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testland/jest-fake-timers

Wraps Jest's built-in modern fake-timers (built on @sinonjs/fake-timers since Jest 27): jest.useFakeTimers(), jest.setSystemTime(), jest.advanceTimersByTime(), jest.runAllTimers(), and jest.useRealTimers() for selective restoration. Use when testing JS/TS code in Jest where setTimeout / setInterval / Date / Date.now need deterministic control.

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Files

advanced-scenarios.mdreferences/

jest fake-timers advanced scenarios

Selective faking, DST/timezone tests, and fake-timer + mocked fetch interplay extracted from the core skill. Enable / advance / run-all usage stays in SKILL.md.

Selective faking

jest.useFakeTimers({
  doNotFake: ['nextTick', 'queueMicrotask'],
  now: new Date('2026-05-20T14:30:00Z').getTime(),
});

DST tests

Set the runtime zone before enabling fake timers, then set system time to the UTC instant of the transition.

beforeAll(() => {
  process.env.TZ = 'America/New_York';
  jest.useFakeTimers();
});

test('spring-forward behaviour', () => {
  jest.setSystemTime(new Date('2026-03-08T06:30:00Z'));  // 02:30 EDT - invalid local
  expect(new Date().toString()).toMatch(/03:30/);  // Browser/Node normalises
});

Mix fake-timers with real fetch

If fetch is mocked separately, ensure the mock awaits a faked timer too:

test('debounce + fetch', async () => {
  global.fetch = jest.fn().mockResolvedValue({ json: () => ({ ok: true }) });

  myDebouncedFetch();

  await jest.advanceTimersByTimeAsync(300);
  expect(fetch).toHaveBeenCalled();
});

references

advanced-scenarios.md

SKILL.md

tile.json