Stage 1 Draft / March 12, 2026

MaybeDeferredResolvePromise (name TBD)

You'll notice the spec looks a bit 'playful' -- this is because I don't want to give too much of an impression that this is perfection, done, or even entirely thought through deeply. This spec text is a sketch, intended for discussion not implementation.

1 MaybeDeferredPromiseResolve Abstract Operations

These operations provide a mechanism for resolving promises without risk of synchronously executing arbitrary user code. When the resolution value is an object that could trigger user code (such as a Proxy, a thenable, or an object with accessor properties), the actual resolution is deferred to a future job.

1.1 RequiresDeferredPromiseResolution ( value )

The abstract operation RequiresDeferredPromiseResolution takes argument value (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It determines whether resolving a promise with value might execute user code that should be deferred to avoid running user code It performs the following steps when called:

  1. Let o be value.
  2. While o is defined, Repeat:
    1. If value is not an Object, return false.
    2. If value has a [[ProxyHandler]] internal slot, return true.
    3. Let desc be o.[[GetOwnProperty]]("then").
    4. If desc is not undefined and IsAccessorDescriptor(desc) is true, return true.
    5. Set o to o.[[GetPrototypeOf]]().
  3. Let constructorDesc be ? value.[[GetOwnProperty]]("constructor").
  4. If constructorDesc is not undefined, then
    1. If IsAccessorDescriptor(constructorDesc) is true, return true.
  5. Return false.
 Note

I've sketched this assuming that [[GetPrototypeOf]] cannot trap on a getter, but I'm not 100% sure of this

Also, the lookup of "constructor" needs also to be a loop of similar shape to the lookup for 'then'

1.2 DoTheRestOfThePromiseResolveStuffWithoutAccidentallyCreatingAnotherJob ( promiseToResolve, resolution )

The abstract operation DoTheRestOfThePromiseResolveStuffWithoutAccidentallyCreatingAnotherJob takes arguments promiseToResolve (a Promise) and resolution (an ECMAScript language value). Basically a nicely layered version of PromiseResolve that doesn't accidentally force another tick. It performs the following steps when called:

  1. Do ? PromiseResolve(promiseToResolve, resolution), but without accidentally introducing another tick.

1.3 NewDeferredSafeResolveJob ( promiseToResolve, resolution, realm )

The abstract operation NewDeferredSafeResolveJob takes arguments promiseToResolve (a Promise), resolution (an ECMAScript language value), and realm (a Realm Record) and returns a Record with fields [[Job]] (a Job Abstract Closure) and [[Realm]] (a Realm Record). It returns a Job that performs deferred promise resolution in a microtask. It performs the following steps when called:

  1. Let job be a new Job Abstract Closure with no parameters that captures promiseToResolve and resolution and performs the following steps when called:
    1. Return ? DoTheRestOfThePromiseResolveStuffWithoutAccidentallyCreatingAnotherJob(promiseToResolve, resolution).
  2. Return the Record { [[Job]]: job, [[Realm]]: realm }.

1.4 MaybeDeferredPromiseResolve ( promise, resolution )

The abstract operation MaybeDeferredPromiseResolve takes arguments promise (a Promise) and resolution (an ECMAScript language value) and returns either a normal completion containing undefined or a throw completion. It resolves promise with resolution, deferring to a microtask if resolution might execute user code. It performs the following steps when called:

  1. Assert: promise is a Promise.
  2. Assert: promise.[[PromiseState]] is pending.
  3. Let requiresDeferred be ? RequiresDeferredPromiseResolution(resolution).
  4. If requiresDeferred is true, then
    1. Set promise.[[PromiseState]] to deferred.
    2. Let callerRealm be the current Realm Record.
    3. Let job be NewDeferredSafeResolveJob(promise, resolution, callerRealm).
    4. Perform HostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
    5. Return undefined.
  5. Return ? PromiseResolve(promise, resolution).
 Note

deferred is a new state that I've not run through the rest of the spec, but the intention is that it prevents future resolutions from overriding the resolution applied here.

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