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Normative: Add Optional Chaining #1646

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Dec 12, 2019
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Normative: Add Optional Chaining #1646

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232 changes: 220 additions & 12 deletions spec.html
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Original file line number Diff line number Diff line change
Expand Up @@ -10973,7 +10973,11 @@ <h2>Syntax</h2>
<h1>Punctuators</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
Punctuator :: one of
OptionalChainingPunctuator ::
`?.` [lookahead &lt;! DecimalDigit]
OtherPunctuator

OtherPunctuator :: one of
`{` `(` `)` `[` `]`
`.` `...` `;` `,`
`&lt;` `&gt;` `&lt;=` `&gt;=`
Expand All @@ -10988,6 +10992,10 @@ <h2>Syntax</h2>
`=` `+=` `-=` `*=` `%=` `**=` `&lt;&lt;=` `&gt;&gt;=` `&gt;&gt;&gt;=` `&amp;=` `|=` `^=`
`=&gt;`

Punctuator ::
OptionalChainingPunctuator
OtherPunctuator

DivPunctuator ::
`/`
`/=`
Expand Down Expand Up @@ -13387,9 +13395,25 @@ <h2>Syntax</h2>
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] :
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`?.` Arguments[?Yield, ?Await]
`?.` `[` Expression[+In, ?Yield, ?Await] `]`
`?.` IdentifierName
`?.` TemplateLiteral[?Yield, ?Await, +Tagged]
OptionalChain[?Yield, ?Await] Arguments[?Yield, ?Await]
OptionalChain[?Yield, ?Await] `[` Expression[+In, ?Yield, ?Await] `]`
OptionalChain[?Yield, ?Await] `.` IdentifierName
OptionalChain[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, +Tagged]

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 <emu-grammar>CallExpression : CoverCallExpressionAndAsyncArrowHead</emu-grammar> the interpretation of |CoverCallExpressionAndAsyncArrowHead| is refined using the following grammar:</p>
Expand All @@ -13401,6 +13425,33 @@ <h2>Supplemental Syntax</h2>
<emu-clause id="sec-static-semantics">
<h1>Static Semantics</h1>

<emu-clause id="sec-left-hand-side-expressions-static-semantics-early-errors">
<h1>Static Semantics: Early Errors</h1>
<emu-grammar>
OptionalChain[Yield, Await] :
`?.` TemplateLiteral[?Yield, ?Await, +Tagged]
OptionalChain[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, +Tagged]
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</emu-grammar>
<ul>
<li>
It is a Syntax Error if any code matches this production.
</li>
</ul>
<emu-note>
<p>This production exists in order to prevent automatic semicolon insertion rules (<emu-xref href="#sec-automatic-semicolon-insertion"></emu-xref>) from being applied to the following code:</p>
<pre><code class="javascript">
a?.b
`c`
</code></pre>
<p>so that it would be interpreted as two valid statements. The purpose is to maintain consistency with similar code without optional chaining operator:</p>
<pre><code class="javascript">
a.b
`c`
</code></pre>
<p>which is a valid statement and where automatic semicolon insertion does not apply.</p>
</emu-note>
</emu-clause>

<emu-clause id="sec-left-hand-side-expressions-static-semantics-coveredcallexpression">
<h1>Static Semantics: CoveredCallExpression</h1>
<emu-grammar>
Expand Down Expand Up @@ -13436,6 +13487,19 @@ <h1>Static Semantics: Contains</h1>
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>OptionalChain : `?.` IdentifierName</emu-grammar>
<emu-alg>
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>OptionalChain : OptionalChain `.` IdentifierName</emu-grammar>
<emu-alg>
1. If |OptionalChain| 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>

<emu-clause id="sec-static-semantics-static-semantics-isfunctiondefinition">
Expand All @@ -13455,6 +13519,7 @@ <h1>Static Semantics: IsFunctionDefinition</h1>

LeftHandSideExpression :
CallExpression
OptionalExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
Expand Down Expand Up @@ -13483,6 +13548,7 @@ <h1>Static Semantics: IsDestructuring</h1>

LeftHandSideExpression :
CallExpression
OptionalExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
Expand All @@ -13506,6 +13572,7 @@ <h1>Static Semantics: IsIdentifierRef</h1>

LeftHandSideExpression :
CallExpression
OptionalExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
Expand Down Expand Up @@ -13545,6 +13612,9 @@ <h1>Static Semantics: AssignmentTargetType</h1>

NewTarget :
`new` `.` `target`

LeftHandSideExpression :
OptionalExpression
</emu-grammar>
<emu-alg>
1. Return ~invalid~.
Expand Down Expand Up @@ -13592,29 +13662,53 @@ <h1>Runtime Semantics: Evaluation</h1>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |MemberExpression|.
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_.
1. Return ? EvaluatePropertyAccessWithExpressionKey(_baseValue_, |Expression|, _strict_).
</emu-alg>
<emu-grammar>MemberExpression : MemberExpression `.` IdentifierName</emu-grammar>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |MemberExpression|.
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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_.
1. Return ? EvaluatePropertyAccessWithIdentifierKey(_baseValue_, |IdentifierName|, _strict_).
</emu-alg>
<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>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |CallExpression|.
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. If the code matched by this |CallExpression| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return ? EvaluatePropertyAccessWithExpressionKey(_baseValue_, |Expression|, _strict_).
</emu-alg>
<emu-grammar>CallExpression : CallExpression `.` IdentifierName</emu-grammar>
<p>Is evaluated in exactly the same manner as <emu-grammar>MemberExpression : MemberExpression `.` IdentifierName</emu-grammar> except that the contained |CallExpression| is evaluated in step 1.</p>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |CallExpression|.
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. If the code matched by this |CallExpression| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return ? EvaluatePropertyAccessWithIdentifierKey(_baseValue_, |IdentifierName|, _strict_).
</emu-alg>
</emu-clause>
</emu-clause>

<emu-clause id="sec-evaluate-expression-key-property-access" aoid="EvaluatePropertyAccessWithExpressionKey">
<h1>Runtime Semantics: EvaluatePropertyAccessWithExpressionKey( _baseValue_, _expression_, _strict_ )</h1>
<p>The abstract operation EvaluatePropertyAccessWithExpressionKey takes as arguments a value _baseValue_, a Parse Node _expression_, and a Boolean argument _strict_. It performs the following steps:</p>
<emu-alg>
1. Let _propertyNameReference_ be the result of evaluating _expression_.
1. Let _propertyNameValue_ be ? GetValue(_propertyNameReference_).
1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
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1. Let _propertyKey_ be ? ToPropertyKey(_propertyNameValue_).
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-clause>
<emu-clause id="sec-evaluate-identifier-key-property-access" aoid="EvaluatePropertyAccessWithIdentifierKey">
<h1>Runtime Semantics: EvaluatePropertyAccessWithIdentifierKey( _baseValue_, _identifierName_, _strict_ )</h1>
<p>The abstract operation EvaluatePropertyAccessWithIdentifierKey takes as arguments a value _baseValue_, a Parse Node _identifierName_, and a Boolean argument _strict_. It performs the following steps:</p>
<emu-alg>
1. Assert: _identifierName_ is an |IdentifierName|
1. Let _bv_ be ? RequireObjectCoercible(_baseValue_).
1. Let _propertyNameString_ be StringValue of _identifierName_.
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-clause>

<emu-clause id="sec-new-operator">
<h1>The `new` Operator</h1>

Expand Down Expand Up @@ -13659,7 +13753,7 @@ <h1>Runtime Semantics: Evaluation</h1>
1. Let _arguments_ be the |Arguments| of _expr_.
1. Let _ref_ be the result of evaluating _memberExpr_.
1. Let _func_ be ? GetValue(_ref_).
1. If Type(_ref_) is Reference and IsPropertyReference(_ref_) is *false* and GetReferencedName(_ref_) is *"eval"*, then
1. If Type(_ref_) is Reference, IsPropertyReference(_ref_) is *false*, and GetReferencedName(_ref_) 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*.
Expand Down Expand Up @@ -13822,6 +13916,92 @@ <h1>Runtime Semantics: ArgumentListEvaluation</h1>
</emu-clause>
</emu-clause>

<emu-clause id="sec-optional-chains">
<h1>Optional Chains</h1>
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<emu-note>An optional chain is a chain of one or more property accesses and function calls, the first of which begins with the token `?.`.</emu-note>
<emu-clause id="sec-optional-chaining-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>
OptionalExpression :
MemberExpression OptionalChain
</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 *undefined*.
1. Return the result of performing ChainEvaluation of |OptionalChain| with arguments _baseValue_ and _baseReference_.
</emu-alg>
<emu-grammar>
OptionalExpression :
CallExpression OptionalChain
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</emu-grammar>
<emu-alg>
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1. Let _baseReference_ be the result of evaluating |CallExpression|.
1. Let _baseValue_ be ? GetValue(_baseReference_).
1. If _baseValue_ is *undefined* or *null*, then
1. Return *undefined*.
1. Return the result of performing ChainEvaluation of |OptionalChain| with arguments _baseValue_ and _baseReference_.
</emu-alg>
<emu-grammar>
OptionalExpression :
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OptionalExpression OptionalChain
</emu-grammar>
<emu-alg>
1. Let _baseReference_ be the result of evaluating |OptionalExpression|.
1. Let _baseValue_ be ? GetValue(_baseReference_).
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1. If _baseValue_ is *undefined* or *null*, then
1. Return *undefined*.
1. Return the result of performing ChainEvaluation of |OptionalChain| with arguments _baseValue_ and _baseReference_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-optional-chaining-chain-evaluation">
<h1>Runtime Semantics: ChainEvaluation</h1>
<p>With parameters _baseValue_ and _baseReference_.</p>
<emu-grammar>OptionalChain : `?.` Arguments</emu-grammar>
<emu-alg>
1. Let _thisChain_ be this |OptionalChain|.
1. Let _tailCall_ be IsInTailPosition(_thisChain_).
1. Return ? EvaluateCall(_baseValue_, _baseReference_, |Arguments|, _tailCall_).
</emu-alg>
<emu-grammar>OptionalChain : `?.` `[` Expression `]`</emu-grammar>
<emu-alg>
1. If the code matched by this |OptionalChain| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return ? EvaluatePropertyAccessWithExpressionKey(_baseValue_, |Expression|, _strict_).
</emu-alg>
<emu-grammar>OptionalChain : `?.` IdentifierName</emu-grammar>
<emu-alg>
1. If the code matched by this |OptionalChain| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return ? EvaluatePropertyAccessWithIdentifierKey(_baseValue_, |IdentifierName|, _strict_).
</emu-alg>
<emu-grammar>OptionalChain : OptionalChain Arguments</emu-grammar>
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<emu-alg>
1. Let _optionalChain_ be |OptionalChain|.
1. Let _newReference_ be ? ChainEvaluation of _optionalChain_ with arguments _baseValue_ and _baseReference_.
1. Let _newValue_ be ? GetValue(_newReference_).
1. Let _thisChain_ be this |OptionalChain|.
1. Let _tailCall_ be IsInTailPosition(_thisChain_).
1. Return ? EvaluateCall(_newValue_, _newReference_, |Arguments|, _tailCall_).
</emu-alg>
<emu-grammar>OptionalChain : OptionalChain `[` Expression `]`</emu-grammar>
<emu-alg>
1. Let _optionalChain_ be |OptionalChain|.
1. Let _newReference_ be ? ChainEvaluation of _optionalChain_ with arguments _baseValue_ and _baseReference_.
1. Let _newValue_ be ? GetValue(_newReference_).
1. If the code matched by this |OptionalChain| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return ? EvaluatePropertyAccessWithExpressionKey(_newValue_, |Expression|, _strict_).
</emu-alg>
<emu-grammar>OptionalChain : OptionalChain `.` IdentifierName</emu-grammar>
<emu-alg>
1. Let _optionalChain_ be |OptionalChain|.
1. Let _newReference_ be ? ChainEvaluation of _optionalChain_ with arguments _baseValue_ and _baseReference_.
1. Let _newValue_ be ? GetValue(_newReference_).
1. If the code matched by this |OptionalChain| is strict mode code, let _strict_ be *true*, else let _strict_ be *false*.
1. Return ? EvaluatePropertyAccessWithIdentifierKey(_newValue_, |IdentifierName|, _strict_).
</emu-alg>
</emu-clause>
</emu-clause>

<emu-clause id="sec-import-calls">
<h1>Import Calls</h1>

Expand Down Expand Up @@ -21758,6 +21938,34 @@ <h1>Expression Rules</h1>
1. If this |CallExpression| is _call_, return *true*.
1. Return *false*.
</emu-alg>
<emu-grammar>
OptionalExpression :
MemberExpression OptionalChain
CallExpression OptionalChain
OptionalExpression OptionalChain
</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |OptionalChain| with argument _call_.
</emu-alg>
<emu-grammar>
OptionalChain :
`?.` `[` Expression `]`
`?.` IdentifierName
OptionalChain `[` Expression `]`
OptionalChain `.` IdentifierName
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
<emu-grammar>
OptionalChain :
`?.` Arguments
OptionalChain Arguments
</emu-grammar>
<emu-alg>
1. If this |OptionalChain| is _call_, return *true*.
1. Return *false*.
</emu-alg>
<emu-grammar>
MemberExpression :
MemberExpression TemplateLiteral
Expand Down