27.2 Promise Objects

A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.

Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:

  • A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
  • A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
  • A promise is pending if it is neither fulfilled nor rejected.

A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.

A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.

27.2.1 Promise Abstract Operations

27.2.1.1 PromiseCapability Records

A PromiseCapability Record is a Record value used to encapsulate a promise object along with the functions that are capable of resolving or rejecting that promise object. PromiseCapability Records are produced by the NewPromiseCapability abstract operation.

PromiseCapability Records have the fields listed in Table 70.

Table 70: PromiseCapability Record Fields
Field Name Value Meaning
[[Promise]] An object An object that is usable as a promise.
[[Resolve]] A function object The function that is used to resolve the given promise object.
[[Reject]] A function object The function that is used to reject the given promise object.

27.2.1.1.1 IfAbruptRejectPromise ( value, capability )

IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use a PromiseCapability Record. An algorithm step of the form:

  1. IfAbruptRejectPromise(value, capability).

means the same thing as:

  1. If value is an abrupt completion, then
    1. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]] »).
    2. Return capability.[[Promise]].
  2. Else if value is a Completion Record, set value to value.[[Value]].

27.2.1.2 PromiseReaction Records

The PromiseReaction 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.

PromiseReaction records have the fields listed in Table 71.

Table 71: PromiseReaction Record Fields
Field Name Value Meaning
[[Capability]] A PromiseCapability Record, or undefined The capabilities of the promise for which this record provides a reaction handler.
[[Type]] Fulfill | Reject The [[Type]] is used when [[Handler]] is empty to allow for behaviour specific to the settlement type.
[[Handler]] A JobCallback Record | empty. 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.

27.2.1.3 CreateResolvingFunctions ( promise )

The abstract operation CreateResolvingFunctions takes argument promise. It performs the following steps when called:

  1. Let alreadyResolved be the Record { [[Value]]: false }.
  2. Let stepsResolve be the algorithm steps defined in Promise Resolve Functions.
  3. Let lengthResolve be the number of non-optional parameters of the function definition in Promise Resolve Functions.
  4. Let resolve be ! CreateBuiltinFunction(stepsResolve, lengthResolve, "", « [[Promise]], [[AlreadyResolved]] »).
  5. Set resolve.[[Promise]] to promise.
  6. Set resolve.[[AlreadyResolved]] to alreadyResolved.
  7. Let stepsReject be the algorithm steps defined in Promise Reject Functions.
  8. Let lengthReject be the number of non-optional parameters of the function definition in Promise Reject Functions.
  9. Let reject be ! CreateBuiltinFunction(stepsReject, lengthReject, "", « [[Promise]], [[AlreadyResolved]] »).
  10. Set reject.[[Promise]] to promise.
  11. Set reject.[[AlreadyResolved]] to alreadyResolved.
  12. Return the Record { [[Resolve]]: resolve, [[Reject]]: reject }.

27.2.1.3.1 Promise Reject Functions

A promise reject function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise reject function is called with argument reason, the following steps are taken:

  1. Let F be the active function object.
  2. Assert: F has a [[Promise]] internal slot whose value is an Object.
  3. Let promise be F.[[Promise]].
  4. Let alreadyResolved be F.[[AlreadyResolved]].
  5. If alreadyResolved.[[Value]] is true, return undefined.
  6. Set alreadyResolved.[[Value]] to true.
  7. Return RejectPromise(promise, reason).

The "length" property of a promise reject function is 1𝔽.

27.2.1.3.2 Promise Resolve Functions

A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise resolve function is called with argument resolution, the following steps are taken:

  1. Let F be the active function object.
  2. Assert: F has a [[Promise]] internal slot whose value is an Object.
  3. Let promise be F.[[Promise]].
  4. Let alreadyResolved be F.[[AlreadyResolved]].
  5. If alreadyResolved.[[Value]] is true, return undefined.
  6. Set alreadyResolved.[[Value]] to true.
  7. If SameValue(resolution, promise) is true, then
    1. Let selfResolutionError be a newly created TypeError object.
    2. Return RejectPromise(promise, selfResolutionError).
  8. If Type(resolution) is not Object, then
    1. Return FulfillPromise(promise, resolution).
  9. Let then be Get(resolution, "then").
  10. If then is an abrupt completion, then
    1. Return RejectPromise(promise, then.[[Value]]).
  11. Let thenAction be then.[[Value]].
  12. If IsCallable(thenAction) is false, then
    1. Return FulfillPromise(promise, resolution).
  13. Let thenJobCallback be HostMakeJobCallback(thenAction).
  14. Let job be NewPromiseResolveThenableJob(promise, resolution, thenJobCallback).
  15. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  16. Return undefined.

The "length" property of a promise resolve function is 1𝔽.

27.2.1.4 FulfillPromise ( promise, value )

The abstract operation FulfillPromise takes arguments promise and value. It performs the following steps when called:

  1. Assert: The value of promise.[[PromiseState]] is pending.
  2. Let reactions be promise.[[PromiseFulfillReactions]].
  3. Set promise.[[PromiseResult]] to value.
  4. Set promise.[[PromiseFulfillReactions]] to undefined.
  5. Set promise.[[PromiseRejectReactions]] to undefined.
  6. Set promise.[[PromiseState]] to fulfilled.
  7. Return TriggerPromiseReactions(reactions, value).

27.2.1.5 NewPromiseCapability ( C )

The abstract operation NewPromiseCapability takes argument C. It attempts to use C as a constructor in the fashion of the built-in Promise constructor to create a Promise object and extract its resolve and reject functions. The Promise object plus the resolve and reject functions are used to initialize a new PromiseCapability Record. It performs the following steps when called:

  1. If IsConstructor(C) is false, throw a TypeError exception.
  2. NOTE: C is assumed to be a constructor function that supports the parameter conventions of the Promise constructor (see 27.2.3.1).
  3. Let promiseCapability be the PromiseCapability Record { [[Promise]]: undefined, [[Resolve]]: undefined, [[Reject]]: undefined }.
  4. Let steps be the algorithm steps defined in GetCapabilitiesExecutor Functions.
  5. Let length be the number of non-optional parameters of the function definition in GetCapabilitiesExecutor Functions.
  6. Let executor be ! CreateBuiltinFunction(steps, length, "", « [[Capability]] »).
  7. Set executor.[[Capability]] to promiseCapability.
  8. Let promise be ? Construct(C, « executor »).
  9. If IsCallable(promiseCapability.[[Resolve]]) is false, throw a TypeError exception.
  10. If IsCallable(promiseCapability.[[Reject]]) is false, throw a TypeError exception.
  11. Set promiseCapability.[[Promise]] to promise.
  12. Return promiseCapability.
Note

This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise constructor to any subclass.

27.2.1.5.1 GetCapabilitiesExecutor Functions

A GetCapabilitiesExecutor function is an anonymous built-in function that has a [[Capability]] internal slot.

When a GetCapabilitiesExecutor function is called with arguments resolve and reject, the following steps are taken:

  1. Let F be the active function object.
  2. Assert: F has a [[Capability]] internal slot whose value is a PromiseCapability Record.
  3. Let promiseCapability be F.[[Capability]].
  4. If promiseCapability.[[Resolve]] is not undefined, throw a TypeError exception.
  5. If promiseCapability.[[Reject]] is not undefined, throw a TypeError exception.
  6. Set promiseCapability.[[Resolve]] to resolve.
  7. Set promiseCapability.[[Reject]] to reject.
  8. Return undefined.

The "length" property of a GetCapabilitiesExecutor function is 2𝔽.

27.2.1.6 IsPromise ( x )

The abstract operation IsPromise takes argument x. It checks for the promise brand on an object. It performs the following steps when called:

  1. If Type(x) is not Object, return false.
  2. If x does not have a [[PromiseState]] internal slot, return false.
  3. Return true.

27.2.1.7 RejectPromise ( promise, reason )

The abstract operation RejectPromise takes arguments promise and reason. It performs the following steps when called:

  1. Assert: The value of promise.[[PromiseState]] is pending.
  2. Let reactions be promise.[[PromiseRejectReactions]].
  3. Set promise.[[PromiseResult]] to reason.
  4. Set promise.[[PromiseFulfillReactions]] to undefined.
  5. Set promise.[[PromiseRejectReactions]] to undefined.
  6. Set promise.[[PromiseState]] to rejected.
  7. If promise.[[PromiseIsHandled]] is false, perform HostPromiseRejectionTracker(promise, "reject").
  8. Return TriggerPromiseReactions(reactions, reason).

27.2.1.8 TriggerPromiseReactions ( reactions, argument )

The abstract operation TriggerPromiseReactions takes arguments reactions (a collection of PromiseReaction Records) and argument. It enqueues a new Job for each record in reactions. Each such Job processes the [[Type]] and [[Handler]] of the PromiseReaction Record, and if the [[Handler]] is not empty, calls it passing the given argument. If the [[Handler]] is empty, the behaviour is determined by the [[Type]]. It performs the following steps when called:

  1. For each element reaction of reactions, in original insertion order, do
    1. Let job be NewPromiseReactionJob(reaction, argument).
    2. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  2. Return undefined.

27.2.1.9 HostPromiseRejectionTracker ( promise, operation )

The host-defined abstract operation HostPromiseRejectionTracker takes arguments promise (a Promise) and operation ("reject" or "handle"). It allows host environments to track promise rejections.

An implementation of HostPromiseRejectionTracker must complete normally in all cases. The default implementation of HostPromiseRejectionTracker is to unconditionally return an empty normal completion.

Note 1

HostPromiseRejectionTracker is called in two scenarios:

  • When a promise is rejected without any handlers, it is called with its operation argument set to "reject".
  • When a handler is added to a rejected promise for the first time, it is called with its operation argument set to "handle".

A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.

Note 2

If operation is "handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is "reject", since it is expected that rejections will be rare and not on hot code paths.

27.2.2 Promise Jobs

27.2.2.1 NewPromiseReactionJob ( reaction, argument )

The abstract operation NewPromiseReactionJob takes arguments reaction and argument. 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. It performs the following steps when called:

  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. Assert: reaction is a PromiseReaction Record.
    2. Let promiseCapability be reaction.[[Capability]].
    3. Let type be reaction.[[Type]].
    4. Let handler be reaction.[[Handler]].
    5. If handler is empty, then
      1. If type is Fulfill, let handlerResult be NormalCompletion(argument).
      2. Else,
        1. Assert: type is Reject.
        2. Let handlerResult be ThrowCompletion(argument).
    6. Else, let handlerResult be HostCallJobCallback(handler, undefined, « argument »).
    7. If promiseCapability is undefined, then
      1. Assert: handlerResult is not an abrupt completion.
      2. Return NormalCompletion(empty).
    8. Assert: promiseCapability is a PromiseCapability Record.
    9. If handlerResult is an abrupt completion, then
      1. Let status be Call(promiseCapability.[[Reject]], undefined, « handlerResult.[[Value]] »).
    10. Else,
      1. Let status be Call(promiseCapability.[[Resolve]], undefined, « handlerResult.[[Value]] »).
    11. Return Completion(status).
  2. Let handlerRealm be null.
  3. If reaction.[[Handler]] is not empty, then
    1. Let getHandlerRealmResult be GetFunctionRealm(reaction.[[Handler]].[[Callback]]).
    2. If getHandlerRealmResult is a normal completion, set handlerRealm to getHandlerRealmResult.[[Value]].
    3. Else, set handlerRealm to the current Realm Record.
    4. 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.
  4. Return the Record { [[Job]]: job, [[Realm]]: handlerRealm }.

27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve, thenable, then )

The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve, thenable, and then. It performs the following steps when called:

  1. Let job be a new Job Abstract Closure with no parameters that captures promiseToResolve, thenable, and then and performs the following steps when called:
    1. Let resolvingFunctions be CreateResolvingFunctions(promiseToResolve).
    2. Let thenCallResult be HostCallJobCallback(then, thenable, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »).
    3. If thenCallResult is an abrupt completion, then
      1. Let status be Call(resolvingFunctions.[[Reject]], undefined, « thenCallResult.[[Value]] »).
      2. Return Completion(status).
    4. Return Completion(thenCallResult).
  2. Let getThenRealmResult be GetFunctionRealm(then.[[Callback]]).
  3. If getThenRealmResult is a normal completion, let thenRealm be getThenRealmResult.[[Value]].
  4. Else, let thenRealm be the current Realm Record.
  5. NOTE: thenRealm is never null. When then.[[Callback]] is a revoked Proxy and no code runs, thenRealm is used to create error objects.
  6. Return the Record { [[Job]]: job, [[Realm]]: thenRealm }.
Note

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.

27.2.3 The Promise Constructor

The Promise constructor:

  • is %Promise%.
  • is the initial value of the "Promise" property of the global object.
  • creates and initializes a new Promise object when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promise constructor to create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.

27.2.3.1 Promise ( executor )

When the Promise function is called with argument executor, the following steps are taken:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If IsCallable(executor) is false, throw a TypeError exception.
  3. Let promise be ? OrdinaryCreateFromConstructor(NewTarget, "%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
  4. Set promise.[[PromiseState]] to pending.
  5. Set promise.[[PromiseFulfillReactions]] to a new empty List.
  6. Set promise.[[PromiseRejectReactions]] to a new empty List.
  7. Set promise.[[PromiseIsHandled]] to false.
  8. Let resolvingFunctions be CreateResolvingFunctions(promise).
  9. Let completion be Call(executor, undefined, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »).
  10. If completion is an abrupt completion, then
    1. Perform ? Call(resolvingFunctions.[[Reject]], undefined, « completion.[[Value]] »).
  11. Return promise.
Note

The executor argument must be a function object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise object. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.

The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.

The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.

The resolve and reject functions passed to an executor function by the Promise constructor have the capability to actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes in customized values for resolve and reject.

27.2.4 Properties of the Promise Constructor

The Promise constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

27.2.4.1 Promise.all ( iterable )

The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be GetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be GetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be PerformPromiseAll(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return Completion(result).
Note

The all function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.1.1 GetPromiseResolve ( promiseConstructor )

The abstract operation GetPromiseResolve takes argument promiseConstructor. It performs the following steps when called:

  1. Assert: IsConstructor(promiseConstructor) is true.
  2. Let promiseResolve be ? Get(promiseConstructor, "resolve").
  3. If IsCallable(promiseResolve) is false, throw a TypeError exception.
  4. Return promiseResolve.

27.2.4.1.2 PerformPromiseAll ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAll takes arguments iteratorRecord, constructor, resultCapability (a PromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert: IsConstructor(constructor) is true.
  2. Assert: IsCallable(promiseResolve) is true.
  3. Let values be a new empty List.
  4. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  5. Let index be 0.
  6. Repeat,
    1. Let next be IteratorStep(iteratorRecord).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] is 0, then
        1. Let valuesArray be ! CreateArrayFromList(values).
        2. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue be IteratorValue(next).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Append undefined to values.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let steps be the algorithm steps defined in Promise.all Resolve Element Functions.
    11. Let length be the number of non-optional parameters of the function definition in Promise.all Resolve Element Functions.
    12. Let onFulfilled be ! CreateBuiltinFunction(steps, length, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    13. Set onFulfilled.[[AlreadyCalled]] to false.
    14. Set onFulfilled.[[Index]] to index.
    15. Set onFulfilled.[[Values]] to values.
    16. Set onFulfilled.[[Capability]] to resultCapability.
    17. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
    18. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
    19. Perform ? Invoke(nextPromise, "then", « onFulfilled, resultCapability.[[Reject]] »).
    20. Set index to index + 1.

27.2.4.1.3 Promise.all Resolve Element Functions

A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.all resolve element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. If F.[[AlreadyCalled]] is true, return undefined.
  3. Set F.[[AlreadyCalled]] to true.
  4. Let index be F.[[Index]].
  5. Let values be F.[[Values]].
  6. Let promiseCapability be F.[[Capability]].
  7. Let remainingElementsCount be F.[[RemainingElements]].
  8. Set values[index] to x.
  9. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  10. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
  11. Return undefined.

The "length" property of a Promise.all resolve element function is 1𝔽.

27.2.4.2 Promise.allSettled ( iterable )

The allSettled function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be GetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be GetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be PerformPromiseAllSettled(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return Completion(result).
Note

The allSettled function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.2.1 PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord, constructor, resultCapability (a PromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert: ! IsConstructor(constructor) is true.
  2. Assert: IsCallable(promiseResolve) is true.
  3. Let values be a new empty List.
  4. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  5. Let index be 0.
  6. Repeat,
    1. Let next be IteratorStep(iteratorRecord).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] is 0, then
        1. Let valuesArray be ! CreateArrayFromList(values).
        2. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue be IteratorValue(next).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Append undefined to values.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let stepsFulfilled be the algorithm steps defined in Promise.allSettled Resolve Element Functions.
    11. Let lengthFulfilled be the number of non-optional parameters of the function definition in Promise.allSettled Resolve Element Functions.
    12. Let onFulfilled be ! CreateBuiltinFunction(stepsFulfilled, lengthFulfilled, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    13. Let alreadyCalled be the Record { [[Value]]: false }.
    14. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
    15. Set onFulfilled.[[Index]] to index.
    16. Set onFulfilled.[[Values]] to values.
    17. Set onFulfilled.[[Capability]] to resultCapability.
    18. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
    19. Let stepsRejected be the algorithm steps defined in Promise.allSettled Reject Element Functions.
    20. Let lengthRejected be the number of non-optional parameters of the function definition in Promise.allSettled Reject Element Functions.
    21. Let onRejected be ! CreateBuiltinFunction(stepsRejected, lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    22. Set onRejected.[[AlreadyCalled]] to alreadyCalled.
    23. Set onRejected.[[Index]] to index.
    24. Set onRejected.[[Values]] to values.
    25. Set onRejected.[[Capability]] to resultCapability.
    26. Set onRejected.[[RemainingElements]] to remainingElementsCount.
    27. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
    28. Perform ? Invoke(nextPromise, "then", « onFulfilled, onRejected »).
    29. Set index to index + 1.

27.2.4.2.2 Promise.allSettled Resolve Element Functions

A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. Let alreadyCalled be F.[[AlreadyCalled]].
  3. If alreadyCalled.[[Value]] is true, return undefined.
  4. Set alreadyCalled.[[Value]] to true.
  5. Let index be F.[[Index]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(obj, "status", "fulfilled").
  11. Perform ! CreateDataPropertyOrThrow(obj, "value", x).
  12. Set values[index] to obj.
  13. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  14. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
  15. Return undefined.

The "length" property of a Promise.allSettled resolve element function is 1𝔽.

27.2.4.2.3 Promise.allSettled Reject Element Functions

A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled reject element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. Let alreadyCalled be F.[[AlreadyCalled]].
  3. If alreadyCalled.[[Value]] is true, return undefined.
  4. Set alreadyCalled.[[Value]] to true.
  5. Let index be F.[[Index]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(obj, "status", "rejected").
  11. Perform ! CreateDataPropertyOrThrow(obj, "reason", x).
  12. Set values[index] to obj.
  13. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  14. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
  15. Return undefined.

The "length" property of a Promise.allSettled reject element function is 1𝔽.

27.2.4.3 Promise.any ( iterable )

The any function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be GetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be GetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be PerformPromiseAny(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return Completion(result).
Note

The any function requires its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.3.1 PerformPromiseAny ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAny takes arguments iteratorRecord, constructor, resultCapability (a PromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert: ! IsConstructor(constructor) is true.
  2. Assert: ! IsCallable(promiseResolve) is true.
  3. Let errors be a new empty List.
  4. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  5. Let index be 0.
  6. Repeat,
    1. Let next be IteratorStep(iteratorRecord).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] is 0, then
        1. Let error be a newly created AggregateError object.
        2. Perform ! DefinePropertyOrThrow(error, "errors", PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]: false, [[Writable]]: true, [[Value]]: ! CreateArrayFromList(errors) }).
        3. Return ThrowCompletion(error).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue be IteratorValue(next).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Append undefined to errors.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let stepsRejected be the algorithm steps defined in Promise.any Reject Element Functions.
    11. Let lengthRejected be the number of non-optional parameters of the function definition in Promise.any Reject Element Functions.
    12. Let onRejected be ! CreateBuiltinFunction(stepsRejected, lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Errors]], [[Capability]], [[RemainingElements]] »).
    13. Set onRejected.[[AlreadyCalled]] to false.
    14. Set onRejected.[[Index]] to index.
    15. Set onRejected.[[Errors]] to errors.
    16. Set onRejected.[[Capability]] to resultCapability.
    17. Set onRejected.[[RemainingElements]] to remainingElementsCount.
    18. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
    19. Perform ? Invoke(nextPromise, "then", « resultCapability.[[Resolve]], onRejected »).
    20. Set index to index + 1.

27.2.4.3.2 Promise.any Reject Element Functions

A Promise.any reject element function is an anonymous built-in function that is used to reject a specific Promise.any element. Each Promise.any reject element function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.any reject element function is called with argument x, the following steps are taken:

  1. Let F be the active function object.
  2. If F.[[AlreadyCalled]] is true, return undefined.
  3. Set F.[[AlreadyCalled]] to true.
  4. Let index be F.[[Index]].
  5. Let errors be F.[[Errors]].
  6. Let promiseCapability be F.[[Capability]].
  7. Let remainingElementsCount be F.[[RemainingElements]].
  8. Set errors[index] to x.
  9. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  10. If remainingElementsCount.[[Value]] is 0, then
    1. Let error be a newly created AggregateError object.
    2. Perform ! DefinePropertyOrThrow(error, "errors", PropertyDescriptor { [[Configurable]]: true, [[Enumerable]]: false, [[Writable]]: true, [[Value]]: ! CreateArrayFromList(errors) }).
    3. Return ? Call(promiseCapability.[[Reject]], undefined, « error »).
  11. Return undefined.

The "length" property of a Promise.any reject element function is 1𝔽.

27.2.4.4 Promise.prototype

The initial value of Promise.prototype is the Promise prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

27.2.4.5 Promise.race ( iterable )

The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve be GetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord be GetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result be PerformPromiseRace(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is an abrupt completion, then
    1. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. Return Completion(result).
Note 1

If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.

Note 2

The race function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor. It also expects that its this value provides a resolve method.

27.2.4.5.1 PerformPromiseRace ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseRace takes arguments iteratorRecord, constructor, resultCapability (a PromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert: IsConstructor(constructor) is true.
  2. Assert: IsCallable(promiseResolve) is true.
  3. Repeat,
    1. Let next be IteratorStep(iteratorRecord).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
      2. Return resultCapability.[[Promise]].
    5. Let nextValue be IteratorValue(next).
    6. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    7. ReturnIfAbrupt(nextValue).
    8. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    9. Perform ? Invoke(nextPromise, "then", « resultCapability.[[Resolve]], resultCapability.[[Reject]] »).

27.2.4.6 Promise.reject ( r )

The reject function returns a new promise rejected with the passed argument.

  1. Let C be the this value.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
  4. Return promiseCapability.[[Promise]].
Note

The reject function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.7 Promise.resolve ( x )

The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this constructor.

  1. Let C be the this value.
  2. If Type(C) is not Object, throw a TypeError exception.
  3. Return ? PromiseResolve(C, x).
Note

The resolve function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.

27.2.4.7.1 PromiseResolve ( C, x )

The abstract operation PromiseResolve takes arguments C (a constructor) and x (an ECMAScript language value). It returns a new promise resolved with x. It performs the following steps when called:

  1. Assert: Type(C) is Object.
  2. If IsPromise(x) is true, then
    1. Let xConstructor be ? Get(x, "constructor").
    2. If SameValue(xConstructor, C) is true, return x.
  3. Let promiseCapability be ? NewPromiseCapability(C).
  4. Perform ? Call(promiseCapability.[[Resolve]], undefined, « x »).
  5. Return promiseCapability.[[Promise]].

27.2.4.8 get Promise [ @@species ]

Promise[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

Promise prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.

27.2.5 Properties of the Promise Prototype Object

The Promise prototype object:

  • is %Promise.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.

27.2.5.1 Promise.prototype.catch ( onRejected )

When the catch method is called with argument onRejected, the following steps are taken:

  1. Let promise be the this value.
  2. Return ? Invoke(promise, "then", « undefined, onRejected »).

27.2.5.2 Promise.prototype.constructor

The initial value of Promise.prototype.constructor is %Promise%.

27.2.5.3 Promise.prototype.finally ( onFinally )

When the finally method is called with argument onFinally, the following steps are taken:

  1. Let promise be the this value.
  2. If Type(promise) is not Object, throw a TypeError exception.
  3. Let C be ? SpeciesConstructor(promise, %Promise%).
  4. Assert: IsConstructor(C) is true.
  5. If IsCallable(onFinally) is false, then
    1. Let thenFinally be onFinally.
    2. Let catchFinally be onFinally.
  6. Else,
    1. Let stepsThenFinally be the algorithm steps defined in Then Finally Functions.
    2. Let lengthThenFinally be the number of non-optional parameters of the function definition in Then Finally Functions.
    3. Let thenFinally be ! CreateBuiltinFunction(stepsThenFinally, lengthThenFinally, "", « [[Constructor]], [[OnFinally]] »).
    4. Set thenFinally.[[Constructor]] to C.
    5. Set thenFinally.[[OnFinally]] to onFinally.
    6. Let stepsCatchFinally be the algorithm steps defined in Catch Finally Functions.
    7. Let lengthCatchFinally be the number of non-optional parameters of the function definition in Catch Finally Functions.
    8. Let catchFinally be ! CreateBuiltinFunction(stepsCatchFinally, lengthCatchFinally, "", « [[Constructor]], [[OnFinally]] »).
    9. Set catchFinally.[[Constructor]] to C.
    10. Set catchFinally.[[OnFinally]] to onFinally.
  7. Return ? Invoke(promise, "then", « thenFinally, catchFinally »).

27.2.5.3.1 Then Finally Functions

A Then Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot. The value of the [[Constructor]] internal slot is a Promise-like constructor function object, and the value of the [[OnFinally]] internal slot is a function object.

When a Then Finally function is called with argument value, the following steps are taken:

  1. Let F be the active function object.
  2. Let onFinally be F.[[OnFinally]].
  3. Assert: IsCallable(onFinally) is true.
  4. Let result be ? Call(onFinally, undefined).
  5. Let C be F.[[Constructor]].
  6. Assert: IsConstructor(C) is true.
  7. Let promise be ? PromiseResolve(C, result).
  8. Let valueThunk be equivalent to a function that returns value.
  9. Return ? Invoke(promise, "then", « valueThunk »).

The "length" property of a Then Finally function is 1𝔽.

27.2.5.3.2 Catch Finally Functions

A Catch Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot. The value of the [[Constructor]] internal slot is a Promise-like constructor function object, and the value of the [[OnFinally]] internal slot is a function object.

When a Catch Finally function is called with argument reason, the following steps are taken:

  1. Let F be the active function object.
  2. Let onFinally be F.[[OnFinally]].
  3. Assert: IsCallable(onFinally) is true.
  4. Let result be ? Call(onFinally, undefined).
  5. Let C be F.[[Constructor]].
  6. Assert: IsConstructor(C) is true.
  7. Let promise be ? PromiseResolve(C, result).
  8. Let thrower be equivalent to a function that throws reason.
  9. Return ? Invoke(promise, "then", « thrower »).

The "length" property of a Catch Finally function is 1𝔽.

27.2.5.4 Promise.prototype.then ( onFulfilled, onRejected )

When the then method is called with arguments onFulfilled and onRejected, the following steps are taken:

  1. Let promise be the this value.
  2. If IsPromise(promise) is false, throw a TypeError exception.
  3. Let C be ? SpeciesConstructor(promise, %Promise%).
  4. Let resultCapability be ? NewPromiseCapability(C).
  5. Return PerformPromiseThen(promise, onFulfilled, onRejected, resultCapability).

27.2.5.4.1 PerformPromiseThen ( promise, onFulfilled, onRejected [ , resultCapability ] )

The abstract operation PerformPromiseThen takes arguments promise, onFulfilled, and onRejected and optional argument resultCapability (a PromiseCapability Record). 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. It performs the following steps when called:

  1. Assert: IsPromise(promise) is true.
  2. If resultCapability is not present, then
    1. Set resultCapability to undefined.
  3. If IsCallable(onFulfilled) is false, then
    1. Let onFulfilledJobCallback be empty.
  4. Else,
    1. Let onFulfilledJobCallback be HostMakeJobCallback(onFulfilled).
  5. If IsCallable(onRejected) is false, then
    1. Let onRejectedJobCallback be empty.
  6. Else,
    1. Let onRejectedJobCallback be HostMakeJobCallback(onRejected).
  7. Let fulfillReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]: Fulfill, [[Handler]]: onFulfilledJobCallback }.
  8. Let rejectReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]: Reject, [[Handler]]: onRejectedJobCallback }.
  9. If promise.[[PromiseState]] is pending, then
    1. Append fulfillReaction as the last element of the List that is promise.[[PromiseFulfillReactions]].
    2. Append rejectReaction as the last element of the List that is promise.[[PromiseRejectReactions]].
  10. Else if promise.[[PromiseState]] is fulfilled, then
    1. Let value be promise.[[PromiseResult]].
    2. Let fulfillJob be NewPromiseReactionJob(fulfillReaction, value).
    3. Perform HostEnqueuePromiseJob(fulfillJob.[[Job]], fulfillJob.[[Realm]]).
  11. Else,
    1. Assert: The value of promise.[[PromiseState]] is rejected.
    2. Let reason be promise.[[PromiseResult]].
    3. If promise.[[PromiseIsHandled]] is false, perform HostPromiseRejectionTracker(promise, "handle").
    4. Let rejectJob be NewPromiseReactionJob(rejectReaction, reason).
    5. Perform HostEnqueuePromiseJob(rejectJob.[[Job]], rejectJob.[[Realm]]).
  12. Set promise.[[PromiseIsHandled]] to true.
  13. If resultCapability is undefined, then
    1. Return undefined.
  14. Else,
    1. Return resultCapability.[[Promise]].

27.2.5.5 Promise.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Promise".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

27.2.6 Properties of Promise Instances

Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic, %Promise.prototype%). Promise instances are initially created with the internal slots described in Table 72.

Table 72: Internal Slots of Promise Instances
Internal Slot Description
[[PromiseState]] One of pending, fulfilled, or rejected. Governs how a promise will react to incoming calls to its then method.
[[PromiseResult]] The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not pending.
[[PromiseFulfillReactions]] A List of PromiseReaction records to be processed when/if the promise transitions from the pending state to the fulfilled state.
[[PromiseRejectReactions]] A List of PromiseReaction records to be processed when/if the promise transitions from the pending state to the rejected state.
[[PromiseIsHandled]] A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.