Extendable Promise

A promise that can be delayed (extended) via repeated calls to waitUntil.

"Extendable" here refers to the ability to extend the duration of the promise, not inheritance/subtypes/etc. The name and API are modelled after the ExtendableEvent interface that was introduced to the web part of the Service Worker API. As such, it provides a waitUntil method that takes a promise as argument that extends settlement of the parent promise until all promises that were added in this way have settled. You can think about it like Promise.allSettled, except you can add promises to the list after the fact. Example:

const p = new ExtendablePromise(timeout(200).then(() => 1));
await timeout(100)
p.waitUntil(timeout(300).then(() => Promise.reject(Error('2'))));

const res = await p; // total duration ~400ms (100 + 300)

assertEquals(res, [
  { status: "fulfilled", value: 1 },
  { status: "rejected", reason: Error('2') },

After all promises have settled, further calls to waitUntil become no-ops.

The constructor takes a promise as argument, which sets the minimum duration for the extendable promise.

If an extendable promise is created without supplying an initial promise, it is "idle" (awaiting it pauses execution) until the first call to waitUntil "primes" it.

Initializing it with a synchronous value or a settled promise immediately settles the extendable promise with that value and all calls to waitUntil are no-ops.