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Normative: PromiseResolve check proto instead of constructor #3689

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Normative: PromiseResolve check proto instead of constructor #3689

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da0dd08
Normative: PromiseResolve check proto instead of constructor
mhofman Sep 11, 2025
53b2ab1
Normative: resolve function adopts promises
mhofman Sep 11, 2025
ed27ba3
fixup! Normative: PromiseResolve check proto instead of constructor
mhofman Sep 11, 2025
02dc66b
fixup! Normative: resolve function adopts promises
mhofman Sep 11, 2025
23f3711
fixup! Normative: PromiseResolve check proto instead of constructor
mhofman Sep 16, 2025
0de5715
fixup! Normative: resolve function adopts promises
mhofman Sep 16, 2025
ed3e660
fixup! Normative: PromiseResolve check proto instead of constructor
mhofman Sep 16, 2025
74c0260
revert Normative: resolve function adopts promises
mhofman Sep 19, 2025
5c666fe
Merge branch main
mhofman Sep 19, 2025
d1e26b9
Merge branch 'main' into adopt-promises
mhofman Sep 23, 2025
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54 changes: 44 additions & 10 deletions spec.html
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Original file line number Diff line number Diff line change
Expand Up @@ -3890,6 +3890,16 @@ <h1>Well-Known Intrinsic Objects</h1>
The Promise constructor (<emu-xref href="#sec-promise-constructor"></emu-xref>)
</td>
</tr>
<tr>
<td>
%PromiseThenAction%
</td>
<td>
</td>
<td>
An internal function object for PerformPromiseThen
</td>
</tr>
<tr>
<td>
%Proxy%
Expand Down Expand Up @@ -48472,21 +48482,43 @@ <h1>Promise Resolve Functions</h1>
1. If _resolution_ is not an Object, then
1. Perform FulfillPromise(_promise_, _resolution_).
1. Return *undefined*.
1. Let _then_ be Completion(Get(_resolution_, *"then"*)).
1. If _then_ is an abrupt completion, then
1. Perform RejectPromise(_promise_, _then_.[[Value]]).
1. Return *undefined*.
1. Let _thenAction_ be _then_.[[Value]].
1. If IsCallable(_thenAction_) is *false*, then
1. Perform FulfillPromise(_promise_, _resolution_).
1. Return *undefined*.
1. Let _thenAction_ be *null*.
1. If IsPromise(_resolution_) is *true*, then
1. Let _proto_ be ! _resolution_.[[GetPrototypeOf]]().
1. If SameValue(_proto_, %Promise.prototype%) is *true*, set _thenAction_ to %PromiseThenAction%.
1. If _thenAction_ is *null*, then
1. Let _then_ be Completion(Get(_resolution_, *"then"*)).
1. If _then_ is an abrupt completion, then
1. Perform RejectPromise(_promise_, _then_.[[Value]]).
1. Return *undefined*.
1. Set _thenAction_ to _then_.[[Value]].
1. If IsCallable(_thenAction_) is *false*, then
1. Perform FulfillPromise(_promise_, _resolution_).
1. Return *undefined*.
1. Let _thenJobCallback_ be HostMakeJobCallback(_thenAction_).
1. NOTE: _thenAction_ may be %PromiseThenAction%, a value never directly accessible to ECMAScript code.
1. Let _job_ be NewPromiseResolveThenableJob(_promise_, _resolution_, _thenJobCallback_).
1. Perform HostEnqueuePromiseJob(_job_.[[Job]], _job_.[[Realm]]).
1. Return *undefined*.
</emu-alg>
<p>The *"length"* property of a promise resolve function is *1*<sub>𝔽</sub>.</p>
</emu-clause>

<emu-clause id="sec-%promisethenaction%">
<h1>%PromiseThenAction% ( _onFulfilled_, _onRejected_ )</h1>
<p>The <dfn>%PromiseThenAction%</dfn> intrinsic function:</p>
<ul>
<li>is an anonymous built-in function object that is defined once for each realm.</li>
<li>is never directly accessible to ECMAScript code.</li>
<li>is used by the promise resolve functions to adopt the state of the promise passed as the receiver.</li>
<li>performs the following steps when called:</li>
</ul>
<emu-alg>
1. Let _promise_ be the *this* value.
1. Assert: IsPromise(_promise_) is *true*.
1. Return PerformPromiseThen(_promise_, _onFulfilled_, _onRejected_).
</emu-alg>
</emu-clause>
</emu-clause>

<emu-clause id="sec-fulfillpromise" type="abstract operation">
Expand Down Expand Up @@ -49141,8 +49173,10 @@ <h1>
</dl>
<emu-alg>
1. If IsPromise(_x_) is *true*, then
1. Let _xConstructor_ be ? Get(_x_, *"constructor"*).
1. If SameValue(_xConstructor_, _C_) is *true*, return _x_.
1. Let _xProto_ be ! _x_.[[GetPrototypeOf]]().

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1. Let _CPrototype_ be ? Get(_C_, *"prototype"*).

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@jridgewell jridgewell Sep 11, 2025

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If we wanted to be really prescriptive about this, we could check if _C_ is a constructor/function. I don't believe the .prototype of a function can ever be an accessor? So we could turn this into ! instead of ?.

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@mhofman mhofman Sep 11, 2025

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Some functions don't have a .prototype, which could be installed manually. Also C could be a proxy for a constructor / function.

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@jridgewell jridgewell Sep 12, 2025

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If it’s not a constructor, then we can’t new C, so NewPromiseCapability(C) would just throw. I don’t think I we need to consider those.

I’m kinda ambivalent on whether we need to support proxies of the promise constructor. Up to you.

1. NOTE: When _C_ is the intrinsic %Promise%, the above step cannot throw.
1. If SameValue(_xProto_, _CPrototype_) is *true*, return _x_.

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1. Let _promiseCapability_ be ? NewPromiseCapability(_C_).
1. Perform ? Call(_promiseCapability_.[[Resolve]], *undefined*, « _x_ »).
1. Return _promiseCapability_.[[Promise]].
Expand Down