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<!doctype html>
<meta charset="utf8">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/styles/github.min.css">
<pre class="metadata">
title: AsyncContext
stage: 1
contributors: Chengzhong Wu, Justin Ridgewell
</pre>
<emu-biblio href="./shadowrealm-biblio.json"></emu-biblio>
<emu-clause id="sec-ecmascript-data-types-and-values">
<h1>ECMAScript Data Types and Values</h1>
<emu-clause id="sec-ecmascript-specification-types">
<h1>ECMAScript Specification Types</h1>
<ins class="block">
<emu-clause id="sec-asynccontext-mapping-record-specification-type">
<h1>The Async Context Mapping Record Specification Type</h1>
<p>The <dfn variants="Async Context Mapping Records">Async Context Mapping Record</dfn> type is used to represent an AsyncContext.Variable value mapping in the surrounding Agent's [[AsyncContextMapping]].</p>
<p>An Async Context Mapping Record's fields are defined by <emu-xref href="#table-asynccontext-mapping-record-fields"></emu-xref>.</p>
<emu-table id="table-asynccontext-mapping-record-fields" caption="Async Context Mapping Record Fields">
<table>
<tr>
<th>
Field Name
</th>
<th>
Value
</th>
<th>
Meaning
</th>
</tr>
<tr>
<td>
[[AsyncContextKey]]
</td>
<td>
an AsyncContext.Variable instance
</td>
<td>
The AsyncContext.Variable instance as the key in the mapping.
</td>
</tr>
<tr>
<td>
[[AsyncContextValue]]
</td>
<td>
an ECMAScript language value
</td>
<td>
The value of the AsyncContext.Variable instance in the mapping.
</td>
</tr>
</table>
</emu-table>
</emu-clause>
</ins>
</emu-clause>
</emu-clause>
<emu-clause id="sec-executable-code-and-execution-contexts">
<h1>Executable Code and Execution Contexts</h1>
<emu-clause id="sec-agents">
<h1>Agents</h1>
<p>This proposal adds a new field to the Agent Record as the following table:</p>
<emu-table id="table-agent-record" caption="Agent Record Fields">
<table>
<tr>
<th>Field Name</th>
<th>Value</th>
<th>Meaning</th>
</tr>
<tr>
<td>[[LittleEndian]]</td>
<td>a Boolean</td>
<td>The default value computed for the <em>isLittleEndian</em> parameter when it is needed by the algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-defined and should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.</td>
</tr>
<tr>
<td>[[CanBlock]]</td>
<td>a Boolean</td>
<td>Determines whether the agent can block or not.</td>
</tr>
<tr>
<td>[[Signifier]]</td>
<td>an agent signifier</td>
<td>Uniquely identifies the agent within its agent cluster.</td>
</tr>
<tr>
<td>[[IsLockFree1]]</td>
<td>a Boolean</td>
<td>*true* if atomic operations on one-<emu-not-ref>byte values</emu-not-ref> are lock-free, *false* otherwise.</td>
</tr>
<tr>
<td>[[IsLockFree2]]</td>
<td>a Boolean</td>
<td>*true* if atomic operations on two-<emu-not-ref>byte values</emu-not-ref> are lock-free, *false* otherwise.</td>
</tr>
<tr>
<td>[[IsLockFree8]]</td>
<td>a Boolean</td>
<td>*true* if atomic operations on eight-<emu-not-ref>byte values</emu-not-ref> are lock-free, *false* otherwise.</td>
</tr>
<tr>
<td>[[CandidateExecution]]</td>
<td>a candidate execution Record</td>
<td>See the memory model.</td>
</tr>
<tr>
<td>[[KeptAlive]]</td>
<td>a List of Objects</td>
<td>Initially a new empty List, representing the list of objects to be kept alive until the end of the current Job</td>
</tr>
<tr>
<td>
<ins>[[AsyncContextMapping]]</ins>
</td>
<td>
<ins>a List of Async Context Mapping Records</ins>
</td>
<td>
<ins>A map from the AsyncContext.Variable instances to the saved ECMAScript language value. Every Record in the List contains a unique [[AsyncContextKey]]. The map is initially empty.</ins>
</td>
</tr>
<tr>
<td>
<ins>[[ThrowAsyncContextMapping]]</ins>
</td>
<td>
<ins>~unused~ or a List of Async Context Mapping Records</ins>
</td>
<td>
<ins>The value of [[AsyncContextMapping]] at the time of the last time an exception was thrown, if any. Used by hosts to provide Async Context Mapping information when reporting uncaught exceptions.</ins>
</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-cleanup-finalization-registry" type="abstract operation">
<h1>
CleanupFinalizationRegistry (
_finalizationRegistry_: a FinalizationRegistry,
): either a normal completion containing ~unused~ or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. <del>Assert: _finalizationRegistry_ has [[Cells]] and [[CleanupCallback]] internal slots.</del>
1. <ins>Assert: _finalizationRegistry_ has [[Cells]], [[CleanupCallback]], and [[FinalizationRegistryAsyncContextSnapshot]] internal slots.</ins>
1. Let _callback_ be _finalizationRegistry_.[[CleanupCallback]].
1. While _finalizationRegistry_.[[Cells]] contains a Record _cell_ such that _cell_.[[WeakRefTarget]] is ~empty~, an implementation may perform the following steps:
1. Choose any such _cell_.
1. Remove _cell_ from _finalizationRegistry_.[[Cells]].
1. <del>Perform ? HostCallJobCallback(_callback_, *undefined*, « _cell_.[[HeldValue]] »).</del>
1. <ins>Let _previousContextMapping_ be AsyncContextSwap(_finalizationRegistry_.[[FinalizationRegistryAsyncContextSnapshot]]).</ins>
1. <ins>Let _result_ be Completion(HostCallJobCallback(_callback_, *undefined*, « _cell_.[[HeldValue]] »)).</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_, _result_).</ins>
1. <ins>Perform ? _result_.</ins>
1. Return ~unused~.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ordinary-and-exotic-objects-behaviours">
<h1>Ordinary and Exotic Objects Behaviours</h1>
<emu-clause id="sec-built-in-function-objects">
<h1>Built-in Function Objects</h1>
<emu-clause id="sec-createbuiltinfunction" type="abstract operation">
<h1>
CreateBuiltinFunction (
_behaviour_: an Abstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification,
optional _length_: a non-negative integer or +∞,
optional _name_: a property key or a Private Name,
optional _additionalInternalSlotsList_: a List of names of internal slots,
optional _realm_: a Realm Record,
optional _prototype_: an Object or *null*,
optional _prefix_: a String,
): a function object
</h1>
<dl class="header">
<dt>description</dt>
<dd>_additionalInternalSlotsList_ contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object.</dd>
</dl>
<emu-alg>
1. <ins>If _additionalInternalSlotsList_ is not present, set _additionalInternalSlotsList_ to a new empty List.</ins>
1. If _realm_ is not present, set _realm_ to the current Realm Record.
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. Set _func_.[[Prototype]] to _prototype_.
1. Set _func_.[[Extensible]] to *true*.
1. Set _func_.[[Realm]] to _realm_.
1. Set _func_.[[InitialName]] to *null*.
1. <ins>If _length_ is present, then</ins>
1. Perform SetFunctionLength(_func_, _length_).
1. <ins>If _name_ is present, then</ins>
1. If _prefix_ is not present, then
1. Perform SetFunctionName(_func_, _name_).
1. Else,
1. Perform SetFunctionName(_func_, _name_, _prefix_).
1. Return _func_.
</emu-alg>
<p>Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.</p>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ecmascript-language-statements-and-declarations">
<h1>ECMAScript Language: Statements and Declarations</h1>
<emu-clause id="sec-try-statement">
<h1>The `try` Statement</h1>
<emu-clause id="sec-try-statement-runtime-semantics-evaluation" type="sdo">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>TryStatement : `try` Block Catch</emu-grammar>
<emu-alg>
1. Let _B_ be Completion(Evaluation of |Block|).
1. <del>If _B_ is a throw completion, let _C_ be Completion(CatchClauseEvaluation of |Catch| with argument _B_.[[Value]]).</del>
1. <del>Else, let _C_ be _B_.</del>
1. <ins>If _B_ is a throw completion, then</ins>
1. <ins>Let _agentRecord_ be the surrounding agent's Agent Record.</ins>
1. <ins>Set _agentRecord_.[[ThrowAsyncContextMapping]] to ~empty~.</ins>
1. <ins>Let _C_ be Completion(CatchClauseEvaluation of |Catch| with argument _B_.[[Value]]).</ins>
1. <ins>Else,</ins>
1. <ins>Let _C_ be _B_.</ins>
1. Return ? UpdateEmpty(_C_, *undefined*).
</emu-alg>
<emu-grammar>TryStatement : `try` Block Finally</emu-grammar>
<emu-alg>
1. <ins>Let _agentRecord_ be the surrounding agent's AgentRecord.</ins>
1. Let _B_ be Completion(Evaluation of |Block|).
1. <ins>Let _throwAsyncContextMapping_ be _agentRecord_.[[ThrowAsyncContextMapping]].</ins>
1. Let _F_ be Completion(Evaluation of |Finally|).
1. <del>If _F_ is a normal completion, set _F_ to _B_.</del>
1. <ins>If _F_ is a normal completion, then</ins>
1. <ins>Set _agentRecord_.[[ThrowAsyncContextMapping]] to _throwAsyncContextMapping_.</ins>
1. <ins>Set _F_ to _B_.</ins>
1. Return ? UpdateEmpty(_F_, *undefined*).
</emu-alg>
<emu-grammar>TryStatement : `try` Block Catch Finally</emu-grammar>
<emu-alg>
1. <ins>Let _agentRecord_ be the surrounding agent's Agent Record.</ins>
1. Let _B_ be Completion(Evaluation of |Block|).
1. <del>If _B_ is a throw completion, let _C_ be Completion(CatchClauseEvaluation of |Catch| with argument _B_.[[Value]]).</del>
1. <del>Else, let _C_ be _B_.</del>
1. <ins>If _B_ is a throw completion, then</ins>
1. <ins>Set _agentRecord_.[[ThrowAsyncContextMapping]] to ~empty~.</ins>
1. <ins>Let _C_ be Completion(CatchClauseEvaluation of |Catch| with argument _B_.[[Value]]).</ins>
1. <ins>Else,</ins>
1. <ins>Let _C_ be _B_.</ins>
1. <ins>Let _throwAsyncContextMapping_ be _agentRecord_.[[ThrowAsyncContextMapping]].</ins>
1. Let _F_ be Completion(Evaluation of |Finally|).
1. <del>If _F_ is a normal completion, set _F_ to _C_.</del>
1. <ins>If _F_ is a normal completion, then</ins>
1. <ins>Set _agentRecord_.[[ThrowAsyncContextMapping]] to _throwAsyncContextMapping_.</ins>
1. <ins>Set _F_ to _C_.</ins>
1. Return ? UpdateEmpty(_F_, *undefined*).
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-ecmascript-language-functions-and-classes">
<h1>ECMAScript Language: Functions and Classes</h1>
<emu-clause id="sec-generator-function-definitions">
<h1>Generator Function Definitions</h1>
<emu-clause id="sec-runtime-semantics-evaluategeneratorbody" oldids="sec-generator-function-definitions-runtime-semantics-evaluatebody" type="sdo">
<h1>
Runtime Semantics: EvaluateGeneratorBody (
_functionObject_: a function object,
_argumentsList_: a List of ECMAScript language values,
): a throw completion or a return completion
</h1>
<dl class="header">
</dl>
<emu-grammar>GeneratorBody : FunctionBody</emu-grammar>
<emu-alg>
1. Perform ? FunctionDeclarationInstantiation(_functionObject_, _argumentsList_).
1. Let _G_ be ? OrdinaryCreateFromConstructor(_functionObject_, *"%GeneratorFunction.prototype.prototype%"*, « [[GeneratorState]], [[GeneratorContext]], <ins>[[GeneratorAsyncContextMapping]],</ins> [[GeneratorBrand]] »).
1. Set _G_.[[GeneratorBrand]] to ~empty~.
1. Perform GeneratorStart(_G_, |FunctionBody|).
1. Return Completion Record { [[Type]]: ~return~, [[Value]]: _G_, [[Target]]: ~empty~ }.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-async-generator-function-definitions">
<h1>Async Generator Function Definitions</h1>
<emu-clause id="sec-runtime-semantics-evaluateasyncgeneratorbody" oldids="sec-asyncgenerator-definitions-evaluatebody" type="sdo">
<h1>
Runtime Semantics: EvaluateAsyncGeneratorBody (
_functionObject_: a function object,
_argumentsList_: a List of ECMAScript language values,
): a throw completion or a return completion
</h1>
<dl class="header">
</dl>
<emu-grammar>
AsyncGeneratorBody : FunctionBody
</emu-grammar>
<emu-alg>
1. Perform ? FunctionDeclarationInstantiation(_functionObject_, _argumentsList_).
1. Let _generator_ be ? OrdinaryCreateFromConstructor(_functionObject_, *"%AsyncGeneratorFunction.prototype.prototype%"*, « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], <ins>[[AsyncGeneratorAsyncContextMapping]],</ins> [[GeneratorBrand]] »).
1. Set _generator_.[[GeneratorBrand]] to ~empty~.
1. Perform AsyncGeneratorStart(_generator_, |FunctionBody|).
1. Return Completion Record { [[Type]]: ~return~, [[Value]]: _generator_, [[Target]]: ~empty~ }.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-control-abstraction-objects">
<h1>Control Abstraction Objects</h1>
<emu-clause id="sec-promise-objects">
<h1>Promise Objects</h1>
<emu-clause id="sec-promise-abstract-operations">
<h1>Promise Abstract Operations</h1>
<emu-clause id="sec-promisereaction-records">
<h1>PromiseReaction Records</h1>
<p>A <dfn variants="PromiseReaction Records">PromiseReaction Record</dfn> is a Record value used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction Records are created by the PerformPromiseThen abstract operation, and are used by the Abstract Closure returned by NewPromiseReactionJob.</p>
<p>PromiseReaction Records have the fields listed in <emu-xref href="#table-promisereaction-record-fields"></emu-xref>.</p>
<emu-table id="table-promisereaction-record-fields" caption="PromiseReaction Record Fields" oldids="table-58">
<table>
<tr>
<th>
Field Name
</th>
<th>
Value
</th>
<th>
Meaning
</th>
</tr>
<tr>
<td>
[[Capability]]
</td>
<td>
a PromiseCapability Record or *undefined*
</td>
<td>
The capabilities of the promise for which this record provides a reaction handler.
</td>
</tr>
<tr>
<td>
[[Type]]
</td>
<td>
~fulfill~ or ~reject~
</td>
<td>
The [[Type]] is used when [[Handler]] is ~empty~ to allow for behaviour specific to the settlement type.
</td>
</tr>
<tr>
<td>
[[Handler]]
</td>
<td>
a JobCallback Record or ~empty~
</td>
<td>
The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is ~empty~, a function that depends on the value of [[Type]] will be used instead.
</td>
</tr>
<tr>
<td>
<ins>[[PromiseAsyncContextSnapshot]]</ins>
</td>
<td>
<ins>a List of Async Context Mapping Records</ins>
</td>
<td>
<ins>A map from the AsyncContext.Variable instances to the saved ECMAScript language value. Every Record in the List contains a unique [[AsyncContextKey]].</ins>
</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-host-promise-rejection-tracker" type="host-defined abstract operation">
<h1>
HostPromiseRejectionTracker (
_promise_: a Promise,
_operation_: *"reject"* or *"handle"*,
): ~unused~
</h1>
<dl class="header">
<dt>description</dt>
<dd>It allows host environments to track promise rejections.</dd>
</dl>
<p>An implementation of HostPromiseRejectionTracker must conform to the following requirements:</p>
<ul>
<li>It must complete normally (i.e. not return an abrupt completion).</li>
</ul>
<p>The default implementation of HostPromiseRejectionTracker is to return ~unused~.</p>
<ins class="block">
<emu-note>
<p>An implementation of HostPromiseRejectionTracker that delays notifying developers of unhandled rejections must conform to the following requirements</p>
<ul>
<li>It must perform AsyncContextSnapshot() at the call of HostPromiseRejectionTracker,</li>
<li>It must perform AsyncContextSwap before the event notification, with the result of the AsyncContextSnapshot operation,</li>
<li>It must perform AsyncContextSwap after the event notification, with the result of the earlier AsyncContextSwap operation.</li>
</ul>
</emu-note>
</ins>
</emu-clause>
</emu-clause>
<emu-clause id="sec-promise-jobs">
<h1>Promise Jobs</h1>
<emu-clause id="sec-newpromisereactionjob" type="abstract operation" oldids="sec-promisereactionjob">
<h1>
NewPromiseReactionJob (
_reaction_: a PromiseReaction Record,
_argument_: an ECMAScript language value,
): a Record with fields [[Job]] (a Job Abstract Closure) and [[Realm]] (a Realm Record or *null*)
</h1>
<dl class="header">
<dt>description</dt>
<dd>It returns a new Job Abstract Closure that applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler.</dd>
</dl>
<emu-alg>
1. Let _job_ be a new Job Abstract Closure with no parameters that captures _reaction_ and _argument_ and performs the following steps when called:
1. Let _promiseCapability_ be _reaction_.[[Capability]].
1. Let _type_ be _reaction_.[[Type]].
1. Let _handler_ be _reaction_.[[Handler]].
1. <ins>Let _previousContextMapping_ be AsyncContextSwap(_reaction_.[[PromiseAsyncContextSnapshot]]).</ins>
1. If _handler_ is ~empty~, then
1. If _type_ is ~fulfill~, then
1. let _handlerResult_ be NormalCompletion(_argument_).
1. Else,
1. Assert: _type_ is ~reject~.
1. Let _handlerResult_ be ThrowCompletion(_argument_).
1. Else,
1. let _handlerResult_ be Completion(HostCallJobCallback(_handler_, *undefined*, « _argument_ »)).
1. If _promiseCapability_ is *undefined*, then
1. Assert: _handlerResult_ is not an abrupt completion.
1. <ins>AsyncContextSwap(_previousContextMapping_).</ins>
1. Return ~empty~.
1. Assert: _promiseCapability_ is a PromiseCapability Record.
1. If _handlerResult_ is an abrupt completion, then
1. <del>Return ? Call(_promiseCapability_.[[Reject]], *undefined*, « _handlerResult_.[[Value]] »).</del>
1. <ins>Let _resolvingFunctionResult_ be Completion(Call(_promiseCapability_.[[Reject]], *undefined*, « _handlerResult_.[[Value]] »)).</ins>
1. Else,
1. <del>Return ? Call(_promiseCapability_.[[Resolve]], *undefined*, « _handlerResult_.[[Value]] »).</del>
1. <ins>Let _resolvingFunctionResult_ be Completion(Call(_promiseCapability_.[[Resolve]], *undefined*, « _handlerResult_.[[Value]] »)).</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_, _resolvingFunctionResult_).</ins>
1. <ins>Return _resolvingFunctionResult_.</ins>
1. Let _handlerRealm_ be *null*.
1. If _reaction_.[[Handler]] is not ~empty~, then
1. Let _getHandlerRealmResult_ be Completion(GetFunctionRealm(_reaction_.[[Handler]].[[Callback]])).
1. If _getHandlerRealmResult_ is a normal completion, set _handlerRealm_ to _getHandlerRealmResult_.[[Value]].
1. Else, set _handlerRealm_ to the current Realm Record.
1. NOTE: _handlerRealm_ is never *null* unless the handler is *undefined*. When the handler is a revoked Proxy and no ECMAScript code runs, _handlerRealm_ is used to create error objects.
1. Return the Record { [[Job]]: _job_, [[Realm]]: _handlerRealm_ }.
</emu-alg>
</emu-clause>
<emu-clause id="sec-newpromiseresolvethenablejob" type="abstract operation" oldids="sec-promiseresolvethenablejob">
<h1>
NewPromiseResolveThenableJob (
_promiseToResolve_: a Promise,
_thenable_: an Object,
_then_: a JobCallback Record,
): a Record with fields [[Job]] (a Job Abstract Closure) and [[Realm]] (a Realm Record)
</h1>
<dl class="header">
</dl>
<emu-alg>
1. <ins>Let _snapshot_ be AsyncContextSnapshot().</ins>
1. Let _job_ be a new Job Abstract Closure with no parameters that captures _promiseToResolve_, _thenable_, _then_, <ins>and _snapshot_</ins> and performs the following steps when called:
1. Let _resolvingFunctions_ be CreateResolvingFunctions(_promiseToResolve_).
1. <ins>Let _previousContextMapping_ be AsyncContextSwap(_snapshot_).</ins>
1. Let _thenCallResult_ be Completion(HostCallJobCallback(_then_, _thenable_, « _resolvingFunctions_.[[Resolve]], _resolvingFunctions_.[[Reject]] »)).
1. If _thenCallResult_ is an abrupt completion, then
1. <del>Return ? Call(_resolvingFunctions_.[[Reject]], *undefined*, « _thenCallResult_.[[Value]] »).</del>
1. <ins>Let _rejectResult_ be Completion(Call(_resolvingFunctions_.[[Reject]], *undefined*, « _thenCallResult_.[[Value]] »)).</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_, _rejectResult_).</ins>
1. <ins>Return _rejectResult_.</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_).</ins>
1. Return ? _thenCallResult_.
1. Let _getThenRealmResult_ be Completion(GetFunctionRealm(_then_.[[Callback]])).
1. If _getThenRealmResult_ is a normal completion, let _thenRealm_ be _getThenRealmResult_.[[Value]].
1. Else, let _thenRealm_ be the current Realm Record.
1. NOTE: _thenRealm_ is never *null*. When _then_.[[Callback]] is a revoked Proxy and no code runs, _thenRealm_ is used to create error objects.
1. Return the Record { [[Job]]: _job_, [[Realm]]: _thenRealm_ }.
</emu-alg>
<emu-note>
<p>This Job uses the supplied thenable and its `then` method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the `then` method occurs after evaluation of any surrounding code has completed.</p>
</emu-note>
</emu-clause>
</emu-clause>
<emu-clause id="sec-properties-of-the-promise-prototype-object">
<h1>Properties of the Promise Prototype Object</h1>
<p>The <dfn>Promise prototype object</dfn>:</p>
<ul>
<li>is <dfn>%Promise.prototype%</dfn>.</li>
<li>has a [[Prototype]] internal slot whose value is %Object.prototype%.</li>
<li>is an ordinary object.</li>
<li>does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.</li>
</ul>
<emu-clause id="sec-promise.prototype.then">
<h1>Promise.prototype.then ( _onFulfilled_, _onRejected_ )</h1>
<p>This method performs the following steps when called:</p>
<emu-alg>
1. Let _promise_ be the *this* value.
1. If IsPromise(_promise_) is *false*, throw a *TypeError* exception.
1. Let _C_ be ? SpeciesConstructor(_promise_, %Promise%).
1. Let _resultCapability_ be ? NewPromiseCapability(_C_).
1. Return PerformPromiseThen(_promise_, _onFulfilled_, _onRejected_, _resultCapability_).
</emu-alg>
<emu-clause id="sec-performpromisethen" type="abstract operation">
<h1>
PerformPromiseThen (
_promise_: a Promise,
_onFulfilled_: an ECMAScript language value,
_onRejected_: an ECMAScript language value,
optional _resultCapability_: a PromiseCapability Record,
): an ECMAScript language value
</h1>
<dl class="header">
<dt>description</dt>
<dd>It performs the “then” operation on _promise_ using _onFulfilled_ and _onRejected_ as its settlement actions. If _resultCapability_ is passed, the result is stored by updating _resultCapability_'s promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the result does not matter.</dd>
</dl>
<emu-alg>
1. Assert: IsPromise(_promise_) is *true*.
1. If _resultCapability_ is not present, then
1. Set _resultCapability_ to *undefined*.
1. If IsCallable(_onFulfilled_) is *false*, then
1. Let _onFulfilledJobCallback_ be ~empty~.
1. Else,
1. Let _onFulfilledJobCallback_ be HostMakeJobCallback(_onFulfilled_).
1. If IsCallable(_onRejected_) is *false*, then
1. Let _onRejectedJobCallback_ be ~empty~.
1. Else,
1. Let _onRejectedJobCallback_ be HostMakeJobCallback(_onRejected_).
1. <ins>Let _snapshot_ be AsyncContextSnapshot().</ins>
1. Let _fulfillReaction_ be the PromiseReaction Record { [[Capability]]: _resultCapability_, [[Type]]: ~fulfill~, [[Handler]]: _onFulfilledJobCallback_, <ins>[[PromiseAsyncContextSnapshot]]: _snapshot_</ins> }.
1. Let _rejectReaction_ be the PromiseReaction Record { [[Capability]]: _resultCapability_, [[Type]]: ~reject~, [[Handler]]: _onRejectedJobCallback_, <ins>[[PromiseAsyncContextSnapshot]]: _snapshot_</ins>}.
1. If _promise_.[[PromiseState]] is ~pending~, then
1. Append _fulfillReaction_ to _promise_.[[PromiseFulfillReactions]].
1. Append _rejectReaction_ to _promise_.[[PromiseRejectReactions]].
1. Else if _promise_.[[PromiseState]] is ~fulfilled~, then
1. Let _value_ be _promise_.[[PromiseResult]].
1. Let _fulfillJob_ be NewPromiseReactionJob(_fulfillReaction_, _value_).
1. Perform HostEnqueuePromiseJob(_fulfillJob_.[[Job]], _fulfillJob_.[[Realm]]).
1. Else,
1. Assert: The value of _promise_.[[PromiseState]] is ~rejected~.
1. Let _reason_ be _promise_.[[PromiseResult]].
1. If _promise_.[[PromiseIsHandled]] is *false*, perform HostPromiseRejectionTracker(_promise_, *"handle"*).
1. Let _rejectJob_ be NewPromiseReactionJob(_rejectReaction_, _reason_).
1. Perform HostEnqueuePromiseJob(_rejectJob_.[[Job]], _rejectJob_.[[Realm]]).
1. Set _promise_.[[PromiseIsHandled]] to *true*.
1. If _resultCapability_ is *undefined*, then
1. Return *undefined*.
1. Else,
1. Return _resultCapability_.[[Promise]].
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-generator-objects">
<h1>Generator Objects</h1>
<emu-clause id="sec-properties-of-generator-instances">
<h1>Properties of Generator Instances</h1>
<p>Generator instances are initially created with the internal slots described in <emu-xref href="#table-internal-slots-of-generator-instances"></emu-xref>.</p>
<emu-table id="table-internal-slots-of-generator-instances" caption="Internal Slots of Generator Instances" oldids="table-56">
<table>
<tr>
<th>
Internal Slot
</th>
<th>
Type
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
[[GeneratorState]]
</td>
<td>
*undefined*, ~suspendedStart~, ~suspendedYield~, ~executing~, or ~completed~
</td>
<td>
The current execution state of the generator.
</td>
</tr>
<tr>
<td>
[[GeneratorContext]]
</td>
<td>
an execution context
</td>
<td>
The execution context that is used when executing the code of this generator.
</td>
</tr>
<tr>
<td>
<ins>[[GeneratorAsyncContextMapping]]</ins>
</td>
<td>
<ins>a List of Async Context Mapping Records or ~empty~</ins>
</td>
<td>
<ins>The value of the agent's [[AsyncContextMapping]] to use the next time this generator is resumed.</ins>
</td>
</tr>
<tr>
<td>
[[GeneratorBrand]]
</td>
<td>
a String or ~empty~
</td>
<td>
A brand used to distinguish different kinds of generators. The [[GeneratorBrand]] of generators declared by ECMAScript source text is always ~empty~.
</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-generator-abstract-operations">
<h1>Generator Abstract Operations</h1>
<emu-clause id="sec-generatorstart" type="abstract operation">
<h1>
GeneratorStart (
_generator_: a Generator,
_generatorBody_: a |FunctionBody| Parse Node or an Abstract Closure with no parameters,
): ~unused~
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Assert: The value of _generator_.[[GeneratorState]] is *undefined*.
1. Let _genContext_ be the running execution context.
1. Set the Generator component of _genContext_ to _generator_.
1. Let _closure_ be a new Abstract Closure with no parameters that captures _generatorBody_ and performs the following steps when called:
1. Let _acGenContext_ be the running execution context.
1. Let _acGenerator_ be the Generator component of _acGenContext_.
1. If _generatorBody_ is a Parse Node, then
1. Let _result_ be Completion(Evaluation of _generatorBody_).
1. Else,
1. Assert: _generatorBody_ is an Abstract Closure with no parameters.
1. Let _result_ be _generatorBody_().
1. Assert: If we return here, the generator either threw an exception or performed either an implicit or explicit return.
1. Remove _acGenContext_ from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
1. Set _acGenerator_.[[GeneratorState]] to ~completed~.
1. NOTE: Once a generator enters the ~completed~ state it never leaves it and its associated execution context is never resumed. Any execution state associated with _acGenerator_ can be discarded at this point.
1. If _result_.[[Type]] is ~normal~, then
1. Let _resultValue_ be *undefined*.
1. Else if _result_.[[Type]] is ~return~, then
1. Let _resultValue_ be _result_.[[Value]].
1. Else,
1. Assert: _result_.[[Type]] is ~throw~.
1. Return ? _result_.
1. Return CreateIterResultObject(_resultValue_, *true*).
1. Set the code evaluation state of _genContext_ such that when evaluation is resumed for that execution context, _closure_ will be called with no arguments.
1. Set _generator_.[[GeneratorContext]] to _genContext_.
1. <ins>If _generatorBody_ is a |FunctionBody| Parse Node, then</ins>
1. <ins>Set _generator_.[[GeneratorAsyncContextMapping]] to AsyncContextSnapshot().</ins>
1. <ins>Else,</ins>
1. <ins>Set _generator_.[[GeneratorAsyncContextMapping]] to ~empty~.</ins>
1. Set _generator_.[[GeneratorState]] to ~suspended-start~.
1. Return ~unused~.
</emu-alg>
</emu-clause>
<emu-clause id="sec-generatorvalidate" type="abstract operation">
<h1>
GeneratorValidate (
_generator_: an ECMAScript language value,
_generatorBrand_: a String or ~empty~,
): either a normal completion containing one of ~suspendedStart~, ~suspendedYield~, or ~completed~, or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Perform ? RequireInternalSlot(_generator_, [[GeneratorState]]).
1. Perform ? RequireInternalSlot(_generator_, [[GeneratorBrand]]).
1. If _generator_.[[GeneratorBrand]] is not _generatorBrand_, throw a *TypeError* exception.
1. <del>Assert: _generator_ also has a [[GeneratorContext]] internal slot.</del>
1. <ins>Assert: _generator_ also has [[GeneratorContext]] and [[GeneratorAsyncContextMapping]] internal slots.</ins>
1. Let _state_ be _generator_.[[GeneratorState]].
1. If _state_ is ~executing~, throw a *TypeError* exception.
1. Return _state_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-generatorresume" type="abstract operation">
<h1>
GeneratorResume (
_generator_: an ECMAScript language value,
_value_: an ECMAScript language value or ~empty~,
_generatorBrand_: a String or ~empty~,
): either a normal completion containing an ECMAScript language value or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Let _state_ be ? GeneratorValidate(_generator_, _generatorBrand_).
1. If _state_ is ~completed~, return CreateIterResultObject(*undefined*, *true*).
1. Assert: _state_ is either ~suspended-start~ or ~suspended-yield~.
1. Let _genContext_ be _generator_.[[GeneratorContext]].
1. Let _methodContext_ be the running execution context.
1. Suspend _methodContext_.
1. Set _generator_.[[GeneratorState]] to ~executing~.
1. <ins>If _generator_.[[GeneratorAsyncContextMapping]] is ~empty~, then</ins>
1. <ins>Let _previousContextMapping_ be ~empty~.</ins>
1. <ins>Else,</ins>
1. <ins>Let _previousContextMapping_ be AsyncContextSwap(_generator_.[[GeneratorAsyncContextMapping]]).</ins>
1. Push _genContext_ onto the execution context stack; _genContext_ is now the running execution context.
1. <emu-meta effects="user-code">Resume the suspended evaluation of _genContext_</emu-meta> using NormalCompletion(_value_) as the result of the operation that suspended it. Let _result_ be the value returned by the resumed computation.
1. Assert: When we return here, _genContext_ has already been removed from the execution context stack and _methodContext_ is the currently running execution context.
1. <ins>If _previousContextMapping_ is not ~empty~, then</ins>
1. <ins>Assert: The result of AsyncContextSnapshot() is _generator_.[[GeneratorAsyncContextMapping]].</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_, _result_).</ins>
1. Return ? _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-generatorresumeabrupt" type="abstract operation">
<h1>
GeneratorResumeAbrupt (
_generator_: an ECMAScript language value,
_abruptCompletion_: a return completion or a throw completion,
_generatorBrand_: a String or ~empty~,
): either a normal completion containing an ECMAScript language value or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Let _state_ be ? GeneratorValidate(_generator_, _generatorBrand_).
1. If _state_ is ~suspended-start~, then
1. Set _generator_.[[GeneratorState]] to ~completed~.
1. NOTE: Once a generator enters the ~completed~ state it never leaves it and its associated execution context is never resumed. Any execution state associated with _generator_ can be discarded at this point.
1. Set _state_ to ~completed~.
1. If _state_ is ~completed~, then
1. If _abruptCompletion_.[[Type]] is ~return~, then
1. Return CreateIterResultObject(_abruptCompletion_.[[Value]], *true*).
1. Return ? _abruptCompletion_.
1. Assert: _state_ is ~suspended-yield~.
1. Let _genContext_ be _generator_.[[GeneratorContext]].
1. Let _methodContext_ be the running execution context.
1. Suspend _methodContext_.
1. Set _generator_.[[GeneratorState]] to ~executing~.
1. <ins>If _generator_.[[GeneratorAsyncContextMapping]] is ~empty~, then</ins>
1. <ins>Let _previousContextMapping_ be ~empty~.</ins>
1. <ins>Else,</ins>
1. <ins>Let _previousContextMapping_ be AsyncContextSwap(_generator_.[[GeneratorAsyncContextMapping]]).</ins>
1. Push _genContext_ onto the execution context stack; _genContext_ is now the running execution context.
1. <emu-meta effects="user-code">Resume the suspended evaluation of _genContext_</emu-meta> using _abruptCompletion_ as the result of the operation that suspended it. Let _result_ be the Completion Record returned by the resumed computation.
1. Assert: When we return here, _genContext_ has already been removed from the execution context stack and _methodContext_ is the currently running execution context.
1. <ins>If _previousContextMapping_ is not ~empty~, then</ins>
1. <ins>Assert: The result of AsyncContextSnapshot() is _generator_.[[GeneratorAsyncContextMapping]].</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_, _result_).</ins>
1. Return ? _result_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-createiteratorfromclosure" type="abstract operation">
<h1>
CreateIteratorFromClosure (
_closure_: an Abstract Closure with no parameters,
_generatorBrand_: a String or ~empty~,
_generatorPrototype_: an Object,
): a Generator
</h1>
<dl class="header">
</dl>
<emu-alg>
1. NOTE: _closure_ can contain uses of the Yield operation to yield an IteratorResult object.
1. Let _internalSlotsList_ be « [[GeneratorState]], [[GeneratorContext]], <ins>[[GeneratorAsyncContextMapping]],</ins> [[GeneratorBrand]] ».
1. Let _generator_ be OrdinaryObjectCreate(_generatorPrototype_, _internalSlotsList_).
1. Set _generator_.[[GeneratorBrand]] to _generatorBrand_.
1. Set _generator_.[[GeneratorState]] to *undefined*.
1. Let _callerContext_ be the running execution context.
1. Let _calleeContext_ be a new execution context.
1. Set the Function of _calleeContext_ to *null*.
1. Set the Realm of _calleeContext_ to the current Realm Record.
1. Set the ScriptOrModule of _calleeContext_ to _callerContext_'s ScriptOrModule.
1. If _callerContext_ is not already suspended, suspend _callerContext_.
1. Push _calleeContext_ onto the execution context stack; _calleeContext_ is now the running execution context.
1. Perform GeneratorStart(_generator_, _closure_).
1. Remove _calleeContext_ from the execution context stack and restore _callerContext_ as the running execution context.
1. Return _generator_.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-asyncgenerator-objects">
<h1>AsyncGenerator Objects</h1>
<emu-clause id="sec-properties-of-asyncgenerator-intances">
<h1>Properties of AsyncGenerator Instances</h1>
<p>AsyncGenerator instances are initially created with the internal slots described below:</p>
<emu-table id="table-internal-slots-of-asyncgenerator-instances" caption="Internal Slots of AsyncGenerator Instances">
<table>
<tr>
<th>Internal Slot</th>
<th>Type</th>
<th>Description</th>
</tr>
<tr>
<td>[[AsyncGeneratorState]]</td>
<td>*undefined*, ~suspendedStart~, ~suspendedYield~, ~executing~, ~awaiting-return~, or ~completed~</td>
<td>The current execution state of the async generator.</td>
</tr>
<tr>
<td>[[AsyncGeneratorContext]]</td>
<td>an execution context</td>
<td>The execution context that is used when executing the code of this async generator.</td>
</tr>
<tr>
<td>[[AsyncGeneratorQueue]]</td>
<td>a List of AsyncGeneratorRequest Records</td>
<td>Records which represent requests to resume the async generator. Except during state transitions, it is non-empty if and only if [[AsyncGeneratorState]] is either ~executing~ or ~awaiting-return~.</td>
</tr>
<tr>
<td><ins>[[AsyncGeneratorAsyncContextMapping]]</ins></td>
<td><ins>a List of Async Context Mapping Records or ~empty~</ins></td>
<td><ins>The value of the agent's [[AsyncContextMapping]] to use the next time this generator is resumed.</ins></td>
</tr>
<tr>
<td>[[GeneratorBrand]]</td>
<td>a String or ~empty~</td>
<td>A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is always ~empty~.</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-asyncgenerator-abstract-operations">
<h1>AsyncGenerator Abstract Operations</h1>
<emu-clause id="sec-asyncgeneratorstart" type="abstract operation">
<h1>
AsyncGeneratorStart (
_generator_: an AsyncGenerator,
_generatorBody_: a |FunctionBody| Parse Node or an Abstract Closure with no parameters,
): ~unused~
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Assert: _generator_.[[AsyncGeneratorState]] is *undefined*.
1. Let _genContext_ be the running execution context.
1. Set the Generator component of _genContext_ to _generator_.
1. Let _closure_ be a new Abstract Closure with no parameters that captures _generatorBody_ and performs the following steps when called:
1. Let _acGenContext_ be the running execution context.
1. Let _acGenerator_ be the Generator component of _acGenContext_.
1. If _generatorBody_ is a Parse Node, then
1. Let _result_ be Completion(Evaluation of _generatorBody_).
1. Else,
1. Assert: _generatorBody_ is an Abstract Closure with no parameters.
1. Let _result_ be Completion(_generatorBody_()).
1. Assert: If we return here, the async generator either threw an exception or performed either an implicit or explicit return.
1. Remove _acGenContext_ from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
1. Set _acGenerator_.[[AsyncGeneratorState]] to ~completed~.
1. If _result_.[[Type]] is ~normal~, set _result_ to NormalCompletion(*undefined*).
1. If _result_.[[Type]] is ~return~, set _result_ to NormalCompletion(_result_.[[Value]]).
1. Perform AsyncGeneratorCompleteStep(_acGenerator_, _result_, *true*).
1. Perform AsyncGeneratorDrainQueue(_acGenerator_).
1. Return *undefined*.
1. Set the code evaluation state of _genContext_ such that when evaluation is resumed for that execution context, _closure_ will be called with no arguments.
1. Set _generator_.[[AsyncGeneratorContext]] to _genContext_.
1. Set _generator_.[[AsyncGeneratorState]] to ~suspended-start~.
1. Set _generator_.[[AsyncGeneratorQueue]] to a new empty List.
1. <ins>If _generatorBody_ is a |FunctionBody| Parse Node, then</ins>
1. <ins>Set _generator_.[[AsyncGeneratorAsyncContextMapping]] to AsyncContextSnapshot().</ins>
1. <ins>Else,</ins>
1. <ins>Set _generator_.[[AsyncGeneratorAsyncContextMapping]] to ~empty~.</ins>
1. Return ~unused~.
</emu-alg>
</emu-clause>
<emu-clause id="sec-asyncgeneratorvalidate" type="abstract operation">
<h1>
AsyncGeneratorValidate (
_generator_: an ECMAScript language value,
_generatorBrand_: a String or ~empty~,
): either a normal completion containing ~unused~ or a throw completion
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Perform ? RequireInternalSlot(_generator_, [[AsyncGeneratorContext]]).
1. Perform ? RequireInternalSlot(_generator_, [[AsyncGeneratorState]]).
1. Perform ? RequireInternalSlot(_generator_, [[AsyncGeneratorQueue]]).
1. <ins>Perform ? RequireInternalSlot(_generator_, [[AsyncGeneratorAsyncContextMapping]]).</ins>
1. If _generator_.[[GeneratorBrand]] is not _generatorBrand_, throw a *TypeError* exception.
1. Return ~unused~.
</emu-alg>
</emu-clause>
<emu-clause id="sec-asyncgeneratorresume" type="abstract operation">
<h1>
AsyncGeneratorResume (
_generator_: an AsyncGenerator,
_completion_: a Completion Record,
): ~unused~
</h1>
<dl class="header">
</dl>
<emu-alg>
1. Assert: _generator_.[[AsyncGeneratorState]] is either ~suspended-start~ or ~suspended-yield~.
1. Let _genContext_ be _generator_.[[AsyncGeneratorContext]].
1. Let _callerContext_ be the running execution context.
1. Suspend _callerContext_.
1. Set _generator_.[[AsyncGeneratorState]] to ~executing~.
1. <ins>If _generator_.[[AsyncGeneratorAsyncContextMapping]] is ~empty~, then</ins>
1. <ins>Let _previousContextMapping_ be ~empty~.</ins>
1. <ins>Else,</ins>
1. <ins>Let _previousContextMapping_ be AsyncContextSwap(_generator_.[[AsyncGeneratorAsyncContextMapping]]).</ins>
1. Push _genContext_ onto the execution context stack; _genContext_ is now the running execution context.
1. <emu-meta effects="user-code">Resume the suspended evaluation of _genContext_</emu-meta> using _completion_ as the result of the operation that suspended it. Let _result_ be the Completion Record returned by the resumed computation.
1. Assert: _result_ is never an abrupt completion.
1. Assert: When we return here, _genContext_ has already been removed from the execution context stack and _callerContext_ is the currently running execution context.
1. <ins>If _previousContextMapping_ is not ~empty~, then</ins>
1. <ins>Assert: The result of AsyncContextSnapshot() is _generator_.[[AsyncGeneratorAsyncContextMapping]].</ins>
1. <ins>AsyncContextSwap(_previousContextMapping_).</ins>
1. Return ~unused~.
</emu-alg>
</emu-clause>
<emu-clause id="sec-createasynciteratorfromclosure" type="abstract operation">
<h1>
CreateAsyncIteratorFromClosure (
_closure_: an Abstract Closure with no parameters,
_generatorBrand_: a String or ~empty~,
_generatorPrototype_: an Object,
): an AsyncGenerator
</h1>
<dl class="header">
</dl>
<emu-alg>
1. NOTE: _closure_ can contain uses of the Await operation and uses of the Yield operation to yield an IteratorResult object.
1. Let _internalSlotsList_ be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], <ins>[[AsyncGeneratorAsyncContextMapping]],</ins> [[GeneratorBrand]] ».
1. Let _generator_ be OrdinaryObjectCreate(_generatorPrototype_, _internalSlotsList_).
1. Set _generator_.[[GeneratorBrand]] to _generatorBrand_.
1. Set _generator_.[[AsyncGeneratorState]] to *undefined*.
1. Let _callerContext_ be the running execution context.
1. Let _calleeContext_ be a new execution context.
1. Set the Function of _calleeContext_ to *null*.
1. Set the Realm of _calleeContext_ to the current Realm Record.
1. Set the ScriptOrModule of _calleeContext_ to _callerContext_'s ScriptOrModule.
1. If _callerContext_ is not already suspended, suspend _callerContext_.
1. Push _calleeContext_ onto the execution context stack; _calleeContext_ is now the running execution context.
1. Perform AsyncGeneratorStart(_generator_, _closure_).
1. Remove _calleeContext_ from the execution context stack and restore _callerContext_ as the running execution context.
1. Return _generator_.
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>
<ins class="block">
<emu-clause id="sec-asynccontext-object">
<h1>The AsyncContext Object</h1>
<p>The AsyncContext object:</p>
<ul>
<li>is the intrinsic object <dfn>%AsyncContext%</dfn>.</li>
<li>is the initial value of the *"AsyncContext"* property of the global object.</li>
<li>is an ordinary object.</li>
<li>has a [[Prototype]] internal slot whose value is %Object.prototype%.</li>
<li>is not a function object.</li>
<li>does not have a [[Construct]] internal method; it cannot be used as a constructor with the `new` operator.</li>