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spec.emu
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<!doctype html>
<meta charset="utf8">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/styles/github.min.css">
<link rel="spec" href="es2015" />
<pre class="metadata">
title: Extractors
status: proposal
stage: 1
contributors: Ron Buckton, Ecma International
</pre>
<emu-biblio href="node_modules/@tc39/ecma262-biblio/biblio.json"></emu-biblio>
<emu-intro id="intro">
<h1>Introduction</h1>
<p>See <a href="https://github.com/tc39/proposal-extractors">the proposal repository</a> for background material and discussion.</p>
<emu-note>
<p>Draft Note: Portions of this proposal are intended to align with <a href="https://github.com/tc39/proposal-pattern-matching">Pattern Matching</a>. Where appropriate, functionality shared with Pattern Matching will be called out with a Draft Note.</p>
</emu-note>
</emu-intro>
<emu-clause id="sec-ecmascript-data-types-and-values" aoid="Type" number="6">
<h1>ECMAScript Data Types and Values</h1>
<emu-clause id="sec-ecmascript-language-types">
<h1>ECMAScript Language Types</h1>
<emu-clause id="sec-ecmascript-language-types-symbol-type" number="5">
<h1>The Symbol Type</h1>
<emu-clause id="sec-well-known-symbols">
<h1>Well-Known Symbols</h1>
<emu-table id="table-1" caption="Well-known Symbols">
<table>
<tbody>
<tr>
<th>
Specification Name
</th>
<th>
[[Description]]
</th>
<th>
Value and Purpose
</th>
</tr>
<tr>
<td>
<ins><dfn>@@customMatcher</dfn></ins>
</td>
<td>
<ins>`"Symbol.customMatcher"`</ins>
</td>
<td>
<ins>A method that performs custom pattern matching and destructuring semantics. Called by the semantics of the pattern-matching and destructuring features.</ins>
</td>
</tr>
</tbody>
</table>
</emu-table>
<emu-note type="editor">
<p>Draft Note: @@customMatcher is also part of the <a href="https://github.com/tc39/proposal-pattern-matching">Pattern Matching</a> proposal.</p>
</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-algorithm-conventions-syntax-directed-operations">
<h1>Syntax-Directed Operations</h1>
<emu-clause id="sec-syntax-directed-operations-scope-analysis">
<h1>Scope Analysis</h1>
<emu-clause id="sec-static-semantics-boundnames" oldids="sec-identifiers-static-semantics-boundnames,sec-let-and-const-declarations-static-semantics-boundnames,sec-variable-statement-static-semantics-boundnames,sec-destructuring-binding-patterns-static-semantics-boundnames,sec-for-in-and-for-of-statements-static-semantics-boundnames,sec-function-definitions-static-semantics-boundnames,sec-arrow-function-definitions-static-semantics-boundnames,sec-generator-function-definitions-static-semantics-boundnames,sec-async-generator-function-definitions-static-semantics-boundnames,sec-class-definitions-static-semantics-boundnames,sec-async-function-definitions-static-semantics-BoundNames,sec-async-arrow-function-definitions-static-semantics-BoundNames,sec-imports-static-semantics-boundnames,sec-exports-static-semantics-boundnames" type="sdo">
<h1>Static Semantics: BoundNames ( ): a List of Strings</h1>
<dl class="header">
</dl>
<emu-note id="note-star-default-star">
<p>*"\*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 <emu-xref href="#sec-resolveexport" title></emu-xref> 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.</p>
</emu-note>
<emu-grammar>BindingIdentifier : Identifier</emu-grammar>
<emu-alg>
1. Return a List whose sole element is the StringValue of |Identifier|.
</emu-alg>
<emu-grammar>BindingIdentifier : `yield`</emu-grammar>
<emu-alg>
1. Return « *"yield"* ».
</emu-alg>
<emu-grammar>BindingIdentifier : `await`</emu-grammar>
<emu-alg>
1. Return « *"await"* ».
</emu-alg>
<emu-grammar>LexicalDeclaration : LetOrConst BindingList `;`</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingList|.
</emu-alg>
<emu-grammar>BindingList : BindingList `,` LexicalBinding</emu-grammar>
<emu-alg>
1. Let _names1_ be the BoundNames of |BindingList|.
1. Let _names2_ be the BoundNames of |LexicalBinding|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>LexicalBinding : BindingIdentifier Initializer?</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>LexicalBinding : BindingPattern Initializer</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingPattern|.
</emu-alg>
<emu-grammar>VariableDeclarationList : VariableDeclarationList `,` VariableDeclaration</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |VariableDeclarationList|.
1. Let _names2_ be BoundNames of |VariableDeclaration|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>VariableDeclaration : BindingIdentifier Initializer?</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>VariableDeclaration : BindingPattern Initializer</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingPattern|.
</emu-alg>
<emu-grammar>ObjectBindingPattern : `{` `}`</emu-grammar>
<emu-alg>
1. Return a new empty List.
</emu-alg>
<emu-grammar>ObjectBindingPattern : `{` BindingPropertyList `,` BindingRestProperty `}`</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |BindingPropertyList|.
1. Let _names2_ be BoundNames of |BindingRestProperty|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` Elision? `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` Elision? `)`</ins>
</emu-grammar>
<emu-alg>
1. Return a new empty List.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` Elision? BindingRestElement `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` Elision? BindingRestElement `)`</ins>
</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingRestElement|.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` BindingElementList `,` Elision? `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` BindingElementList `,` Elision? `)`</ins>
</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingElementList|.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` BindingElementList `,` Elision? BindingRestElement `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` BindingElementList `,` Elision? BindingRestElement `)`</ins>
</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |BindingElementList|.
1. Let _names2_ be BoundNames of |BindingRestElement|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>BindingPropertyList : BindingPropertyList `,` BindingProperty</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |BindingPropertyList|.
1. Let _names2_ be BoundNames of |BindingProperty|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>BindingElementList : BindingElementList `,` BindingElisionElement</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |BindingElementList|.
1. Let _names2_ be BoundNames of |BindingElisionElement|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>BindingElisionElement : Elision? BindingElement</emu-grammar>
<emu-alg>
1. Return BoundNames of |BindingElement|.
</emu-alg>
<emu-grammar>BindingProperty : PropertyName `:` BindingElement</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingElement|.
</emu-alg>
<emu-grammar>SingleNameBinding : BindingIdentifier Initializer?</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>BindingElement : BindingPattern Initializer?</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingPattern|.
</emu-alg>
<emu-grammar>ForDeclaration : LetOrConst ForBinding</emu-grammar>
<emu-alg>
1. Return the BoundNames of |ForBinding|.
</emu-alg>
<emu-grammar>FunctionDeclaration : `function` BindingIdentifier `(` FormalParameters `)` `{` FunctionBody `}`</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>FunctionDeclaration : `function` `(` FormalParameters `)` `{` FunctionBody `}`</emu-grammar>
<emu-alg>
1. Return « *"\*default\*"* ».
</emu-alg>
<emu-grammar>FormalParameters : [empty]</emu-grammar>
<emu-alg>
1. Return a new empty List.
</emu-alg>
<emu-grammar>FormalParameters : FormalParameterList `,` FunctionRestParameter</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |FormalParameterList|.
1. Let _names2_ be BoundNames of |FunctionRestParameter|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>FormalParameterList : FormalParameterList `,` FormalParameter</emu-grammar>
<emu-alg>
1. Let _names1_ be BoundNames of |FormalParameterList|.
1. Let _names2_ be BoundNames of |FormalParameter|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _formals_ be the |ArrowFormalParameters| that is covered by |CoverParenthesizedExpressionAndArrowParameterList|.
1. Return the BoundNames of _formals_.
</emu-alg>
<emu-grammar>GeneratorDeclaration : `function` `*` BindingIdentifier `(` FormalParameters `)` `{` GeneratorBody `}`</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>GeneratorDeclaration : `function` `*` `(` FormalParameters `)` `{` GeneratorBody `}`</emu-grammar>
<emu-alg>
1. Return « *"\*default\*"* ».
</emu-alg>
<emu-grammar>AsyncGeneratorDeclaration : `async` `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>AsyncGeneratorDeclaration : `async` `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
<emu-alg>
1. Return « *"\*default\*"* ».
</emu-alg>
<emu-grammar>ClassDeclaration : `class` BindingIdentifier ClassTail</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>ClassDeclaration : `class` ClassTail</emu-grammar>
<emu-alg>
1. Return « *"\*default\*"* ».
</emu-alg>
<emu-grammar>
AsyncFunctionDeclaration : `async` `function` BindingIdentifier `(` FormalParameters `)` `{` AsyncFunctionBody `}`
</emu-grammar>
<emu-alg>
1. Return the BoundNames of |BindingIdentifier|.
</emu-alg>
<emu-grammar>
AsyncFunctionDeclaration : `async` `function` `(` FormalParameters `)` `{` AsyncFunctionBody `}`
</emu-grammar>
<emu-alg>
1. Return « *"\*default\*"* ».
</emu-alg>
<emu-grammar>
CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments
</emu-grammar>
<emu-alg>
1. Let _head_ be the |AsyncArrowHead| that is covered by |CoverCallExpressionAndAsyncArrowHead|.
1. Return the BoundNames of _head_.
</emu-alg>
<emu-grammar>ImportDeclaration : `import` ImportClause FromClause `;`</emu-grammar>
<emu-alg>
1. Return the BoundNames of |ImportClause|.
</emu-alg>
<emu-grammar>ImportDeclaration : `import` ModuleSpecifier `;`</emu-grammar>
<emu-alg>
1. Return a new empty List.
</emu-alg>
<emu-grammar>ImportClause : ImportedDefaultBinding `,` NameSpaceImport</emu-grammar>
<emu-alg>
1. Let _names1_ be the BoundNames of |ImportedDefaultBinding|.
1. Let _names2_ be the BoundNames of |NameSpaceImport|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>ImportClause : ImportedDefaultBinding `,` NamedImports</emu-grammar>
<emu-alg>
1. Let _names1_ be the BoundNames of |ImportedDefaultBinding|.
1. Let _names2_ be the BoundNames of |NamedImports|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>NamedImports : `{` `}`</emu-grammar>
<emu-alg>
1. Return a new empty List.
</emu-alg>
<emu-grammar>ImportsList : ImportsList `,` ImportSpecifier</emu-grammar>
<emu-alg>
1. Let _names1_ be the BoundNames of |ImportsList|.
1. Let _names2_ be the BoundNames of |ImportSpecifier|.
1. Return the list-concatenation of _names1_ and _names2_.
</emu-alg>
<emu-grammar>ImportSpecifier : ModuleExportName `as` ImportedBinding</emu-grammar>
<emu-alg>
1. Return the BoundNames of |ImportedBinding|.
</emu-alg>
<emu-grammar>
ExportDeclaration :
`export` ExportFromClause FromClause `;`
`export` NamedExports `;`
</emu-grammar>
<emu-alg>
1. Return a new empty List.
</emu-alg>
<emu-grammar>ExportDeclaration : `export` VariableStatement</emu-grammar>
<emu-alg>
1. Return the BoundNames of |VariableStatement|.
</emu-alg>
<emu-grammar>ExportDeclaration : `export` Declaration</emu-grammar>
<emu-alg>
1. Return the BoundNames of |Declaration|.
</emu-alg>
<emu-grammar>ExportDeclaration : `export` `default` HoistableDeclaration</emu-grammar>
<emu-alg>
1. Let _declarationNames_ be the BoundNames of |HoistableDeclaration|.
1. If _declarationNames_ does not include the element *"\*default\*"*, append *"\*default\*"* to _declarationNames_.
1. Return _declarationNames_.
</emu-alg>
<emu-grammar>ExportDeclaration : `export` `default` ClassDeclaration</emu-grammar>
<emu-alg>
1. Let _declarationNames_ be the BoundNames of |ClassDeclaration|.
1. If _declarationNames_ does not include the element *"\*default\*"*, append *"\*default\*"* to _declarationNames_.
1. Return _declarationNames_.
</emu-alg>
<emu-grammar>ExportDeclaration : `export` `default` AssignmentExpression `;`</emu-grammar>
<emu-alg>
1. Return « *"\*default\*"* ».
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-syntax-directed-operations-miscellaneous">
<h1>Miscellaneous</h1>
<emu-clause id="sec-runtime-semantics-bindinginitialization" oldids="sec-identifiers-runtime-semantics-bindinginitialization,sec-destructuring-binding-patterns-runtime-semantics-bindinginitialization" type="sdo">
<h1>
Runtime Semantics: BindingInitialization (
_value_: an ECMAScript language value,
_environment_: an Environment Record or *undefined*,
): either a normal completion containing ~unused~ or an abrupt completion
</h1>
<dl class="header">
</dl>
<emu-note>
<p>*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 <emu-xref href="#sec-functiondeclarationinstantiation"></emu-xref>). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.</p>
</emu-note>
<emu-grammar>BindingIdentifier : Identifier</emu-grammar>
<emu-alg>
1. Let _name_ be StringValue of |Identifier|.
1. Return ? InitializeBoundName(_name_, _value_, _environment_).
</emu-alg>
<emu-grammar>BindingIdentifier : `yield`</emu-grammar>
<emu-alg>
1. Return ? InitializeBoundName(*"yield"*, _value_, _environment_).
</emu-alg>
<emu-grammar>BindingIdentifier : `await`</emu-grammar>
<emu-alg>
1. Return ? InitializeBoundName(*"await"*, _value_, _environment_).
</emu-alg>
<emu-grammar>BindingPattern : ObjectBindingPattern</emu-grammar>
<emu-alg>
1. Perform ? RequireObjectCoercible(_value_).
1. Return ? BindingInitialization of |ObjectBindingPattern| with arguments _value_ and _environment_.
</emu-alg>
<emu-grammar>BindingPattern : ArrayBindingPattern</emu-grammar>
<emu-alg>
1. Let _iteratorRecord_ be ? GetIterator(_value_, ~sync~).
1. Let _result_ be Completion(IteratorBindingInitialization of |ArrayBindingPattern| with arguments _iteratorRecord_ and _environment_).
1. If _iteratorRecord_.[[Done]] is *false*, return ? IteratorClose(_iteratorRecord_, _result_).
1. Return ? _result_.
</emu-alg>
<ins class="block">
<emu-grammar>
ExtractorBindingPattern :
ExtractorMemberExpression `(` Elision? BindingRestElement? `)`
ExtractorMemberExpression `(` BindingElementList `)`
ExtractorMemberExpression `(` BindingElementList `,` Elision? BindingRestElement? `)`
</emu-grammar>
<emu-alg>
1. Let _expr_ be the |LeftHandSideExpression| that is covered by |ExtractorMemberExpression|.
1. Let _ref_ be ? Evaluation of _expr_.
1. Let _extractor_ be ? GetValue(_ref_).
1. If _ref_ is a Reference Record and IsPropertyReference(_ref_) is *true*, let _receiver_ be GetThisValue(_ref_).
1. Else, let _receiver_ be *null*.
1. Let _iteratorRecord_ be ? InvokeCustomMatcherOrThrow(_extractor_, _value_, _receiver_).
1. Let _result_ be ? IteratorBindingInitialization of this |ExtractorBindingPattern| with arguments _iteratorRecord_ and _environment_.
1. If _iteratorRecord_.[[Done]] is *false*, return ? IteratorClose(_iteratorRecord_, _result_).
1. Return ? _result_.
</emu-alg>
<emu-note type="editor">
<p>Draft Note: _receiver_ is part of the Pattern Matching proposal but does not have consensus in the Pattern Matching champion group yet.</p>
</emu-note>
</ins>
<emu-grammar>ObjectBindingPattern : `{` `}`</emu-grammar>
<emu-alg>
1. Return ~unused~.
</emu-alg>
<emu-grammar>
ObjectBindingPattern :
`{` BindingPropertyList `}`
`{` BindingPropertyList `,` `}`
</emu-grammar>
<emu-alg>
1. Perform ? PropertyBindingInitialization of |BindingPropertyList| with arguments _value_ and _environment_.
1. Return ~unused~.
</emu-alg>
<emu-grammar>ObjectBindingPattern : `{` BindingRestProperty `}`</emu-grammar>
<emu-alg>
1. Let _excludedNames_ be a new empty List.
1. Return ? RestBindingInitialization of |BindingRestProperty| with arguments _value_, _environment_, and _excludedNames_.
</emu-alg>
<emu-grammar>ObjectBindingPattern : `{` BindingPropertyList `,` BindingRestProperty `}`</emu-grammar>
<emu-alg>
1. Let _excludedNames_ be ? PropertyBindingInitialization of |BindingPropertyList| with arguments _value_ and _environment_.
1. Return ? RestBindingInitialization of |BindingRestProperty| with arguments _value_, _environment_, and _excludedNames_.
</emu-alg>
<ins class="block">
<emu-clause id="sec-invokecustommatcherorthrow" type="abstract operation">
<h1>
<ins>
InvokeCustomMatcherOrThrow (
_matcher_: an ECMAScript language value,
_subject_: an ECMAScript language value,
_receiver_: an ECMAScript language value,
): either a normal completion containing an Object, or a throw completion
</ins>
</h1>
<dl class="header"></dl>
<emu-alg>
1. If _matcher_ is not an Object, throw a *TypeError* exception.
1. Let _f_ be ? GetMethod(_matcher_, @@customMatcher).
1. If _f_ is *undefined*, throw a *TypeError* exception.
1. Let _result_ be ? Call(_f_, _matcher_, « _subject_, `"list"`, _receiver_ »).
1. If _result_ is not an Object, throw a *TypeError* exception.
1. Let _iteratorRecord_ be ? GetIterator(_result_, ~sync~).
1. Return _iteratorRecord_.
</emu-alg>
<emu-note type="editor">
<p>Draft Note: The InvokeCustomMatcherOrThrow abstract operation is derived from <a href="https://tc39.es/proposal-pattern-matching/#sec-invoke-custom-matcher">InvokeCustomMatcher</a> in the Pattern Matching proposal.</p>
</emu-note>
<emu-note type="editor">
<p>Draft Note: The value `"list"` is related to two different modes (`"boolean"` and `"list"`) supported by custom matchers in the Pattern Matching proposal and is provided here to satisfy cross-cutting concerns between the two proposals.</p>
</emu-note>
<emu-note type="editor">
<p>Draft Note: The _receiver_ parameter is not a consensus in the Pattern Matching champion group yet. This design is to keep the `this` value when calling the custom matchers. Not everyone in the champion group agrees we need to keep the `this` value.</p>
</emu-note>
<emu-note type="editor">
<p>Draft Note: This proposal currently uses iterators for destructuring, but may choose to use Array objects explicitly in the future due to performance concerns related to iterators.</p>
</emu-note>
</emu-clause>
</ins>
</emu-clause>
<emu-clause id="sec-runtime-semantics-iteratorbindinginitialization" oldids="sec-destructuring-binding-patterns-runtime-semantics-iteratorbindinginitialization,sec-function-definitions-runtime-semantics-iteratorbindinginitialization,sec-arrow-function-definitions-runtime-semantics-iteratorbindinginitialization,sec-async-arrow-function-definitions-IteratorBindingInitialization" type="sdo">
<h1>
Runtime Semantics: IteratorBindingInitialization (
_iteratorRecord_: an Iterator Record,
_environment_: an Environment Record or *undefined*,
): either a normal completion containing ~unused~ or an abrupt completion
</h1>
<dl class="header">
</dl>
<emu-note>
<p>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.</p>
</emu-note>
<emu-grammar>
ArrayBindingPattern : `[` `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` `)`</ins>
</emu-grammar>
<emu-alg>
1. Return ~unused~.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` Elision `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` Elision `)`</ins>
</emu-grammar>
<emu-alg>
1. Return ? IteratorDestructuringAssignmentEvaluation of |Elision| with argument _iteratorRecord_.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` Elision? BindingRestElement `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` Elision? BindingRestElement `)`</ins>
</emu-grammar>
<emu-alg>
1. If |Elision| is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of |Elision| with argument _iteratorRecord_.
1. Return ? IteratorBindingInitialization of |BindingRestElement| with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` BindingElementList `,` Elision `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` BindingElementList `,` Elision `)`</ins>
</emu-grammar>
<emu-alg>
1. Perform ? IteratorBindingInitialization of |BindingElementList| with arguments _iteratorRecord_ and _environment_.
1. Return ? IteratorDestructuringAssignmentEvaluation of |Elision| with argument _iteratorRecord_.
</emu-alg>
<emu-grammar>
ArrayBindingPattern : `[` BindingElementList `,` Elision? BindingRestElement `]`
<ins class="block">ExtractorBindingPattern : ExtractorMemberExpression `(` BindingElementList `,` Elision? BindingRestElement `)`</ins>
</emu-grammar>
<emu-alg>
1. Perform ? IteratorBindingInitialization of |BindingElementList| with arguments _iteratorRecord_ and _environment_.
1. If |Elision| is present, then
1. Perform ? IteratorDestructuringAssignmentEvaluation of |Elision| with argument _iteratorRecord_.
1. Return ? IteratorBindingInitialization of |BindingRestElement| with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>BindingElementList : BindingElementList `,` BindingElisionElement</emu-grammar>
<emu-alg>
1. Perform ? IteratorBindingInitialization of |BindingElementList| with arguments _iteratorRecord_ and _environment_.
1. Return ? IteratorBindingInitialization of |BindingElisionElement| with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>BindingElisionElement : Elision BindingElement</emu-grammar>
<emu-alg>
1. Perform ? IteratorDestructuringAssignmentEvaluation of |Elision| with argument _iteratorRecord_.
1. Return ? IteratorBindingInitialization of |BindingElement| with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>SingleNameBinding : BindingIdentifier Initializer?</emu-grammar>
<emu-alg>
1. Let _bindingId_ be StringValue of |BindingIdentifier|.
1. Let _lhs_ be ? ResolveBinding(_bindingId_, _environment_).
1. Let _v_ be *undefined*.
1. If _iteratorRecord_.[[Done]] is *false*, then
1. Let _next_ be ? IteratorStepValue(_iteratorRecord_).
1. If _next_ is not ~done~, then
1. Set _v_ to _next_.
1. If |Initializer| is present and _v_ is *undefined*, then
1. If IsAnonymousFunctionDefinition(|Initializer|) is *true*, then
1. Set _v_ to ? NamedEvaluation of |Initializer| with argument _bindingId_.
1. Else,
1. Let _defaultValue_ be ? Evaluation of |Initializer|.
1. Set _v_ to ? GetValue(_defaultValue_).
1. If _environment_ is *undefined*, return ? PutValue(_lhs_, _v_).
1. Return ? InitializeReferencedBinding(_lhs_, _v_).
</emu-alg>
<emu-grammar>BindingElement : BindingPattern Initializer?</emu-grammar>
<emu-alg>
1. Let _v_ be *undefined*.
1. If _iteratorRecord_.[[Done]] is *false*, then
1. Let _next_ be ? IteratorStepValue(_iteratorRecord_).
1. If _next_ is not ~done~, then
1. Set _v_ to _next_.
1. If |Initializer| is present and _v_ is *undefined*, then
1. Let _defaultValue_ be ? Evaluation of |Initializer|.
1. Set _v_ to ? GetValue(_defaultValue_).
1. Return ? BindingInitialization of |BindingPattern| with arguments _v_ and _environment_.
</emu-alg>
<emu-grammar>BindingRestElement : `...` BindingIdentifier</emu-grammar>
<emu-alg>
1. Let _lhs_ be ? ResolveBinding(StringValue of |BindingIdentifier|, _environment_).
1. Let _A_ be ! ArrayCreate(0).
1. Let _n_ be 0.
1. Repeat,
1. Let _next_ be ~done~.
1. If _iteratorRecord_.[[Done]] is *false*, then
1. Set _next_ to ? IteratorStepValue(_iteratorRecord_).
1. If _next_ is ~done~, then
1. If _environment_ is *undefined*, return ? PutValue(_lhs_, _A_).
1. Return ? InitializeReferencedBinding(_lhs_, _A_).
1. Perform ! CreateDataPropertyOrThrow(_A_, ! ToString(𝔽(_n_)), _next_).
1. Set _n_ to _n_ + 1.
</emu-alg>
<emu-grammar>BindingRestElement : `...` BindingPattern</emu-grammar>
<emu-alg>
1. Let _A_ be ! ArrayCreate(0).
1. Let _n_ be 0.
1. Repeat,
1. Let _next_ be ~done~.
1. If _iteratorRecord_.[[Done]] is *false*, then
1. Set _next_ to ? IteratorStepValue(_iteratorRecord_).
1. If _next_ is ~done~, then
1. Return ? BindingInitialization of |BindingPattern| with arguments _A_ and _environment_.
1. Perform ! CreateDataPropertyOrThrow(_A_, ! ToString(𝔽(_n_)), _next_).
1. Set _n_ to _n_ + 1.
</emu-alg>
<emu-grammar>FormalParameters : [empty]</emu-grammar>
<emu-alg>
1. Return ~unused~.
</emu-alg>
<emu-grammar>FormalParameters : FormalParameterList `,` FunctionRestParameter</emu-grammar>
<emu-alg>
1. Perform ? IteratorBindingInitialization of |FormalParameterList| with arguments _iteratorRecord_ and _environment_.
1. Return ? IteratorBindingInitialization of |FunctionRestParameter| with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>FormalParameterList : FormalParameterList `,` FormalParameter</emu-grammar>
<emu-alg>
1. Perform ? IteratorBindingInitialization of |FormalParameterList| with arguments _iteratorRecord_ and _environment_.
1. Return ? IteratorBindingInitialization of |FormalParameter| with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>ArrowParameters : BindingIdentifier</emu-grammar>
<emu-alg>
1. Let _v_ be *undefined*.
1. Assert: _iteratorRecord_.[[Done]] is *false*.
1. Let _next_ be ? IteratorStepValue(_iteratorRecord_).
1. If _next_ is not ~done~, then
1. Set _v_ to _next_.
1. Return ? BindingInitialization of |BindingIdentifier| with arguments _v_ and _environment_.
</emu-alg>
<emu-grammar>ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _formals_ be the |ArrowFormalParameters| that is covered by |CoverParenthesizedExpressionAndArrowParameterList|.
1. Return ? IteratorBindingInitialization of _formals_ with arguments _iteratorRecord_ and _environment_.
</emu-alg>
<emu-grammar>
AsyncArrowBindingIdentifier : BindingIdentifier
</emu-grammar>
<emu-alg>
1. Let _v_ be *undefined*.
1. Assert: _iteratorRecord_.[[Done]] is *false*.
1. Let _next_ be ? IteratorStepValue(_iteratorRecord_).
1. If _next_ is not ~done~, then
1. Set _v_ to _next_.
1. Return ? BindingInitialization of |BindingIdentifier| with arguments _v_ and _environment_.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ecmascript-language-expressions">
<h1>ECMAScript Language: Expressions</h1>
<emu-clause id="sec-left-hand-side-expressions">
<h1>Left-Hand-Side Expressions</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
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] #callcover
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]
</emu-grammar>
<h2>Supplemental Syntax</h2>
<p>
When processing an instance of the production<br>
<emu-grammar>CallExpression : CoverCallExpressionAndAsyncArrowHead</emu-grammar><br>
the interpretation of |CoverCallExpressionAndAsyncArrowHead| is refined using the following grammar:
</p>
<emu-grammar type="definition">
CallMemberExpression[Yield, Await] :
MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]
</emu-grammar>
<emu-clause id="sec-function-calls">
<h1>Function Calls</h1>
<emu-clause id="sec-function-calls-runtime-semantics-evaluation" type="sdo">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>CallExpression : CoverCallExpressionAndAsyncArrowHead</emu-grammar>
<emu-alg>
1. Let _expr_ be the |CallMemberExpression| that is covered by |CoverCallExpressionAndAsyncArrowHead|.
1. Let _memberExpr_ be the |MemberExpression| of _expr_.
1. Let _arguments_ be the |Arguments| of _expr_.
1. Let _ref_ be ? Evaluation of _memberExpr_.
1. Let _func_ be ? GetValue(_ref_).
1. If _ref_ is a Reference Record, IsPropertyReference(_ref_) is *false*, and _ref_.[[ReferencedName]] is *"eval"*, then
1. If SameValue(_func_, %eval%) is *true*, then
1. Let _argList_ be ? ArgumentListEvaluation of _arguments_.
1. If _argList_ has no elements, return *undefined*.
1. Let _evalArg_ be the first element of _argList_.
1. If IsStrict(this |CallExpression|) is *true*, let _strictCaller_ be *true*. Otherwise let _strictCaller_ be *false*.
1. [id="step-callexpression-evaluation-direct-eval"] Return ? PerformEval(_evalArg_, _strictCaller_, *true*).
1. Let _thisCall_ be this |CallExpression|.
1. Let _tailCall_ be IsInTailPosition(_thisCall_).
1. Return ? EvaluateCall(_func_, _ref_, _arguments_, _tailCall_).
</emu-alg>
<p>A |CallExpression| evaluation that executes step <emu-xref href="#step-callexpression-evaluation-direct-eval"></emu-xref> is a <dfn variants="direct evals">direct eval</dfn>.</p>
<emu-grammar>
CallExpression : CallExpression Arguments
</emu-grammar>
<emu-alg>
1. Let _ref_ be ? Evaluation of |CallExpression|.
1. Let _func_ be ? GetValue(_ref_).
1. Let _thisCall_ be this |CallExpression|.
1. Let _tailCall_ be IsInTailPosition(_thisCall_).
1. Return ? EvaluateCall(_func_, _ref_, |Arguments|, _tailCall_).
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-assignment-operators">
<h1>Assignment Operators</h1>
<emu-clause id="sec-assignment-operators-static-semantics-early-errors">
<h1>Static Semantics: Early Errors</h1>
<emu-grammar>AssignmentExpression : LeftHandSideExpression `=` AssignmentExpression</emu-grammar>
<ul>
<li>
If |LeftHandSideExpression| is either an |ObjectLiteral|<del> or</del><ins>,</ins> an |ArrayLiteral|<ins>, or a |CoverCallExpressionAndAsyncArrowHead|</ins>, |LeftHandSideExpression| must cover an |AssignmentPattern|.
</li>
<li>
If |LeftHandSideExpression| is neither an |ObjectLiteral|<del> nor</del><ins>,</ins> an |ArrayLiteral|<ins>, nor a |CoverCallExpressionAndAsyncArrowHead|</ins>, it is a Syntax Error if the AssignmentTargetType of |LeftHandSideExpression| is not ~simple~.
</li>
</ul>
<emu-grammar>
AssignmentExpression :
LeftHandSideExpression AssignmentOperator AssignmentExpression
LeftHandSideExpression `&&=` AssignmentExpression
LeftHandSideExpression `||=` AssignmentExpression
LeftHandSideExpression `??=` AssignmentExpression
</emu-grammar>
<ul>
<li>
It is a Syntax Error if the AssignmentTargetType of |LeftHandSideExpression| is not ~simple~.
</li>
</ul>
</emu-clause>
<emu-clause id="sec-assignment-operators-runtime-semantics-evaluation" type="sdo">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>AssignmentExpression : LeftHandSideExpression `=` AssignmentExpression</emu-grammar>
<emu-alg>
1. If |LeftHandSideExpression| is neither an |ObjectLiteral|<del> nor</del><ins>,</ins> an |ArrayLiteral|<ins>, nor a |CoverCallExpressionAndAsyncArrowHead|</ins>, then
1. Let _lref_ be ? Evaluation of |LeftHandSideExpression|.
1. If IsAnonymousFunctionDefinition(|AssignmentExpression|) and IsIdentifierRef of |LeftHandSideExpression| are both *true*, then
1. Let _rval_ be ? NamedEvaluation of |AssignmentExpression| with argument _lref_.[[ReferencedName]].
1. Else,
1. Let _rref_ be ? Evaluation of |AssignmentExpression|.
1. Let _rval_ be ? GetValue(_rref_).
1. [id="step-assignmentexpression-evaluation-simple-putvalue"] Perform ? PutValue(_lref_, _rval_).
1. Return _rval_.
1. Let _assignmentPattern_ be the |AssignmentPattern| that is covered by |LeftHandSideExpression|.
1. Let _rref_ be ? Evaluation of |AssignmentExpression|.
1. Let _rval_ be ? GetValue(_rref_).
1. Perform ? DestructuringAssignmentEvaluation of _assignmentPattern_ with argument _rval_.
1. Return _rval_.
</emu-alg>
<emu-grammar>AssignmentExpression : LeftHandSideExpression AssignmentOperator AssignmentExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be ? Evaluation of |LeftHandSideExpression|.
1. [id="step-assignmentexpression-evaluation-compound-getvalue"] Let _lval_ be ? GetValue(_lref_).
1. Let _rref_ be ? Evaluation of |AssignmentExpression|.
1. Let _rval_ be ? GetValue(_rref_).
1. Let _assignmentOpText_ be the source text matched by |AssignmentOperator|.
1. Let _opText_ be the sequence of Unicode code points associated with _assignmentOpText_ in the following table:
<figure>
<!-- emu-format ignore -->
<table class="lightweight-table">
<tr><th> _assignmentOpText_ </th><th> _opText_ </th></tr>
<tr><td> `**=` </td><td> `**` </td></tr>
<tr><td> `*=` </td><td> `*` </td></tr>
<tr><td> `/=` </td><td> `/` </td></tr>
<tr><td> `%=` </td><td> `%` </td></tr>
<tr><td> `+=` </td><td> `+` </td></tr>
<tr><td> `-=` </td><td> `-` </td></tr>
<tr><td> `<<=` </td><td> `<<` </td></tr>
<tr><td> `>>=` </td><td> `>>` </td></tr>
<tr><td> `>>>=` </td><td> `>>>` </td></tr>
<tr><td> `&=` </td><td> `&` </td></tr>
<tr><td> `^=` </td><td> `^` </td></tr>
<tr><td> `|=` </td><td> `|` </td></tr>
</table>
</figure>
1. Let _r_ be ? ApplyStringOrNumericBinaryOperator(_lval_, _opText_, _rval_).
1. [id="step-assignmentexpression-evaluation-compound-putvalue"] Perform ? PutValue(_lref_, _r_).
1. Return _r_.
</emu-alg>
<emu-grammar>AssignmentExpression : LeftHandSideExpression `&&=` AssignmentExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be ? Evaluation of |LeftHandSideExpression|.
1. [id="step-assignmentexpression-evaluation-lgcl-and-getvalue"] Let _lval_ be ? GetValue(_lref_).
1. Let _lbool_ be ToBoolean(_lval_).
1. If _lbool_ is *false*, return _lval_.
1. If IsAnonymousFunctionDefinition(|AssignmentExpression|) is *true* and IsIdentifierRef of |LeftHandSideExpression| is *true*, then
1. Let _rval_ be ? NamedEvaluation of |AssignmentExpression| with argument _lref_.[[ReferencedName]].
1. Else,
1. Let _rref_ be ? Evaluation of |AssignmentExpression|.
1. Let _rval_ be ? GetValue(_rref_).
1. [id="step-assignmentexpression-evaluation-lgcl-and-putvalue"] Perform ? PutValue(_lref_, _rval_).
1. Return _rval_.
</emu-alg>
<emu-grammar>AssignmentExpression : LeftHandSideExpression `||=` AssignmentExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be ? Evaluation of |LeftHandSideExpression|.
1. [id="step-assignmentexpression-evaluation-lgcl-or-getvalue"] Let _lval_ be ? GetValue(_lref_).
1. Let _lbool_ be ToBoolean(_lval_).
1. If _lbool_ is *true*, return _lval_.
1. If IsAnonymousFunctionDefinition(|AssignmentExpression|) is *true* and IsIdentifierRef of |LeftHandSideExpression| is *true*, then
1. Let _rval_ be ? NamedEvaluation of |AssignmentExpression| with argument _lref_.[[ReferencedName]].
1. Else,
1. Let _rref_ be ? Evaluation of |AssignmentExpression|.
1. Let _rval_ be ? GetValue(_rref_).
1. [id="step-assignmentexpression-evaluation-lgcl-or-putvalue"] Perform ? PutValue(_lref_, _rval_).
1. Return _rval_.
</emu-alg>
<emu-grammar>AssignmentExpression : LeftHandSideExpression `??=` AssignmentExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be ? Evaluation of |LeftHandSideExpression|.
1. [id="step-assignmentexpression-evaluation-lgcl-nullish-getvalue"] Let _lval_ be ? GetValue(_lref_).
1. If _lval_ is neither *undefined* nor *null*, return _lval_.
1. If IsAnonymousFunctionDefinition(|AssignmentExpression|) is *true* and IsIdentifierRef of |LeftHandSideExpression| is *true*, then
1. Let _rval_ be ? NamedEvaluation of |AssignmentExpression| with argument _lref_.[[ReferencedName]].
1. Else,
1. Let _rref_ be ? Evaluation of |AssignmentExpression|.
1. Let _rval_ be ? GetValue(_rref_).
1. [id="step-assignmentexpression-evaluation-lgcl-nullish-putvalue"] Perform ? PutValue(_lref_, _rval_).
1. Return _rval_.
</emu-alg>
<emu-note>
<p>When this expression occurs within strict mode code, it is a runtime error if _lref_ in step <emu-xref href="#step-assignmentexpression-evaluation-simple-putvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-compound-getvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-lgcl-and-getvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-lgcl-or-getvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-lgcl-nullish-getvalue"></emu-xref> is an unresolvable reference. If it is, a *ReferenceError* exception is thrown. Additionally, it is a runtime error if the _lref_ in step <emu-xref href="#step-assignmentexpression-evaluation-compound-putvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-lgcl-and-putvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-lgcl-or-putvalue"></emu-xref>, <emu-xref href="#step-assignmentexpression-evaluation-lgcl-nullish-putvalue"></emu-xref> 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.</p>
</emu-note>
</emu-clause>
<emu-clause id="sec-destructuring-assignment">
<h1>Destructuring Assignment</h1>
<h2>Supplemental Syntax</h2>
<p>
In certain circumstances when processing an instance of the production<br>
<emu-grammar>AssignmentExpression : LeftHandSideExpression `=` AssignmentExpression</emu-grammar><br>
the interpretation of |LeftHandSideExpression| is refined using the following grammar:
</p>
<emu-grammar type="definition">
AssignmentPattern[Yield, Await] :
ObjectAssignmentPattern[?Yield, ?Await]
ArrayAssignmentPattern[?Yield, ?Await]
<ins>ExtractorAssignmentPattern[?Yield, ?Await]</ins>
ObjectAssignmentPattern[Yield, Await] :
`{` `}`
`{` AssignmentRestProperty[?Yield, ?Await] `}`
`{` AssignmentPropertyList[?Yield, ?Await] `}`
`{` AssignmentPropertyList[?Yield, ?Await] `,` AssignmentRestProperty[?Yield, ?Await]? `}`
ArrayAssignmentPattern[Yield, Await] :
`[` Elision? AssignmentRestElement[?Yield, ?Await]? `]`
`[` AssignmentElementList[?Yield, ?Await] `]`
`[` AssignmentElementList[?Yield, ?Await] `,` Elision? AssignmentRestElement[?Yield, ?Await]? `]`
<ins class="block">
ExtractorAssignmentPattern[Yield, Await] :
ExtractorMemberExpression[?Yield, ?Await] `(` Elision? AssignmentRestElement[?Yield, ?Await]? `)`
ExtractorMemberExpression[?Yield, ?Await] `(` AssignmentElementList[?Yield, ?Await] `)`
ExtractorMemberExpression[?Yield, ?Await] `(` AssignmentElementList[?Yield, ?Await] `,` Elision? AssignmentRestElement[?Yield, ?Await]? `)`
</ins>
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] :
Elision? AssignmentElement[?Yield, ?Await]
AssignmentProperty[Yield, Await] :
IdentifierReference[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]?
PropertyName[?Yield, ?Await] `:` AssignmentElement[?Yield, ?Await]
AssignmentElement[Yield, Await] :
DestructuringAssignmentTarget[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]?
AssignmentRestElement[Yield, Await] :
`...` DestructuringAssignmentTarget[?Yield, ?Await]
DestructuringAssignmentTarget[Yield, Await] :
LeftHandSideExpression[?Yield, ?Await]
</emu-grammar>
<emu-clause id="sec-destructuring-assignment-static-semantics-early-errors">
<h1>Static Semantics: Early Errors</h1>
<emu-grammar>AssignmentProperty : IdentifierReference Initializer?</emu-grammar>
<ul>
<li>
It is a Syntax Error if AssignmentTargetType of |IdentifierReference| is not ~simple~.
</li>
</ul>
<emu-grammar>AssignmentRestProperty : `...` DestructuringAssignmentTarget</emu-grammar>
<ul>
<li>
It is a Syntax Error if |DestructuringAssignmentTarget| is either an |ArrayLiteral|<del> or</del><ins>,</ins> an |ObjectLiteral|<ins>, or a |CoverCallExpressionAndAsyncArrowHead|</ins>.
</li>
</ul>
<emu-grammar>DestructuringAssignmentTarget : LeftHandSideExpression</emu-grammar>
<ul>
<li>
If |LeftHandSideExpression| is either an |ObjectLiteral|<del> or</del><ins>,</ins> an |ArrayLiteral|<ins>, or a |CoverCallExpressionAndAsyncArrowHead|</ins>, |LeftHandSideExpression| must cover an |AssignmentPattern|.
</li>
<li>
If |LeftHandSideExpression| is neither an |ObjectLiteral|<del> nor</del><ins>,</ins> an |ArrayLiteral|<ins>, nor a |CoverCallExpressionAndAsyncArrowHead|</ins>, it is a Syntax Error if the AssignmentTargetType of |LeftHandSideExpression| is not ~simple~.
</li>
</ul>
</emu-clause>
<emu-clause id="sec-runtime-semantics-destructuringassignmentevaluation" type="sdo">
<h1>
Runtime Semantics: DestructuringAssignmentEvaluation (
_value_: an ECMAScript language value,
): either a normal completion containing ~unused~ or an abrupt completion
</h1>
<dl class="header">
</dl>
<emu-grammar>ObjectAssignmentPattern : `{` `}`</emu-grammar>
<emu-alg>
1. Perform ? RequireObjectCoercible(_value_).
1. Return ~unused~.
</emu-alg>
<emu-grammar>
ObjectAssignmentPattern :
`{` AssignmentPropertyList `}`
`{` AssignmentPropertyList `,` `}`
</emu-grammar>
<emu-alg>
1. Perform ? RequireObjectCoercible(_value_).
1. Perform ? PropertyDestructuringAssignmentEvaluation of |AssignmentPropertyList| with argument _value_.
1. Return ~unused~.
</emu-alg>
<emu-grammar>ObjectAssignmentPattern : `{` AssignmentRestProperty `}`</emu-grammar>
<emu-alg>
1. Perform ? RequireObjectCoercible(_value_).
1. Let _excludedNames_ be a new empty List.
1. Return ? RestDestructuringAssignmentEvaluation of |AssignmentRestProperty| with arguments _value_ and _excludedNames_.
</emu-alg>
<emu-grammar>ObjectAssignmentPattern : `{` AssignmentPropertyList `,` AssignmentRestProperty `}`</emu-grammar>
<emu-alg>
1. Perform ? RequireObjectCoercible(_value_).
1. Let _excludedNames_ be ? PropertyDestructuringAssignmentEvaluation of |AssignmentPropertyList| with argument _value_.