom]] contains either (E, D) or (D, E),
then
1. 1. 1. Return true.
2. 2. 2. Return false.
29.9 DATA RACES
For an execution execution, two events E and D in
SharedDataBlockEventSet(execution) are in a data race if the following algorithm
returns true.
1. 1. 1. If E and D are in a race in execution, then
1. a. a. If E.[[Order]] is not SeqCst or D.[[Order]] is not SeqCst, then
1. i. i. Return true.
2. b. b. If E and D have overlapping ranges, then
1. i. i. Return true.
2. 2. 2. Return false.
29.10 DATA RACE FREEDOM
An execution execution is data race free if there are no two events in
SharedDataBlockEventSet(execution) that are in a data race.
A program is data race free if all its executions are data race free.
The memory model guarantees sequential consistency of all events for data race
free programs.
29.11 SHARED MEMORY GUIDELINES
Note 1
The following are guidelines for ECMAScript programmers working with shared
memory.
We recommend programs be kept data race free, i.e., make it so that it is
impossible for there to be concurrent non-atomic operations on the same memory
location. Data race free programs have interleaving semantics where each step in
the evaluation semantics of each agent are interleaved with each other. For data
race free programs, it is not necessary to understand the details of the memory
model. The details are unlikely to build intuition that will help one to better
write ECMAScript.
More generally, even if a program is not data race free it may have predictable
behaviour, so long as atomic operations are not involved in any data races and
the operations that race all have the same access size. The simplest way to
arrange for atomics not to be involved in races is to ensure that different
memory cells are used by atomic and non-atomic operations and that atomic
accesses of different sizes are not used to access the same cells at the same
time. Effectively, the program should treat shared memory as strongly typed as
much as possible. One still cannot depend on the ordering and timing of
non-atomic accesses that race, but if memory is treated as strongly typed the
racing accesses will not "tear" (bits of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler
transformations for programs using shared memory.
It is desirable to allow most program transformations that are valid in a
single-agent setting in a multi-agent setting, to ensure that the performance of
each agent in a multi-agent program is as good as it would be in a single-agent
setting. Frequently these transformations are hard to judge. We outline some
rules about program transformations that are intended to be taken as normative
(in that they are implied by the memory model or stronger than what the memory
model implies) but which are likely not exhaustive. These rules are intended to
apply to program transformations that precede the introductions of the events
that make up the agent-order.
Let an agent-order slice be the subset of the agent-order pertaining to a single
agent.
Let possible read values of a read event be the set of all values of
ValueOfReadEvent for that event across all valid executions.
Any transformation of an agent-order slice that is valid in the absence of
shared memory is valid in the presence of shared memory, with the following
exceptions.
* Atomics are carved in stone: Program transformations must not cause the
SeqCst events in an agent-order slice to be reordered with its Unordered
operations, nor its SeqCst operations to be reordered with each other, nor
may a program transformation remove a SeqCst operation from the agent-order.
(In practice, the prohibition on reorderings forces a compiler to assume that
every SeqCst operation is a synchronization and included in the final
memory-order, which it would usually have to assume anyway in the absence of
inter-agent program analysis. It also forces the compiler to assume that
every call where the callee's effects on the memory-order are unknown may
contain SeqCst operations.)
* Reads must be stable: Any given shared memory read must only observe a single
value in an execution.
(For example, if what is semantically a single read in the program is
executed multiple times then the program is subsequently allowed to observe
only one of the values read. A transformation known as rematerialization can
violate this rule.)
* Writes must be stable: All observable writes to shared memory must follow
from program semantics in an execution.
(For example, a transformation may not introduce certain observable writes,
such as by using read-modify-write operations on a larger location to write a
smaller datum, writing a value to memory that the program could not have
written, or writing a just-read value back to the location it was read from,
if that location could have been overwritten by another agent after the
read.)
* Possible read values must be non-empty: Program transformations cannot cause
the possible read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes,
because writes have force in memory model insofar as to be read by read
events. For example, writes may be moved and coalesced and sometimes
reordered between two SeqCst operations, but the transformation may not
remove every write that updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic
reads from the same location, reordering non-atomic reads, introducing
speculative non-atomic reads, merging multiple non-atomic writes to the same
location, reordering non-atomic writes to different locations, and hoisting
non-atomic reads out of loops even if that affects termination. Note in general
that aliased TypedArrays make it hard to prove that locations are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code
for shared memory accesses.
For architectures with memory models no weaker than those of ARM or Power,
non-atomic stores and loads may be compiled to bare stores and loads on the
target architecture. Atomic stores and loads may be compiled down to
instructions that guarantee sequential consistency. If no such instructions
exist, memory barriers are to be employed, such as placing barriers on both
sides of a bare store or load. Read-modify-write operations may be compiled to
read-modify-write instructions on the target architecture, such as LOCK-prefixed
instructions on x86, load-exclusive/store-exclusive instructions on ARM, and
load-link/store-conditional instructions on Power.
Specifically, the memory model is intended to allow code generation as follows.
* Every atomic operation in the program is assumed to be necessary.
* Atomic operations are never rearranged with each other or with non-atomic
operations.
* Functions are always assumed to perform atomic operations.
* Atomic operations are never implemented as read-modify-write operations on
larger data, but as non-lock-free atomics if the platform does not have
atomic operations of the appropriate size. (We already assume that every
platform has normal memory access operations of every interesting size.)
Naive code generation uses these patterns:
* Regular loads and stores compile to single load and store instructions.
* Lock-free atomic loads and stores compile to a full (sequentially consistent)
fence, a regular load or store, and a full fence.
* Lock-free atomic read-modify-write accesses compile to a full fence, an
atomic read-modify-write instruction sequence, and a full fence.
* Non-lock-free atomics compile to a spinlock acquire, a full fence, a series
of non-atomic load and store instructions, a full fence, and a spinlock
release.
That mapping is correct so long as an atomic operation on an address range does
not race with a non-atomic write or with an atomic operation of different size.
However, that is all we need: the memory model effectively demotes the atomic
operations involved in a race to non-atomic status. On the other hand, the naive
mapping is quite strong: it allows atomic operations to be used as sequentially
consistent fences, which the memory model does not actually guarantee.
Local improvements to those basic patterns are also allowed, subject to the
constraints of the memory model. For example:
* There are obvious platform-dependent improvements that remove redundant
fences. For example, on x86 the fences around lock-free atomic loads and
stores can always be omitted except for the fence following a store, and no
fence is needed for lock-free read-modify-write instructions, as these all
use LOCK-prefixed instructions. On many platforms there are fences of several
strengths, and weaker fences can be used in certain contexts without
destroying sequential consistency.
* Most modern platforms support lock-free atomics for all the data sizes
required by ECMAScript atomics. Should non-lock-free atomics be needed, the
fences surrounding the body of the atomic operation can usually be folded
into the lock and unlock steps. The simplest solution for non-lock-free
atomics is to have a single lock word per SharedArrayBuffer.
* There are also more complicated platform-dependent local improvements,
requiring some code analysis. For example, two back-to-back fences often have
the same effect as a single fence, so if code is generated for two atomic
operations in sequence, only a single fence need separate them. On x86, even
a single fence separating atomic stores can be omitted, as the fence
following a store is only needed to separate the store from a subsequent
load.
A GRAMMAR SUMMARY
A.1 LEXICAL GRAMMAR
SourceCharacter :: any Unicode code point InputElementDiv :: WhiteSpace
LineTerminator Comment CommonToken DivPunctuator RightBracePunctuator
InputElementRegExp :: WhiteSpace LineTerminator Comment CommonToken
RightBracePunctuator RegularExpressionLiteral InputElementRegExpOrTemplateTail
:: WhiteSpace LineTerminator Comment CommonToken RegularExpressionLiteral
TemplateSubstitutionTail InputElementTemplateTail :: WhiteSpace LineTerminator
Comment CommonToken DivPunctuator TemplateSubstitutionTail
InputElementHashbangOrRegExp :: WhiteSpace LineTerminator Comment CommonToken
HashbangComment RegularExpressionLiteral WhiteSpace :: <TAB> <VT> <FF> <ZWNBSP>
<USP> LineTerminator :: <LF> <CR> <LS> <PS> LineTerminatorSequence :: <LF> <CR>
[lookahead ≠ <LF>] <LS> <PS> <CR> <LF> Comment :: MultiLineComment
SingleLineComment MultiLineComment :: /* MultiLineCommentCharsopt */
MultiLineCommentChars :: MultiLineNotAsteriskChar MultiLineCommentCharsopt *
PostAsteriskCommentCharsopt PostAsteriskCommentChars ::
MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt *
PostAsteriskCommentCharsopt MultiLineNotAsteriskChar :: SourceCharacter but not
* MultiLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or
* SingleLineComment :: // SingleLineCommentCharsopt SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt SingleLineCommentChar ::
SourceCharacter but not LineTerminator HashbangComment :: #!
SingleLineCommentCharsopt CommonToken :: IdentifierName PrivateIdentifier
Punctuator NumericLiteral StringLiteral Template PrivateIdentifier :: #
IdentifierName IdentifierName :: IdentifierStart IdentifierName IdentifierPart
IdentifierStart :: IdentifierStartChar \ UnicodeEscapeSequence IdentifierPart ::
IdentifierPartChar \ UnicodeEscapeSequence IdentifierStartChar :: UnicodeIDStart
$ _ IdentifierPartChar :: UnicodeIDContinue $ <ZWNJ> <ZWJ> AsciiLetter :: one of
a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N
O P Q R S T U V W X Y Z UnicodeIDStart :: any Unicode code point with the
Unicode property “ID_Start” UnicodeIDContinue :: any Unicode code point with the
Unicode property “ID_Continue” ReservedWord :: one of await break case catch
class const continue debugger default delete do else enum export extends false
finally for function if import in instanceof new null return super switch this
throw true try typeof var void while with yield Punctuator ::
OptionalChainingPunctuator OtherPunctuator OptionalChainingPunctuator :: ?.
[lookahead ∉ DecimalDigit] OtherPunctuator :: one of { ( ) [ ] . ... ; , < > <=
>= == != === !== + - * % ** ++ -- << >> >>> & | ^ ! ~ && || ?? ? : = += -= *= %=
**= <<= >>= >>>= &= |= ^= &&= ||= ??= => DivPunctuator :: / /=
RightBracePunctuator :: } NullLiteral :: null BooleanLiteral :: true false
NumericLiteralSeparator :: _ NumericLiteral :: DecimalLiteral
DecimalBigIntegerLiteral NonDecimalIntegerLiteral[+Sep]
NonDecimalIntegerLiteral[+Sep] BigIntLiteralSuffix LegacyOctalIntegerLiteral
DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix NonZeroDigit
DecimalDigits[+Sep]opt BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator
DecimalDigits[+Sep] BigIntLiteralSuffix NonDecimalIntegerLiteral[Sep] ::
BinaryIntegerLiteral[?Sep] OctalIntegerLiteral[?Sep] HexIntegerLiteral[?Sep]
BigIntLiteralSuffix :: n DecimalLiteral :: DecimalIntegerLiteral .
DecimalDigits[+Sep]opt ExponentPart[+Sep]opt . DecimalDigits[+Sep]
ExponentPart[+Sep]opt DecimalIntegerLiteral ExponentPart[+Sep]opt
DecimalIntegerLiteral :: 0 NonZeroDigit NonZeroDigit NumericLiteralSeparatoropt
DecimalDigits[+Sep] NonOctalDecimalIntegerLiteral DecimalDigits[Sep] ::
DecimalDigit DecimalDigits[?Sep] DecimalDigit [+Sep] DecimalDigits[+Sep]
NumericLiteralSeparator DecimalDigit DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9
NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9 ExponentPart[Sep] :: ExponentIndicator
SignedInteger[?Sep] ExponentIndicator :: one of e E SignedInteger[Sep] ::
DecimalDigits[?Sep] + DecimalDigits[?Sep] - DecimalDigits[?Sep]
BinaryIntegerLiteral[Sep] :: 0b BinaryDigits[?Sep] 0B BinaryDigits[?Sep]
BinaryDigits[Sep] :: BinaryDigit BinaryDigits[?Sep] BinaryDigit [+Sep]
BinaryDigits[+Sep] NumericLiteralSeparator BinaryDigit BinaryDigit :: one of 0 1
OctalIntegerLiteral[Sep] :: 0o OctalDigits[?Sep] 0O OctalDigits[?Sep]
OctalDigits[Sep] :: OctalDigit OctalDigits[?Sep] OctalDigit [+Sep]
OctalDigits[+Sep] NumericLiteralSeparator OctalDigit LegacyOctalIntegerLiteral
:: 0 OctalDigit LegacyOctalIntegerLiteral OctalDigit
NonOctalDecimalIntegerLiteral :: 0 NonOctalDigit
LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit NonOctalDecimalIntegerLiteral
DecimalDigit LegacyOctalLikeDecimalIntegerLiteral :: 0 OctalDigit
LegacyOctalLikeDecimalIntegerLiteral OctalDigit OctalDigit :: one of 0 1 2 3 4 5
6 7 NonOctalDigit :: one of 8 9 HexIntegerLiteral[Sep] :: 0x HexDigits[?Sep] 0X
HexDigits[?Sep] HexDigits[Sep] :: HexDigit HexDigits[?Sep] HexDigit [+Sep]
HexDigits[+Sep] NumericLiteralSeparator HexDigit HexDigit :: one of 0 1 2 3 4 5
6 7 8 9 a b c d e f A B C D E F StringLiteral :: " DoubleStringCharactersopt " '
SingleStringCharactersopt ' DoubleStringCharacters :: DoubleStringCharacter
DoubleStringCharactersopt SingleStringCharacters :: SingleStringCharacter
SingleStringCharactersopt DoubleStringCharacter :: SourceCharacter but not one
of " or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation
SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator
<LS> <PS> \ EscapeSequence LineContinuation LineContinuation :: \
LineTerminatorSequence EscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉
DecimalDigit] LegacyOctalEscapeSequence NonOctalDecimalEscapeSequence
HexEscapeSequence UnicodeEscapeSequence CharacterEscapeSequence ::
SingleEscapeCharacter NonEscapeCharacter SingleEscapeCharacter :: one of ' " \ b
f n r t v NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter
or LineTerminator EscapeCharacter :: SingleEscapeCharacter DecimalDigit x u
LegacyOctalEscapeSequence :: 0 [lookahead ∈ { 8, 9 }] NonZeroOctalDigit
[lookahead ∉ OctalDigit] ZeroToThree OctalDigit [lookahead ∉ OctalDigit]
FourToSeven OctalDigit ZeroToThree OctalDigit OctalDigit NonZeroOctalDigit ::
OctalDigit but not 0 ZeroToThree :: one of 0 1 2 3 FourToSeven :: one of 4 5 6 7
NonOctalDecimalEscapeSequence :: one of 8 9 HexEscapeSequence :: x HexDigit
HexDigit UnicodeEscapeSequence :: u Hex4Digits u{ CodePoint } Hex4Digits ::
HexDigit HexDigit HexDigit HexDigit RegularExpressionLiteral :: /
RegularExpressionBody / RegularExpressionFlags RegularExpressionBody ::
RegularExpressionFirstChar RegularExpressionChars RegularExpressionChars ::
[empty] RegularExpressionChars RegularExpressionChar RegularExpressionFirstChar
:: RegularExpressionNonTerminator but not one of * or \ or / or [
RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionChar
:: RegularExpressionNonTerminator but not one of \ or / or [
RegularExpressionBackslashSequence RegularExpressionClass
RegularExpressionBackslashSequence :: \ RegularExpressionNonTerminator
RegularExpressionNonTerminator :: SourceCharacter but not LineTerminator
RegularExpressionClass :: [ RegularExpressionClassChars ]
RegularExpressionClassChars :: [empty] RegularExpressionClassChars
RegularExpressionClassChar RegularExpressionClassChar ::
RegularExpressionNonTerminator but not one of ] or \
RegularExpressionBackslashSequence RegularExpressionFlags :: [empty]
RegularExpressionFlags IdentifierPartChar Template :: NoSubstitutionTemplate
TemplateHead NoSubstitutionTemplate :: ` TemplateCharactersopt ` TemplateHead ::
` TemplateCharactersopt ${ TemplateSubstitutionTail :: TemplateMiddle
TemplateTail TemplateMiddle :: } TemplateCharactersopt ${ TemplateTail :: }
TemplateCharactersopt ` TemplateCharacters :: TemplateCharacter
TemplateCharactersopt TemplateCharacter :: $ [lookahead ≠ {] \
TemplateEscapeSequence \ NotEscapeSequence LineContinuation
LineTerminatorSequence SourceCharacter but not one of ` or \ or $ or
LineTerminator TemplateEscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉
DecimalDigit] HexEscapeSequence UnicodeEscapeSequence NotEscapeSequence :: 0
DecimalDigit DecimalDigit but not 0 x [lookahead ∉ HexDigit] x HexDigit
[lookahead ∉ HexDigit] u [lookahead ∉ HexDigit] [lookahead ≠ {] u HexDigit
[lookahead ∉ HexDigit] u HexDigit HexDigit [lookahead ∉ HexDigit] u HexDigit
HexDigit HexDigit [lookahead ∉ HexDigit] u { [lookahead ∉ HexDigit] u {
NotCodePoint [lookahead ∉ HexDigit] u { CodePoint [lookahead ∉ HexDigit]
[lookahead ≠ }] NotCodePoint :: HexDigits[~Sep] but only if MV of HexDigits >
0x10FFFF CodePoint :: HexDigits[~Sep] but only if MV of HexDigits ≤ 0x10FFFF
A.2 EXPRESSIONS
IdentifierReference[Yield, Await] : Identifier [~Yield] yield [~Await] await
BindingIdentifier[Yield, Await] : Identifier yield await LabelIdentifier[Yield,
Await] : Identifier [~Yield] yield [~Await] await Identifier : IdentifierName
but not ReservedWord PrimaryExpression[Yield, Await] : this
IdentifierReference[?Yield, ?Await] Literal ArrayLiteral[?Yield, ?Await]
ObjectLiteral[?Yield, ?Await] FunctionExpression ClassExpression[?Yield, ?Await]
GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression
RegularExpressionLiteral TemplateLiteral[?Yield, ?Await, ~Tagged]
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList[Yield, Await] : (
Expression[+In, ?Yield, ?Await] ) ( Expression[+In, ?Yield, ?Await] , ) ( ) (
... BindingIdentifier[?Yield, ?Await] ) ( ... BindingPattern[?Yield, ?Await] ) (
Expression[+In, ?Yield, ?Await] , ... BindingIdentifier[?Yield, ?Await] ) (
Expression[+In, ?Yield, ?Await] , ... BindingPattern[?Yield, ?Await] )
When processing an instance of the production
PrimaryExpression[Yield, Await] :
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is
refined using the following grammar:
ParenthesizedExpression[Yield, Await] : ( Expression[+In, ?Yield, ?Await] )
Literal : NullLiteral BooleanLiteral NumericLiteral StringLiteral
ArrayLiteral[Yield, Await] : [ Elisionopt ] [ ElementList[?Yield, ?Await] ] [
ElementList[?Yield, ?Await] , Elisionopt ] ElementList[Yield, Await] :
Elisionopt AssignmentExpression[+In, ?Yield, ?Await] Elisionopt
SpreadElement[?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt
AssignmentExpression[+In, ?Yield, ?Await] ElementList[?Yield, ?Await] ,
Elisionopt SpreadElement[?Yield, ?Await] Elision : , Elision ,
SpreadElement[Yield, Await] : ... AssignmentExpression[+In, ?Yield, ?Await]
ObjectLiteral[Yield, Await] : { } { PropertyDefinitionList[?Yield, ?Await] } {
PropertyDefinitionList[?Yield, ?Await] , } PropertyDefinitionList[Yield, Await]
: PropertyDefinition[?Yield, ?Await] PropertyDefinitionList[?Yield, ?Await] ,
PropertyDefinition[?Yield, ?Await] PropertyDefinition[Yield, Await] :
IdentifierReference[?Yield, ?Await] CoverInitializedName[?Yield, ?Await]
PropertyName[?Yield, ?Await] : AssignmentExpression[+In, ?Yield, ?Await]
MethodDefinition[?Yield, ?Await] ... AssignmentExpression[+In, ?Yield, ?Await]
PropertyName[Yield, Await] : LiteralPropertyName ComputedPropertyName[?Yield,
?Await] LiteralPropertyName : IdentifierName StringLiteral NumericLiteral
ComputedPropertyName[Yield, Await] : [ AssignmentExpression[+In, ?Yield, ?Await]
] CoverInitializedName[Yield, Await] : IdentifierReference[?Yield, ?Await]
Initializer[+In, ?Yield, ?Await] Initializer[In, Yield, Await] : =
AssignmentExpression[?In, ?Yield, ?Await] TemplateLiteral[Yield, Await, Tagged]
: NoSubstitutionTemplate SubstitutionTemplate[?Yield, ?Await, ?Tagged]
SubstitutionTemplate[Yield, Await, Tagged] : TemplateHead Expression[+In,
?Yield, ?Await] TemplateSpans[?Yield, ?Await, ?Tagged] TemplateSpans[Yield,
Await, Tagged] : TemplateTail TemplateMiddleList[?Yield, ?Await, ?Tagged]
TemplateTail TemplateMiddleList[Yield, Await, Tagged] : TemplateMiddle
Expression[+In, ?Yield, ?Await] TemplateMiddleList[?Yield, ?Await, ?Tagged]
TemplateMiddle Expression[+In, ?Yield, ?Await] MemberExpression[Yield, Await] :
PrimaryExpression[?Yield, ?Await] MemberExpression[?Yield, ?Await] [
Expression[+In, ?Yield, ?Await] ] MemberExpression[?Yield, ?Await] .
IdentifierName MemberExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await,
+Tagged] SuperProperty[?Yield, ?Await] MetaProperty new MemberExpression[?Yield,
?Await] Arguments[?Yield, ?Await] MemberExpression[?Yield, ?Await] .
PrivateIdentifier SuperProperty[Yield, Await] : super [ Expression[+In, ?Yield,
?Await] ] super . IdentifierName MetaProperty : NewTarget ImportMeta NewTarget :
new . target ImportMeta : import . meta NewExpression[Yield, Await] :
MemberExpression[?Yield, ?Await] new NewExpression[?Yield, ?Await]
CallExpression[Yield, Await] : CoverCallExpressionAndAsyncArrowHead[?Yield,
?Await] SuperCall[?Yield, ?Await] ImportCall[?Yield, ?Await]
CallExpression[?Yield, ?Await] Arguments[?Yield, ?Await] CallExpression[?Yield,
?Await] [ Expression[+In, ?Yield, ?Await] ] CallExpression[?Yield, ?Await] .
IdentifierName CallExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await,
+Tagged] CallExpression[?Yield, ?Await] . PrivateIdentifier
When processing an instance of the production
CallExpression[Yield, Await] : CoverCallExpressionAndAsyncArrowHead[?Yield,
?Await]
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the
following grammar:
CallMemberExpression[Yield, Await] : MemberExpression[?Yield, ?Await]
Arguments[?Yield, ?Await]
SuperCall[Yield, Await] : super Arguments[?Yield, ?Await] ImportCall[Yield,
Await] : import ( AssignmentExpression[+In, ?Yield, ?Await] ) Arguments[Yield,
Await] : ( ) ( ArgumentList[?Yield, ?Await] ) ( ArgumentList[?Yield, ?Await] , )
ArgumentList[Yield, Await] : AssignmentExpression[+In, ?Yield, ?Await] ...
AssignmentExpression[+In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] ,
AssignmentExpression[+In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , ...
AssignmentExpression[+In, ?Yield, ?Await] OptionalExpression[Yield, Await] :
MemberExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await]
CallExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await]
OptionalExpression[?Yield, ?Await] OptionalChain[?Yield, ?Await]
OptionalChain[Yield, Await] : ?. Arguments[?Yield, ?Await] ?. [ Expression[+In,
?Yield, ?Await] ] ?. IdentifierName ?. TemplateLiteral[?Yield, ?Await, +Tagged]
?. PrivateIdentifier OptionalChain[?Yield, ?Await] Arguments[?Yield, ?Await]
OptionalChain[?Yield, ?Await] [ Expression[+In, ?Yield, ?Await] ]
OptionalChain[?Yield, ?Await] . IdentifierName OptionalChain[?Yield, ?Await]
TemplateLiteral[?Yield, ?Await, +Tagged] OptionalChain[?Yield, ?Await] .
PrivateIdentifier LeftHandSideExpression[Yield, Await] : NewExpression[?Yield,
?Await] CallExpression[?Yield, ?Await] OptionalExpression[?Yield, ?Await]
UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await]
LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] ++
LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] -- ++
UnaryExpression[?Yield, ?Await] -- UnaryExpression[?Yield, ?Await]
UnaryExpression[Yield, Await] : UpdateExpression[?Yield, ?Await] delete
UnaryExpression[?Yield, ?Await] void UnaryExpression[?Yield, ?Await] typeof
UnaryExpression[?Yield, ?Await] + UnaryExpression[?Yield, ?Await] -
UnaryExpression[?Yield, ?Await] ~ UnaryExpression[?Yield, ?Await] !
UnaryExpression[?Yield, ?Await] [+Await] AwaitExpression[?Yield]
ExponentiationExpression[Yield, Await] : UnaryExpression[?Yield, ?Await]
UpdateExpression[?Yield, ?Await] ** ExponentiationExpression[?Yield, ?Await]
MultiplicativeExpression[Yield, Await] : ExponentiationExpression[?Yield,
?Await] MultiplicativeExpression[?Yield, ?Await] MultiplicativeOperator
ExponentiationExpression[?Yield, ?Await] MultiplicativeOperator : one of * / %
AdditiveExpression[Yield, Await] : MultiplicativeExpression[?Yield, ?Await]
AdditiveExpression[?Yield, ?Await] + MultiplicativeExpression[?Yield, ?Await]
AdditiveExpression[?Yield, ?Await] - MultiplicativeExpression[?Yield, ?Await]
ShiftExpression[Yield, Await] : AdditiveExpression[?Yield, ?Await]
ShiftExpression[?Yield, ?Await] << AdditiveExpression[?Yield, ?Await]
ShiftExpression[?Yield, ?Await] >> AdditiveExpression[?Yield, ?Await]
ShiftExpression[?Yield, ?Await] >>> AdditiveExpression[?Yield, ?Await]
RelationalExpression[In, Yield, Await] : ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] < ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] > ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] <= ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] >= ShiftExpression[?Yield, ?Await]
RelationalExpression[?In, ?Yield, ?Await] instanceof ShiftExpression[?Yield,
?Await] [+In] RelationalExpression[+In, ?Yield, ?Await] in
ShiftExpression[?Yield, ?Await] [+In] PrivateIdentifier in
ShiftExpression[?Yield, ?Await] EqualityExpression[In, Yield, Await] :
RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In, ?Yield,
?Await] == RelationalExpression[?In, ?Yield, ?Await] EqualityExpression[?In,
?Yield, ?Await] != RelationalExpression[?In, ?Yield, ?Await]
EqualityExpression[?In, ?Yield, ?Await] === RelationalExpression[?In, ?Yield,
?Await] EqualityExpression[?In, ?Yield, ?Await] !== RelationalExpression[?In,
?Yield, ?Await] BitwiseANDExpression[In, Yield, Await] : EqualityExpression[?In,
?Yield, ?Await] BitwiseANDExpression[?In, ?Yield, ?Await] &
EqualityExpression[?In, ?Yield, ?Await] BitwiseXORExpression[In, Yield, Await] :
BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseXORExpression[?In, ?Yield,
?Await] ^ BitwiseANDExpression[?In, ?Yield, ?Await] BitwiseORExpression[In,
Yield, Await] : BitwiseXORExpression[?In, ?Yield, ?Await]
BitwiseORExpression[?In, ?Yield, ?Await] | BitwiseXORExpression[?In, ?Yield,
?Await] LogicalANDExpression[In, Yield, Await] : BitwiseORExpression[?In,
?Yield, ?Await] LogicalANDExpression[?In, ?Yield, ?Await] &&
BitwiseORExpression[?In, ?Yield, ?Await] LogicalORExpression[In, Yield, Await] :
LogicalANDExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield,
?Await] || LogicalANDExpression[?In, ?Yield, ?Await] CoalesceExpression[In,
Yield, Await] : CoalesceExpressionHead[?In, ?Yield, ?Await] ??
BitwiseORExpression[?In, ?Yield, ?Await] CoalesceExpressionHead[In, Yield,
Await] : CoalesceExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In,
?Yield, ?Await] ShortCircuitExpression[In, Yield, Await] :
LogicalORExpression[?In, ?Yield, ?Await] CoalesceExpression[?In, ?Yield, ?Await]
ConditionalExpression[In, Yield, Await] : ShortCircuitExpression[?In, ?Yield,
?Await] ShortCircuitExpression[?In, ?Yield, ?Await] ? AssignmentExpression[+In,
?Yield, ?Await] : AssignmentExpression[?In, ?Yield, ?Await]
AssignmentExpression[In, Yield, Await] : ConditionalExpression[?In, ?Yield,
?Await] [+Yield] YieldExpression[?In, ?Await] ArrowFunction[?In, ?Yield, ?Await]
AsyncArrowFunction[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] =
AssignmentExpression[?In, ?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await]
AssignmentOperator AssignmentExpression[?In, ?Yield, ?Await]
LeftHandSideExpression[?Yield, ?Await] &&= AssignmentExpression[?In, ?Yield,
?Await] LeftHandSideExpression[?Yield, ?Await] ||= AssignmentExpression[?In,
?Yield, ?Await] LeftHandSideExpression[?Yield, ?Await] ??=
AssignmentExpression[?In, ?Yield, ?Await] AssignmentOperator : one of *= /= %=
+= -= <<= >>= >>>= &= ^= |= **=
In certain circumstances when processing an instance of the production
AssignmentExpression[In, Yield, Await] : LeftHandSideExpression[?Yield, ?Await]
= AssignmentExpression[?In, ?Yield, ?Await]
the interpretation of LeftHandSideExpression is refined using the following
grammar:
AssignmentPattern[Yield, Await] : ObjectAssignmentPattern[?Yield, ?Await]
ArrayAssignmentPattern[?Yield, ?Await] ObjectAssignmentPattern[Yield, Await] : {
} { AssignmentRestProperty[?Yield, ?Await] } { AssignmentPropertyList[?Yield,
?Await] } { AssignmentPropertyList[?Yield, ?Await] ,
AssignmentRestProperty[?Yield, ?Await]opt } ArrayAssignmentPattern[Yield, Await]
: [ Elisionopt AssignmentRestElement[?Yield, ?Await]opt ] [
AssignmentElementList[?Yield, ?Await] ] [ AssignmentElementList[?Yield, ?Await]
, Elisionopt AssignmentRestElement[?Yield, ?Await]opt ]
AssignmentRestProperty[Yield, Await] : ... DestructuringAssignmentTarget[?Yield,
?Await] AssignmentPropertyList[Yield, Await] : AssignmentProperty[?Yield,
?Await] AssignmentPropertyList[?Yield, ?Await] , AssignmentProperty[?Yield,
?Await] AssignmentElementList[Yield, Await] : AssignmentElisionElement[?Yield,
?Await] AssignmentElementList[?Yield, ?Await] , AssignmentElisionElement[?Yield,
?Await] AssignmentElisionElement[Yield, Await] : Elisionopt
AssignmentElement[?Yield, ?Await] AssignmentProperty[Yield, Await] :
IdentifierReference[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt
PropertyName[?Yield, ?Await] : AssignmentElement[?Yield, ?Await]
AssignmentElement[Yield, Await] : DestructuringAssignmentTarget[?Yield, ?Await]
Initializer[+In, ?Yield, ?Await]opt AssignmentRestElement[Yield, Await] : ...
DestructuringAssignmentTarget[?Yield, ?Await]
DestructuringAssignmentTarget[Yield, Await] : LeftHandSideExpression[?Yield,
?Await]
Expression[In, Yield, Await] : AssignmentExpression[?In, ?Yield, ?Await]
Expression[?In, ?Yield, ?Await] , AssignmentExpression[?In, ?Yield, ?Await]
A.3 STATEMENTS
Statement[Yield, Await, Return] : BlockStatement[?Yield, ?Await, ?Return]
VariableStatement[?Yield, ?Await] EmptyStatement ExpressionStatement[?Yield,
?Await] IfStatement[?Yield, ?Await, ?Return] BreakableStatement[?Yield, ?Await,
?Return] ContinueStatement[?Yield, ?Await] BreakStatement[?Yield, ?Await]
[+Return] ReturnStatement[?Yield, ?Await] WithStatement[?Yield, ?Await, ?Return]
LabelledStatement[?Yield, ?Await, ?Return] ThrowStatement[?Yield, ?Await]
TryStatement[?Yield, ?Await, ?Return] DebuggerStatement Declaration[Yield,
Await] : HoistableDeclaration[?Yield, ?Await, ~Default] ClassDeclaration[?Yield,
?Await, ~Default] LexicalDeclaration[+In, ?Yield, ?Await]
HoistableDeclaration[Yield, Await, Default] : FunctionDeclaration[?Yield,
?Await, ?Default] GeneratorDeclaration[?Yield, ?Await, ?Default]
AsyncFunctionDeclaration[?Yield, ?Await, ?Default]
AsyncGeneratorDeclaration[?Yield, ?Await, ?Default] BreakableStatement[Yield,
Await, Return] : IterationStatement[?Yield, ?Await, ?Return]
SwitchStatement[?Yield, ?Await, ?Return] BlockStatement[Yield, Await, Return] :
Block[?Yield, ?Await, ?Return] Block[Yield, Await, Return] : {
StatementList[?Yield, ?Await, ?Return]opt } StatementList[Yield, Await, Return]
: StatementListItem[?Yield, ?Await, ?Return] StatementList[?Yield, ?Await,
?Return] StatementListItem[?Yield, ?Await, ?Return] StatementListItem[Yield,
Await, Return] : Statement[?Yield, ?Await, ?Return] Declaration[?Yield, ?Await]
LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield,
?Await] ; LetOrConst : let const BindingList[In, Yield, Await] :
LexicalBinding[?In, ?Yield, ?Await] BindingList[?In, ?Yield, ?Await] ,
LexicalBinding[?In, ?Yield, ?Await] LexicalBinding[In, Yield, Await] :
BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt
BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]
VariableStatement[Yield, Await] : var VariableDeclarationList[+In, ?Yield,
?Await] ; VariableDeclarationList[In, Yield, Await] : VariableDeclaration[?In,
?Yield, ?Await] VariableDeclarationList[?In, ?Yield, ?Await] ,
VariableDeclaration[?In, ?Yield, ?Await] VariableDeclaration[In, Yield, Await] :
BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt
BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]
BindingPattern[Yield, Await] : ObjectBindingPattern[?Yield, ?Await]
ArrayBindingPattern[?Yield, ?Await] ObjectBindingPattern[Yield, Await] : { } {
BindingRestProperty[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] } {
BindingPropertyList[?Yield, ?Await] , BindingRestProperty[?Yield, ?Await]opt }
ArrayBindingPattern[Yield, Await] : [ Elisionopt BindingRestElement[?Yield,
?Await]opt ] [ BindingElementList[?Yield, ?Await] ] [ BindingElementList[?Yield,
?Await] , Elisionopt BindingRestElement[?Yield, ?Await]opt ]
BindingRestProperty[Yield, Await] : ... BindingIdentifier[?Yield, ?Await]
BindingPropertyList[Yield, Await] : BindingProperty[?Yield, ?Await]
BindingPropertyList[?Yield, ?Await] , BindingProperty[?Yield, ?Await]
BindingElementList[Yield, Await] : BindingElisionElement[?Yield, ?Await]
BindingElementList[?Yield, ?Await] , BindingElisionElement[?Yield, ?Await]
BindingElisionElement[Yield, Await] : Elisionopt BindingElement[?Yield, ?Await]
BindingProperty[Yield, Await] : SingleNameBinding[?Yield, ?Await]
PropertyName[?Yield, ?Await] : BindingElement[?Yield, ?Await]
BindingElement[Yield, Await] : SingleNameBinding[?Yield, ?Await]
BindingPattern[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt
SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await]
Initializer[+In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ...
BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await]
EmptyStatement : ; ExpressionStatement[Yield, Await] : [lookahead ∉ { {,
function, async [no LineTerminator here] function, class, let [ }]
Expression[+In, ?Yield, ?Await] ; IfStatement[Yield, Await, Return] : if (
Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else
Statement[?Yield, ?Await, ?Return] if ( Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] [lookahead ≠ else] IterationStatement[Yield,
Await, Return] : DoWhileStatement[?Yield, ?Await, ?Return]
WhileStatement[?Yield, ?Await, ?Return] ForStatement[?Yield, ?Await, ?Return]
ForInOfStatement[?Yield, ?Await, ?Return] DoWhileStatement[Yield, Await, Return]
: do Statement[?Yield, ?Await, ?Return] while ( Expression[+In, ?Yield, ?Await]
) ; WhileStatement[Yield, Await, Return] : while ( Expression[+In, ?Yield,
?Await] ) Statement[?Yield, ?Await, ?Return] ForStatement[Yield, Await, Return]
: for ( [lookahead ≠ let [] Expression[~In, ?Yield, ?Await]opt ; Expression[+In,
?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield,
?Await, ?Return] for ( var VariableDeclarationList[~In, ?Yield, ?Await] ;
Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt )
Statement[?Yield, ?Await, ?Return] for ( LexicalDeclaration[~In, ?Yield, ?Await]
Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt )
Statement[?Yield, ?Await, ?Return] ForInOfStatement[Yield, Await, Return] : for
( [lookahead ≠ let [] LeftHandSideExpression[?Yield, ?Await] in Expression[+In,
?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var
ForBinding[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] in
Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for (
[lookahead ∉ { let, async of }] LeftHandSideExpression[?Yield, ?Await] of
AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
for ( var ForBinding[?Yield, ?Await] of AssignmentExpression[+In, ?Yield,
?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield,
?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await,
?Return] [+Await] for await ( [lookahead ≠ let] LeftHandSideExpression[?Yield,
?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await,
?Return] [+Await] for await ( var ForBinding[?Yield, ?Await] of
AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
[+Await] for await ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[+In,
?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] ForDeclaration[Yield,
Await] : LetOrConst ForBinding[?Yield, ?Await] ForBinding[Yield, Await] :
BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await]
ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here]
LabelIdentifier[?Yield, ?Await] ; BreakStatement[Yield, Await] : break ; break
[no LineTerminator here] LabelIdentifier[?Yield, ?Await] ;
ReturnStatement[Yield, Await] : return ; return [no LineTerminator here]
Expression[+In, ?Yield, ?Await] ; WithStatement[Yield, Await, Return] : with (
Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
SwitchStatement[Yield, Await, Return] : switch ( Expression[+In, ?Yield, ?Await]
) CaseBlock[?Yield, ?Await, ?Return] CaseBlock[Yield, Await, Return] : {
CaseClauses[?Yield, ?Await, ?Return]opt } { CaseClauses[?Yield, ?Await,
?Return]opt DefaultClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await,
?Return]opt } CaseClauses[Yield, Await, Return] : CaseClause[?Yield, ?Await,
?Return] CaseClauses[?Yield, ?Await, ?Return] CaseClause[?Yield, ?Await,
?Return] CaseClause[Yield, Await, Return] : case Expression[+In, ?Yield, ?Await]
: StatementList[?Yield, ?Await, ?Return]opt DefaultClause[Yield, Await, Return]
: default : StatementList[?Yield, ?Await, ?Return]opt LabelledStatement[Yield,
Await, Return] : LabelIdentifier[?Yield, ?Await] : LabelledItem[?Yield, ?Await,
?Return] LabelledItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return]
FunctionDeclaration[?Yield, ?Await, ~Default] ThrowStatement[Yield, Await] :
throw [no LineTerminator here] Expression[+In, ?Yield, ?Await] ;
TryStatement[Yield, Await, Return] : try Block[?Yield, ?Await, ?Return]
Catch[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return]
Finally[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return]
Catch[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] Catch[Yield,
Await, Return] : catch ( CatchParameter[?Yield, ?Await] ) Block[?Yield, ?Await,
?Return] catch Block[?Yield, ?Await, ?Return] Finally[Yield, Await, Return] :
finally Block[?Yield, ?Await, ?Return] CatchParameter[Yield, Await] :
BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await]
DebuggerStatement : debugger ;
A.4 FUNCTIONS AND CLASSES
UniqueFormalParameters[Yield, Await] : FormalParameters[?Yield, ?Await]
FormalParameters[Yield, Await] : [empty] FunctionRestParameter[?Yield, ?Await]
FormalParameterList[?Yield, ?Await] FormalParameterList[?Yield, ?Await] ,
FormalParameterList[?Yield, ?Await] , FunctionRestParameter[?Yield, ?Await]
FormalParameterList[Yield, Await] : FormalParameter[?Yield, ?Await]
FormalParameterList[?Yield, ?Await] , FormalParameter[?Yield, ?Await]
FunctionRestParameter[Yield, Await] : BindingRestElement[?Yield, ?Await]
FormalParameter[Yield, Await] : BindingElement[?Yield, ?Await]
FunctionDeclaration[Yield, Await, Default] : function BindingIdentifier[?Yield,
?Await] ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
[+Default] function ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield,
~Await] } FunctionExpression : function BindingIdentifier[~Yield, ~Await]opt (
FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
FunctionBody[Yield, Await] : FunctionStatementList[?Yield, ?Await]
FunctionStatementList[Yield, Await] : StatementList[?Yield, ?Await, +Return]opt
ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no
LineTerminator here] => ConciseBody[?In] ArrowParameters[Yield, Await] :
BindingIdentifier[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
ConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, ~Await] {
FunctionBody[~Yield, ~Await] } ExpressionBody[In, Await] :
AssignmentExpression[?In, ~Yield, ?Await]
When processing an instance of the production
ArrowParameters[Yield, Await] :
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is
refined using the following grammar:
ArrowFormalParameters[Yield, Await] : ( UniqueFormalParameters[?Yield, ?Await] )
AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here]
AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] =>
AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no
LineTerminator here] => AsyncConciseBody[?In] AsyncConciseBody[In] : [lookahead
≠ {] ExpressionBody[?In, +Await] { AsyncFunctionBody }
AsyncArrowBindingIdentifier[Yield] : BindingIdentifier[?Yield, +Await]
CoverCallExpressionAndAsyncArrowHead[Yield, Await] : MemberExpression[?Yield,
?Await] Arguments[?Yield, ?Await]
When processing an instance of the production
AsyncArrowFunction[In, Yield, Await] :
CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] =>
AsyncConciseBody[?In]
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the
following grammar:
AsyncArrowHead : async [no LineTerminator here] ArrowFormalParameters[~Yield,
+Await]
MethodDefinition[Yield, Await] : ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
GeneratorMethod[?Yield, ?Await] AsyncMethod[?Yield, ?Await]
AsyncGeneratorMethod[?Yield, ?Await] get ClassElementName[?Yield, ?Await] ( ) {
FunctionBody[~Yield, ~Await] } set ClassElementName[?Yield, ?Await] (
PropertySetParameterList ) { FunctionBody[~Yield, ~Await] }
PropertySetParameterList : FormalParameter[~Yield, ~Await]
GeneratorDeclaration[Yield, Await, Default] : function *
BindingIdentifier[?Yield, ?Await] ( FormalParameters[+Yield, ~Await] ) {
GeneratorBody } [+Default] function * ( FormalParameters[+Yield, ~Await] ) {
GeneratorBody } GeneratorExpression : function * BindingIdentifier[+Yield,
~Await]opt ( FormalParameters[+Yield, ~Await] ) { GeneratorBody }
GeneratorMethod[Yield, Await] : * ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[+Yield, ~Await] ) { GeneratorBody } GeneratorBody :
FunctionBody[+Yield, ~Await] YieldExpression[In, Await] : yield yield [no
LineTerminator here] AssignmentExpression[?In, +Yield, ?Await] yield [no
LineTerminator here] * AssignmentExpression[?In, +Yield, ?Await]
AsyncGeneratorDeclaration[Yield, Await, Default] : async [no LineTerminator
here] function * BindingIdentifier[?Yield, ?Await] ( FormalParameters[+Yield,
+Await] ) { AsyncGeneratorBody } [+Default] async [no LineTerminator here]
function * ( FormalParameters[+Yield, +Await] ) { AsyncGeneratorBody }
AsyncGeneratorExpression : async [no LineTerminator here] function *
BindingIdentifier[+Yield, +Await]opt ( FormalParameters[+Yield, +Await] ) {
AsyncGeneratorBody } AsyncGeneratorMethod[Yield, Await] : async [no
LineTerminator here] * ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[+Yield, +Await] ) { AsyncGeneratorBody }
AsyncGeneratorBody : FunctionBody[+Yield, +Await]
AsyncFunctionDeclaration[Yield, Await, Default] : async [no LineTerminator here]
function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, +Await] )
{ AsyncFunctionBody } [+Default] async [no LineTerminator here] function (
FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncFunctionExpression
: async [no LineTerminator here] function BindingIdentifier[~Yield, +Await]opt (
FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncMethod[Yield,
Await] : async [no LineTerminator here] ClassElementName[?Yield, ?Await] (
UniqueFormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncFunctionBody
: FunctionBody[~Yield, +Await] AwaitExpression[Yield] : await
UnaryExpression[?Yield, +Await] ClassDeclaration[Yield, Await, Default] : class
BindingIdentifier[?Yield, ?Await] ClassTail[?Yield, ?Await] [+Default] class
ClassTail[?Yield, ?Await] ClassExpression[Yield, Await] : class
BindingIdentifier[?Yield, ?Await]opt ClassTail[?Yield, ?Await] ClassTail[Yield,
Await] : ClassHeritage[?Yield, ?Await]opt { ClassBody[?Yield, ?Await]opt }
ClassHeritage[Yield, Await] : extends LeftHandSideExpression[?Yield, ?Await]
ClassBody[Yield, Await] : ClassElementList[?Yield, ?Await]
ClassElementList[Yield, Await] : ClassElement[?Yield, ?Await]
ClassElementList[?Yield, ?Await] ClassElement[?Yield, ?Await]
ClassElement[Yield, Await] : MethodDefinition[?Yield, ?Await] static
MethodDefinition[?Yield, ?Await] FieldDefinition[?Yield, ?Await] ; static
FieldDefinition[?Yield, ?Await] ; ClassStaticBlock ; FieldDefinition[Yield,
Await] : ClassElementName[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt
ClassElementName[Yield, Await] : PropertyName[?Yield, ?Await] PrivateIdentifier
ClassStaticBlock : static { ClassStaticBlockBody } ClassStaticBlockBody :
ClassStaticBlockStatementList ClassStaticBlockStatementList :
StatementList[~Yield, +Await, ~Return]opt
A.5 SCRIPTS AND MODULES
Script : ScriptBodyopt ScriptBody : StatementList[~Yield, ~Await, ~Return]
Module : ModuleBodyopt ModuleBody : ModuleItemList ModuleItemList : ModuleItem
ModuleItemList ModuleItem ModuleItem : ImportDeclaration ExportDeclaration
StatementListItem[~Yield, +Await, ~Return] ModuleExportName : IdentifierName
StringLiteral ImportDeclaration : import ImportClause FromClause ; import
ModuleSpecifier ; ImportClause : ImportedDefaultBinding NameSpaceImport
NamedImports ImportedDefaultBinding , NameSpaceImport ImportedDefaultBinding ,
NamedImports ImportedDefaultBinding : ImportedBinding NameSpaceImport : * as
ImportedBinding NamedImports : { } { ImportsList } { ImportsList , } FromClause
: from ModuleSpecifier ImportsList : ImportSpecifier ImportsList ,
ImportSpecifier ImportSpecifier : ImportedBinding ModuleExportName as
ImportedBinding ModuleSpecifier : StringLiteral ImportedBinding :
BindingIdentifier[~Yield, +Await] ExportDeclaration : export ExportFromClause
FromClause ; export NamedExports ; export VariableStatement[~Yield, +Await]
export Declaration[~Yield, +Await] export default HoistableDeclaration[~Yield,
+Await, +Default] export default ClassDeclaration[~Yield, +Await, +Default]
export default [lookahead ∉ { function, async [no LineTerminator here] function,
class }] AssignmentExpression[+In, ~Yield, +Await] ; ExportFromClause : * * as
ModuleExportName NamedExports NamedExports : { } { ExportsList } { ExportsList ,
} ExportsList : ExportSpecifier ExportsList , ExportSpecifier ExportSpecifier :
ModuleExportName ModuleExportName as ModuleExportName
A.6 NUMBER CONVERSIONS
StringNumericLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrNumericLiteral
StrWhiteSpaceopt StrWhiteSpace ::: StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar ::: WhiteSpace LineTerminator StrNumericLiteral :::
StrDecimalLiteral NonDecimalIntegerLiteral[~Sep] StrDecimalLiteral :::
StrUnsignedDecimalLiteral + StrUnsignedDecimalLiteral -
StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral ::: Infinity
DecimalDigits[~Sep] . DecimalDigits[~Sep]opt ExponentPart[~Sep]opt .
DecimalDigits[~Sep] ExponentPart[~Sep]opt DecimalDigits[~Sep]
ExponentPart[~Sep]opt
All grammar symbols not explicitly defined by the StringNumericLiteral grammar
have the definitions used in the Lexical Grammar for numeric literals.
StringIntegerLiteral ::: StrWhiteSpaceopt StrWhiteSpaceopt StrIntegerLiteral
StrWhiteSpaceopt StrIntegerLiteral ::: SignedInteger[~Sep]
NonDecimalIntegerLiteral[~Sep]
A.7 TIME ZONE OFFSET STRING FORMAT
UTCOffset ::: TemporalSign Hour TemporalSign Hour HourSubcomponents[+Extended]
TemporalSign Hour HourSubcomponents[~Extended] TemporalSign ::: ASCIISign
<MINUS> ASCIISign ::: one of + - Hour ::: 0 DecimalDigit 1 DecimalDigit 20 21 22
23 HourSubcomponents[Extended] ::: TimeSeparator[?Extended] MinuteSecond
TimeSeparator[?Extended] MinuteSecond TimeSeparator[?Extended] MinuteSecond
TemporalDecimalFractionopt TimeSeparator[Extended] ::: [+Extended] : [~Extended]
[empty] MinuteSecond ::: 0 DecimalDigit 1 DecimalDigit 2 DecimalDigit 3
DecimalDigit 4 DecimalDigit 5 DecimalDigit TemporalDecimalFraction :::
TemporalDecimalSeparator DecimalDigit TemporalDecimalSeparator DecimalDigit
DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit
TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit
TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit
DecimalDigit TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit
DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator DecimalDigit
DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit
TemporalDecimalSeparator DecimalDigit DecimalDigit DecimalDigit DecimalDigit
DecimalDigit DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator
DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit DecimalDigit
DecimalDigit DecimalDigit DecimalDigit TemporalDecimalSeparator ::: one of . ,
A.8 REGULAR EXPRESSIONS
Pattern[UnicodeMode, N] :: Disjunction[?UnicodeMode, ?N]
Disjunction[UnicodeMode, N] :: Alternative[?UnicodeMode, ?N]
Alternative[?UnicodeMode, ?N] | Disjunction[?UnicodeMode, ?N]
Alternative[UnicodeMode, N] :: [empty] Alternative[?UnicodeMode, ?N]
Term[?UnicodeMode, ?N] Term[UnicodeMode, N] :: Assertion[?UnicodeMode, ?N]
Atom[?UnicodeMode, ?N] Atom[?UnicodeMode, ?N] Quantifier Assertion[UnicodeMode,
N] :: ^ $ \b \B (?= Disjunction[?UnicodeMode, ?N] ) (?!
Disjunction[?UnicodeMode, ?N] ) (?<= Disjunction[?UnicodeMode, ?N] ) (?<!
Disjunction[?UnicodeMode, ?N] ) Quantifier :: QuantifierPrefix QuantifierPrefix
? QuantifierPrefix :: * + ? { DecimalDigits[~Sep] } { DecimalDigits[~Sep] ,} {
DecimalDigits[~Sep] , DecimalDigits[~Sep] } Atom[UnicodeMode, N] ::
PatternCharacter . \ AtomEscape[?UnicodeMode, ?N] CharacterClass[?UnicodeMode] (
GroupSpecifier[?UnicodeMode]opt Disjunction[?UnicodeMode, ?N] ) (?:
Disjunction[?UnicodeMode, ?N] ) SyntaxCharacter :: one of ^ $ \ . * + ? ( ) [ ]
{ } | PatternCharacter :: SourceCharacter but not SyntaxCharacter
AtomEscape[UnicodeMode, N] :: DecimalEscape CharacterClassEscape[?UnicodeMode]
CharacterEscape[?UnicodeMode] [+N] k GroupName[?UnicodeMode]
CharacterEscape[UnicodeMode] :: ControlEscape c AsciiLetter 0 [lookahead ∉
DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode]
IdentityEscape[?UnicodeMode] ControlEscape :: one of f n r t v
GroupSpecifier[UnicodeMode] :: ? GroupName[?UnicodeMode] GroupName[UnicodeMode]
:: < RegExpIdentifierName[?UnicodeMode] > RegExpIdentifierName[UnicodeMode] ::
RegExpIdentifierStart[?UnicodeMode] RegExpIdentifierName[?UnicodeMode]
RegExpIdentifierPart[?UnicodeMode] RegExpIdentifierStart[UnicodeMode] ::
IdentifierStartChar \ RegExpUnicodeEscapeSequence[+UnicodeMode] [~UnicodeMode]
UnicodeLeadSurrogate UnicodeTrailSurrogate RegExpIdentifierPart[UnicodeMode] ::
IdentifierPartChar \ RegExpUnicodeEscapeSequence[+UnicodeMode] [~UnicodeMode]
UnicodeLeadSurrogate UnicodeTrailSurrogate
RegExpUnicodeEscapeSequence[UnicodeMode] :: [+UnicodeMode] u HexLeadSurrogate \u
HexTrailSurrogate [+UnicodeMode] u HexLeadSurrogate [+UnicodeMode] u
HexTrailSurrogate [+UnicodeMode] u HexNonSurrogate [~UnicodeMode] u Hex4Digits
[+UnicodeMode] u{ CodePoint } UnicodeLeadSurrogate :: any Unicode code point in
the inclusive interval from U+D800 to U+DBFF UnicodeTrailSurrogate :: any
Unicode code point in the inclusive interval from U+DC00 to U+DFFF
Each \u HexTrailSurrogate for which the choice of associated u HexLeadSurrogate
is ambiguous shall be associated with the nearest possible u HexLeadSurrogate
that would otherwise have no corresponding \u HexTrailSurrogate.
HexLeadSurrogate :: Hex4Digits but only if the MV of Hex4Digits is in the
inclusive interval from 0xD800 to 0xDBFF HexTrailSurrogate :: Hex4Digits but
only if the MV of Hex4Digits is in the inclusive interval from 0xDC00 to 0xDFFF
HexNonSurrogate :: Hex4Digits but only if the MV of Hex4Digits is not in the
inclusive interval from 0xD800 to 0xDFFF IdentityEscape[UnicodeMode] ::
[+UnicodeMode] SyntaxCharacter [+UnicodeMode] / [~UnicodeMode] SourceCharacter
but not UnicodeIDContinue DecimalEscape :: NonZeroDigit DecimalDigits[~Sep]opt
[lookahead ∉ DecimalDigit] CharacterClassEscape[UnicodeMode] :: d D s S w W
[+UnicodeMode] p{ UnicodePropertyValueExpression } [+UnicodeMode] P{
UnicodePropertyValueExpression } UnicodePropertyValueExpression ::
UnicodePropertyName = UnicodePropertyValue LoneUnicodePropertyNameOrValue
UnicodePropertyName :: UnicodePropertyNameCharacters
UnicodePropertyNameCharacters :: UnicodePropertyNameCharacter
UnicodePropertyNameCharactersopt UnicodePropertyValue ::
UnicodePropertyValueCharacters LoneUnicodePropertyNameOrValue ::
UnicodePropertyValueCharacters UnicodePropertyValueCharacters ::
UnicodePropertyValueCharacter UnicodePropertyValueCharactersopt
UnicodePropertyValueCharacter :: UnicodePropertyNameCharacter DecimalDigit
UnicodePropertyNameCharacter :: AsciiLetter _ CharacterClass[UnicodeMode] :: [
[lookahead ≠ ^] ClassRanges[?UnicodeMode] ] [^ ClassRanges[?UnicodeMode] ]
ClassRanges[UnicodeMode] :: [empty] NonemptyClassRanges[?UnicodeMode]
NonemptyClassRanges[UnicodeMode] :: ClassAtom[?UnicodeMode]
ClassAtom[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode]
ClassAtom[?UnicodeMode] - ClassAtom[?UnicodeMode] ClassRanges[?UnicodeMode]
NonemptyClassRangesNoDash[UnicodeMode] :: ClassAtom[?UnicodeMode]
ClassAtomNoDash[?UnicodeMode] NonemptyClassRangesNoDash[?UnicodeMode]
ClassAtomNoDash[?UnicodeMode] - ClassAtom[?UnicodeMode]
ClassRanges[?UnicodeMode] ClassAtom[UnicodeMode] :: -
ClassAtomNoDash[?UnicodeMode] ClassAtomNoDash[UnicodeMode] :: SourceCharacter
but not one of \ or ] or - \ ClassEscape[?UnicodeMode] ClassEscape[UnicodeMode]
:: b [+UnicodeMode] - CharacterClassEscape[?UnicodeMode]
CharacterEscape[?UnicodeMode]
B ADDITIONAL ECMASCRIPT FEATURES FOR WEB BROWSERS
The ECMAScript language syntax and semantics defined in this annex are required
when the ECMAScript host is a web browser. The content of this annex is
normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web
browser ECMAScript hosts. All of the language features and behaviours specified
in this annex have one or more undesirable characteristics and in the absence of
legacy usage would be removed from this specification. However, the usage of
these features by large numbers of existing web pages means that web browsers
must continue to support them. The specifications in this annex define the
requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language.
Programmers should not use or assume the existence of these features and
behaviours when writing new ECMAScript code. ECMAScript implementations are
discouraged from implementing these features unless the implementation is part
of a web browser or is required to run the same legacy ECMAScript code that web
browsers encounter.
B.1 ADDITIONAL SYNTAX
B.1.1 HTML-LIKE COMMENTS
The syntax and semantics of 12.4 is extended as follows except that this
extension is not allowed when parsing source text using the goal symbol Module:
SYNTAX
Comment :: MultiLineComment SingleLineComment SingleLineHTMLOpenComment
SingleLineHTMLCloseComment SingleLineDelimitedComment MultiLineComment :: /*
FirstCommentLineopt LineTerminator MultiLineCommentCharsopt */
HTMLCloseCommentopt FirstCommentLine :: SingleLineDelimitedCommentChars
SingleLineHTMLOpenComment :: <!-- SingleLineCommentCharsopt
SingleLineHTMLCloseComment :: LineTerminatorSequence HTMLCloseComment
SingleLineDelimitedComment :: /* SingleLineDelimitedCommentCharsopt */
HTMLCloseComment :: WhiteSpaceSequenceopt SingleLineDelimitedCommentSequenceopt
--> SingleLineCommentCharsopt SingleLineDelimitedCommentChars ::
SingleLineNotAsteriskChar SingleLineDelimitedCommentCharsopt *
SingleLinePostAsteriskCommentCharsopt SingleLineNotAsteriskChar ::
SourceCharacter but not one of * or LineTerminator
SingleLinePostAsteriskCommentChars :: SingleLineNotForwardSlashOrAsteriskChar
SingleLineDelimitedCommentCharsopt * SingleLinePostAsteriskCommentCharsopt
SingleLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or *
or LineTerminator WhiteSpaceSequence :: WhiteSpace WhiteSpaceSequenceopt
SingleLineDelimitedCommentSequence :: SingleLineDelimitedComment
WhiteSpaceSequenceopt SingleLineDelimitedCommentSequenceopt
Similar to a MultiLineComment that contains a line terminator code point, a
SingleLineHTMLCloseComment is considered to be a LineTerminator for purposes of
parsing by the syntactic grammar.
B.1.2 REGULAR EXPRESSIONS PATTERNS
The syntax of 22.2.1 is modified and extended as follows. These changes
introduce ambiguities that are broken by the ordering of grammar productions and
by contextual information. When parsing using the following grammar, each
alternative is considered only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and
semantics of BMP patterns. The following grammar extensions include productions
parameterized with the [UnicodeMode] parameter. However, none of these
extensions change the syntax of Unicode patterns recognized when parsing with
the [UnicodeMode] parameter present on the goal symbol.
SYNTAX
Term[UnicodeMode, N] :: [+UnicodeMode] Assertion[+UnicodeMode, ?N]
[+UnicodeMode] Atom[+UnicodeMode, ?N] Quantifier [+UnicodeMode]
Atom[+UnicodeMode, ?N] [~UnicodeMode] QuantifiableAssertion[?N] Quantifier
[~UnicodeMode] Assertion[~UnicodeMode, ?N] [~UnicodeMode] ExtendedAtom[?N]
Quantifier [~UnicodeMode] ExtendedAtom[?N] Assertion[UnicodeMode, N] :: ^ $ \b
\B [+UnicodeMode] (?= Disjunction[+UnicodeMode, ?N] ) [+UnicodeMode] (?!
Disjunction[+UnicodeMode, ?N] ) [~UnicodeMode] QuantifiableAssertion[?N] (?<=
Disjunction[?UnicodeMode, ?N] ) (?<! Disjunction[?UnicodeMode, ?N] )
QuantifiableAssertion[N] :: (?= Disjunction[~UnicodeMode, ?N] ) (?!
Disjunction[~UnicodeMode, ?N] ) ExtendedAtom[N] :: . \ AtomEscape[~UnicodeMode,
?N] \ [lookahead = c] CharacterClass[~UnicodeMode] (
GroupSpecifier[~UnicodeMode]opt Disjunction[~UnicodeMode, ?N] ) (?:
Disjunction[~UnicodeMode, ?N] ) InvalidBracedQuantifier ExtendedPatternCharacter
InvalidBracedQuantifier :: { DecimalDigits[~Sep] } { DecimalDigits[~Sep] ,} {
DecimalDigits[~Sep] , DecimalDigits[~Sep] } ExtendedPatternCharacter ::
SourceCharacter but not one of ^ $ \ . * + ? ( ) [ | AtomEscape[UnicodeMode, N]
:: [+UnicodeMode] DecimalEscape [~UnicodeMode] DecimalEscape but only if the
CapturingGroupNumber of DecimalEscape is ≤ CountLeftCapturingParensWithin(the
Pattern containing DecimalEscape) CharacterClassEscape[?UnicodeMode]
CharacterEscape[?UnicodeMode, ?N] [+N] k GroupName[?UnicodeMode]
CharacterEscape[UnicodeMode, N] :: ControlEscape c AsciiLetter 0 [lookahead ∉
DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode]
[~UnicodeMode] LegacyOctalEscapeSequence IdentityEscape[?UnicodeMode, ?N]
IdentityEscape[UnicodeMode, N] :: [+UnicodeMode] SyntaxCharacter [+UnicodeMode]
/ [~UnicodeMode] SourceCharacterIdentityEscape[?N]
SourceCharacterIdentityEscape[N] :: [~N] SourceCharacter but not c [+N]
SourceCharacter but not one of c or k ClassAtomNoDash[UnicodeMode, N] ::
SourceCharacter but not one of \ or ] or - \ ClassEscape[?UnicodeMode, ?N] \
[lookahead = c] ClassEscape[UnicodeMode, N] :: b [+UnicodeMode] - [~UnicodeMode]
c ClassControlLetter CharacterClassEscape[?UnicodeMode]
CharacterEscape[?UnicodeMode, ?N] ClassControlLetter :: DecimalDigit _ Note
When the same left-hand sides occurs with both [+UnicodeMode] and [~UnicodeMode]
guards it is to control the disambiguation priority.
B.1.2.1 STATIC SEMANTICS: EARLY ERRORS
The semantics of 22.2.1.1 is extended as follows:
ExtendedAtom :: InvalidBracedQuantifier
* It is a Syntax Error if any source text is matched by this production.
Additionally, the rules for the following productions are modified with the
addition of the highlighted text:
NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
* It is a Syntax Error if IsCharacterClass of the first ClassAtom is true or
IsCharacterClass of the second ClassAtom is true and this production has a
[UnicodeMode] parameter.
* It is a Syntax Error if IsCharacterClass of the first ClassAtom is false,
IsCharacterClass of the second ClassAtom is false, and the CharacterValue of
the first ClassAtom is strictly greater than the CharacterValue of the second
ClassAtom.
NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
* It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is true or
IsCharacterClass of ClassAtom is true and this production has a [UnicodeMode]
parameter.
* It is a Syntax Error if IsCharacterClass of ClassAtomNoDash is false,
IsCharacterClass of ClassAtom is false, and the CharacterValue of
ClassAtomNoDash is strictly greater than the CharacterValue of ClassAtom.
B.1.2.2 STATIC SEMANTICS: COUNTLEFTCAPTURINGPARENSWITHIN AND
COUNTLEFTCAPTURINGPARENSBEFORE
In the definitions of CountLeftCapturingParensWithin and
CountLeftCapturingParensBefore, references to “ Atom :: ( GroupSpecifieropt
Disjunction ) ” are to be interpreted as meaning “ Atom :: ( GroupSpecifieropt
Disjunction ) ” or “ ExtendedAtom :: ( GroupSpecifieropt Disjunction ) ”.
B.1.2.3 STATIC SEMANTICS: ISCHARACTERCLASS
The semantics of 22.2.1.5 is extended as follows:
ClassAtomNoDash :: \ [lookahead = c]
1. 1. 1. Return false.
B.1.2.4 STATIC SEMANTICS: CHARACTERVALUE
The semantics of 22.2.1.6 is extended as follows:
ClassAtomNoDash :: \ [lookahead = c]
1. 1. 1. Return the numeric value of U+005C (REVERSE SOLIDUS).
ClassEscape :: c ClassControlLetter
1. 1. 1. Let ch be the code point matched by ClassControlLetter.
2. 2. 2. Let i be the numeric value of ch.
3. 3. 3. Return the remainder of dividing i by 32.
CharacterEscape :: LegacyOctalEscapeSequence
1. 1. 1. Return the MV of LegacyOctalEscapeSequence (see 12.9.4.3).
B.1.2.5 RUNTIME SEMANTICS: COMPILESUBPATTERN
The semantics of CompileSubpattern is extended as follows:
The rule for Term :: QuantifiableAssertion Quantifier is the same as for Term ::
Atom Quantifier but with QuantifiableAssertion substituted for Atom.
The rule for Term :: ExtendedAtom Quantifier is the same as for Term :: Atom
Quantifier but with ExtendedAtom substituted for Atom.
The rule for Term :: ExtendedAtom is the same as for Term :: Atom but with
ExtendedAtom substituted for Atom.
B.1.2.6 RUNTIME SEMANTICS: COMPILEASSERTION
CompileAssertion rules for the Assertion :: (?= Disjunction ) and Assertion ::
(?! Disjunction ) productions are also used for the QuantifiableAssertion
productions, but with QuantifiableAssertion substituted for Assertion.
B.1.2.7 RUNTIME SEMANTICS: COMPILEATOM
CompileAtom rules for the Atom productions except for Atom :: PatternCharacter
are also used for the ExtendedAtom productions, but with ExtendedAtom
substituted for Atom. The following rules, with parameter direction, are also
added:
ExtendedAtom :: \ [lookahead = c]
1. 1. 1. Let A be the CharSet containing the single character \ U+005C (REVERSE
SOLIDUS).
2. 2. 2. Return CharacterSetMatcher(rer, A, false, direction).
ExtendedAtom :: ExtendedPatternCharacter
1. 1. 1. Let ch be the character represented by ExtendedPatternCharacter.
2. 2. 2. Let A be a one-element CharSet containing the character ch.
3. 3. 3. Return CharacterSetMatcher(rer, A, false, direction).
B.1.2.8 RUNTIME SEMANTICS: COMPILETOCHARSET
The semantics of 22.2.2.9 is extended as follows:
The following two rules replace the corresponding rules of CompileToCharSet.
NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
1. 1. 1. Let A be CompileToCharSet of the first ClassAtom with argument rer.
2. 2. 2. Let B be CompileToCharSet of the second ClassAtom with argument rer.
3. 3. 3. Let C be CompileToCharSet of ClassRanges with argument rer.
4. 4. 4. Let D be CharacterRangeOrUnion(rer, A, B).
5. 5. 5. Return the union of D and C.
NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
1. 1. 1. Let A be CompileToCharSet of ClassAtomNoDash with argument rer.
2. 2. 2. Let B be CompileToCharSet of ClassAtom with argument rer.
3. 3. 3. Let C be CompileToCharSet of ClassRanges with argument rer.
4. 4. 4. Let D be CharacterRangeOrUnion(rer, A, B).
5. 5. 5. Return the union of D and C.
In addition, the following rules are added to CompileToCharSet.
ClassEscape :: c ClassControlLetter
1. 1. 1. Let cv be the CharacterValue of this ClassEscape.
2. 2. 2. Let c be the character whose character value is cv.
3. 3. 3. Return the CharSet containing the single character c.
ClassAtomNoDash :: \ [lookahead = c]
1. 1. 1. Return the CharSet containing the single character \ U+005C (REVERSE
SOLIDUS).
Note
This production can only be reached from the sequence \c within a character
class where it is not followed by an acceptable control character.
B.1.2.8.1 CHARACTERRANGEORUNION ( RER, A, B )
The abstract operation CharacterRangeOrUnion takes arguments rer (a RegExp
Record), A (a CharSet), and B (a CharSet) and returns a CharSet. It performs the
following steps when called:
1. 1. 1. If rer.[[Unicode]] is false, then
1. a. a. If A does not contain exactly one character or B does not contain
exactly one character, then
1. i. i. Let C be the CharSet containing the single character - U+002D
(HYPHEN-MINUS).
2. ii. ii. Return the union of CharSets A, B and C.
2. 2. 2. Return CharacterRange(A, B).
B.1.2.9 STATIC SEMANTICS: PARSEPATTERN ( PATTERNTEXT, U )
The semantics of 22.2.3.4 is extended as follows:
The abstract operation ParsePattern takes arguments patternText (a sequence of
Unicode code points) and u (a Boolean). It performs the following steps when
called:
1. 1. 1. If u is true, then
1. a. a. Let parseResult be ParseText(patternText, Pattern[+UnicodeMode,
+N]).
2. 2. 2. Else,
1. a. a. Let parseResult be ParseText(patternText, Pattern[~UnicodeMode,
~N]).
2. b. b. If parseResult is a Parse Node and parseResult contains a
GroupName, then
1. i. i. Set parseResult to ParseText(patternText, Pattern[~UnicodeMode,
+N]).
3. 3. 3. Return parseResult.
B.2 ADDITIONAL BUILT-IN PROPERTIES
When the ECMAScript host is a web browser the following additional properties of
the standard built-in objects are defined.
B.2.1 ADDITIONAL PROPERTIES OF THE GLOBAL OBJECT
The entries in Table 91 are added to Table 6.
Table 91: Additional Well-known Intrinsic Objects
Intrinsic Name Global Name ECMAScript Language Association %escape% escape The
escape function (B.2.1.1) %unescape% unescape The unescape function (B.2.1.2)
B.2.1.1 ESCAPE ( STRING )
This function is a property of the global object. It computes a new version of a
String value in which certain code units have been replaced by a hexadecimal
escape sequence.
When replacing a code unit of numeric value less than or equal to 0x00FF, a
two-digit escape sequence of the form %xx is used. When replacing a code unit of
numeric value strictly greater than 0x00FF, a four-digit escape sequence of the
form %uxxxx is used.
It is the %escape% intrinsic object.
It performs the following steps when called:
1. 1. 1. Set string to ? ToString(string).
2. 2. 2. Let len be the length of string.
3. 3. 3. Let R be the empty String.
4. 4. 4. Let unescapedSet be the string-concatenation of the ASCII word
characters and "@*+-./".
5. 5. 5. Let k be 0.
6. 6. 6. Repeat, while k < len,
1. a. a. Let C be the code unit at index k within string.
2. b. b. If unescapedSet contains C, then
1. i. i. Let S be C.
3. c. c. Else,
1. i. i. Let n be the numeric value of C.
2. ii. ii. If n < 256, then
1. 1. 1. Let hex be the String representation of n, formatted as an
uppercase hexadecimal number.
2. 2. 2. Let S be the string-concatenation of "%" and ! StringPad(hex,
2𝔽, "0", start).
3. iii. iii. Else,
1. 1. 1. Let hex be the String representation of n, formatted as an
uppercase hexadecimal number.
2. 2. 2. Let S be the string-concatenation of "%u" and
! StringPad(hex, 4𝔽, "0", start).
4. d. d. Set R to the string-concatenation of R and S.
5. e. e. Set k to k + 1.
7. 7. 7. Return R.
Note
The encoding is partly based on the encoding described in RFC 1738, but the
entire encoding specified in this standard is described above without regard to
the contents of RFC 1738. This encoding does not reflect changes to RFC 1738
made by RFC 3986.
B.2.1.2 UNESCAPE ( STRING )
This function is a property of the global object. It computes a new version of a
String value in which each escape sequence of the sort that might be introduced
by the escape function is replaced with the code unit that it represents.
It is the %unescape% intrinsic object.
It performs the following steps when called:
1. 1. 1. Set string to ? ToString(string).
2. 2. 2. Let len be the length of string.
3. 3. 3. Let R be the empty String.
4. 4. 4. Let k be 0.
5. 5. 5. Repeat, while k < len,
1. a. a. Let C be the code unit at index k within string.
2. b. b. If C is the code unit 0x0025 (PERCENT SIGN), then
1. i. i. Let hexDigits be the empty String.
2. ii. ii. Let optionalAdvance be 0.
3. iii. iii. If k + 5 < len and the code unit at index k + 1 within
string is the code unit 0x0075 (LATIN SMALL LETTER U), then
1. 1. 1. Set hexDigits to the substring of string from k + 2 to k + 6.
2. 2. 2. Set optionalAdvance to 5.
4. iv. iv. Else if k + 3 ≤ len, then
1. 1. 1. Set hexDigits to the substring of string from k + 1 to k + 3.
2. 2. 2. Set optionalAdvance to 2.
5. v. v. Let parseResult be ParseText(StringToCodePoints(hexDigits),
HexDigits[~Sep]).
6. vi. vi. If parseResult is a Parse Node, then
1. 1. 1. Let n be the MV of parseResult.
2. 2. 2. Set C to the code unit whose numeric value is n.
3. 3. 3. Set k to k + optionalAdvance.
3. c. c. Set R to the string-concatenation of R and C.
4. d. d. Set k to k + 1.
6. 6. 6. Return R.
B.2.2 ADDITIONAL PROPERTIES OF THE STRING.PROTOTYPE OBJECT
B.2.2.1 STRING.PROTOTYPE.SUBSTR ( START, LENGTH )
This method returns a substring of the result of converting the this value to a
String, starting from index start and running for length code units (or through
the end of the String if length is undefined). If start is negative, it is
treated as sourceLength + start where sourceLength is the length of the String.
The result is a String value, not a String object.
It performs the following steps when called:
1. 1. 1. Let O be ? RequireObjectCoercible(this value).
2. 2. 2. Let S be ? ToString(O).
3. 3. 3. Let size be the length of S.
4. 4. 4. Let intStart be ? ToIntegerOrInfinity(start).
5. 5. 5. If intStart = -∞, set intStart to 0.
6. 6. 6. Else if intStart < 0, set intStart to max(size + intStart, 0).
7. 7. 7. Else, set intStart to min(intStart, size).
8. 8. 8. If length is undefined, let intLength be size; otherwise let
intLength be ? ToIntegerOrInfinity(length).
9. 9. 9. Set intLength to the result of clamping intLength between 0 and size.
10. 10. 10. Let intEnd be min(intStart + intLength, size).
11. 11. 11. Return the substring of S from intStart to intEnd.
Note
This method is intentionally generic; it does not require that its this value be
a String object. Therefore it can be transferred to other kinds of objects for
use as a method.
B.2.2.2 STRING.PROTOTYPE.ANCHOR ( NAME )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "a", "name", name).
B.2.2.2.1 CREATEHTML ( STRING, TAG, ATTRIBUTE, VALUE )
The abstract operation CreateHTML takes arguments string (an ECMAScript language
value), tag (a String), attribute (a String), and value (an ECMAScript language
value) and returns either a normal completion containing a String or a throw
completion. It performs the following steps when called:
1. 1. 1. Let str be ? RequireObjectCoercible(string).
2. 2. 2. Let S be ? ToString(str).
3. 3. 3. Let p1 be the string-concatenation of "<" and tag.
4. 4. 4. If attribute is not the empty String, then
1. a. a. Let V be ? ToString(value).
2. b. b. Let escapedV be the String value that is the same as V except that
each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been
replaced with the six code unit sequence """.
3. c. c. Set p1 to the string-concatenation of:
* p1
* the code unit 0x0020 (SPACE)
* attribute
* the code unit 0x003D (EQUALS SIGN)
* the code unit 0x0022 (QUOTATION MARK)
* escapedV
* the code unit 0x0022 (QUOTATION MARK)
5. 5. 5. Let p2 be the string-concatenation of p1 and ">".
6. 6. 6. Let p3 be the string-concatenation of p2 and S.
7. 7. 7. Let p4 be the string-concatenation of p3, "</", tag, and ">".
8. 8. 8. Return p4.
B.2.2.3 STRING.PROTOTYPE.BIG ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "big", "", "").
B.2.2.4 STRING.PROTOTYPE.BLINK ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "blink", "", "").
B.2.2.5 STRING.PROTOTYPE.BOLD ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "b", "", "").
B.2.2.6 STRING.PROTOTYPE.FIXED ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "tt", "", "").
B.2.2.7 STRING.PROTOTYPE.FONTCOLOR ( COLOR )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "font", "color", color).
B.2.2.8 STRING.PROTOTYPE.FONTSIZE ( SIZE )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "font", "size", size).
B.2.2.9 STRING.PROTOTYPE.ITALICS ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "i", "", "").
B.2.2.10 STRING.PROTOTYPE.LINK ( URL )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "a", "href", url).
B.2.2.11 STRING.PROTOTYPE.SMALL ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "small", "", "").
B.2.2.12 STRING.PROTOTYPE.STRIKE ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "strike", "", "").
B.2.2.13 STRING.PROTOTYPE.SUB ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "sub", "", "").
B.2.2.14 STRING.PROTOTYPE.SUP ( )
This method performs the following steps when called:
1. 1. 1. Let S be the this value.
2. 2. 2. Return ? CreateHTML(S, "sup", "", "").
B.2.2.15 STRING.PROTOTYPE.TRIMLEFT ( )
Note
The property "trimStart" is preferred. The "trimLeft" property is provided
principally for compatibility with old code. It is recommended that the
"trimStart" property be used in new ECMAScript code.
The initial value of the "trimLeft" property is %String.prototype.trimStart%,
defined in 22.1.3.32.
B.2.2.16 STRING.PROTOTYPE.TRIMRIGHT ( )
Note
The property "trimEnd" is preferred. The "trimRight" property is provided
principally for compatibility with old code. It is recommended that the
"trimEnd" property be used in new ECMAScript code.
The initial value of the "trimRight" property is %String.prototype.trimEnd%,
defined in 22.1.3.31.
B.2.3 ADDITIONAL PROPERTIES OF THE DATE.PROTOTYPE OBJECT
B.2.3.1 DATE.PROTOTYPE.GETYEAR ( )
Note
The getFullYear method is preferred for nearly all purposes, because it avoids
the “year 2000 problem.”
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. If t is NaN, return NaN.
3. 3. 3. Return YearFromTime(LocalTime(t)) - 1900𝔽.
B.2.3.2 DATE.PROTOTYPE.SETYEAR ( YEAR )
Note
The setFullYear method is preferred for nearly all purposes, because it avoids
the “year 2000 problem.”
This method performs the following steps when called:
1. 1. 1. Let t be ? thisTimeValue(this value).
2. 2. 2. Let y be ? ToNumber(year).
3. 3. 3. If t is NaN, set t to +0𝔽; otherwise, set t to LocalTime(t).
4. 4. 4. If y is NaN, then
1. a. a. Set the [[DateValue]] internal slot of this Date object to NaN.
2. b. b. Return NaN.
5. 5. 5. Let yi be ! ToIntegerOrInfinity(y).
6. 6. 6. If 0 ≤ yi ≤ 99, let yyyy be 1900𝔽 + 𝔽(yi).
7. 7. 7. Else, let yyyy be y.
8. 8. 8. Let d be MakeDay(yyyy, MonthFromTime(t), DateFromTime(t)).
9. 9. 9. Let date be UTC(MakeDate(d, TimeWithinDay(t))).
10. 10. 10. Set the [[DateValue]] internal slot of this Date object to
TimeClip(date).
11. 11. 11. Return the value of the [[DateValue]] internal slot of this Date
object.
B.2.3.3 DATE.PROTOTYPE.TOGMTSTRING ( )
Note
The toUTCString method is preferred. This method is provided principally for
compatibility with old code.
The initial value of the "toGMTString" property is %Date.prototype.toUTCString%,
defined in 21.4.4.43.
B.2.4 ADDITIONAL PROPERTIES OF THE REGEXP.PROTOTYPE OBJECT
B.2.4.1 REGEXP.PROTOTYPE.COMPILE ( PATTERN, FLAGS )
This method performs the following steps when called:
1. 1. 1. Let O be the this value.
2. 2. 2. Perform ? RequireInternalSlot(O, [[RegExpMatcher]]).
3. 3. 3. If pattern is an Object and pattern has a [[RegExpMatcher]] internal
slot, then
1. a. a. If flags is not undefined, throw a TypeError exception.
2. b. b. Let P be pattern.[[OriginalSource]].
3. c. c. Let F be pattern.[[OriginalFlags]].
4. 4. 4. Else,
1. a. a. Let P be pattern.
2. b. b. Let F be flags.
5. 5. 5. Return ? RegExpInitialize(O, P, F).
Note
This method completely reinitializes the this value RegExp with a new pattern
and flags. An implementation may interpret use of this method as an assertion
that the resulting RegExp object will be used multiple times and hence is a
candidate for extra optimization.
B.3 OTHER ADDITIONAL FEATURES
B.3.1 LABELLED FUNCTION DECLARATIONS
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow
for the association of a statement label with a FunctionDeclaration. However, a
labelled FunctionDeclaration was an allowable extension for non-strict code and
most browser-hosted ECMAScript implementations supported that extension. In
ECMAScript 2015 and later, the grammar production for LabelledStatement permits
use of FunctionDeclaration as a LabelledItem but 14.13.1 includes an Early Error
rule that produces a Syntax Error if that occurs. That rule is modified with the
addition of the highlighted text:
LabelledItem : FunctionDeclaration
* It is a Syntax Error if any source text that is strict mode code is matched
by this production.
Note
The early error rules for WithStatement, IfStatement, and IterationStatement
prevent these statements from containing a labelled FunctionDeclaration in
non-strict code.
B.3.2 BLOCK-LEVEL FUNCTION DECLARATIONS WEB LEGACY COMPATIBILITY SEMANTICS
Prior to ECMAScript 2015, the ECMAScript specification did not define the
occurrence of a FunctionDeclaration as an element of a Block statement's
StatementList. However, support for that form of FunctionDeclaration was an
allowable extension and most browser-hosted ECMAScript implementations permitted
them. Unfortunately, the semantics of such declarations differ among those
implementations. Because of these semantic differences, existing web ECMAScript
source text that uses Block level function declarations is only portable among
browser implementations if the usage only depends upon the semantic intersection
of all of the browser implementations for such declarations. The following are
the use cases that fall within that intersection semantics:
1. A function is declared and only referenced within a single block.
* One or more FunctionDeclarations whose BindingIdentifier is the name f
occur within the function code of an enclosing function g and that
declaration is nested within a Block.
* No other declaration of f that is not a var declaration occurs within the
function code of g.
* All occurrences of f as an IdentifierReference are within the
StatementList of the Block containing the declaration of f.
2. A function is declared and possibly used within a single Block but also
referenced by an inner function definition that is not contained within that
same Block.
* One or more FunctionDeclarations whose BindingIdentifier is the name f
occur within the function code of an enclosing function g and that
declaration is nested within a Block.
* No other declaration of f that is not a var declaration occurs within the
function code of g.
* There may be occurrences of f as an IdentifierReference within the
StatementList of the Block containing the declaration of f.
* There is at least one occurrence of f as an IdentifierReference within
another function h that is nested within g and no other declaration of f
shadows the references to f from within h.
* All invocations of h occur after the declaration of f has been evaluated.
3. A function is declared and possibly used within a single block but also
referenced within subsequent blocks.
* One or more FunctionDeclaration whose BindingIdentifier is the name f
occur within the function code of an enclosing function g and that
declaration is nested within a Block.
* No other declaration of f that is not a var declaration occurs within the
function code of g.
* There may be occurrences of f as an IdentifierReference within the
StatementList of the Block containing the declaration of f.
* There is at least one occurrence of f as an IdentifierReference within the
function code of g that lexically follows the Block containing the
declaration of f.
The first use case is interoperable with the semantics of Block level function
declarations provided by ECMAScript 2015. Any pre-existing ECMAScript source
text that employs that use case will operate using the Block level function
declarations semantics defined by clauses 10, 14, and 15.
ECMAScript 2015 interoperability for the second and third use cases requires the
following extensions to the clause 10, clause 15, clause 19.2.1 and clause
16.1.7 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning
messages, a warning should be produced when code contains a FunctionDeclaration
for which these compatibility semantics are applied and introduce observable
differences from non-compatibility semantics. For example, if a var binding is
not introduced because its introduction would create an early error, a warning
message should not be produced.
B.3.2.1 CHANGES TO FUNCTIONDECLARATIONINSTANTIATION
During FunctionDeclarationInstantiation the following steps are performed in
place of step 29:
29. 29. 29. If strict is false, then
1. a. a. For each FunctionDeclaration f that is directly contained in the
StatementList of a Block, CaseClause, or DefaultClause, do
1. i. i. Let F be StringValue of the BindingIdentifier of f.
2. ii. ii. If replacing the FunctionDeclaration f with a
VariableStatement that has F as a BindingIdentifier would not produce
any Early Errors for func and parameterNames does not contain F, then
1. 1. 1. NOTE: A var binding for F is only instantiated here if it is
neither a VarDeclaredName, the name of a formal parameter, or
another FunctionDeclaration.
2. 2. 2. If initializedBindings does not contain F and F is not
"arguments", then
1. a. a. Perform ! varEnv.CreateMutableBinding(F, false).
2. b. b. Perform ! varEnv.InitializeBinding(F, undefined).
3. c. c. Append F to instantiatedVarNames.
3. 3. 3. When the FunctionDeclaration f is evaluated, perform the
following steps in place of the FunctionDeclaration Evaluation
algorithm provided in 15.2.6:
1. a. a. Let fenv be the running execution context's
VariableEnvironment.
2. b. b. Let benv be the running execution context's
LexicalEnvironment.
3. c. c. Let fobj be ! benv.GetBindingValue(F, false).
4. d. d. Perform ! fenv.SetMutableBinding(F, fobj, false).
5. e. e. Return unused.
B.3.2.2 CHANGES TO GLOBALDECLARATIONINSTANTIATION
During GlobalDeclarationInstantiation the following steps are performed in place
of step 12:
12. 12. 12. Perform the following steps:
1. a. a. Let strict be IsStrict of script.
2. b. b. If strict is false, then
1. i. i. Let declaredFunctionOrVarNames be the list-concatenation of
declaredFunctionNames and declaredVarNames.
2. ii. ii. For each FunctionDeclaration f that is directly contained in
the StatementList of a Block, CaseClause, or DefaultClause Contained
within script, do
1. 1. 1. Let F be StringValue of the BindingIdentifier of f.
2. 2. 2. If replacing the FunctionDeclaration f with a
VariableStatement that has F as a BindingIdentifier would not
produce any Early Errors for script, then
1. a. a. If env.HasLexicalDeclaration(F) is false, then
1. i. i. Let fnDefinable be ? env.CanDeclareGlobalVar(F).
2. ii. ii. If fnDefinable is true, then
1. i. i. NOTE: A var binding for F is only instantiated here
if it is neither a VarDeclaredName nor the name of
another FunctionDeclaration.
2. ii. ii. If declaredFunctionOrVarNames does not contain F,
then
1. i. i. Perform ? env.CreateGlobalVarBinding(F, false).
2. ii. ii. Append F to declaredFunctionOrVarNames.
3. iii. iii. When the FunctionDeclaration f is evaluated,
perform the following steps in place of the
FunctionDeclaration Evaluation algorithm provided in
15.2.6:
1. i. i. Let genv be the running execution context's
VariableEnvironment.
2. ii. ii. Let benv be the running execution context's
LexicalEnvironment.
3. iii. iii. Let fobj be ! benv.GetBindingValue(F,
false).
4. iv. iv. Perform ? genv.SetMutableBinding(F, fobj,
false).
5. v. v. Return unused.
B.3.2.3 CHANGES TO EVALDECLARATIONINSTANTIATION
During EvalDeclarationInstantiation the following steps are performed in place
of step 11:
11. 11. 11. If strict is false, then
1. a. a. Let declaredFunctionOrVarNames be the list-concatenation of
declaredFunctionNames and declaredVarNames.
2. b. b. For each FunctionDeclaration f that is directly contained in the
StatementList of a Block, CaseClause, or DefaultClause Contained within
body, do
1. i. i. Let F be StringValue of the BindingIdentifier of f.
2. ii. ii. If replacing the FunctionDeclaration f with a
VariableStatement that has F as a BindingIdentifier would not produce
any Early Errors for body, then
1. 1. 1. Let bindingExists be false.
2. 2. 2. Let thisEnv be lexEnv.
3. 3. 3. Assert: The following loop will terminate.
4. 4. 4. Repeat, while thisEnv is not varEnv,
1. a. a. If thisEnv is not an Object Environment Record, then
1. i. i. If ! thisEnv.HasBinding(F) is true, then
1. i. i. Let bindingExists be true.
2. b. b. Set thisEnv to thisEnv.[[OuterEnv]].
5. 5. 5. If bindingExists is false and varEnv is a Global Environment
Record, then
1. a. a. If varEnv.HasLexicalDeclaration(F) is false, then
1. i. i. Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
2. b. b. Else,
1. i. i. Let fnDefinable be false.
6. 6. 6. Else,
1. a. a. Let fnDefinable be true.
7. 7. 7. If bindingExists is false and fnDefinable is true, then
1. a. a. If declaredFunctionOrVarNames does not contain F, then
1. i. i. If varEnv is a Global Environment Record, then
1. i. i. Perform ? varEnv.CreateGlobalVarBinding(F, true).
2. ii. ii. Else,
1. i. i. Let bindingExists be ! varEnv.HasBinding(F).
2. ii. ii. If bindingExists is false, then
1. i. i. Perform ! varEnv.CreateMutableBinding(F, true).
2. ii. ii. Perform ! varEnv.InitializeBinding(F,
undefined).
3. iii. iii. Append F to declaredFunctionOrVarNames.
2. b. b. When the FunctionDeclaration f is evaluated, perform the
following steps in place of the FunctionDeclaration Evaluation
algorithm provided in 15.2.6:
1. i. i. Let genv be the running execution context's
VariableEnvironment.
2. ii. ii. Let benv be the running execution context's
LexicalEnvironment.
3. iii. iii. Let fobj be ! benv.GetBindingValue(F, false).
4. iv. iv. Perform ? genv.SetMutableBinding(F, fobj, false).
5. v. v. Return unused.
B.3.2.4 CHANGES TO BLOCK STATIC SEMANTICS: EARLY ERRORS
The rules for the following production in 14.2.1 are modified with the addition
of the highlighted text:
Block : { StatementList }
* It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains
any duplicate entries, unless the source text matched by this production is
not strict mode code and the duplicate entries are only bound by
FunctionDeclarations.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
StatementList also occurs in the VarDeclaredNames of StatementList.
B.3.2.5 CHANGES TO SWITCH STATEMENT STATIC SEMANTICS: EARLY ERRORS
The rules for the following production in 14.12.1 are modified with the addition
of the highlighted text:
SwitchStatement : switch ( Expression ) CaseBlock
* It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any
duplicate entries, unless the source text matched by this production is not
strict mode code and the duplicate entries are only bound by
FunctionDeclarations.
* It is a Syntax Error if any element of the LexicallyDeclaredNames of
CaseBlock also occurs in the VarDeclaredNames of CaseBlock.
B.3.2.6 CHANGES TO BLOCKDECLARATIONINSTANTIATION
During BlockDeclarationInstantiation the following steps are performed in place
of step 3.a.ii.1:
1. 1. 1. If ! env.HasBinding(dn) is false, then
1. a. a. Perform ! env.CreateMutableBinding(dn, false).
During BlockDeclarationInstantiation the following steps are performed in place
of step 3.b.iii:
3. iii. iii. Perform the following steps:
1. 1. 1. If the binding for fn in env is an uninitialized binding, then
1. a. a. Perform ! env.InitializeBinding(fn, fo).
2. 2. 2. Else,
1. a. a. Assert: d is a FunctionDeclaration.
2. b. b. Perform ! env.SetMutableBinding(fn, fo, false).
B.3.3 FUNCTIONDECLARATIONS IN IFSTATEMENT STATEMENT CLAUSES
The following augments the IfStatement production in 14.6:
IfStatement[Yield, Await, Return] : if ( Expression[+In, ?Yield, ?Await] )
FunctionDeclaration[?Yield, ?Await, ~Default] else Statement[?Yield, ?Await,
?Return] if ( Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await,
?Return] else FunctionDeclaration[?Yield, ?Await, ~Default] if ( Expression[+In,
?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] else
FunctionDeclaration[?Yield, ?Await, ~Default] if ( Expression[+In, ?Yield,
?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] [lookahead ≠ else]
This production only applies when parsing non-strict code. Source text matched
by this production is processed as if each matching occurrence of
FunctionDeclaration[?Yield, ?Await, ~Default] was the sole StatementListItem of
a BlockStatement occupying that position in the source text. The semantics of
such a synthetic BlockStatement includes the web legacy compatibility semantics
specified in B.3.2.
B.3.4 VARIABLESTATEMENTS IN CATCH BLOCKS
The content of subclause 14.15.1 is replaced with the following:
Catch : catch ( CatchParameter ) Block
* It is a Syntax Error if BoundNames of CatchParameter contains any duplicate
elements.
* It is a Syntax Error if any element of the BoundNames of CatchParameter also
occurs in the LexicallyDeclaredNames of Block.
* It is a Syntax Error if any element of the BoundNames of CatchParameter also
occurs in the VarDeclaredNames of Block unless CatchParameter is
CatchParameter : BindingIdentifier .
Note
The Block of a Catch clause may contain var declarations that bind a name that
is also bound by the CatchParameter. At runtime, such bindings are instantiated
in the VariableDeclarationEnvironment. They do not shadow the same-named
bindings introduced by the CatchParameter and hence the Initializer for such var
declarations will assign to the corresponding catch parameter rather than the
var binding.
This modified behaviour also applies to var and function declarations introduced
by direct eval calls contained within the Block of a Catch clause. This change
is accomplished by modifying the algorithm of 19.2.1.3 as follows:
Step 3.d.i.2.a.i is replaced by:
1. i. i. If thisEnv is not the Environment Record for a Catch clause, throw a
SyntaxError exception.
Step 11.b.ii.4.a.i.i is replaced by:
1. i. i. If thisEnv is not the Environment Record for a Catch clause, let
bindingExists be true.
B.3.5 INITIALIZERS IN FORIN STATEMENT HEADS
The following augments the ForInOfStatement production in 14.7.5:
ForInOfStatement[Yield, Await, Return] : for ( var BindingIdentifier[?Yield,
?Await] Initializer[~In, ?Yield, ?Await] in Expression[+In, ?Yield, ?Await] )
Statement[?Yield, ?Await, ?Return]
This production only applies when parsing non-strict code.
The static semantics of ContainsDuplicateLabels in 8.3.1 are augmented with the
following:
ForInOfStatement : for ( var BindingIdentifier Initializer in Expression )
Statement
1. 1. 1. Return ContainsDuplicateLabels of Statement with argument labelSet.
The static semantics of ContainsUndefinedBreakTarget in 8.3.2 are augmented with
the following:
ForInOfStatement : for ( var BindingIdentifier Initializer in Expression )
Statement
1. 1. 1. Return ContainsUndefinedBreakTarget of Statement with argument
labelSet.
The static semantics of ContainsUndefinedContinueTarget in 8.3.3 are augmented
with the following:
ForInOfStatement : for ( var BindingIdentifier Initializer in Expression )
Statement
1. 1. 1. Return ContainsUndefinedContinueTarget of Statement with arguments
iterationSet and « ».
The static semantics of IsDestructuring in 14.7.5.2 are augmented with the
following:
BindingIdentifier : Identifier yield await
1. 1. 1. Return false.
The static semantics of VarDeclaredNames in 8.2.6 are augmented with the
following:
ForInOfStatement : for ( var BindingIdentifier Initializer in Expression )
Statement
1. 1. 1. Let names1 be the BoundNames of BindingIdentifier.
2. 2. 2. Let names2 be the VarDeclaredNames of Statement.
3. 3. 3. Return the list-concatenation of names1 and names2.
The static semantics of VarScopedDeclarations in 8.2.7 are augmented with the
following:
ForInOfStatement : for ( var BindingIdentifier Initializer in Expression )
Statement
1. 1. 1. Let declarations1 be « BindingIdentifier ».
2. 2. 2. Let declarations2 be the VarScopedDeclarations of Statement.
3. 3. 3. Return the list-concatenation of declarations1 and declarations2.
The runtime semantics of ForInOfLoopEvaluation in 14.7.5.5 are augmented with
the following:
ForInOfStatement : for ( var BindingIdentifier Initializer in Expression )
Statement
1. 1. 1. Let bindingId be StringValue of BindingIdentifier.
2. 2. 2. Let lhs be ? ResolveBinding(bindingId).
3. 3. 3. If IsAnonymousFunctionDefinition(Initializer) is true, then
1. a. a. Let value be ? NamedEvaluation of Initializer with argument
bindingId.
4. 4. 4. Else,
1. a. a. Let rhs be ? Evaluation of Initializer.
2. b. b. Let value be ? GetValue(rhs).
5. 5. 5. Perform ? PutValue(lhs, value).
6. 6. 6. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
7. 7. 7. Return ? ForIn/OfBodyEvaluation(BindingIdentifier, Statement,
keyResult, enumerate, varBinding, labelSet).
B.3.6 THE [[ISHTMLDDA]] INTERNAL SLOT
An [[IsHTMLDDA]] internal slot may exist on host-defined objects. Objects with
an [[IsHTMLDDA]] internal slot behave like undefined in the ToBoolean and
IsLooselyEqual abstract operations and when used as an operand for the typeof
operator.
Note
Objects with an [[IsHTMLDDA]] internal slot are never created by this
specification. However, the document.all object in web browsers is a
host-defined exotic object with this slot that exists for web compatibility
purposes. There are no other known examples of this type of object and
implementations should not create any with the exception of document.all.
B.3.6.1 CHANGES TO TOBOOLEAN
The following step replaces step 3 of ToBoolean:
3. 3. 3. If argument is an Object and argument has an [[IsHTMLDDA]] internal
slot, return false.
B.3.6.2 CHANGES TO ISLOOSELYEQUAL
The following steps replace step 4 of IsLooselyEqual:
4. 4. 4. Perform the following steps:
1. a. a. If x is an Object, x has an [[IsHTMLDDA]] internal slot, and y is
either null or undefined, return true.
2. b. b. If x is either null or undefined, y is an Object, and y has an
[[IsHTMLDDA]] internal slot, return true.
B.3.6.3 CHANGES TO THE TYPEOF OPERATOR
The following step replaces step 12 of the evaluation semantics for typeof:
12. 12. 12. If val has an [[IsHTMLDDA]] internal slot, return "undefined".
B.3.7 NON-DEFAULT BEHAVIOUR IN HOSTMAKEJOBCALLBACK
The HostMakeJobCallback abstract operation allows hosts which are web browsers
to specify non-default behaviour.
B.3.8 NON-DEFAULT BEHAVIOUR IN HOSTENSURECANADDPRIVATEELEMENT
The HostEnsureCanAddPrivateElement abstract operation allows hosts which are web
browsers to specify non-default behaviour.
C THE STRICT MODE OF ECMASCRIPT
The strict mode restriction and exceptions
* implements, interface, let, package, private, protected, public, static, and
yield are reserved words within strict mode code. (12.7.2).
* A conforming implementation, when processing strict mode code, must disallow
instances of the productions NumericLiteral :: LegacyOctalIntegerLiteral and
DecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral .
* A conforming implementation, when processing strict mode code, must disallow
instances of the productions EscapeSequence :: LegacyOctalEscapeSequence and
EscapeSequence :: NonOctalDecimalEscapeSequence .
* Assignment to an undeclared identifier or otherwise unresolvable reference
does not create a property in the global object. When a simple assignment
occurs within strict mode code, its LeftHandSideExpression must not evaluate
to an unresolvable Reference. If it does a ReferenceError exception is thrown
(6.2.5.6). The LeftHandSideExpression also may not be a reference to a data
property with the attribute value { [[Writable]]: false }, to an accessor
property with the attribute value { [[Set]]: undefined }, nor to a
non-existent property of an object whose [[Extensible]] internal slot is
false. In these cases a TypeError exception is thrown (13.15).
* An IdentifierReference with the StringValue "eval" or "arguments" may not
appear as the LeftHandSideExpression of an Assignment operator (13.15) or of
an UpdateExpression (13.4) or as the UnaryExpression operated upon by a
Prefix Increment (13.4.4) or a Prefix Decrement (13.4.5) operator.
* Arguments objects for strict functions define a non-configurable accessor
property "callee" which throws a TypeError exception on access (10.4.4.6).
* Arguments objects for strict functions do not dynamically share their
array-indexed property values with the corresponding formal parameter
bindings of their functions. (10.4.4).
* For strict functions, if an arguments object is created the binding of the
local identifier arguments to the arguments object is immutable and hence may
not be the target of an assignment expression. (10.2.11).
* It is a SyntaxError if the StringValue of a BindingIdentifier is either
"eval" or "arguments" within strict mode code (13.1.1).
* Strict mode eval code cannot instantiate variables or functions in the
variable environment of the caller to eval. Instead, a new variable
environment is created and that environment is used for declaration binding
instantiation for the eval code (19.2.1).
* If this is evaluated within strict mode code, then the this value is not
coerced to an object. A this value of either undefined or null is not
converted to the global object and primitive values are not converted to
wrapper objects. The this value passed via a function call (including calls
made using Function.prototype.apply and Function.prototype.call) do not
coerce the passed this value to an object (10.2.1.2, 20.2.3.1, 20.2.3.3).
* When a delete operator occurs within strict mode code, a SyntaxError is
thrown if its UnaryExpression is a direct reference to a variable, function
argument, or function name (13.5.1.1).
* When a delete operator occurs within strict mode code, a TypeError is thrown
if the property to be deleted has the attribute { [[Configurable]]: false }
or otherwise cannot be deleted (13.5.1.2).
* Strict mode code may not include a WithStatement. The occurrence of a
WithStatement in such a context is a SyntaxError (14.11.1).
* It is a SyntaxError if a CatchParameter occurs within strict mode code and
BoundNames of CatchParameter contains either eval or arguments (14.15.1).
* It is a SyntaxError if the same BindingIdentifier appears more than once in
the FormalParameters of a strict function. An attempt to create such a
function using a Function, Generator, or AsyncFunction constructor is a
SyntaxError (15.2.1, 20.2.1.1.1).
* An implementation may not extend, beyond that defined in this specification,
the meanings within strict functions of properties named "caller" or
"arguments" of function instances.
D HOST LAYERING POINTS
See 4.2 for the definition of host.
D.1 HOST HOOKS
HostCallJobCallback(...)
HostEnqueueFinalizationRegistryCleanupJob(...)
HostEnqueuePromiseJob(...)
HostEnsureCanCompileStrings(...)
HostFinalizeImportMeta(...)
HostGetImportMetaProperties(...)
HostHasSourceTextAvailable(...)
HostLoadImportedModule(...)
HostMakeJobCallback(...)
HostPromiseRejectionTracker(...)
InitializeHostDefinedRealm(...)
D.2 HOST-DEFINED FIELDS
[[HostDefined]] on Realm Records: See Table 24.
[[HostDefined]] on Script Records: See Table 39.
[[HostDefined]] on Module Records: See Table 40.
[[HostDefined]] on JobCallback Records: See Table 28.
[[HostSynchronizesWith]] on Candidate Executions: See Table 90.
[[IsHTMLDDA]]: See B.3.6.
D.3 HOST-DEFINED OBJECTS
The global object: See clause 19.
D.4 RUNNING JOBS
Preparation steps before, and cleanup steps after, invocation of Job Abstract
Closures. See 9.5.
D.5 INTERNAL METHODS OF EXOTIC OBJECTS
Any of the essential internal methods in Table 4 for any exotic object not
specified within this specification.
D.6 BUILT-IN OBJECTS AND METHODS
Any built-in objects and methods not defined within this specification, except
as restricted in 17.1.
E CORRECTIONS AND CLARIFICATIONS IN ECMASCRIPT 2015 WITH POSSIBLE COMPATIBILITY
IMPACT
9.1.1.4.15-9.1.1.4.18 Edition 5 and 5.1 used a property existence test to
determine whether a global object property corresponding to a new global
declaration already existed. ECMAScript 2015 uses an own property existence
test. This corresponds to what has been most commonly implemented by web
browsers.
10.4.2.1: The 5th Edition moved the capture of the current array length prior to
the integer conversion of the array index or new length value. However, the
captured length value could become invalid if the conversion process has the
side-effect of changing the array length. ECMAScript 2015 specifies that the
current array length must be captured after the possible occurrence of such
side-effects.
21.4.1.17: Previous editions permitted the TimeClip abstract operation to return
either +0𝔽 or -0𝔽 as the representation of a 0 time value. ECMAScript 2015
specifies that +0𝔽 always returned. This means that for ECMAScript 2015 the
time value of a Date is never observably -0𝔽 and methods that return time
values never return -0𝔽.
21.4.1.18: If a UTC offset representation is not present, the local time zone is
used. Edition 5.1 incorrectly stated that a missing time zone should be
interpreted as "z".
21.4.4.36: If the year cannot be represented using the Date Time String Format
specified in 21.4.1.18 a RangeError exception is thrown. Previous editions did
not specify the behaviour for that case.
21.4.4.41: Previous editions did not specify the value returned by
Date.prototype.toString when this time value is NaN. ECMAScript 2015 specifies
the result to be the String value "Invalid Date".
22.2.4.1, 22.2.6.13.1: Any LineTerminator code points in the value of the
"source" property of a RegExp instance must be expressed using an escape
sequence. Edition 5.1 only required the escaping of /.
22.2.6.8, 22.2.6.11: In previous editions, the specifications for
String.prototype.match and String.prototype.replace was incorrect for cases
where the pattern argument was a RegExp value whose global flag is set. The
previous specifications stated that for each attempt to match the pattern, if
lastIndex did not change, it should be incremented by 1. The correct behaviour
is that lastIndex should be incremented by 1 only if the pattern matched the
empty String.
23.1.3.30: Previous editions did not specify how a NaN value returned by a
comparefn was interpreted by Array.prototype.sort. ECMAScript 2015 specifies
that such as value is treated as if +0𝔽 was returned from the comparefn.
ECMAScript 2015 also specifies that ToNumber is applied to the result returned
by a comparefn. In previous editions, the effect of a comparefn result that is
not a Number value was implementation-defined. In practice, implementations call
ToNumber.
F ADDITIONS AND CHANGES THAT INTRODUCE INCOMPATIBILITIES WITH PRIOR EDITIONS
6.2.5: In ECMAScript 2015, Function calls are not allowed to return a Reference
Record.
7.1.4.1: In ECMAScript 2015, ToNumber applied to a String value now recognizes
and converts BinaryIntegerLiteral and OctalIntegerLiteral numeric strings. In
previous editions such strings were converted to NaN.
9.3: In ECMAScript 2018, Template objects are canonicalized based on Parse Node
(source location), instead of across all occurrences of that template literal or
tagged template in a Realm in previous editions.
12.2: In ECMAScript 2016, Unicode 8.0.0 or higher is mandated, as opposed to
ECMAScript 2015 which mandated Unicode 5.1. In particular, this caused U+180E
MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator (Zs) category and
thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf)
category (as of Unicode 6.3.0). This causes whitespace-sensitive methods to
behave differently. For example, "\u180E".trim().length was 0 in previous
editions, but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017
mandated always using the latest version of the Unicode Standard.
12.7: In ECMAScript 2015, the valid code points for an IdentifierName are
specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In
previous editions, the valid IdentifierName or Identifier code points were
specified by enumerating various Unicode code point categories.
12.10.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at
the end of a do-while statement if the semicolon is missing. This change aligns
the specification with the actual behaviour of most existing implementations.
13.2.5.1: In ECMAScript 2015, it is no longer an early error to have duplicate
property names in Object Initializers.
13.15.1: In ECMAScript 2015, strict mode code containing an assignment to an
immutable binding such as the function name of a FunctionExpression does not
produce an early error. Instead it produces a runtime error.
14.2: In ECMAScript 2015, a StatementList beginning with the token let followed
by the input elements LineTerminator then Identifier is the start of a
LexicalDeclaration. In previous editions, automatic semicolon insertion would
always insert a semicolon before the Identifier input element.
14.5: In ECMAScript 2015, a StatementListItem beginning with the token let
followed by the token [ is the start of a LexicalDeclaration. In previous
editions such a sequence would be the start of an ExpressionStatement.
14.6.2: In ECMAScript 2015, the normal result of an IfStatement is never the
value empty. If no Statement part is evaluated or if the evaluated Statement
part produces a normal completion containing empty, the result of the
IfStatement is undefined.
14.7: In ECMAScript 2015, if the ( token of a for statement is immediately
followed by the token sequence let [ then the let is treated as the start of a
LexicalDeclaration. In previous editions such a token sequence would be the
start of an Expression.
14.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately
followed by the token sequence let [ then the let is treated as the start of a
ForDeclaration. In previous editions such a token sequence would be the start of
an LeftHandSideExpression.
14.7: Prior to ECMAScript 2015, an initialization expression could appear as
part of the VariableDeclaration that precedes the in keyword. In ECMAScript
2015, the ForBinding in that same position does not allow the occurrence of such
an initializer. In ECMAScript 2017, such an initializer is permitted only in
non-strict code.
14.7: In ECMAScript 2015, the result of evaluating an IterationStatement is
never a normal completion whose [[Value]] is empty. If the Statement part of an
IterationStatement is not evaluated or if the final evaluation of the Statement
part produces a normal completion whose [[Value]] is empty, the result of
evaluating the IterationStatement is a normal completion whose [[Value]] is
undefined.
14.11.2: In ECMAScript 2015, the result of evaluating a WithStatement is never a
normal completion whose [[Value]] is empty. If evaluation of the Statement part
of a WithStatement produces a normal completion whose [[Value]] is empty, the
result of evaluating the WithStatement is a normal completion whose [[Value]] is
undefined.
14.12.4: In ECMAScript 2015, the result of evaluating a SwitchStatement is never
a normal completion whose [[Value]] is empty. If evaluation of the CaseBlock
part of a SwitchStatement produces a normal completion whose [[Value]] is empty,
the result of evaluating the SwitchStatement is a normal completion whose
[[Value]] is undefined.
14.15: In ECMAScript 2015, it is an early error for a Catch clause to contain a
var declaration for the same Identifier that appears as the Catch clause
parameter. In previous editions, such a variable declaration would be
instantiated in the enclosing variable environment but the declaration's
Initializer value would be assigned to the Catch parameter.
14.15, 19.2.1.3: In ECMAScript 2015, a runtime SyntaxError is thrown if a Catch
clause evaluates a non-strict direct eval whose eval code includes a var or
FunctionDeclaration declaration that binds the same Identifier that appears as
the Catch clause parameter.
14.15.3: In ECMAScript 2015, the result of a TryStatement is never the value
empty. If the Block part of a TryStatement evaluates to a normal completion
containing empty, the result of the TryStatement is undefined. If the Block part
of a TryStatement evaluates to a throw completion and it has a Catch part that
evaluates to a normal completion containing empty, the result of the
TryStatement is undefined if there is no Finally clause or if its Finally clause
evaluates to an empty normal completion.
15.4.5 In ECMAScript 2015, the function objects that are created as the values
of the [[Get]] or [[Set]] attribute of accessor properties in an ObjectLiteral
are not constructor functions and they do not have a "prototype" own property.
In the previous edition, they were constructors and had a "prototype" property.
20.1.2.6: In ECMAScript 2015, if the argument to Object.freeze is not an object
it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.8: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor
is not an object an attempt is made to coerce the argument using ToObject. If
the coercion is successful the result is used in place of the original argument
value. In the previous edition, a non-object argument always causes a TypeError
to be thrown.
20.1.2.10: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is
not an object an attempt is made to coerce the argument using ToObject. If the
coercion is successful the result is used in place of the original argument
value. In the previous edition, a non-object argument always causes a TypeError
to be thrown.
20.1.2.12: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not
an object an attempt is made to coerce the argument using ToObject. If the
coercion is successful the result is used in place of the original argument
value. In the previous edition, a non-object argument always causes a TypeError
to be thrown.
20.1.2.15: In ECMAScript 2015, if the argument to Object.isExtensible is not an
object it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.16: In ECMAScript 2015, if the argument to Object.isFrozen is not an
object it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.17: In ECMAScript 2015, if the argument to Object.isSealed is not an
object it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.18: In ECMAScript 2015, if the argument to Object.keys is not an object
an attempt is made to coerce the argument using ToObject. If the coercion is
successful the result is used in place of the original argument value. In the
previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.19: In ECMAScript 2015, if the argument to Object.preventExtensions is
not an object it is treated as if it was a non-extensible ordinary object with
no own properties. In the previous edition, a non-object argument always causes
a TypeError to be thrown.
20.1.2.21: In ECMAScript 2015, if the argument to Object.seal is not an object
it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound
function is set to the [[GetPrototypeOf]] value of its target function. In the
previous edition, [[Prototype]] was always set to %Function.prototype%.
20.2.4.1: In ECMAScript 2015, the "length" property of function instances is
configurable. In previous editions it was non-configurable.
20.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a NativeError
constructor is the Error constructor. In previous editions it was the Function
prototype object.
21.4.4 In ECMAScript 2015, the Date prototype object is not a Date instance. In
previous editions it was a Date instance whose TimeValue was NaN.
22.1.3.11 In ECMAScript 2015, the String.prototype.localeCompare function must
treat Strings that are canonically equivalent according to the Unicode Standard
as being identical. In previous editions implementations were permitted to
ignore canonical equivalence and could instead use a bit-wise comparison.
22.1.3.27 and 22.1.3.29 In ECMAScript 2015, lowercase/upper conversion
processing operates on code points. In previous editions such the conversion
processing was only applied to individual code units. The only affected code
points are those in the Deseret block of Unicode.
22.1.3.30 In ECMAScript 2015, the String.prototype.trim method is defined to
recognize white space code points that may exist outside of the Unicode BMP.
However, as of Unicode 7 no such code points are defined. In previous editions
such code points would not have been recognized as white space.
22.2.4.1 In ECMAScript 2015, If the pattern argument is a RegExp instance and
the flags argument is not undefined, a new RegExp instance is created just like
pattern except that pattern's flags are replaced by the argument flags. In
previous editions a TypeError exception was thrown when pattern was a RegExp
instance and flags was not undefined.
22.2.6 In ECMAScript 2015, the RegExp prototype object is not a RegExp instance.
In previous editions it was a RegExp instance whose pattern is the empty String.
22.2.6 In ECMAScript 2015, "source", "global", "ignoreCase", and "multiline" are
accessor properties defined on the RegExp prototype object. In previous editions
they were data properties defined on RegExp instances.
25.4.13: In ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to
prevent confusion with Atomics.wait.
27.1.4.4, 27.6.3.6: In ECMAScript 2019, the number of Jobs enqueued by await was
reduced, which could create an observable difference in resolution order between
a then() call and an await expression.
G COLOPHON
This specification is authored on GitHub in a plaintext source format called
Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and
toolset for authoring ECMAScript specifications in plaintext and processing the
specification into a full-featured HTML rendering that follows the editorial
conventions for this document. Ecmarkup builds on and integrates a number of
other formats and technologies including Grammarkdown for defining syntax and
Ecmarkdown for authoring algorithm steps. PDF renderings of this specification
are produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the Ecmarkup
source text that formed the basis of this edition was produced by converting the
ECMAScript 2015 Word document to Ecmarkup using an automated conversion tool.
H BIBLIOGRAPHY
1. IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of
Electrical and Electronic Engineers, New York (2019) Note
There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that
affect the ECMA-262 specification.
2. The Unicode Standard, available at <https://unicode.org/versions/latest>
3. Unicode Technical Note #5: Canonical Equivalence in Applications, available
at <https://unicode.org/notes/tn5/>
4. Unicode Technical Standard #10: Unicode Collation Algorithm, available at
<https://unicode.org/reports/tr10/>
5. Unicode Standard Annex #15, Unicode Normalization Forms, available at
<https://unicode.org/reports/tr15/>
6. Unicode Standard Annex #18: Unicode Regular Expressions, available at
<https://unicode.org/reports/tr18/>
7. Unicode Standard Annex #24: Unicode Script Property, available at
<https://unicode.org/reports/tr24/>
8. Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax,
available at <https://unicode.org/reports/tr31/>
9. Unicode Standard Annex #44: Unicode Character Database, available at
<https://unicode.org/reports/tr44/>
10. Unicode Technical Standard #51: Unicode Emoji, available at
<https://unicode.org/reports/tr51/>
11. IANA Time Zone Database, available at <https://www.iana.org/time-zones>
12. ISO 8601:2004(E) Data elements and interchange formats — Information
interchange — Representation of dates and times
13. RFC 1738 “Uniform Resource Locators (URL)”, available at
<https://tools.ietf.org/html/rfc1738>
14. RFC 2396 “Uniform Resource Identifiers (URI): Generic Syntax”, available at
<https://tools.ietf.org/html/rfc2396>
15. RFC 3629 “UTF-8, a transformation format of ISO 10646”, available at
<https://tools.ietf.org/html/rfc3629>
16. RFC 7231 “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”,
available at <https://tools.ietf.org/html/rfc7231>
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