Properties of the Promise constructor

The Promise constructor:

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

Promise.all

This 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 Completion(GetPromiseResolve(C)).
4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
5. Let iteratorRecord be Completion(GetIterator(iterable, SYNC)).
6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
7. Let result be Completion(PerformPromiseAll(iteratorRecord, C, promiseCapability, promiseResolve)).
8. If result is an abrupt completion, then a. If iteratorRecord.[[Done]] is false, set result to Completion(IteratorClose(iteratorRecord, result)). b. IfAbruptRejectPromise(result, promiseCapability).
9. Return ? result.

GetPromiseResolve

The abstract operation GetPromiseResolve takes argument promiseConstructor (a constructor) and returns either a normal completion containing a function object or a throw completion. It performs the following steps when called:

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

PerformPromiseAll

The abstract operation PerformPromiseAll takes arguments iteratorRecord (an Iterator Record), constructor (a constructor), resultCapability (a PromiseCapability Record), and promiseResolve (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion.

It performs the following steps when called:

1. Let values be a new empty List.
2. Let remainingElementsCount be the Record [[Value]]: 1.
3. Let index be 0.
4. Repeat, a. Let next be ? IteratorStepValue(iteratorRecord). b. If next is DONE, then i. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1. ii. If remainingElementsCount.[[Value]] = 0, then
5. Let valuesArray be CreateArrayFromList(values).
6. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »). iii. Return resultCapability.[[Promise]]. c. Append undefined to values. d. Let nextPromise be ? Call(promiseResolve, constructor, « next »). e. Let steps be the algorithm steps defined in Promise.all Resolve Element Functions. f. Let length be the number of non-optional parameters of the function definition in Promise.all Resolve Element Functions. g. Let onFulfilled be CreateBuiltinFunction(steps, length, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »). h. Set onFulfilled.[[AlreadyCalled]] to false. i. Set onFulfilled.[[Index]] to index. j. Set onFulfilled.[[Values]] to values. k. Set onFulfilled.[[Capability]] to resultCapability. l. Set onFulfilled.[[RemainingElements]] to remainingElementsCount. m. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1. n. Perform ? Invoke(nextPromise, "then", « onFulfilled, resultCapability.[[Reject]] »). o. Set index to index + 1.

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]] = 0, then a. Let valuesArray be CreateArrayFromList(values). b. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
11. Return undefined. The "length" property of a Promise.all resolve element function is 1𝔽.

Promise.allSettled

This 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 Completion(GetPromiseResolve(C)).
4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
5. Let iteratorRecord be Completion(GetIterator(iterable, SYNC)).
6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
7. Let result be Completion(PerformPromiseAllSettled(iteratorRecord, C, promiseCapability, promiseResolve)).
8. If result is an abrupt completion, then a. If iteratorRecord.[[Done]] is false, set result to Completion(IteratorClose(iteratorRecord, result)). b. IfAbruptRejectPromise(result, promiseCapability).
9. Return ? result.

PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord (an Iterator Record), constructor (a constructor), resultCapability (a PromiseCapability Record), and promiseResolve (a function object) and returns either a normal completion containing an ECMAScript language value or a throw completion.

It performs the following steps when called:

1. Let values be a new empty List.
2. Let remainingElementsCount be the Record [[Value]]: 1.
3. Let index be 0.
4. Repeat, a. Let next be ? IteratorStepValue(iteratorRecord). b. If next is DONE, then i. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1. ii. If remainingElementsCount.[[Value]] = 0, then
5. Let valuesArray be CreateArrayFromList(values).
6. Perform ? Call(resultCapability.[[Resolve]], undefined, « valuesArray »). iii. Return resultCapability.[[Promise]]. c. Append undefined to values. d. Let nextPromise be ? Call(promiseResolve, constructor, « next »). e. Let stepsFulfilled be the algorithm steps defined in Promise.allSettled Resolve Element Functions. f. Let lengthFulfilled be the number of non-optional parameters of the function definition in Promise.allSettled Resolve Element Functions. g. Let onFulfilled be CreateBuiltinFunction(stepsFulfilled, lengthFulfilled, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »). h. Let alreadyCalled be the Record [[Value]]: false. i. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled. j. Set onFulfilled.[[Index]] to index. k. Set onFulfilled.[[Values]] to values. l. Set onFulfilled.[[Capability]] to resultCapability. m. Set onFulfilled.[[RemainingElements]] to remainingElementsCount. n. Let stepsRejected be the algorithm steps defined in Promise.allSettled Reject Element Functions. o. Let lengthRejected be the number of non-optional parameters of the function definition in Promise.allSettled Reject Element Functions. p. Let onRejected be CreateBuiltinFunction(stepsRejected, lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »). q. Set onRejected.[[AlreadyCalled]] to alreadyCalled. r. Set onRejected.[[Index]] to index. s. Set onRejected.[[Values]] to values. t. Set onRejected.[[Capability]] to resultCapability. u. Set onRejected.[[RemainingElements]] to remainingElementsCount. v. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1. w. Perform ? Invoke(nextPromise, "then", « onFulfilled, onRejected »). x. Set index to index + 1.

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]] = 0, then a. Let valuesArray be CreateArrayFromList(values). b. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
15. Return undefined. The "length" property of a Promise.allSettled resolve element function is 1𝔽.

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]] = 0, then a. Let valuesArray be CreateArrayFromList(values). b. Return ? Call(promiseCapability.[[Resolve]], undefined, « valuesArray »).
15. Return undefined.

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

Promise.any

Promise.prototype

Promise.race

Promise.reject

Promise.resolve

Promise.withResolvers

get Promise[@@species]