jest.useRealTimers() has no effect

[mjesun]

When timers is set to fake in Jest configuration, top level calls to useRealTimers have no effect on the test behavior. Files to reproduce:

Given the following package.json file:

{
  "dependencies": {
    "jest": "^20.0.4"
  },
  "jest": {
    "timers": "fake"
  }
}

And the following test:

jest.useRealTimers();

describe('foo', () => {
  it('bar', () => {
    jest.runAllTimers();
  });
});

You can observe how executing this test will not throw any warnings on calling runAllTimers with (supposedly) real timers enabled. This is due to the fact that, after executing useRealTimers at the top level, before entering the test execution, a useFakeTimers is executed on a beforeEach (see here, on line 67).

Temporary solution was to move useRealTimers calls inside a beforeEach, but IMO it would be nice to fix it for keeping compatibility with previous versions :)

[rogeliog]

Maybe related to #3814

[thymikee]

If the "timers": "fake" option is set, Jest runs jest.useFakeTimers() before every test, so what you need to do is run jest.useRealTimers(); in specific test function or in beforeEach hook:

describe('foo', () => {
  beforeEach(() => jest.useRealTimers());
  it('bar', () => {
    jest.runAllTimers();
  });
});

Maybe we could document it better?

[thymikee]

Discard my previous comment, I haven't read your final thoughts. PR is on it's way :)

[mjesun]

No worries! Thanks for looking at it @thymikee 😊

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