jest: "Modern" fake-timer implementation doesn't work with PromiseJS.

I have the same error with nock 12.0.3 and jest 26.1.0

The test case

test('nock', async () => {
    jest.useFakeTimers('modern');
    nock('http://modern').get("/").reply(200);
    await fetch(`http://modern`, { method: "GET" });
});

Failing :

 : Timeout - Async callback was not invoked within the 5000 ms timeout specified by jest.setTimeout.Timeout - Async callback was not invoked within the 5000 ms timeout specified by jest.setTimeout.Error:
  > 18 |         test('nock', async () => {
    |         ^
    19 |             jest.useFakeTimers('modern');
    20 |             nock('http://modern').get("/").reply(200);
    21 |             await fetch(`http://modern`, { method: "GET" });

It’s working when I replace

• ‘modern’ by ‘legacy’ or
• fetch(…) by Promise.resolve() (nock is not actually used)

Is this moving along? As issue still persists.

I created a pull request to add the options object with an ignore property (notToFake) 100% inspired by the comment above. Any feedback is welcome and greatly appreciated.

The added code is kept deliberately simple and pragmatic, it can still be changed easily to reflect any input. https://github.com/facebook/jest/pull/11661

@tomdaniel0x01 thanks, I think I will use fake timers from sinon for now, as they seemed to work. But I’d would rather get rid of another dependency and use it in jest. Its the only I am really missing in jest.

I have to add to this issue for clarity: It does not only affect non-native global.promise replacements, it also makes native promises in Node.JS with TypeScript stop working.

@SimenB any progress on this? It seems like the problem and solution are both known. Is there any reason a change should not be made that would stop mocking nextTick? Is this just waiting on someone to make the change?

As a test, can you modify your node_modules/@jest/fake-timers/build/modernFakeTimers.js (or wherever it’s located) from the line const toFake = Object.keys(this._fakeTimers.timers); to const toFake = Object.keys(this._fakeTimers.timers).filter(name => name !== 'nextTick');.

Yep—when I do this, it works! Looks like https://github.com/sinonjs/fake-timers/pull/323 is related.

As an aside, one should never replace global.Promise - that suggestion is outdated. E.g. when using async-await you will always get the native Promise regardless of the value of global.Promise.

Thanks for the heads-up! Yeah, a priori, I’m not really interested in replacing global.Promise. But I’m using React Native, with preset: 'react-native', so it’s being done whether I like it or not (RN has been doing this since https://github.com/facebook/react-native/commit/3ff39870ce776c653823e1733363be0401896294). I suppose I could make a PR to react-native and fork it in the meantime, if it came to that.

Hmm, I wonder if this is because we fake process.nextTick as well. As a test, can you modify your node_modules/@jest/fake-timers/build/modernFakeTimers.js (or wherever it’s located) from the line const toFake = Object.keys(this._fakeTimers.timers); to const toFake = Object.keys(this._fakeTimers.timers).filter(name => name !== 'nextTick');. That should exclude it. This is the default behavior of @sinonjs/fake-timers, which I’ve changed in Jest’s implementation.

As an aside, one should never replace global.Promise - that suggestion is outdated. E.g. when using async-await you will always get the native Promise regardless of the value of global.Promise.

Finally fixed in #12572, available in https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.8.

facing same issue. Have we fixed this issue or it’s still in progress?

Or the other way : keep everything mocked by default, and a way to exclude some methods ?

it('does not mock nextTick', () => {
  jest.useFakeTimers('modern')
  // `nextTick` is mocked by default
})

it('mocks nextTick', () => {
  jest.useFakeTimers('modern', { toNotFake: ['nextTick'] })
  // nextTick is not mocked when requested
})

Possible workaround for the time being is using https://github.com/sinonjs/fake-timers, as it was mention before. But here’s the example for those who stumbles across this thread:

import FakeTimers, { InstalledClock } from "@sinonjs/fake-timers";

const now = new Date("2021-11-25 11:00:00");

let clock: InstalledClock;
beforeAll(() => {
  clock = FakeTimers.install({
    now: now,
    shouldAdvanceTime: true,
    toFake: ["Date"],
  });
});

afterAll(() => {
  clock.uninstall();
});

What’s the status? Will it be fixed?

@SimenB

If we stop mocking nextTick by default we need to allow people to mock it if they want (right now we mock everything, which I think is ideal, but it sometimes breaks other modules).

So it would be acceptable to stop mocking nextTick by default, as long as there is a way to make sure it gets mocked? Would an API similar to sinon’s for choosing what to mock work here?

The user’s code would then look something like this:

it('does not mock nextTick', () => {
  jest.useFakeTimers('modern')
  // `nextTick` is not mocked by default
})

it('mocks nextTick', () => {
  jest.useFakeTimers('modern', { toFake: ['nextTick'] })
  // nextTick is mocked when requested
})

For most cases here I assume a strategically placed jest.runAllTicks() would solve the issue, but it’s non-obvious

Is this just waiting on someone to make the change?

This, essentially 🙂 However, the change to make is not obvious. If we stop mocking nextTick by default we need to allow people to mock it if they want (right now we mock everything, which I think is ideal, but it sometimes breaks other modules).