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<!DOCTYPE html>
<meta charset="utf-8">
<link rel="icon" href="favicon.ico">
<link href="ecmarkup.css" rel="stylesheet">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/styles/github.min.css">
<script src="ecmarkup.js"></script>
<script>
if (location.hostname.endsWith('.github.io') && location.protocol !== 'https:') {
location.protocol = 'https:';
}
</script>
<pre class=metadata>
title: Optional Chaining
status: proposal
stage: 1
location: https://claudepache.github.io/es-nil
copyright: false
contributors: Claude Pache, ECMA International
</pre>
<emu-clause id="sec-scope">
<h1>Scope</h1>
<p>This is the spec text proposal for introducing Optional Chaining feature (aka Null Propagation, aka Existential Operator) in ECMAScript.</p>
<p>For the syntax, we use the `?.` token, with a lookahead at the level of the lexical grammar that allows to discriminate between `a?.b` (optional chaining) and `a?.3:0` (conditional operator, whose meaning cannot be changed due to backward compatibility constraints).</p>
<p>Modified content are marked <ins>like this</ins>. In order to avoid distraction, we don’t mark mere editorial amendments.</p>
<div style="border: medium double lightgray; padding: 0 1em; background-color: lightyellow">
<h2>Deprecation notice</h2>
<p>A <a href="https://github.com/tc39/proposal-optional-chaining/blob/master/ShortCircuitingWithoutNil.md">major rewrite</a> is in progress, that will eliminate the need of Nil reference. The observed behaviour will change in edge or rare cases only.</p>
</div>
</emu-clause>
<emu-clause id="sec-nil-reference">
<h1>The Nil Reference</h1>
<!-- es6num="6.2.3" -->
<emu-clause id="sec-reference-specification-type">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-reference-specification-type">6.2.4</a>) The Reference Specification Type</h1>
<emu-note>
<p>The Reference 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.</p>
</emu-note>
<p>A <dfn>Reference</dfn> is a resolved name or property binding. A Reference consists of three components, the base value component, the referenced name component, and the Boolean-valued strict reference flag. The base value component is either *undefined*, an Object, a Boolean, a String, a Symbol, a Number, or an Environment Record. A base value component of *undefined* indicates that the Reference could not be resolved to a binding. The referenced name component is a String or Symbol value.</p>
<p>A <dfn id="super-reference">Super Reference</dfn> is a Reference that is used to represents a name binding that was expressed using the super keyword. A Super Reference has an additional thisValue component, and its base value component will never be an Environment Record.</p>
<p><ins>A <dfn id="nil-reference">Nil Reference</dfn> is a special Reference that is used to propagate an *undefined* value that is produced by the optional chaining special form (`?.`). A Nil Reference has only one component, the base value component, which is always equal to the symbol ~Nil~.</ins></p>
<p>The following abstract operations are used in this specification to operate on references:</p>
<emu-clause id="sec-getbase" aoid="GetBase">
<h1 id="ao-getbase">GetBase ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. Return the base value component of _V_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-getreferencedname" aoid="GetReferencedName">
<h1 id="ao-getreferencedname">GetReferencedName ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. <ins>Assert: IsNilReference(_V_) is false.</ins>
1. Return the referenced name component of _V_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-isstrictreference" aoid="IsStrictReference">
<h1 id="ao-isstrictreference">IsStrictReference ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. <ins>Assert: IsNilReference(_V_) is false.</ins>
1. Return the strict reference flag of _V_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-hasprimitivebase" aoid="HasPrimitiveBase">
<h1 id="ao-hasprimitivebase">HasPrimitiveBase ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. If Type(_V_'s base value component) is Boolean, String, Symbol, or Number, return *true*; otherwise return *false*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-ispropertyreference" aoid="IsPropertyReference">
<h1 id="ao-ispropertyreference">IsPropertyReference ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. If either the base value component of _V_ is an Object or HasPrimitiveBase(_V_) is *true*, return *true*; otherwise return *false*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-isunresolvablereference" aoid="IsUnresolvableReference">
<h1 id="ao-isunresolvablereference">IsUnresolvableReference ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. If the base value component of _V_ is *undefined*, return *true*; otherwise return *false*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-issuperreference" aoid="IsSuperReference">
<h1 id="ao-issuperreference">IsSuperReference ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. If _V_ has a thisValue component, return *true*; otherwise return *false*.
</emu-alg>
</emu-clause>
<ins class="block">
<emu-clause id="sec-isnilreference" aoid="IsNilReference">
<h1 id="ao-issuperreference">IsNilReference ( _V_ )</h1>
<emu-alg>
1. Assert: Type(_V_) is Reference.
1. If the base value component of _V_ is the symbol ~Nil~, return *true*; otherwise, return *false*.
</emu-alg>
</emu-clause>
</ins>
<!-- es6num="6.2.3.1" -->
<emu-clause id="sec-getvalue" aoid="GetValue">
<h1>GetValue ( _V_ )</h1>
<emu-alg>
1. ReturnIfAbrupt(_V_).
1. If Type(_V_) is not Reference, return _V_.
1. Let _base_ be GetBase(_V_).
1. <ins>If IsNilReference(_V_) is *true*, return *undefined*.</ins>
1. If IsUnresolvableReference(_V_) is *true*, throw a *ReferenceError* exception.
1. If IsPropertyReference(_V_) is *true*, then
1. If HasPrimitiveBase(_V_) is *true*, then
1. Assert: In this case, _base_ will never be *undefined* or *null*.
1. Set _base_ to ! ToObject(_base_).
1. Return ? _base_.[[Get]](GetReferencedName(_V_), GetThisValue(_V_)).
1. Else _base_ must be an Environment Record,
1. Return ? _base_.GetBindingValue(GetReferencedName(_V_), IsStrictReference(_V_)) (see <emu-xref href="#sec-environment-records"></emu-xref>).
</emu-alg>
<emu-note>
<p>The object that may be created in step 5.a.ii 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.</p>
</emu-note>
</emu-clause>
<!-- es6num="6.2.3.2" -->
<emu-clause id="sec-putvalue" aoid="PutValue">
<h1>PutValue ( _V_, _W_ )</h1>
<emu-alg>
1. ReturnIfAbrupt(_V_).
1. ReturnIfAbrupt(_W_).
1. If Type(_V_) is not Reference, throw a *ReferenceError* exception.
1. Let _base_ be GetBase(_V_).
1. <ins>Assert: IsNilReference(_V_) is *false*.</ins>
1. If IsUnresolvableReference(_V_) is *true*, then
1. If IsStrictReference(_V_) is *true*, then
1. Throw a *ReferenceError* exception.
1. Let _globalObj_ be GetGlobalObject().
1. Return ? Set(_globalObj_, GetReferencedName(_V_), _W_, *false*).
1. Else if IsPropertyReference(_V_) is *true*, then
1. If HasPrimitiveBase(_V_) is *true*, then
1. Assert: In this case, _base_ will never be *undefined* or *null*.
1. Set _base_ to ! ToObject(_base_).
1. Let _succeeded_ be ? _base_.[[Set]](GetReferencedName(_V_), _W_, GetThisValue(_V_)).
1. If _succeeded_ is *false* and IsStrictReference(_V_) is *true*, throw a *TypeError* exception.
1. Return.
1. Else _base_ must be an Environment Record,
1. Return ? _base_.SetMutableBinding(GetReferencedName(_V_), _W_, IsStrictReference(_V_)) (see <emu-xref href="#sec-environment-records"></emu-xref>).
</emu-alg>
<emu-note>
<p>The object that may be created in step 6.a.ii is not accessible outside of the above algorithm and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.</p>
</emu-note>
</emu-clause>
<!-- es6num="6.2.3.3" -->
<emu-clause id="sec-getthisvalue" aoid="GetThisValue">
<h1>GetThisValue ( _V_ )</h1>
<emu-alg>
1. Assert: IsPropertyReference(_V_) is *true*.
1. If IsSuperReference(_V_) is *true*, then
1. Return the value of the thisValue component of the reference _V_.
1. Return GetBase(_V_).
</emu-alg>
</emu-clause>
<!-- es6num="6.2.3.4" -->
<emu-clause id="sec-initializereferencedbinding" aoid="InitializeReferencedBinding">
<h1>InitializeReferencedBinding ( _V_, _W_ )</h1>
<emu-alg>
1. ReturnIfAbrupt(_V_).
1. ReturnIfAbrupt(_W_).
1. Assert: Type(_V_) is Reference.
1. <ins>Assert: IsNilReference(_V_) is *false*.</ins>
1. Assert: IsUnresolvableReference(_V_) is *false*.
1. Let _base_ be GetBase(_V_).
1. Assert: _base_ is an Environment Record.
1. Return _base_.InitializeBinding(GetReferencedName(_V_), _W_).
</emu-alg>
</emu-clause>
</emu-clause>
<!-- es6num="12.5.4" -->
<emu-clause id="sec-delete-operator">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-delete-operator">12.5.3</a>) The `delete` Operator</h1>
<!-- es6num="12.5.4.1" -->
<emu-clause id="sec-delete-operator-static-semantics-early-errors">
<h1>Static Semantics: Early Errors</h1>
<emu-grammar>UnaryExpression : `delete` UnaryExpression</emu-grammar>
<ul>
<li>
It is a Syntax Error if the |UnaryExpression| is contained in strict mode code and the derived |UnaryExpression| is <emu-grammar>PrimaryExpression : IdentifierReference</emu-grammar>.
</li>
<li>
<p>It is a Syntax Error if the derived |UnaryExpression| is
<br>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<br>
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.</p>
</li>
</ul>
<emu-note>
<p>The last rule means that expressions such as `delete (((foo)))` produce early errors because of recursive application of the first rule.</p>
</emu-note>
</emu-clause>
<!-- es6num="12.5.4.2" -->
<emu-clause id="sec-delete-operator-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>UnaryExpression : `delete` UnaryExpression</emu-grammar>
<emu-alg>
1. Let _ref_ be the result of evaluating |UnaryExpression|.
1. ReturnIfAbrupt(_ref_).
1. If Type(_ref_) is not Reference, return *true*.
1. <ins>If IsNilReference(_ref_) is *true*, return *true*.</ins>
1. If IsUnresolvableReference(_ref_) is *true*, then
1. Assert: IsStrictReference(_ref_) is *false*.
1. Return *true*.
1. If IsPropertyReference(_ref_) is *true*, then
1. If IsSuperReference(_ref_) is *true*, throw a *ReferenceError* exception.
1. Let _baseObj_ be ! ToObject(GetBase(_ref_)).
1. Let _deleteStatus_ be ? _baseObj_.[[Delete]](GetReferencedName(_ref_)).
1. If _deleteStatus_ is *false* and IsStrictReference(_ref_) is *true*, throw a *TypeError* exception.
1. Return _deleteStatus_.
1. Else _ref_ is a Reference to an Environment Record binding,
1. Let _bindings_ be GetBase(_ref_).
1. Return ? _bindings_.DeleteBinding(GetReferencedName(_ref_)).
</emu-alg>
<emu-note>
<p>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* }, a *TypeError* exception is thrown.</p>
</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
<!-- es6num="11.7" -->
<emu-clause id="sec-punctuators">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-punctuators">11.7</a>) Lexical Grammar: Punctuators</h1>
<h2>Syntax</h2>
<emu-grammar>
<ins class="block">
OptionalChainingOperator ::
`?.` [lookahead <! DecimalDigit
</ins>
OtherPunctuator :: one of
`{` `(` `)` `[` `]`
`.` `...` `;` `,`
`<` `>` `<=` `>=`
`==` `!=` `===` `!==`
`+` `-` `*` `%` `**`
`++` `--`
`<<` `>>` `>>>`
`&` `|` `^`
`!` `~`
`&&` `||`
`?` `:`
`=` `+=` `-=` `*=` `%=` `**=` `<<=` `>>=` `>>>=` `&=` `|=` `^=`
`=>`
Punctuator ::
<ins>OptionalChainingOperator</ins>
OtherPunctuator
DivPunctuator ::
`/`
`/=`
RightBracePunctuator ::
`}`
</emu-grammar>
</emu-clause>
<!-- es6num="12.2" -->
<emu-clause id="sec-primary-expression">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-primary-expressions">12.2</a>) Primary Expression</h1>
<h2>Syntax</h2>
<emu-grammar>
PrimaryExpression[Yield, Await] :
`this`
IdentifierReference[?Yield, ?Await]
Literal
ArrayLiteral[?Yield, ?Await]
ObjectLiteral[?Yield, ?Await]
FunctionExpression
ClassExpression[?Yield, ?Await]
GeneratorExpression
AsyncFunctionExpression
RegularExpressionLiteral
TemplateLiteral[?Yield, ?Await]
CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await] #parencover
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] `)`
</emu-grammar>
<h2>Supplemental Syntax</h2>
<p>When processing an instance of the production
<br>
<emu-grammar>PrimaryExpression[Yield, Await] : CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]</emu-grammar>
<br>
the interpretation of |CoverParenthesizedExpressionAndArrowParameterList| is refined using the following grammar:</p>
<emu-grammar>
ParenthesizedExpression[Yield, Await] :
`(` Expression[+In, ?Yield, ?Await] `)`
</emu-grammar>
<!-- es6num="12.2.1" -->
<emu-clause id="sec-primary-expression-semantics">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-primary-expression-semantics">12.2.1</a>) Semantics</h1>
<!-- es6num="12.2.1.1" -->
<emu-clause id="sec-static-semantics-coveredparenthesizedexpression">
<h1>Static Semantics: CoveredParenthesizedExpression</h1>
<emu-grammar>CoverParenthesizedExpressionAndArrowParameterList[Yield, Await] : `(` Expression[+In, ?Yield, ?Await] `)`</emu-grammar>
<emu-alg>
1. Return the result of parsing the lexical token stream matched by |CoverParenthesizedExpressionAndArrowParameterList| using |ParenthesizedExpression| as the goal symbol with its <sub>[Yield]</sub> and <sub>[Await]</sub> parameters set to the values used when parsing this |CoverParenthesizedExpressionAndArrowParameterList|.
</emu-alg>
</emu-clause>
<!-- es6num="12.2.1.2" -->
<emu-clause id="sec-semantics-static-semantics-hasname">
<h1>Static Semantics: HasName</h1>
<emu-see-also-para op="HasName"></emu-see-also-para>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _expr_ be CoveredParenthesizedExpression of |CoverParenthesizedExpressionAndArrowParameterList|.
1. If IsFunctionDefinition of _expr_ is *false*, return *false*.
1. Return HasName of _expr_.
</emu-alg>
</emu-clause>
<!-- es6num="12.2.1.3" -->
<emu-clause id="sec-semantics-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>
PrimaryExpression :
`this`
IdentifierReference
Literal
ArrayLiteral
ObjectLiteral
RegularExpressionLiteral
TemplateLiteral
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _expr_ be CoveredParenthesizedExpression of |CoverParenthesizedExpressionAndArrowParameterList|.
1. Return IsFunctionDefinition of _expr_.
</emu-alg>
</emu-clause>
<!-- es6num="12.2.1.4" -->
<emu-clause id="sec-semantics-static-semantics-isidentifierref">
<h1>Static Semantics: IsIdentifierRef</h1>
<emu-see-also-para op="IsIdentifierRef"></emu-see-also-para>
<emu-grammar>PrimaryExpression : IdentifierReference</emu-grammar>
<emu-alg>
1. Return *true*.
</emu-alg>
<emu-grammar>
PrimaryExpression :
`this`
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
AsyncFunctionExpression
RegularExpressionLiteral
TemplateLiteral
CoverParenthesizedExpressionAndArrowParameterList
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<!-- es6num="12.2.1.5" -->
<emu-clause id="sec-semantics-static-semantics-isvalidsimpleassignmenttarget">
<h1>Static Semantics: IsValidSimpleAssignmentTarget</h1>
<emu-see-also-para op="IsValidSimpleAssignmentTarget"></emu-see-also-para>
<emu-grammar>
PrimaryExpression :
`this`
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
AsyncFunctionExpression
RegularExpressionLiteral
TemplateLiteral
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _expr_ be CoveredParenthesizedExpression of |CoverParenthesizedExpressionAndArrowParameterList|.
1. Return IsValidSimpleAssignmentTarget of _expr_.
</emu-alg>
</emu-clause>
<ins class="block">
<emu-clause id="sec-semantics-static-semantics-mayproducenil">
<h1>Static Semantics: MayProduceNil</h1>
<emu-see-also-para op="MayProduceNil"></emu-see-also-para>
<emu-grammar>
PrimaryExpression :
`this`
Literal
ArrayLiteral
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
AsyncFunctionExpression
RegularExpressionLiteral
TemplateLiteral
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _expr_ be CoveredParenthesizedExpression of |CoverParenthesizedExpressionAndArrowParameterList|.
1. Return MayProduceNil of _expr_.
</emu-alg>
</emu-clause>
</ins>
</emu-clause>
<!-- es6num="12.2.2" -->
<emu-clause id="sec-this-keyword">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-this-keyword">12.2.2</a>) The `this` Keyword</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.3" -->
<emu-clause id="sec-identifier-reference">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-identifier-reference">12.2.3</a>) Identifier Reference</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.4" -->
<emu-clause id="sec-primary-expression-literals">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-literals">12.2.4</a>) Literals</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.5" -->
<emu-clause id="sec-array-initializer">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-array-initializer">12.2.5</a>) Array Initializer</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.6" -->
<emu-clause id="sec-object-initializer">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-object-initializer">12.2.6</a>) Object Initializer</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.7" -->
<emu-clause id="sec-function-defining-expressions">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-defining-expressions">12.2.7</a>) Function Defining Expressions</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.8" -->
<emu-clause id="sec-primary-expression-regular-expression-literals">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-regular-expression-literals">12.2.8</a>) Regular Expression Literals</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.9" -->
<emu-clause id="sec-template-literals">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-template-literals">12.2.9</a>) Template Literals</h1>
<p><i>(not modified)</i></p>
</emu-clause>
<!-- es6num="12.2.10" -->
<emu-clause id="sec-grouping-operator">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-grouping-operator">12.2.10</a>) The Grouping Operator</h1>
<!-- es6num="12.2.10.1" -->
<emu-clause id="sec-grouping-operator-static-semantics-early-errors">
<h1>Static Semantics: Early Errors</h1>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<ul>
<li>
It is a Syntax Error if the lexical token sequence matched by |CoverParenthesizedExpressionAndArrowParameterList| cannot be parsed with no tokens left over using |ParenthesizedExpression| as the goal symbol.
</li>
<li>
All Early Error rules for |ParenthesizedExpression| and its derived productions also apply to CoveredParenthesizedExpression of |CoverParenthesizedExpressionAndArrowParameterList|.
</li>
</ul>
</emu-clause>
<!-- es6num="12.2.10.2" -->
<emu-clause id="sec-grouping-operator-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>ParenthesizedExpression : `(` Expression `)`</emu-grammar>
<emu-alg>
1. Return IsFunctionDefinition of |Expression|.
</emu-alg>
</emu-clause>
<!-- es6num="12.2.10.3" -->
<emu-clause id="sec-grouping-operator-static-semantics-isvalidsimpleassignmenttarget">
<h1>Static Semantics: IsValidSimpleAssignmentTarget</h1>
<emu-see-also-para op="IsValidSimpleAssignmentTarget"></emu-see-also-para>
<emu-grammar>ParenthesizedExpression : `(` Expression `)`</emu-grammar>
<emu-alg>
1. Return IsValidSimpleAssignmentTarget of |Expression|.
</emu-alg>
</emu-clause>
<ins class="block">
<emu-clause id="sec-grouping-operator-static-semantics-mayproducenil">
<h1>Static Semantics: MayProduceNil</h1>
<emu-see-also-para op="MayProduceNil"></emu-see-also-para>
<emu-grammar>ParenthesizedExpression : `(` Expression `)`</emu-grammar>
<emu-alg>
1. Return MayProduceNil of |Expression|.
</emu-alg>
</emu-clause>
</ins>
<!-- es6num="12.2.10.4" -->
<emu-clause id="sec-grouping-operator-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList</emu-grammar>
<emu-alg>
1. Let _expr_ be CoveredParenthesizedExpression of |CoverParenthesizedExpressionAndArrowParameterList|.
1. Return the result of evaluating _expr_.
</emu-alg>
<emu-grammar>ParenthesizedExpression : `(` Expression `)`</emu-grammar>
<emu-alg>
1. Return the result of evaluating |Expression|. This may be of type Reference.
</emu-alg>
<emu-note>
<p>This algorithm does not apply GetValue to the result of evaluating |Expression|. The principal motivation for this is so that operators such as `delete` and `typeof` may be applied to parenthesized expressions.</p>
</emu-note>
</emu-clause>
</emu-clause>
</emu-clause>
<!-- es6num="12.3" -->
<emu-clause id="sec-left-hand-side-expressions">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-left-hand-side-expressions">12.3</a>) Left-Hand-Side Expressions</h1>
<h2>Syntax</h2>
<emu-grammar>
MemberExpression[Yield, Await] :
PrimaryExpression[?Yield, ?Await]
MemberExpression[?Yield, ?Await] `[` Expression[+In, ?Yield, ?Await] `]`
<ins>MemberExpression[?Yield, ?Await] OptionalChainingOperator `[` Expression[+In, ?Yield, ?Await] `]`</ins>
MemberExpression[?Yield, ?Await] `.` IdentifierName
<ins>MemberExpression[?Yield, ?Await] OptionalChainingOperator IdentifierName</ins>
MemberExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await]
SuperProperty[?Yield, ?Await]
MetaProperty
`new` MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]
<ins>`new` MemberExpression[?Yield, ?Await] OptionalChainingOperator Arguments[?Yield, ?Await]</ins>
SuperProperty[Yield, Await] :
`super` `[` Expression[+In, ?Yield, ?Await] `]`
`super` `.` IdentifierName
MetaProperty :
NewTarget
NewTarget :
`new` `.` `target`
NewExpression[Yield, Await] :
MemberExpression[?Yield, ?Await]
`new` NewExpression[?Yield, ?Await]
CallExpression[Yield, Await] :
CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] #callcover
<ins>MemberExpression[?Yield, ?Await] OptionalChainingOperator Arguments[?Yield]</ins>
SuperCall[?Yield, ?Await]
CallExpression[?Yield, ?Await] Arguments[?Yield, ?Await]
<ins>CallExpression[?Yield, ?Await] OptionalChainingOperator Arguments[?Yield, ?Await]</ins>
CallExpression[?Yield, ?Await] `[` Expression[+In, ?Yield, ?Await] `]`
<ins>CallExpression[?Yield, ?Await] OptionalChainingOperator `[` Expression[+In, ?Yield, ?Await] `]`</ins>
CallExpression[?Yield, ?Await] `.` IdentifierName
<ins>CallExpression[?Yield, ?Await] OptionalChainingOperator IdentifierName</ins>
CallExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await]
SuperCall[Yield, Await] :
`super` Arguments[?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]
LeftHandSideExpression[Yield, Await] :
NewExpression[?Yield, ?Await]
CallExpression[?Yield, ?Await]
</emu-grammar>
<h2>Supplemental Syntax</h2>
<p>When processing an instance of the production <emu-grammar>CallExpression : CoverCallExpressionAndAsyncArrowHead</emu-grammar> the interpretation of |CoverCallExpressionAndAsyncArrowHead| is refined using the following grammar:</p>
<emu-grammar>
CallMemberExpression[Yield, Await] :
MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]
</emu-grammar>
<!-- es6num="12.3.1" -->
<emu-clause id="sec-static-semantics">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-static-semantics">12.3.1</a>) Static Semantics</h1>
<emu-clause id="sec-left-hand-side-expressions-static-semantics-coveredcallexpression">
<h1>Static Semantics: CoveredCallExpression</h1>
<emu-grammar>
CallExpression : CoverCallExpressionAndAsyncArrowHead
</emu-grammar>
<emu-alg>
1. Return the result of parsing the lexical token stream matched by |CoverCallExpressionAndAsyncArrowHead| using |CallMemberExpression| as the goal symbol with its <sub>[Yield]</sub> and <sub>[Await]</sub> parameters set to the values used when parsing |CoverCallExpressionAndAsyncArrowHead|.
</emu-alg>
</emu-clause>
<!-- es6num="12.3.1.1" -->
<emu-clause id="sec-static-semantics-static-semantics-contains">
<h1>Static Semantics: Contains</h1>
<p>With parameter _symbol_.</p>
<emu-see-also-para op="Contains"></emu-see-also-para>
<emu-grammar>
MemberExpression :
MemberExpression `.` IdentifierName
<ins>MemberExpression OptionalChainingOperator IdentifierName</ins>
</emu-grammar>
<emu-alg>
1. If |MemberExpression| Contains _symbol_ is *true*, return *true*.
1. If _symbol_ is a |ReservedWord|, return *false*.
1. If _symbol_ is an |Identifier| and StringValue of _symbol_ is the same value as the StringValue of |IdentifierName|, return *true*.
1. Return *false*.
</emu-alg>
<emu-grammar>SuperProperty : `super` `.` IdentifierName</emu-grammar>
<emu-alg>
1. If _symbol_ is the |ReservedWord| `super`, return *true*.
1. If _symbol_ is a |ReservedWord|, return *false*.
1. If _symbol_ is an |Identifier| and StringValue of _symbol_ is the same value as the StringValue of |IdentifierName|, return *true*.
1. Return *false*.
</emu-alg>
<emu-grammar>
CallExpression :
CallExpression `.` IdentifierName
<ins>CallExpression OptionalChainingOperator IdentifierName</ins>
</emu-grammar>
<emu-alg>
1. If |CallExpression| Contains _symbol_ is *true*, return *true*.
1. If _symbol_ is a |ReservedWord|, return *false*.
1. If _symbol_ is an |Identifier| and StringValue of _symbol_ is the same value as the StringValue of |IdentifierName|, return *true*.
1. Return *false*.
</emu-alg>
</emu-clause>
<!-- es6num="12.3.1.2" -->
<emu-clause id="sec-static-semantics-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>
MemberExpression :
MemberExpression `[` Expression `]`
<ins>MemberExpression OptionalChainingOperator `[` Expression `]`</ins>
MemberExpression `.` IdentifierName
<ins>MemberExpression OptionalChainingOperator IdentifierName</ins>
MemberExpression TemplateLiteral
SuperProperty
MetaProperty
`new` MemberExpression Arguments
<ins>`new` MemberExpression OptionalChainingOperator Arguments</ins>
NewExpression :
`new` NewExpression
LeftHandSideExpression :
CallExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<!-- es6num="12.3.1.3" -->
<emu-clause id="sec-static-semantics-static-semantics-isdestructuring">
<h1>Static Semantics: IsDestructuring</h1>
<emu-see-also-para op="IsDestructuring"></emu-see-also-para>
<emu-grammar>MemberExpression : PrimaryExpression</emu-grammar>
<emu-alg>
1. If |PrimaryExpression| is either an |ObjectLiteral| or an |ArrayLiteral|, return *true*.
1. Return *false*.
</emu-alg>
<emu-grammar>
MemberExpression :
MemberExpression `[` Expression `]`
<ins>MemberExpression OptionalChainingOperator `[` Expression `]`</ins>
MemberExpression `.` IdentifierName
<ins>MemberExpression OptionalChainingOperator IdentifierName</ins>
MemberExpression TemplateLiteral
SuperProperty
MetaProperty
`new` MemberExpression Arguments
<ins>`new` MemberExpression OptionalChainingOperator Arguments</ins>
NewExpression :
`new` NewExpression
LeftHandSideExpression :
CallExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<!-- es6num="12.3.1.4" -->
<emu-clause id="sec-static-semantics-static-semantics-isidentifierref">
<h1>Static Semantics: IsIdentifierRef</h1>
<emu-see-also-para op="IsIdentifierRef"></emu-see-also-para>
<emu-grammar>
MemberExpression :
MemberExpression `[` Expression `]`
<ins>MemberExpression OptionalChainingOperator `[` Expression `]`</ins>
MemberExpression `.` IdentifierName
<ins>MemberExpression OptionalChainingOperator IdentifierName</ins>
MemberExpression TemplateLiteral
SuperProperty
MetaProperty
`new` MemberExpression Arguments
<ins>`new` MemberExpression OptionalChainingOperator Arguments</ins>
NewExpression :
`new` NewExpression
LeftHandSideExpression :
CallExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<!-- es6num="12.3.1.5" -->
<emu-clause id="sec-static-semantics-static-semantics-isvalidsimpleassignmenttarget">
<h1>Static Semantics: IsValidSimpleAssignmentTarget</h1>
<emu-see-also-para op="IsValidSimpleAssignmentTarget"></emu-see-also-para>
<emu-grammar>
CallExpression :
CallExpression `[` Expression `]`
CallExpression `.` IdentifierName
</emu-grammar>
<emu-alg>
1. <ins>If MayProduceNil of |CallExpression| is *true*, return *false*.</ins>
2. Return *true*.
</emu-alg>
<emu-grammar>
MemberExpression :
MemberExpression `[` Expression `]`
MemberExpression `.` IdentifierName
</emu-grammar>
<emu-alg>
1. <ins>If MayProduceNil of |MemberExpression| is *true*, return *false*.</ins>
2. Return *true*.
</emu-alg>
<emu-grammar>
MemberExpression :
SuperProperty
</emu-grammar>
<emu-alg>
1. Return *true*.
</emu-alg>
<emu-grammar>
CallExpression :
CoverCallExpressionAndAsyncArrowHead
<ins>MemberExpression OptionalChainingOperator Arguments</ins>
SuperCall
CallExpression Arguments
<ins>CallExpression OptionalChainingOperator Arguments</ins>
<ins>CallExpression OptionalChainingOperator `[` Expression `]`</ins>
<ins>MemberExpression OptionalChainingOperator IdentifierName</ins>
CallExpression TemplateLiteral
NewExpression :
`new` NewExpression
MemberExpression :
MemberExpression TemplateLiteral
`new` MemberExpression Arguments
<ins>`new` MemberExpression OptionalChainingOperator Arguments</ins>
NewTarget :
`new` `.` `target`
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<ins class="block">
<emu-clause id="sec-static_semantics-static-semantics-mayproducenil">
<h1>Static Semantics: MayProduceNil</h1>
<emu-see-also-para op="MayProduceNil"></emu-see-also-para>
<emu-grammar>
MemberExpression :
MemberExpression OptionalChainingOperator `[` Expression `]`
MemberExpression OptionalChainingOperator IdentifierName
`new` MemberExpression OptionalChainingOperator Arguments
CallExpression :
MemberExpression OptionalChainingOperator Arguments
CallExpression OptionalChainingOperator Arguments
CallExpression OptionalChainingOperator `[` Expression `]`
CallExpression OptionalChainingOperator IdentifierName
</emu-grammar>
<emu-alg>
1. Return *true*.
</emu-alg>
<emu-grammar>
MemberExpression :
SuperProperty
MetaProperty
CallExpression :
SuperCall
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
<emu-grammar>
MemberExpression :
MemberExpression `[` Expression `]`
MemberExpression `.` IdentifierName
MemberExpression TemplateLiteral
`new` MemberExpression Arguments
CallMemberExpression :
MemberExpression Arguments
</emu-grammar>
<emu-alg>
1. Return MayProduceNil of |MemberExpression|.
</emu-alg>
<emu-grammar>
CallExpression :
CallExpression Arguments
CallExpression `[` Expression `]`
CallExpression `.` IdentifierName
CallExpression TemplateLiteral
</emu-grammar>
<emu-alg>
1. Return MayProduceNil of |CallExpression|.
</emu-alg>
<emu-grammar>
NewExpression :
`new` NewExpression
</emu-grammar>
<emu-alg>
1. Return MayProduceNil of |NewExpression|.
</emu-alg>
</emu-clause>
</ins>
</emu-clause>
<!-- es6num="12.3.2" -->
<emu-clause id="sec-property-accessors">
<h1>(<a href="https://tc39.github.io/ecma262/#sec-property-accessors">12.3.2</a>) Property Accessors</h1>
<emu-note>
<p>Properties are accessed by name, using either the dot notation:</p>
<div class="rhs">
|MemberExpression| `.` |IdentifierName|
<br>
|CallExpression| `.` |IdentifierName|
</div>
<p>or the bracket notation:</p>
<div class="rhs">
|MemberExpression| `[` |Expression| `]`
<br>
|CallExpression| `[` |Expression| `]`
</div>
<p>The dot notation is explained by the following syntactic conversion:</p>
<div class="rhs">
|MemberExpression| `.` |IdentifierName|
</div>
<p>is identical in its behaviour to</p>
<div class="rhs">
|MemberExpression| `[` <<i>identifier-name-string</i>> `]`
</div>
<p>and similarly</p>
<div class="rhs">
|CallExpression| `.` |IdentifierName|
</div>
<p>is identical in its behaviour to</p>
<div class="rhs">
|CallExpression| `[` <<i>identifier-name-string</i>> `]`
</div>
<p>where <<i>identifier-name-string</i>> is the result of evaluating StringValue of |IdentifierName|.</p>
</emu-note>
<!-- es6num="12.3.2.1" -->
<emu-clause id="sec-property-accessors-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>MemberExpression : MemberExpression `[` Expression `]`</emu-grammar>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |MemberExpression|.
1. ReturnIfAbrupt(_baseReference_).
1. <ins>If _baseReference_ is a Reference and IsNilReference(_baseReference_) is *true*, then</ins>
1. <ins>Return _baseReference_.</ins>
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. Let _propertyNameReference_ be the result of evaluating |Expression|.
1. Let _propertyNameValue_ be ? GetValue(_propertyNameReference_).
1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
1. Let _propertyKey_ be ? ToPropertyKey(_propertyNameValue_).
1. If the code matched by this |MemberExpression| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return a value of type Reference whose base value component is _bv_, whose referenced name component is _propertyKey_, and whose strict reference flag is _strict_.
</emu-alg>
<ins class="block">
<emu-grammar>MemberExpression : MemberExpression OptionalChainingOperator `[` Expression `]`</emu-grammar>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |MemberExpression|.
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. If _baseValue_ is *undefined* or *null*, then
1. Return a value of type Reference whose base value is ~Nil~.
1. Let _propertyNameReference_ be the result of evaluating |Expression|.
1. Let _propertyNameValue_ be ? GetValue(_propertyNameReference_).
1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
1. Let _propertyKey_ be ? ToPropertyKey(_propertyNameValue_).
1. If the code matched by this |MemberExpression| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return a value of type Reference whose base value component is _bv_, whose referenced name component is _propertyKey_, and whose strict reference flag is _strict_.
</emu-alg>
</ins>
<emu-grammar>MemberExpression : MemberExpression `.` IdentifierName</emu-grammar>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |MemberExpression|.
1. ReturnIfAbrupt(_baseReference_).
1. <ins>If _baseReference_ is a Reference and IsNilReference(_baseReference_) is *true*, then</ins>
1. <ins>Return _baseReference_.</ins>
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
1. Let _propertyNameString_ be StringValue of |IdentifierName|.
1. If the code matched by this |MemberExpression| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return a value of type Reference whose base value component is _bv_, whose referenced name component is _propertyNameString_, and whose strict reference flag is _strict_.
</emu-alg>
<ins class="block">
<emu-grammar>MemberExpression : MemberExpression OptionalChainingOperator IdentifierName</emu-grammar>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |MemberExpression|.
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. If _baseValue_ is *undefined* or *null*, then
1. Return a value of type Reference whose base value is ~Nil~.
1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
1. Let _propertyNameString_ be StringValue of |IdentifierName|.
1. If the code matched by this |MemberExpression| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return a value of type Reference whose base value component is _bv_, whose referenced name component is _propertyNameString_, and whose strict reference flag is _strict_.
</emu-alg>
</ins>
<emu-grammar>CallExpression : CallExpression `[` Expression `]`</emu-grammar>
<p>Is evaluated in exactly the same manner as <emu-grammar>MemberExpression : MemberExpression `[` Expression `]`</emu-grammar> except that the contained |CallExpression| is evaluated in step 1.</p>
<ins class="block">
<emu-grammar>CallExpression : CallExpression OptionalChainingOperator `[` Expression `]`</emu-grammar>
<p>Is evaluated in exactly the same manner as <emu-grammar>MemberExpression : MemberExpression OptionalChainingOperator `[` Expression `]`</emu-grammar> except that the contained |CallExpression| is evaluated in step 1.</p>
</ins>
<emu-grammar>CallExpression : CallExpression `.` IdentifierName</emu-grammar>