Why this isn't possible is implicitly (well, somewhat explicitly) addressed in my comment.

  const foo = async () => {
    ... // sync stuff A
    await someLibrary.expensiveComputation()
    ... // sync stuff B
  }

Unsubscribing to the completion listener is never enough. Implementing cancellation in your outer promise is never enough.

> The javascript promise provides the minimum language implementation upon which more elaborate Promise implementations can be constructed.

I'll reiterate: there is no way to write promise cancellation in JS userspace. It's just not possible (for all the reasons outlined in my long-ass comment above). No matter how elaborate your implementation is, you need collaboration from every single promise that might get called in the call stack.

The proposed `unpromise` implementation would not help either. JS would need all promises to expose a sort of `AbortController` that is explicitly connected across all cancellable await points inwards which would introduce cancel-safety issues.

So you'd need something like this to make promises actually cancelable:

  const cancelableFoo = async (signal) => {
    if (signal.aborted) {
      throw new AbortError()
    }

    ... // sync stuff A

    if (signal.aborted) {
      // possibly cleanup for sync stuff A
      throw new AbortError()
    }

    await someLibrary.expensiveComputation(signal)

    if (signal.aborted) {
      // possibly cleanup for sync stuff A
      throw new AbortError()
    }

    ... // sync stuff B

    if (signal.aborted) {
      // possibly cleanup for sync stuff A
      // possibly cleanup for sync stuff B
      throw new AbortError()
    }
  }

  const controller = new AbortController()
  const signal = abortController.signal

  Promise.cancelableRace(
    controller, // cancelableRace will call controller.abort() if any promise completes
    [
      cancellableFoo(signal),
      deletePost(signal),
      timeout(3000, signal),
    ]
  )