How do you test code that uses timers?

Real timers make tests slow and flaky: a debounce of one second would freeze the suite, and a background interval can fire after the test finishes. Fake timers replace setTimeout, setInterval, Date and related APIs with a virtual clock that the test controls.

beforeEach(() => {
  jest.useFakeTimers();
});

afterEach(() => {
  jest.useRealTimers();
});

test("shows the saved indicator after the debounce", () => {
  const onSave = jest.fn();
  const save = debounce(onSave, 500);

  save("draft");
  expect(onSave).not.toHaveBeenCalled();

  jest.advanceTimersByTime(500);
  expect(onSave).toHaveBeenCalledWith("draft");
});

The timer API

• jest.advanceTimersByTime(ms) moves the virtual clock forward
• jest.runAllTimers() drains the queue, which can loop on recursive timers
• jest.runOnlyPendingTimers() runs what is currently queued
• jest.setSystemTime(date) fixes Date.now for deterministic dates
• jest.useFakeTimers() can be scoped with jest.useFakeTimers({ doNotFake: [...] })

Modern fake timers also fake Date and performance, which is usually what you want for deterministic snapshots and time-based logic. They also affect queueMicrotask and process.nextTick depending on configuration. If a test only needs a fixed date, jest.setSystemTime is often simpler than faking every timer.