Stage 1 Draft / June 12, 2024

Discard Bindings

Introduction

See the proposal repository for background material and discussion.

8 Syntax-Directed Operations

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. Return a List whose sole element is the StringValue of Identifier.
 BindingIdentifier : yield
  1. Return « "yield" ».
 BindingIdentifier : await
  1. Return « "await" ».
 LexicalDeclaration : LetOrConst BindingList ;
  1. Return the BoundNames of BindingList.
 UsingDeclaration : using BindingList ; AwaitUsingDeclaration : CoverAwaitExpressionAndAwaitUsingDeclarationHead BindingList ;
  1. Return the BoundNames of BindingList.
 BindingList : BindingList , LexicalBinding
  1. Let names1 be the BoundNames of BindingList.
  2. Let names2 be the BoundNames of LexicalBinding.
  3. Return the list-concatenation of names1 and names2.
 LexicalBinding : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 LexicalBinding : BindingPattern Initializer
  1. Return the BoundNames of BindingPattern.
 LexicalBinding : void Initializer
  1. Return a new empty List.
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Let names1 be BoundNames of VariableDeclarationList.
  2. Let names2 be BoundNames of VariableDeclaration.
  3. Return the list-concatenation of names1 and names2.
 VariableDeclaration : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 VariableDeclaration : BindingPattern Initializer
  1. Return the BoundNames of BindingPattern.
 ObjectBindingPattern : { }
  1. Return a new empty List.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Let names1 be BoundNames of BindingPropertyList.
  2. Let names2 be BoundNames of BindingRestProperty.
  3. Return the list-concatenation of names1 and names2.
 ArrayBindingPattern : [ Elisionopt ]
  1. Return a new empty List.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. Return the BoundNames of BindingRestElement.
 ArrayBindingPattern : [ BindingElementList , Elisionopt ]
  1. Return the BoundNames of BindingElementList.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Let names1 be BoundNames of BindingElementList.
  2. Let names2 be BoundNames of BindingRestElement.
  3. Return the list-concatenation of names1 and names2.
 BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let names1 be BoundNames of BindingPropertyList.
  2. Let names2 be BoundNames of BindingProperty.
  3. Return the list-concatenation of names1 and names2.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Let names1 be BoundNames of BindingElementList.
  2. Let names2 be BoundNames of BindingElisionElement.
  3. Return the list-concatenation of names1 and names2.
 BindingElisionElement : Elisionopt BindingElement
  1. Return BoundNames of BindingElement.
 BindingProperty : PropertyName : BindingElement
  1. Return the BoundNames of BindingElement.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 BindingElement : BindingPattern Initializeropt
  1. Return the BoundNames of BindingPattern.
 BindingElement : void
  1. Return a new empty List.
 ForDeclaration : LetOrConst ForBinding
  1. Return the BoundNames of ForBinding.
 FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Return the BoundNames of BindingIdentifier.
 FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
  1. Return « "*default*" ».
 FormalParameters : [empty]
  1. Return a new empty List.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Let names1 be BoundNames of FormalParameterList.
  2. Let names2 be BoundNames of FunctionRestParameter.
  3. Return the list-concatenation of names1 and names2.
 FormalParameterList : FormalParameterList , FormalParameter
  1. Let names1 be BoundNames of FormalParameterList.
  2. Let names2 be BoundNames of FormalParameter.
  3. Return the list-concatenation of names1 and names2.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return the BoundNames of formals.
 GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody }
  1. Return the BoundNames of BindingIdentifier.
 GeneratorDeclaration : function * ( FormalParameters ) { GeneratorBody }
  1. Return « "*default*" ».
 AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }
  1. Return the BoundNames of BindingIdentifier.
 AsyncGeneratorDeclaration : async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Return « "*default*" ».
 ClassDeclaration : class BindingIdentifier ClassTail
  1. Return the BoundNames of BindingIdentifier.
 ClassDeclaration : class ClassTail
  1. Return « "*default*" ».
 AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody }
  1. Return the BoundNames of BindingIdentifier.
 AsyncFunctionDeclaration : async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return « "*default*" ».
 CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments
  1. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
  2. Return the BoundNames of head.
 ImportDeclaration : import ImportClause FromClause ;
  1. Return the BoundNames of ImportClause.
 ImportDeclaration : import ModuleSpecifier ;
  1. Return a new empty List.
 ImportClause : ImportedDefaultBinding , NameSpaceImport
  1. Let names1 be the BoundNames of ImportedDefaultBinding.
  2. Let names2 be the BoundNames of NameSpaceImport.
  3. Return the list-concatenation of names1 and names2.
 ImportClause : ImportedDefaultBinding , NamedImports
  1. Let names1 be the BoundNames of ImportedDefaultBinding.
  2. Let names2 be the BoundNames of NamedImports.
  3. Return the list-concatenation of names1 and names2.
 NamedImports : { }
  1. Return a new empty List.
 ImportsList : ImportsList , ImportSpecifier
  1. Let names1 be the BoundNames of ImportsList.
  2. Let names2 be the BoundNames of ImportSpecifier.
  3. Return the list-concatenation of names1 and names2.
 ImportSpecifier : ModuleExportName as ImportedBinding
  1. Return the BoundNames of ImportedBinding.
 ExportDeclaration : export ExportFromClause FromClause ; export NamedExports ;
  1. Return a new empty List.
 ExportDeclaration : export VariableStatement
  1. Return the BoundNames of VariableStatement.
 ExportDeclaration : export Declaration
  1. Return the BoundNames of Declaration.
 ExportDeclaration : export default HoistableDeclaration
  1. Let declarationNames be the BoundNames of HoistableDeclaration.
  2. If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
  3. Return declarationNames.
 ExportDeclaration : export default ClassDeclaration
  1. Let declarationNames be the BoundNames of ClassDeclaration.
  2. If declarationNames does not include the element "*default*", append "*default*" to declarationNames.
  3. Return declarationNames.
 ExportDeclaration : export default AssignmentExpression ;
  1. Return « "*default*" ».

8.6 Miscellaneous

8.6.1 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. Let name be StringValue of Identifier.
  2. Return ? InitializeBoundName(name, value, environment).
 BindingIdentifier : yield
  1. Return ? InitializeBoundName("yield", value, environment).
 BindingIdentifier : await
  1. Return ? InitializeBoundName("await", value, environment).
 BindingPattern : ObjectBindingPattern
  1. Perform ? RequireObjectCoercible(value).
  2. Return ? BindingInitialization of ObjectBindingPattern with arguments value and environment.
 BindingPattern : ArrayBindingPattern
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Let result be Completion(IteratorBindingInitialization of ArrayBindingPattern with arguments iteratorRecord and environment).
  3. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
  4. Return ? result.
 ObjectBindingPattern : { }
  1. Return unused.
 ObjectBindingPattern : { BindingPropertyList } { BindingPropertyList , }
  1. Perform ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Return unused.
 ObjectBindingPattern : { BindingRestProperty }
  1. Let excludedNames be a new empty List.
  2. Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Let excludedNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.

13 ECMAScript Language: Expressions

13.2 Primary Expression

13.2.4 Array Initializer

Note 1

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] CoverDiscardElement ElementList[?Yield, ?Await] , Elisionopt AssignmentExpression[+In, ?Yield, ?Await] ElementList[?Yield, ?Await] , Elisionopt SpreadElement[?Yield, ?Await] ElementList[?Yield, ?Await] , CoverDiscardElement Elision : , Elision , SpreadElement[Yield, Await] : ... AssignmentExpression[+In, ?Yield, ?Await] CoverDiscardElement : Elisionopt void Note 2

In certain contexts, ArrayLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverDiscardElement 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 ArrayLiteral is expected.

13.2.4.1 Static Semantics: Early Errors

In addition to describing an actual array initializer the ArrayLiteral productions are also used as a cover grammar for ArrayAssignmentPattern and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When ArrayLiteral appears in a context where ArrayAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or CoverCallExpressionAndAsyncArrowHead.

CoverDiscardElement : Elisionopt void
  • It is a Syntax Error if any source text is matched by this produciton.
Note

This production exists so that ArrayLiteral can serve as a cover grammar for ArrayAssignmentPattern. It cannot occur in an actual array initializer.

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] CoverDiscardProperty[?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] CoverDiscardProperty[Yield, Await] : PropertyName[?Yield, ?Await] : void 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 and CoverDiscardProperty productions are necessary to fully cover these secondary grammars. However, use of this productionthese productions 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

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.

 PropertyDefinition : CoverDiscardProperty
  • It is a Syntax Error if any source text is matched by this production.
Note 2

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 , } Note 3

The List returned by PropertyNameList does not include property names defined using a ComputedPropertyName.

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] CoverDiscardArgument ArgumentList[?Yield, ?Await] , AssignmentExpression[+In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , ... AssignmentExpression[+In, ?Yield, ?Await] ArgumentList[?Yield, ?Await] , CoverDiscardArgument 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] Note 1

In certain contexts, Arguments is used as a cover grammar for a more restricted secondary grammar. The CoverDiscardArgument 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 Arguments is expected.

 Note 2

DRAFT NOTE: The above changes are only applied when considering how this proposal interacts with the Extractors proposal.

13.3.1 Static Semantics: Early Errors

In addition to describing an actual arguments list the Arguments productions are also used as a cover grammar for ExtractorAssignmentPattern and may be recognized as part of a CoverParenthesizedExpressionAndArrowParameterList. When Arguments appears in a context where ExtractorAssignmentPattern is required the following Early Error rules are not applied. In addition, they are not applied when initially parsing a CoverParenthesizedExpressionAndArrowParameterList or CoverCallExpressionAndAsyncArrowHead.

CoverDiscardArgument : Elisionopt void
  • It is a Syntax Error if any source text is matched by this produciton.
Note 1

This production exists so that Arguments can serve as a cover grammar for ExtractorAssignmentPattern. It cannot occur in an actual arguments list.

 Note 2

DRAFT NOTE: The above changes are only applied when considering how this proposal interacts with the Extractors proposal.

13.15 Assignment Operators

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 void AssignmentRestElement[Yield, Await] : ... DestructuringAssignmentTarget[?Yield, ?Await] DestructuringAssignmentTarget[Yield, Await] : LeftHandSideExpression[?Yield, ?Await]

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. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement with argument iteratorRecord.
 AssignmentElementList : AssignmentElementList , AssignmentElisionElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of AssignmentElementList with argument iteratorRecord.
  2. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElisionElement with argument iteratorRecord.
 AssignmentElisionElement : AssignmentElement
  1. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElement with argument iteratorRecord.
 AssignmentElisionElement : Elision AssignmentElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. Return ? IteratorDestructuringAssignmentEvaluation of AssignmentElement with argument iteratorRecord.
 Elision : ,
  1. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
  2. Return unused.
 Elision : Elision ,
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, set iteratorRecord.[[Done]] to true.
  3. Return unused.
 AssignmentElement : DestructuringAssignmentTarget Initializeropt
  1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. Let value be undefined.
  3. If iteratorRecord.[[Done]] is false, then
    1. Let next be ? IteratorStepValue(iteratorRecord).
    2. If next is not done, then
      1. Set value to next.
  4. If Initializer is present and value is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true and IsIdentifierRef of DestructuringAssignmentTarget is true, then
      1. Let v be ? NamedEvaluation of Initializer with argument lref.[[ReferencedName]].
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Let v be ? GetValue(defaultValue).
  5. Else,
    1. Let v be value.
  6. If DestructuringAssignmentTarget is either an ObjectLiteral or an ArrayLiteral, then
    1. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
    2. Return ? DestructuringAssignmentEvaluation of nestedAssignmentPattern with argument v.
  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.

 AssignmentElement : void
  1. If iteratorRecord.[[Done]] is false, then
    1. Perform ? IteratorStepValue(iteratorRecord).
  2. Return unused.
 AssignmentRestElement : ... DestructuringAssignmentTarget
  1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. Let A be ! ArrayCreate(0).
  3. Let n be 0.
  4. Repeat, while iteratorRecord.[[Done]] is false,
    1. Let next be ? IteratorStepValue(iteratorRecord).
    2. If next is not done, then
      1. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), next).
      2. Set n to n + 1.
  5. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Return ? PutValue(lref, A).
  6. Let nestedAssignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
  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. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be ? Evaluation of DestructuringAssignmentTarget.
  2. Let v be ? GetV(value, propertyName).
  3. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) and IsIdentifierRef of DestructuringAssignmentTarget are both true, then
      1. Let rhsValue be ? NamedEvaluation of Initializer with argument lref.[[ReferencedName]].
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Let rhsValue be ? GetValue(defaultValue).
  4. Else,
    1. Let rhsValue be v.
  5. If DestructuringAssignmentTarget is either an ObjectLiteral or an ArrayLiteral, then
    1. Let assignmentPattern be the AssignmentPattern that is covered by DestructuringAssignmentTarget.
    2. Return ? DestructuringAssignmentEvaluation of assignmentPattern with argument rhsValue.
  6. Return ? PutValue(lref, rhsValue).
 AssignmentElement : void
  1. Return unused.

14 ECMAScript Language: Statements and Declarations

14.3 Declarations and the Variable Statement

14.3.1 Let, Const, Using, and Await Using Declarations

Syntax

LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield, ?Await, +Pattern] ; UsingDeclaration[?In, ?Yield, ?Await] [+Await] AwaitUsingDeclaration[?In, ?Yield] LetOrConst : let const UsingDeclaration[In, Yield, Await] : using [no LineTerminator here] BindingList[?In, ?Yield, ?Await, ~Pattern] ; AwaitUsingDeclaration[In, Yield] : CoverAwaitExpressionAndAwaitUsingDeclarationHead[?Yield] [no LineTerminator here] BindingList[?In, ?Yield, +Await, ~Pattern] ; BindingList[In, Yield, Await, Pattern] : LexicalBinding[?In, ?Yield, ?Await, ?Pattern] BindingList[?In, ?Yield, ?Await, ?Pattern] , LexicalBinding[?In, ?Yield, ?Await, ?Pattern] LexicalBinding[In, Yield, Await, Pattern] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt [+Pattern] BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await] [~Pattern] void Initializer[?In, ?Yield, ?Await]

14.3.1.2 Runtime Semantics: BindingEvaluation

The syntax-directed operation BindingEvaluation takes argument hint (one of normal, sync-dispose, or async-dispose) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

BindingList : BindingList , LexicalBinding
  1. Perform ? BindingEvaluation of BindingList with argument hint.
  2. Return ? BindingEvaluation of LexicalBinding with argument hint.
 LexicalBinding : BindingIdentifier
  1. Assert: hint is normal.
  2. Let lhs be ! ResolveBinding(StringValue of BindingIdentifier).
  3. Perform ! InitializeReferencedBinding(lhs, undefined, normal).
  4. Return unused.
 Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const, using, or await using declaration.

 LexicalBinding : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ! ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be ? Evaluation of Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ? InitializeReferencedBinding(lhs, value, hint).
  6. Return unused.
 LexicalBinding : BindingPattern Initializer
  1. Assert: hint is normal.
  2. Let rhs be ? Evaluation of Initializer.
  3. Let value be ? GetValue(rhs).
  4. Let env be the running execution context's LexicalEnvironment.
  5. Return ? BindingInitialization of BindingPattern with arguments value and env.
 LexicalBinding : void Initializer
  1. Assert: hint is not normal.
  2. Let env be the running execution context's LexicalEnvironment.
  3. Assert: env is an Environment Record.
  4. Let rhs be ? Evaluation of Initializer.
  5. Let value be ? GetValue(rhs).
  6. Perform ? AddDisposableResource(env.[[DisposeCapability]], value, hint).
  7. Return unused.

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 void SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await]

A Copyright & Software License

Copyright Notice

© 2024 Ron Buckton, Ecma International

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