Stage 1 Draft / April 21, 2026

MaybeDeferredPromiseResolve (name TBD)

This spec text is not reviewed yet and may have issues.

1 MaybeDeferredPromiseResolve Abstract Operations

These operations provide a mechanism for resolving a promise such that, if resolution would synchronously execute arbitrary user code, the user-code-running portion is deferred to a subsequent microtask. On return from MaybeDeferredPromiseResolve, the caller's PromiseCapability Record is guaranteed to be latched: any subsequent call to its [[Resolve]] or [[Reject]] is a no-op, regardless of whether the deferred job has yet run.

1.1 IsGetterDescriptor ( Desc )

The abstract operation IsGetterDescriptor takes argument Desc (a Property Descriptor) and returns a Boolean. It performs the following steps when called:

  1. If Desc has a [[Get]] field, return true.
  2. Return false.

1.2 HasPropertyWhichCouldRunUserCode ( o, propertyKey )

The abstract operation HasPropertyWhichCouldRunUserCode takes arguments o (an Object) and propertyKey (a property key) and returns a Boolean. It walks the prototype chain of o looking for propertyKey, returning true if retrieving the property could run user code: that is, the property is a getter, or a Proxy is encountered on the chain. It performs the following steps when called:

  1. Repeat, while o is not null,
    1. If o has a [[ProxyHandler]] internal slot, return true.
    2. Let desc be o.[[GetOwnProperty]](propertyKey).
    3. If desc is not undefined, return IsGetterDescriptor(desc).
    4. Set o to o.[[GetPrototypeOf]]().
  2. Return false.
 Note

This relies on ordinary [[GetPrototypeOf]] never running user code. A Proxy is handled by short-circuiting with true before any of its MOP is invoked. Roughly corresponds to SpiderMonkey's LookupOwnPropertyPure.

1.3 RequiresDeferredPromiseResolution ( value )

The abstract operation RequiresDeferredPromiseResolution takes argument value (an ECMAScript language value) and returns a Boolean. It determines whether the synchronous steps of resolving a promise with value might execute user code. It performs the following steps when called:

  1. If value is not an Object, return false.
  2. If HasPropertyWhichCouldRunUserCode(value, "then") is true, return true.
  3. Let thenValue be ! value.[[Get]]("then").
  4. If IsCallable(thenValue) is true, return true.
  5. Return false.

1.4 PerformPromiseResolution ( promise, resolution )

The abstract operation PerformPromiseResolution takes arguments promise (a Promise) and resolution (an ECMAScript language value) and returns unused. It performs the value-inspecting steps of resolving promise with resolution. It performs the following steps when called:

  1. If resolution is not an Object, then
    1. Perform FulfillPromise(promise, resolution).
    2. Return unused.
  2. If SameValue(resolution, promise) is true, then
    1. Let selfResolutionError be a newly created TypeError object.
    2. Perform RejectPromise(promise, selfResolutionError).
    3. Return unused.
  3. Let then be Completion(Get(resolution, "then")).
  4. If then is an abrupt completion, then
    1. Perform RejectPromise(promise, then.[[Value]]).
    2. Return unused.
  5. Let thenAction be then.[[Value]].
  6. If IsCallable(thenAction) is false, then
    1. Perform FulfillPromise(promise, resolution).
    2. Return unused.
  7. Let thenJobCallback be HostMakeJobCallback(thenAction).
  8. Let job be NewPromiseResolveThenableJob(promise, resolution, thenJobCallback).
  9. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  10. Return unused.
 Note

This overlaps heavily with the Abstract Closure inside CreateResolvingFunctions. Later work could resolve this duplication

1.5 CreateResolvingFunctions ( toResolve )

The abstract operation CreateResolvingFunctions takes argument toResolve (a Promise) and returns a Record with fields [[Resolve]] (a function object) and [[Reject]] (a function object). This is a modification of the existing CreateResolvingFunctions. The resolve function takes an additional parameter doSafeResolve; when true, and resolution would synchronously execute user code, the user-code-running steps are deferred via a null-prototype wrapper thenable. It performs the following steps when called:

  1. Let promiseOrEmpty be the Record { [[Value]]: toResolve }.
  2. Let resolveSteps be a new Abstract Closure with parameters (resolution, doSafeResolve) that captures promiseOrEmpty and performs the following steps when called:
    1. If promiseOrEmpty.[[Value]] is empty, return undefined.
    2. Let promise be promiseOrEmpty.[[Value]].
    3. Set promiseOrEmpty.[[Value]] to empty.
    4. If doSafeResolve is true and RequiresDeferredPromiseResolution(resolution) is true, then
      1. Let deferredSteps be a new Abstract Closure with parameters (onFulfilled, onRejected) that captures promise and resolution and performs the following steps when called:
        1. Perform PerformPromiseResolution(promise, resolution).
        2. Return undefined.
      2. Let deferredThen be CreateBuiltinFunction(deferredSteps, 2, "", « »).
      3. Let wrapper be OrdinaryObjectCreate(null).
      4. Perform ! CreateDataPropertyOrThrow(wrapper, "then", deferredThen).
      5. Set resolution to wrapper.
    5. If SameValue(resolution, promise) is true, then
      1. Let selfResolutionError be a newly created TypeError object.
      2. Perform RejectPromise(promise, selfResolutionError).
      3. Return undefined.
    6. If resolution is not an Object, then
      1. Perform FulfillPromise(promise, resolution).
      2. Return undefined.
    7. Let then be Completion(Get(resolution, "then")).
    8. If then is an abrupt completion, then
      1. Perform RejectPromise(promise, then.[[Value]]).
      2. Return undefined.
    9. Let thenAction be then.[[Value]].
    10. If IsCallable(thenAction) is false, then
      1. Perform FulfillPromise(promise, resolution).
      2. Return undefined.
    11. Let thenJobCallback be HostMakeJobCallback(thenAction).
    12. Let job be NewPromiseResolveThenableJob(promise, resolution, thenJobCallback).
    13. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
    14. Return undefined.
  3. Let resolve be CreateBuiltinFunction(resolveSteps, 12, "", « »).
  4. Let rejectSteps be a new Abstract Closure with parameters (reason) that captures promiseOrEmpty and performs the following steps when called:
    1. If promiseOrEmpty.[[Value]] is empty, return undefined.
    2. Let promise be promiseOrEmpty.[[Value]].
    3. Set promiseOrEmpty.[[Value]] to empty.
    4. Perform RejectPromise(promise, reason).
    5. Return undefined.
  5. Let reject be CreateBuiltinFunction(rejectSteps, 1, "", « »).
  6. Return the Record { [[Resolve]]: resolve, [[Reject]]: reject }.
 Note

When doSafeResolve is false (or omitted by an external caller invoking the built-in as a normal function), the behaviour is identical to the current CreateResolvingFunctions: resolution is not inspected for user-code hazards. Internal callers that want the deferred behaviour pass true.

Latching of promiseOrEmpty happens before the deferral branch, so the resolving functions are no-ops on any subsequent call, regardless of whether the deferred job has yet run.

1.6 MaybeDeferredPromiseResolve ( promiseCapability, resolution )

The abstract operation MaybeDeferredPromiseResolve takes arguments promiseCapability (a PromiseCapability Record) and resolution (an ECMAScript language value) and returns either a normal completion containing undefined or a throw completion. It resolves promiseCapability.[[Promise]] with resolution, deferring the user-code-running steps to a subsequent microtask if resolution might execute user code synchronously. In either branch, promiseCapability's resolving functions are latched on return. It performs the following steps when called:

  1. If RequiresDeferredPromiseResolution(resolution) is false, then
    1. Return ? Call(promiseCapability.[[Resolve]], undefined, « resolution »).
  2. Let promise be promiseCapability.[[Promise]].
  3. Let deferredSteps be a new Abstract Closure with parameters (onFulfilled, onRejected) that captures promise and resolution and performs the following steps when called:
    1. Perform PerformPromiseResolution(promise, resolution).
    2. Return undefined.
  4. Let deferredThen be CreateBuiltinFunction(deferredSteps, 2, "", « »).
  5. Let wrapper be OrdinaryObjectCreate(null).
  6. Perform ! CreateDataPropertyOrThrow(wrapper, "then", deferredThen).
  7. Return ? Call(promiseCapability.[[Resolve]], undefined, « wrapper »).
 Note

This idea is thanks to Justin Ridgewell who suggested the wrapper idea.

The wrapper object is never exposed to user code. It is constructed solely so that resolving promise with it uses the existing thenable support for good rather than evil; we mark the promise as not eligible for overriding resolve, and a job is created for running the 'then' code.

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