[Feature]: Allow mocking the Date without mocking timers

[glenjamin]

🚀 Feature Proposal

Currently jest.useFakeTimers() enables mocking of all time and timer related functions.

I'd like to propose allowing callers to be more selective about what timer functions are actually mocked

This could be a case of exposing the toFake option from @sinonjs/fake-timers, or it could take the form of a jest-specific API.

Motivation

I've run into a case where I'm doing a somewhat integrationy test, but I want to control the current time to make my assertion simpler, however, when I do

      jest.useFakeTimers();
      jest.setSystemTime(new Date('2022-02-21T17:35:00Z'));

Then my code hangs, because somewhere in the depths of the stuff that's running, there must be a timer somewhere.

Example

No response

Pitch

Currently timer mocking is part of the jest core API by wrapping sinon, but there's no way to control the options passed to sinon.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 30 days.

[glenjamin]

Not stale, a PR was opened 7 days ago

[mrazauskas]

@glenjamin In case if you would like to simply mock the value returned by Date.now(), all you need is:

jest.spyOn(Date, 'now').mockImplementation(() => 123456);

The thing is that fake timers are sort of freezing the time. The clock has to be advanced manually, code can hang because of this.

The mocked value works in some cases, but that is not a replacement for a ticking clock. Another possible solution (part of the same #12572, hence not yet working) could be:

jest.useFakeTimers({advanceTimers: true, now: new Date('2015-09-25')});

[glenjamin]

Yep, in my case I was also using new Date(), so mocking now was insufficient.

I ended up doing this while I wait for #12572

      const frozen = new Date('2022-02-21T17:35:00Z');
      jest.spyOn(global, 'Date').mockReturnValue(frozen);
      jest.spyOn(Date, 'now').mockReturnValue(frozen.valueOf());

[SimenB]

https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.8

[github-actions]

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