Why doesn't Jest.useFakeTimers work in beforeAll blocks?

user_1267625 0
Viewed 292

I wanted to mock timeout functions globally so I added a call to jest.useFakeTimers in a beforeAll block. When I ran my tests, jest.advanceTimersByTime didn't execute the scheduled code. I used spies to confirm that the mock was indeed being called. I noticed however, that if I moved the jest.advanceTimersByTime call into the test function, or into a beforeEach block, it worked.

Here's a simple example that reproduces the problem.

describe('test', () => {

    beforeEach(() => {
        // Only beforeEach block works, NOT beforeAll
        jest.useFakeTimers();
    })

    it('setTimeout calls callback', () => {
        const callback = jest.fn();

        setTimeout(() => {
            callback()
        }, 3000);

        expect(callback).not.toHaveBeenCalled();

        jest.advanceTimersByTime(3000);

        expect(callback).toHaveBeenCalledTimes(1);
    })

})

I didn't see anything in the docs that explains this. I'm using Jest 26.6.2.

jestjs typescript settimeout javascript
2 Answers

Maybe you could check your jest setup file(like setupFilesAfterEnv config). Say, If I put those codes in the setup file, then it will run into the problem you said above:

beforeEach(() => {
  // reset to real timers
  jest.useRealTimers();
})

Because as default behavior, beforeEach's callback is always executed after beforeAll's, even if it is defined before beforeAll.

user_53575000

Seems like jest.useFakeTimers in the documentation replaces setTimeout in product code, not within the test code.

user_99063250
Related