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Editorial: support built-in async functions #2942

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Editorial: support built-in async functions
michaelficarra Oct 24, 2022
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michaelficarra May 22, 2025
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44 changes: 41 additions & 3 deletions spec.html
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Expand Up @@ -13991,8 +13991,8 @@ <h1>Built-in Function Objects</h1>
<li>[[InitialName]], a String that is the initial name of the function. It is used by <emu-xref href="#sec-function.prototype.tostring"></emu-xref>.</li>
</ul>
<p>The initial value of a built-in function object's [[Prototype]] internal slot is %Function.prototype%, unless otherwise specified.</p>
<p>A built-in function object must have a [[Call]] internal method that conforms to the definition in <emu-xref href="#sec-built-in-function-objects-call-thisargument-argumentslist"></emu-xref>.</p>
<p>A built-in function object has a [[Construct]] internal method if and only if it is described as a “constructor”, or some algorithm in this specification explicitly sets its [[Construct]] internal method. Such a [[Construct]] internal method must conform to the definition in <emu-xref href="#sec-built-in-function-objects-construct-argumentslist-newtarget"></emu-xref>.</p>
<p>A built-in function object must have a [[Call]] internal method that, unless otherwise specified, conforms to the definition in <emu-xref href="#sec-built-in-function-objects-call-thisargument-argumentslist"></emu-xref>.</p>
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Rather than leaving it vaguely open-ended like this, wouldn't it help the reader more to say that [[Call]] must conform to one of two explicit possibilities?

<p>A built-in function object has a [[Construct]] internal method if and only if it is described as a “constructor”, or some algorithm in this specification explicitly sets its [[Construct]] internal method. Unless otherwise specified, such a [[Construct]] internal method must conform to the definition in <emu-xref href="#sec-built-in-function-objects-construct-argumentslist-newtarget"></emu-xref>.</p>
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What is this change for? AFAICT, the current spec has no built-in that specifies an alternative [[Construct]], and this PR isn't adding one, so is this change for the benefit of a future PR? If so, maybe defer this change to that.

<p>An implementation may provide additional built-in function objects that are not defined in this specification.</p>

<emu-clause id="sec-built-in-function-objects-call-thisargument-argumentslist" type="internal method">
Expand Down Expand Up @@ -14081,7 +14081,10 @@ <h1>
1. If _prototype_ is not present, set _prototype_ to _realm_.[[Intrinsics]].[[%Function.prototype%]].
1. Let _internalSlotsList_ be a List containing the names of all the internal slots that <emu-xref href="#sec-built-in-function-objects"></emu-xref> requires for the built-in function object that is about to be created.
1. Append to _internalSlotsList_ the elements of _additionalInternalSlotsList_.
1. Let _func_ be a new built-in function object that, when called, performs the action described by _behaviour_ using the provided arguments as the values of the corresponding parameters specified by _behaviour_. The new function object has internal slots whose names are the elements of _internalSlotsList_, and an [[InitialName]] internal slot.
1. If _behaviour_ is described as async, then
1. Let _func_ be a new built-in async function object that, when called, performs the action described by _behaviour_ using the provided arguments as the values of the corresponding parameters specified by _behaviour_. The new function object has internal slots whose names are the elements of _internalSlotsList_, and an [[InitialName]] internal slot.
1. Else,
1. Let _func_ be a new built-in function object that, when called, performs the action described by _behaviour_ using the provided arguments as the values of the corresponding parameters specified by _behaviour_. The new function object has internal slots whose names are the elements of _internalSlotsList_, and an [[InitialName]] internal slot.
1. Set _func_.[[Prototype]] to _prototype_.
1. Set _func_.[[Extensible]] to *true*.
1. Set _func_.[[Realm]] to _realm_.
Expand All @@ -14097,6 +14100,41 @@ <h1>
</emu-clause>
</emu-clause>

<emu-clause id="sec-built-in-async-function-objects">
<h1>Built-in Async Function Objects</h1>
<p><dfn variants="built-in async function object,built-in async function objects">Built-in async function objects</dfn> are built-in function objects that provide a [[Call]] internal method that conforms to the following definition:</p>

<emu-clause id="sec-built-in-async-function-objects-call" type="internal method">
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Presumably you will need to add this to the possible definitions of [[Call]] for an ordinary object as defined in 6.1.7.2.

<h1>
[[Call]] (
_thisArgument_: an ECMAScript language value,
_argumentsList_: a List of ECMAScript language values,
): a normal completion containing a Promise
</h1>
<dl class="header">
<dt>for</dt>
<dd>a built-in async function object _F_</dd>
</dl>
<emu-alg>
1. Let _callerContext_ be the running execution context.
1. If _callerContext_ is not already suspended, suspend _callerContext_.
1. Let _calleeContext_ be a new execution context.
1. Set the Function of _calleeContext_ to _F_.
1. Let _calleeRealm_ be _F_.[[Realm]].
1. Set the Realm of _calleeContext_ to _calleeRealm_.
1. Set the ScriptOrModule of _calleeContext_ to *null*.
1. Perform any necessary implementation-defined initialization of _calleeContext_.
1. Push _calleeContext_ onto the execution context stack; _calleeContext_ is now the running execution context.
1. Let _promiseCapability_ be ! NewPromiseCapability(%Promise%).
1. Let _resultsClosure_ be a new Abstract Closure with no parameters that captures _F_, _thisArgument_, and _argumentsList_ and performs the following steps when called:
1. Return the Completion Record that is the result of evaluating _F_ in a manner that conforms to the specification of _F_. _thisArgument_ provides the *this* value, _argumentsList_ provides the named parameters, and the NewTarget value is *undefined*.
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The corresponding step in BuiltinCallOrConstruct wraps the result of evaluating in an <emu-meta> element. Does that apply here too?

1. Perform AsyncFunctionStart(_promiseCapability_, _resultsClosure_).
1. Remove _calleeContext_ from the execution context stack and restore _callerContext_ as the running execution context.
1. Return _promiseCapability_.[[Promise]].
</emu-alg>
</emu-clause>
</emu-clause>

<emu-clause id="sec-built-in-exotic-object-internal-methods-and-slots">
<h1>Built-in Exotic Object Internal Methods and Slots</h1>
<p>This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:</p>
Expand Down