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Add an extended attribute to allow propagating TC39's AsyncContext #1568

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Add an extended attribute to allow propagating TC39's AsyncContext
andreubotella Jan 9, 2026
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Original file line number Diff line number Diff line change
Expand Up @@ -91,6 +91,15 @@ urlPrefix: https://tc39.es/ecma262/; spec: ecmascript
url: sec-object-type
text: is an Object
text: is not an Object
urlPrefix: https://tc39.es/proposal-async-context/; spec: PROPOSAL-ASYNCCONTEXT
type: dfn
text: Async Context Mapping; url: async-context-mapping
type: abstract-op
text: AsyncContextSnapshot; url: sec-asynccontextsnapshot

# Link to HTML AsyncContext PR so the spec preview works
urlPrefix: https://whatpr.org/html/12152/infrastructure.html; spec: HTML
type: dfn; text: run; for: Async Context Mapping
</pre>

<pre class=biblio>
Expand All @@ -99,6 +108,11 @@ urlPrefix: https://tc39.es/ecma262/; spec: ecmascript
"publisher": "Ecma",
"href": "https://tc39.es/proposal-float16array/",
"title": "Proposal to add float16 TypedArrays to JavaScript"
},
"PROPOSAL-ASYNCCONTEXT": {
"publisher": "Ecma",
"href": "https://tc39.es/proposal-async-context/",
"title": "AsyncContext"
}
}
</pre>
Expand Down Expand Up @@ -6173,10 +6187,11 @@ the [=callback function=], the values can be references to objects that are not
An IDL value of the callback function type is represented by a tuple of an object
reference and a [=callback context=].

Note: As with [=callback interface types=], the [=callback context=] is used to hold a
reference to the <a spec="HTML">incumbent settings object</a> at
the time a JavaScript Object value is converted to an IDL
callback function type value. See [[#js-callback-function]].
Note: For JavaScript objects, the [=callback context=] is used to hold a
reference to the <a spec="HTML">incumbent settings object</a>, as with
[=callback interface types=], as well as a nullable reference to an
[=Async Context Mapping=], at the time a JavaScript Object value is converted to an
IDL callback function type value. See [[#js-callback-function]].

There is no way to represent a constant [=callback function=]
value in IDL.
Expand Down Expand Up @@ -8099,9 +8114,14 @@ IDL [=callback function types=] are represented by JavaScript [=function objects
[=callback function=]
that is annotated with [{{LegacyTreatNonObjectAsNull}}],
then [=JavaScript/throw=] a <l spec=ecmascript>{{TypeError}}</l>.
1. If the conversion is to an IDL type
[=extended attributes associated with|associated with=] the
[{{PropagatesAsyncContext}}] [=extended attribute=], then let |snapshot|
be [$AsyncContextSnapshot$](). Otherwise, let |snapshot| be null.
1. Return the IDL [=callback function type=] value
that represents a reference to the same object that |V| represents, with the
<a spec="HTML">incumbent settings object</a> as the [=callback context=].
that represents a reference to the same object that |V| represents, with
(the <a spec="HTML">incumbent settings object</a>, |snapshot|) as the
[=callback context=].
</div>

<p id="callback-function-to-js">
Expand Down Expand Up @@ -10380,6 +10400,28 @@ a [=promise type=].
</div>


<h4 id="PropagatesAsyncContext" extended-attribute lt="PropagatesAsyncContext">[PropagatesAsyncContext]</h4>

If the [{{PropagatesAsyncContext}}] [=extended attribute=] appears on a [=callback function=] type,
it creates a new IDL type such that when a JavaScript function object is converted to the IDL type,
the current [=Async Context Mapping=] is stored in the IDL type as part of its [=callback context=].

The [{{PropagatesAsyncContext}}]
extended attribute must
[=takes no arguments|take no arguments=].

A type annotated with the [{{PropagatesAsyncContext}}] extended attribute must only appear in
operation arguments. A type that is not a [=callback function=] must not be
[=extended attributes associated with|associated with=] the [{{PropagatesAsyncContext}}] extended
attribute.

See the rules for converting JavaScript values to callback function
types in [[#js-callback-function]], as well as the rules for invoking
callback functions in [[#js-invoking-callback-functions]],
for the specific requirements that the use of
[{{PropagatesAsyncContext}}] entails.


<h4 id="PutForwards" extended-attribute lt="PutForwards">[PutForwards]</h4>

If the [{{PutForwards}}]
Expand Down Expand Up @@ -14796,44 +14838,49 @@ described in the previous section).
<emu-val>undefined</emu-val> to the callback function's return type.
1. Let |realm| be |F|'s [=associated realm=].
1. Let |relevant settings| be |realm|'s [=realm/settings object=].
1. Let |stored settings| be |callable|'s [=callback context=].
1. [=Prepare to run script=] with |relevant settings|.
1. [=Prepare to run a callback=] with |stored settings|.
1. Let |jsArgs| be the result of [=Web IDL arguments list/converting=] |args| to a JavaScript
arguments list. If this throws an exception, set |completion| to the completion value
representing the thrown exception and jump to the step labeled
<a href="#invoke-return"><i>return</i></a>.
1. Let |callResult| be <a abstract-op>Completion</a>(<a abstract-op>Call</a>(|F|, |thisArg|, |jsArgs|)).
1. If |callResult| is an [=abrupt completion=], set |completion| to
|callResult| and jump to the step labeled <a href="#invoke-return"><i>return</i></a>.
1. Set |completion| to the result of [=converted to an IDL value|converting=]
|callResult|.\[[Value]] to an IDL value of the same type as |callable|'s
return type. If this throws an exception, set |completion| to the completion value
representing the thrown exception.
1. <i id="invoke-return">Return:</i> at this
point |completion| will be set to an IDL value or an [=abrupt completion=].
1. [=Clean up after running a callback=] with |stored settings|.
1. [=Clean up after running script=] with |relevant settings|.
1. If |completion| is an IDL value, return |completion|.
1. [=Assert=]: |completion| is an [=abrupt completion=].
1. If |exceptionBehavior| is "<code>rethrow</code>", throw |completion|.\[[Value]].
1. Otherwise, if |exceptionBehavior| is "<code>report</code>":
1. [=Assert=]: |callable|'s [=return type=] is {{undefined}} or {{any}}.
1. [=Report an exception=] |completion|.\[[Value]] for |realm|'s [=realm/global object=].
1. Return the unique {{undefined}} IDL value.
1. [=Assert=]: |callable|'s [=return type=] is a [=promise type=].
1. Let |rejectedPromise| be [=!=] <a abstract-op>Call</a>({{%Promise.reject%}},
{{%Promise%}}, «|completion|.\[[Value]]»).
1. Return the result of [=converted to an IDL value|converting=]
|rejectedPromise| to the callback function's return type.
1. Let (|stored settings|, |acSnapshot|) be |callable|'s [=callback context=].
1. If |acSnapshot| is not null, then return the result of [=Async Context Mapping/running=]
the following steps with [=Async Context Mapping=] |acSnapshot|. Otherwise, return the
result of running the following steps:
1. [=Prepare to run script=] with |relevant settings|.
1. [=Prepare to run a callback=] with |stored settings|.
1. Let |jsArgs| be the result of [=Web IDL arguments list/converting=] |args| to a
JavaScript arguments list. If this throws an exception, set |completion| to the
completion value representing the thrown exception and jump to the step labeled
<a href="#invoke-return"><i>return</i></a>.
1. Let |callResult| be <a abstract-op>Completion</a>(<a abstract-op>Call</a>(|F|,
|thisArg|, |jsArgs|)).
1. If |callResult| is an [=abrupt completion=], set |completion| to
|callResult| and jump to the step labeled <a href="#invoke-return"><i>return</i></a>.
1. Set |completion| to the result of [=converted to an IDL value|converting=]
|callResult|.\[[Value]] to an IDL value of the same type as |callable|'s
return type. If this throws an exception, set |completion| to the completion value
representing the thrown exception.
1. <i id="invoke-return">Return:</i> at this
point |completion| will be set to an IDL value or an [=abrupt completion=].
1. [=Clean up after running a callback=] with |stored settings|.
1. [=Clean up after running script=] with |relevant settings|.
1. If |completion| is an IDL value, return |completion|.
1. [=Assert=]: |completion| is an [=abrupt completion=].
1. If |exceptionBehavior| is "<code>rethrow</code>", throw |completion|.\[[Value]].
1. Otherwise, if |exceptionBehavior| is "<code>report</code>":
1. [=Assert=]: |callable|'s [=return type=] is {{undefined}} or {{any}}.
1. [=Report an exception=] |completion|.\[[Value]] for |realm|'s
[=realm/global object=].
1. Return the unique {{undefined}} IDL value.
1. [=Assert=]: |callable|'s [=return type=] is a [=promise type=].
1. Let |rejectedPromise| be [=!=] <a abstract-op>Call</a>({{%Promise.reject%}},
{{%Promise%}}, «|completion|.\[[Value]]»).
1. Return the result of [=converted to an IDL value|converting=]
|rejectedPromise| to the callback function's return type.
</div>

Some callback functions are instead used as [=constructors=]. Such callback functions must not have
a return type that is a [=promise type=].

<div algorithm>

To <dfn id="construct-a-callback-function" export>construct</dfn> a
To <dfn id="construct-a-callback-function" export>construct</dfn> aq
[=callback function type=] value |callable| with a [=Web IDL arguments list=] |args|,
perform the following steps.
These steps will either return an IDL value or throw an exception.
Expand All @@ -14844,26 +14891,29 @@ a return type that is a [=promise type=].
<l spec=ecmascript>{{TypeError}}</l> exception.
1. Let |realm| be |F|'s [=associated realm=].
1. Let |relevant settings| be |realm|'s [=realm/settings object=].
1. Let |stored settings| be |callable|'s [=callback context=].
1. [=Prepare to run script=] with |relevant settings|.
1. [=Prepare to run a callback=] with |stored settings|.
1. Let |jsArgs| be the result of [=Web IDL arguments list/converting=] |args| to a JavaScript
arguments list. If this throws an exception, set |completion| to the completion value
representing the thrown exception and jump to the step labeled
<a href="#construct-return"><i>return</i></a>.
1. Let |callResult| be <a abstract-op>Completion</a>(<a abstract-op>Construct</a>(|F|, |jsArgs|)).
1. If |callResult| is an [=abrupt completion=], set |completion| to
|callResult| and jump to the step labeled <a href="#construct-return"><i>return</i></a>.
1. Set |completion| to the result of [=converted to an IDL value|converting=]
|callResult|.\[[Value]] to an IDL value of the same type as |callable|'s
return type. If this throws an exception, set |completion| to the completion value
representing the thrown exception.
1. <i id="construct-return">Return:</i> at this
point |completion| will be set to an IDL value or an [=abrupt completion=].
1. [=Clean up after running a callback=] with |stored settings|.
1. [=Clean up after running script=] with |relevant settings|.
1. If |completion| is an [=abrupt completion=], throw |completion|.\[[Value]].
1. Return |completion|.
1. Let (|stored settings|, |acSnapshot|) be |callable|'s [=callback context=].
1. If |acSnapshot| is not null, then return the result of [=Async Context Mapping/running=]
the following steps with [=Async Context Mapping=] |acSnapshot|. Otherwise, return the
result of running the following steps:
1. [=Prepare to run script=] with |relevant settings|.
1. [=Prepare to run a callback=] with |stored settings|.
1. Let |jsArgs| be the result of [=Web IDL arguments list/converting=] |args| to a JavaScript
arguments list. If this throws an exception, set |completion| to the completion value
representing the thrown exception and jump to the step labeled
<a href="#construct-return"><i>return</i></a>.
1. Let |callResult| be <a abstract-op>Completion</a>(<a abstract-op>Construct</a>(|F|, |jsArgs|)).
1. If |callResult| is an [=abrupt completion=], set |completion| to
|callResult| and jump to the step labeled <a href="#construct-return"><i>return</i></a>.
1. Set |completion| to the result of [=converted to an IDL value|converting=]
|callResult|.\[[Value]] to an IDL value of the same type as |callable|'s
return type. If this throws an exception, set |completion| to the completion value
representing the thrown exception.
1. <i id="construct-return">Return:</i> at this
point |completion| will be set to an IDL value or an [=abrupt completion=].
1. [=Clean up after running a callback=] with |stored settings|.
1. [=Clean up after running script=] with |relevant settings|.
1. If |completion| is an [=abrupt completion=], throw |completion|.\[[Value]].
1. Return |completion|.
</div>


Expand Down