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Normative: Add Nullish Coalescing #1644

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Dec 12, 2019
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Normative: Add Nullish Coalescing #1644

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47 changes: 39 additions & 8 deletions spec.html
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Original file line number Diff line number Diff line change
Expand Up @@ -10987,7 +10987,7 @@ <h2>Syntax</h2>
`&lt;&lt;` `&gt;&gt;` `&gt;&gt;&gt;`
`&amp;` `|` `^`
`!` `~`
`&amp;&amp;` `||`
`&amp;&amp;` `||` `??`
`?` `:`
`=` `+=` `-=` `*=` `%=` `**=` `&lt;&lt;=` `&gt;&gt;=` `&gt;&gt;&gt;=` `&amp;=` `|=` `^=`
`=&gt;`
Expand Down Expand Up @@ -15123,6 +15123,17 @@ <h2>Syntax</h2>
LogicalORExpression[In, Yield, Await] :
LogicalANDExpression[?In, ?Yield, ?Await]
LogicalORExpression[?In, ?Yield, ?Await] `||` LogicalANDExpression[?In, ?Yield, ?Await]

CoalesceExpression[In, Yield, Await] :
CoalesceExpressionHead[?In, ?Yield, ?Await] `??` BitwiseORExpression[?In, ?Yield, ?Await]

CoalesceExpressionHead[In, Yield, Await] :
CoalesceExpression[?In, ?Yield, ?Await]
BitwiseORExpression[?In, ?Yield, ?Await]

ShortCircuitExpression[In, Yield, Await] :
LogicalORExpression[?In, ?Yield, ?Await]
CoalesceExpression[?In, ?Yield, ?Await]
</emu-grammar>
<emu-note>
<p>The value produced by a `&amp;&amp;` or `||` operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.</p>
Expand All @@ -15135,6 +15146,8 @@ <h1>Static Semantics: IsFunctionDefinition</h1>
LogicalANDExpression : LogicalANDExpression `&amp;&amp;` BitwiseORExpression

LogicalORExpression : LogicalORExpression `||` LogicalANDExpression

CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression
</emu-grammar>
<emu-alg>
1. Return *false*.
Expand All @@ -15148,6 +15161,8 @@ <h1>Static Semantics: AssignmentTargetType</h1>
LogicalANDExpression : LogicalANDExpression `&amp;&amp;` BitwiseORExpression

LogicalORExpression : LogicalORExpression `||` LogicalANDExpression

CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression
</emu-grammar>
<emu-alg>
1. Return ~invalid~.
Expand All @@ -15174,6 +15189,15 @@ <h1>Runtime Semantics: Evaluation</h1>
1. Let _rref_ be the result of evaluating |LogicalANDExpression|.
1. Return ? GetValue(_rref_).
</emu-alg>
<emu-grammar>CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be the result of evaluating |CoalesceExpressionHead|.
1. Let _lval_ be ? GetValue(_lref_).
1. If _lval_ is *undefined* or *null*,
1. Let _rref_ be the result of evaluating |BitwiseORExpression|.
1. Return ? GetValue(_rref_).
1. Otherwise, return _lval_.
</emu-alg>
</emu-clause>
</emu-clause>

Expand All @@ -15182,8 +15206,8 @@ <h1>Conditional Operator ( `? :` )</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
ConditionalExpression[In, Yield, Await] :
LogicalORExpression[?In, ?Yield, ?Await]
LogicalORExpression[?In, ?Yield, ?Await] `?` AssignmentExpression[+In, ?Yield, ?Await] `:` AssignmentExpression[?In, ?Yield, ?Await]
ShortCircuitExpression[?In, ?Yield, ?Await]
ShortCircuitExpression[?In, ?Yield, ?Await] `?` AssignmentExpression[+In, ?Yield, ?Await] `:` AssignmentExpression[?In, ?Yield, ?Await]
</emu-grammar>
<emu-note>
<p>The grammar for a |ConditionalExpression| in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an |Expression| but restrict the third expression to be a |ConditionalExpression|. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.</p>
Expand All @@ -15192,7 +15216,7 @@ <h2>Syntax</h2>
<emu-clause id="sec-conditional-operator-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-grammar>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
Expand All @@ -15201,17 +15225,17 @@ <h1>Static Semantics: IsFunctionDefinition</h1>
<emu-clause oldids="sec-conditional-operator-static-semantics-isvalidsimpleassignmenttarget" id="sec-conditional-operator-static-semantics-assignmenttargettype">
<h1>Static Semantics: AssignmentTargetType</h1>
<emu-see-also-para op="AssignmentTargetType"></emu-see-also-para>
<emu-grammar>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-grammar>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-alg>
1. Return ~invalid~.
</emu-alg>
</emu-clause>

<emu-clause id="sec-conditional-operator-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-grammar>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be the result of evaluating |LogicalORExpression|.
1. Let _lref_ be the result of evaluating |ShortCircuitExpression|.
1. Let _lval_ be ! ToBoolean(? GetValue(_lref_)).
1. If _lval_ is *true*, then
1. Let _trueRef_ be the result of evaluating the first |AssignmentExpression|.
Expand Down Expand Up @@ -21914,7 +21938,7 @@ <h1>Expression Rules</h1>
<emu-alg>
1. Return HasCallInTailPosition of |AssignmentExpression| with argument _call_.
</emu-alg>
<emu-grammar>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-grammar>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</emu-grammar>
<emu-alg>
1. Let _has_ be HasCallInTailPosition of the first |AssignmentExpression| with argument _call_.
1. If _has_ is *true*, return *true*.
Expand All @@ -21928,6 +21952,10 @@ <h1>Expression Rules</h1>
<emu-alg>
1. Return HasCallInTailPosition of |LogicalANDExpression| with argument _call_.
</emu-alg>
<emu-grammar>CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |BitwiseORExpression| with argument _call_.
</emu-alg>
<emu-grammar>
CallExpression :
CoverCallExpressionAndAsyncArrowHead
Expand Down Expand Up @@ -41417,6 +41445,9 @@ <h1>Expressions</h1>
<emu-prodref name=BitwiseORExpression></emu-prodref>
<emu-prodref name=LogicalANDExpression></emu-prodref>
<emu-prodref name=LogicalORExpression></emu-prodref>
<emu-prodref name=CoalesceExpression></emu-prodref>
<emu-prodref name=CoalesceExpressionHead></emu-prodref>
<emu-prodref name=ShortCircuitExpression></emu-prodref>
<emu-prodref name=ConditionalExpression></emu-prodref>
<emu-prodref name=AssignmentExpression></emu-prodref>
<p>In certain circumstances when processing an instance of the production <emu-prodref name=AssignmentExpression a=assignment></emu-prodref> the following grammar is used to refine the interpretation of |LeftHandSideExpression|:</p>
Expand Down