Stage 1 Draft / November 7, 2025

Await Dictionary

27 Control Abstraction Objects

27.2 Promise Objects

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 CreateKeyedPromiseCombinatorResultObject ( keys, values )

The abstract operation CreateKeyedPromiseCombinatorResultObject takes arguments keys (a List of property keys) and values (a List of ECMAScript language values) and returns an Object. It performs the following steps when called:

  1. Assert: The number of elements in keys is the same as the number of elements in values.
  2. Let obj be OrdinaryObjectCreate(null).
  3. For each integer i such that 0 ≤ i < the number of elements in keys, in ascending order, do
    1. Perform ! CreateDataPropertyOrThrow(obj, keys[i], values[i]).
  4. Return obj.

27.2.4.2 Promise.allKeyed ( promises )

This method performs the following steps when called:

  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. If promises is not an Object, then
    1. Let error be a newly created TypeError object.
    2. Perform ? Call(promiseCapability.[[Reject]], undefined, « error »).
    3. Return promiseCapability.[[Promise]].
  6. Let result be Completion(PerformPromiseAllKeyed(promises, C, promiseCapability, promiseResolve)).
  7. IfAbruptRejectPromise(result, promiseCapability).
  8. Return promiseCapability.[[Promise]].
 Note

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

27.2.4.2.1 PerformPromiseAllKeyed ( promises, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAllKeyed takes arguments promises (an Object), 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 allKeys be ? promises.[[OwnPropertyKeys]]().
  2. Let keys be a new empty List.
  3. Let values be a new empty List.
  4. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  5. Let index be 0.
  6. For each element key of allKeys, do
    1. Let desc be ? promises.[[GetOwnProperty]](key).
    2. If desc is not undefined and desc.[[Enumerable]] is true, then
      1. Let value be ? Get(promises, key).
      2. Append key to keys.
      3. Append undefined to values.
      4. Let nextPromise be ? Call(promiseResolve, constructor, « value »).
      5. Let steps be the algorithm steps defined in Promise.allKeyed Resolve Element Functions.
      6. Let length be the number of non-optional parameters of the function definition in Promise.allKeyed Resolve Element Functions.
      7. Let onFulfilled be CreateBuiltinFunction(steps, length, "", « [[AlreadyCalled]], [[Index]], [[Keys]], [[Values]], [[Capability]], [[RemainingElements]] »).
      8. Set onFulfilled.[[AlreadyCalled]] to false.
      9. Set onFulfilled.[[Index]] to index.
      10. Set onFulfilled.[[Keys]] to keys.
      11. Set onFulfilled.[[Values]] to values.
      12. Set onFulfilled.[[Capability]] to resultCapability.
      13. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
      14. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
      15. Perform ? Invoke(nextPromise, "then", « onFulfilled, resultCapability.[[Reject]] »).
      16. Set index to index + 1.
  7. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  8. If remainingElementsCount.[[Value]] = 0, then
    1. NOTE: This can happen even if keys was non-empty if an ill-behaved thenable synchronously invoked the callback passed to its "then" method.
    2. Let result be CreateKeyedPromiseCombinatorResultObject(keys, values).
    3. Perform ? Call(resultCapability.[[Resolve]], undefined, « result »).
  9. Return resultCapability.[[Promise]].

27.2.4.2.2 Promise.allKeyed Resolve Element Functions

A Promise.all resolve element function is an anonymous built-in function

When a Promise.allKeyed 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 keys be F.[[Keys]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Set values[index] to x.
  10. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  11. If remainingElementsCount.[[Value]] = 0, then
    1. Let result be CreateKeyedPromiseCombinatorResultObject(keys, values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « result »).
  12. Return undefined.

27.2.4.3 Promise.allSettledKeyed ( promises )

This method performs the following steps when called:

  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. If promises is not an Object, then
    1. Let error be a newly created TypeError object.
    2. Perform ? Call(promiseCapability.[[Reject]], undefined, « error »).
    3. Return promiseCapability.[[Promise]].
  6. Let result be Completion(PerformPromiseAllSettledKeyed(promises, C, promiseCapability, promiseResolve)).
  7. IfAbruptRejectPromise(result, promiseCapability).
  8. Return promiseCapability.[[Promise]].
 Note

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

27.2.4.3.1 PerformPromiseAllSettledKeyed ( promises, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAllSettledKeyed takes arguments promises (an Object), 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 allKeys be ? promises.[[OwnPropertyKeys]]().
  2. Let keys be a new empty List.
  3. Let values be a new empty List.
  4. Let remainingElementsCount be the Record { [[Value]]: 1 }.
  5. Let index be 0.
  6. For each element key of allKeys, do
    1. Let desc be ? promises.[[GetOwnProperty]](key).
    2. If desc is not undefined and desc.[[Enumerable]] is true, then
      1. Let value be ? Get(promises, key).
      2. Append key to keys.
      3. Append undefined to values.
      4. Let nextPromise be ? Call(promiseResolve, constructor, « value »).
      5. Let stepsFulfilled be the algorithm steps defined in Promise.allSettledKeyed Resolve Element Functions.
      6. Let lengthFulfilled be the number of non-optional parameters of the function definition in Promise.allSettledKeyed Resolve Element Functions.
      7. Let onFulfilled be CreateBuiltinFunction(stepsFulfilled, lengthFulfilled, "", « [[AlreadyCalled]], [[Index]], [[Keys]], [[Values]], [[Capability]], [[RemainingElements]] »).
      8. Let alreadyCalled be the Record { [[Value]]: false }.
      9. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
      10. Set onFulfilled.[[Index]] to index.
      11. Set onFulfilled.[[Keys]] to keys.
      12. Set onFulfilled.[[Values]] to values.
      13. Set onFulfilled.[[Capability]] to resultCapability.
      14. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
      15. Let stepsRejected be the algorithm steps defined in Promise.allSettledKeyed Reject Element Functions.
      16. Let lengthRejected be the number of non-optional parameters of the function definition in Promise.allSettledKeyed Reject Element Functions.
      17. Let onRejected be CreateBuiltinFunction(stepsRejected, lengthRejected, "", « [[AlreadyCalled]], [[Index]], [[Keys]], [[Values]], [[Capability]], [[RemainingElements]] »).
      18. Set onRejected.[[AlreadyCalled]] to alreadyCalled.
      19. Set onRejected.[[Index]] to index.
      20. Set onRejected.[[Keys]] to keys.
      21. Set onRejected.[[Values]] to values.
      22. Set onRejected.[[Capability]] to resultCapability.
      23. Set onRejected.[[RemainingElements]] to remainingElementsCount.
      24. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
      25. Perform ? Invoke(nextPromise, "then", « onFulfilled, onRejected »).
      26. Set index to index + 1.
  7. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  8. If remainingElementsCount.[[Value]] = 0, then
    1. NOTE: This can happen even if keys was non-empty if an ill-behaved thenable synchronously invoked the callback passed to its "then" method.
    2. Let result be CreateKeyedPromiseCombinatorResultObject(keys, values).
    3. Perform ? Call(resultCapability.[[Resolve]], undefined, « result »).
  9. Return resultCapability.[[Promise]].

27.2.4.3.2 Promise.allSettledKeyed Resolve Element Functions

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

When a Promise.allSettledKeyed 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 keys be F.[[Keys]].
  7. Let values be F.[[Values]].
  8. Let promiseCapability be F.[[Capability]].
  9. Let remainingElementsCount be F.[[RemainingElements]].
  10. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  11. Perform ! CreateDataPropertyOrThrow(obj, "status", "fulfilled").
  12. Perform ! CreateDataPropertyOrThrow(obj, "value", x).
  13. Set values[index] to obj.
  14. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  15. If remainingElementsCount.[[Value]] = 0, then
    1. Let result be CreateKeyedPromiseCombinatorResultObject(keys, values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « result »).
  16. Return undefined.

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

27.2.4.3.3 Promise.allSettledKeyed Reject Element Functions

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

When a Promise.allSettledKeyed 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 keys be F.[[Keys]].
  7. Let values be F.[[Values]].
  8. Let promiseCapability be F.[[Capability]].
  9. Let remainingElementsCount be F.[[RemainingElements]].
  10. Let obj be OrdinaryObjectCreate(%Object.prototype%).
  11. Perform ! CreateDataPropertyOrThrow(obj, "status", "rejected").
  12. Perform ! CreateDataPropertyOrThrow(obj, "reason", x).
  13. Set values[index] to obj.
  14. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  15. If remainingElementsCount.[[Value]] = 0, then
    1. Let result be CreateKeyedPromiseCombinatorResultObject(keys, values).
    2. Return ? Call(promiseCapability.[[Resolve]], undefined, « result »).
  16. Return undefined.

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

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