How properly test setTimeout after Promise resolve

Question

How to propertly test this function.

1. Set timeout
2. Callback after timeout
3. And recursive call

export function initScheduler(timeout: number, callback: () => Promise<void>): void {
  setTimeout(() => {
    callback().then(() => {
      initScheduler(timeout, callback);
    });
  }, timeout);
}

I tried something like

describe('initScheduler', () => {
  it('should call on schedule', () => {
    jest.useFakeTimers();

    const timeout: number = 60000;
    const callback: jest.Mock = jest.fn().mockResolvedValue(undefined);

    initScheduler(timeout, callback);

    expect(setTimeout).toHaveBeenCalledTimes(1);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), timeout);
    expect(callback).not.toBeCalled();

    jest.advanceTimersByTime(timeout);

    expect(callback).toBeCalled();
    expect(setTimeout).toHaveBeenCalledTimes(2);
  });
});

But last expectation returns 1

Answer 1

Don't concentrate on assertions against setTimeout. Concentrate on what your function is expected to do.

describe('', () => {
  jest.useFakeTimers();
  const timeout = 300;
  const callback = jest.fn();

  beforeEach(() => {
    jest.clearAllTimers();
    callback
      .mockClear()
      .mockReturnValue(Promise.resolve()); 
  });

  it('runs callback only after delay given', () => {
    initScheduler(timeout, callback);
    jest.advanceTimersByTime(timeout - 1);
    expect(callback).not.toHaveBeenCalled();
    jest.advanceTimersByTime(2);
    expect(callback).toHaveBeenCalledTimes(1);
  });

  it('reruns scheduler if callback been resolved successfully', async () => {
    initScheduler(timeout, callback);
    expect(callback).not.toHaveBeenCalled();
    jest.advanceTimersByTime(timeout);
    await Promise.resolve();
    jest.advanceTimersByTime(timeout);
    await Promise.resolve();
    jest.advanceTimersByTime(timeout);
    await Promise.resolve();
    expect(callback).toHaveBeenCalledTimes(3);
  });

  it('stops scheduler if callback rejected', async () => {
    callback.mockReturnValue(Promise.reject());
    initScheduler(timeout, callback);
    jest.advanceTimersByTime(timeout);
    await Promise.resolve();
    jest.advanceTimersByTime(timeout);
    await Promise.resolve();
    expect(callback).toHaveBeenCalledTimes(1);
  });
});

Some details.

1. I'm surprised but timers are not cleared between it() to me without jest.clearAllTimers(). Probably, it depends on jsdom implementation or jest version, I don't know.
2. without await Promise.resolve() your mock for callback dos not run .then part. Actually it could be await <anything else>, I just see await Promise.resolve(); looking less magic than await 42;. Anyway, its purpose to flush microtasks queue while jest itself does not provide straightforward API on that.