Promise.withResolvers() implementation / mocks

[TheBlindHawk]

Hello, I've been trying to use Promise.WithResolvers in my repository, but when I ran the tests I got
TypeError: Promise.withResolvers is not a function
I am guessing that since it is a new feature (as of March 2024 according to MDN) it still hasn't been implemented on vitest, so I tried creating mocks for it.

// with spyOn (Error: withResolvers does not exist)
vi.spyOn(window.Promise.prototype, 'withResolvers').mockImplementation(() => {
  let resolve, reject
  const promise = new Promise((res, rej) => { resolve = res, reject = rej })
  return { promise, resolve, reject }
})

// with mock (TypeError: Cannot read properties of undefined (reading 'replace'))
vi.mock(window.Promise.withResolvers, () => {
  let resolve, reject
  const promise = new Promise((res, rej) => { resolve = res, reject = rej })
  return { promise, resolve, reject }
})

I am guessing spyOn isn't going to work since it only works on functions that already exists.
As for mock, I've been looking for some examples but I only see examples using a file path string, so it's probably not meant to be used this way.
But besides the two I couldn't find anything that could help me mock Promise.withResolvers.

vite: "^5.2.8",
vitest: "^1.4.0"

Is there any way currently to mock Promise.withResolvers?
Will Promise.WithResolvers mocks be coming in with new versions?
if so when are they approximately planned for?
Any help is appreciated, thanks!

[hi-ogawa]

The feature like Promise.withResolvers is simply a runtime feature and it's not something "Vitest implements". As seen at the bottom of the same MDN page, it looks like Node simply doesn't have it yet.

I'm not sure whether "mocking" is relevant for your use case, but I think typically people would simply monkey-patch (i.e. polyfill) non-existing global. Have you tried something like this?

Promise.withResolvers = () => {
  let resolve, reject
  const promise = new Promise((res, rej) => { resolve = res, reject = rej })
  return { promise, resolve, reject }
}

If you want to "spy" its function calls, then you could probably do:

import { vi } from "vitest"
Promise.withResolvers = vi.fn(() => {
  // ... same thing here ...
});

Note that vi.mock is for "module mocking" so it's not for patching some global object.

[TheBlindHawk]

Thank you for the detailed explanation!
The example you provided worked perfectly.
I'll make sure to check further what the issue is beforehand next time, I completely forgot to check node.js compatibility even tho I send the link myself...

[anton-x-t]

Thank you very much! Worked very good inside vitest.setup.ts.