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spec.html
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<!doctype html>
<meta charset="utf8">
<pre class=metadata>
title: Nullish Coalescing Operator
stage: 3
contributors: Daniel Ehrenberg, Daniel Rosenwasser
</pre>
<emu-intro id=sec-intro>
<h1>Introduction</h1>
<p>This document specifies the nullish coalescing operator `??`. See <a href="https://github.com/gisenberg/proposal-nullish-coalescing">the explainer</a> for an introduction.</p>
<p>The main design decisions made in this specification are:
<ol>
<li>The right argument of `??` is evaluated only if needed ("short circuiting").</li>
<li>`??` has lower precedence than `||`.</li>
<li>`??` cannot immediately contain, or be contained within, an `&&` or `||` operation.</li>
<li>The right argument is selected if the left argument is `null` or `undefined`.</li>
</ol>
</p>
</emu-intro>
<emu-clause id="sec-binary-logical-operators">
<h1>Binary Logical Operators</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
<ins>
ShortCircuitExpression[In, Yield, Await] :
LogicalORExpression[?In, ?Yield, ?Await]
CoalesceExpression[?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]
</ins>
</emu-grammar>
<emu-clause id="sec-binary-logical-operators-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>
LogicalANDExpression : LogicalANDExpression `&&` BitwiseORExpression
LogicalORExpression : LogicalORExpression `||` LogicalANDExpression
<ins>CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<emu-clause oldids="sec-binary-logical-operators-static-semantics-isvalidsimpleassignmenttarget,sec-semantics-static-semantics-assignmenttargettype" id="sec-binary-logical-operators-static-semantics-issimpleassignmenttarget">
<h1>Static Semantics: IsSimpleAssignmentTarget</h1>
<emu-see-also-para op="IsSimpleAssignmentTarget"></emu-see-also-para>
<emu-grammar>
LogicalANDExpression : LogicalANDExpression `&&` BitwiseORExpression
LogicalORExpression : LogicalORExpression `||` LogicalANDExpression
<ins>CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-binary-logical-operators-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar><ins>CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression</ins></emu-grammar>
<emu-alg>
1. <ins>Let _lref_ be the result of evaluating |CoalesceExpressionHead|.</ins>
1. <ins>Let _lval_ be ? GetValue(_lref_).</ins>
1. <ins>If _lval_ is *undefined* or *null*,</ins>
1. <ins>Let _rref_ be the result of evaluating |BitwiseORExpression|.</ins>
1. <ins>Return ? GetValue(_rref_).</ins>
1. <ins>Otherwise, return _lval_.</ins>
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-conditional-operator">
<h1>Conditional Operator ( `? :` )</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
ConditionalExpression[In, Yield, Await] :
<del>
LogicalORExpression[?In, ?Yield, ?Await]
LogicalORExpression[?In, ?Yield, ?Await] `?` AssignmentExpression[+In, ?Yield, ?Await] `:` AssignmentExpression[?In, ?Yield, ?Await]
</del>
<ins>
ShortCircuitExpression[?In, ?Yield, ?Await]
ShortCircuitExpression[?In, ?Yield, ?Await] `?` AssignmentExpression[+In, ?Yield, ?Await] `:` AssignmentExpression[+In, ?Yield, ?Await]
</ins>
</emu-grammar>
<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>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<emu-clause oldids="sec-conditional-operator-static-semantics-isvalidsimpleassignmenttarget,sec-semantics-static-semantics-assignmenttargettype" id="sec-conditional-operator-static-semantics-issimpleassignmenttarget">
<h1>Static Semantics: IsSimpleAssignmentTarget</h1>
<emu-see-also-para op="IsSimpleAssignmentTarget"></emu-see-also-para>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-conditional-operator-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. <del>Let _lref_ be the result of evaluating |LogicalORExpression|.</del>
1. <ins>Let _lref_ be the result of evaluating |ShortCircuitExpression|.</ins>
1. Let _lval_ be ToBoolean(? GetValue(_lref_)).
1. If _lval_ is *true*, then
1. Let _trueRef_ be the result of evaluating the first |AssignmentExpression|.
1. Return ? GetValue(_trueRef_).
1. Else,
1. Let _falseRef_ be the result of evaluating the second |AssignmentExpression|.
1. Return ? GetValue(_falseRef_).
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-tail-position-calls">
<h1>Tail Position Calls</h1>
<emu-clause id="sec-static-semantics-hascallintailposition">
<h1>Static Semantics: HasCallInTailPosition</h1>
<p>With parameter _call_.</p>
<emu-clause id="sec-expression-rules">
<h1>Expression Rules</h1>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : ShortCircuitExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. Let _has_ be HasCallInTailPosition of the first |AssignmentExpression| with argument _call_.
1. If _has_ is *true*, return *true*.
1. Return HasCallInTailPosition of the second |AssignmentExpression| with argument _call_.
</emu-alg>
<emu-grammar>LogicalANDExpression : LogicalANDExpression `&&` BitwiseORExpression</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |BitwiseORExpression| with argument _call_.
</emu-alg>
<emu-grammar>LogicalORExpression : LogicalORExpression `||` LogicalANDExpression</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |LogicalANDExpression| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>
CoalesceExpression : CoalesceExpressionHead `??` BitwiseORExpression
</ins>
</emu-grammar>
<emu-alg>
<ins>
1. Return HasCallInTailPosition of |BitwiseORExpression| with argument _call_.
</ins>
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>