PluginPublic

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.

Quality

100%

Does it follow best practices?

Run evals on this skill

Adds up to 20 points to the overall score

View guide

Securityby

Passed

No findings from the security scan

This plugin was archived by the owner on Oct 11, 2026

Reason: Tile archived: Retired from testland/qa (merged into another skill or removed).

name:: jest-fake-timers
description:: 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.

jest-fake-timers

Overview

Per jestjs.io/docs/timer-mocks, Jest 27+ uses modern fake timers built on @sinonjs/fake-timers. The API differs slightly from raw Sinon - Jest exposes jest.advanceTimersByTime instead of clock.tick, and jest.setSystemTime instead of clock.setSystemTime.

When to use

Jest tests for code using setTimeout / setInterval / Date.
Promise-based async timing tests.
Replacing legacy useFakeTimers('legacy') with modern.

Authoring

Enable

beforeAll(() => {
  jest.useFakeTimers();
  jest.setSystemTime(new Date('2026-05-20T14:30:00Z'));
});

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

For a single test, call the same three lines inside the test body and jest.useRealTimers() at the end.

Advance time

test('debounce fires after 300ms', () => {
  let fired = false;
  setTimeout(() => { fired = true; }, 300);

  jest.advanceTimersByTime(299);
  expect(fired).toBe(false);

  jest.advanceTimersByTime(1);
  expect(fired).toBe(true);
});

Promise-based timers

For async code:

test('async debounce', async () => {
  let resolved = false;
  setTimeout(async () => {
    await fetchData();
    resolved = true;
  }, 100);

  await jest.advanceTimersByTimeAsync(100);
  expect(resolved).toBe(true);
});

Per Jest docs, advanceTimersByTimeAsync lets microtasks run between timer ticks.

Run all pending timers

test('chain of timeouts completes', () => {
  let count = 0;
  function recur() {
    if (++count < 5) setTimeout(recur, 100);
  }
  recur();

  jest.runAllTimers();
  expect(count).toBe(5);
});

Run only pending (not recursive)

jest.runOnlyPendingTimers();

Avoids infinite loops for self-scheduling code.

Selective faking, DST, and fetch

Selective faking (doNotFake), DST / timezone tests, and mixing fake timers with a mocked fetch are in references/advanced-scenarios.md.

Anti-patterns

Anti-pattern	Why it fails	Fix
`jest.useFakeTimers('legacy')`	Deprecated; doesn't fake Date	Modern by default since Jest 27
Forget `jest.useRealTimers()`	Subsequent tests inherit fake timers	`afterEach(jest.useRealTimers)`
`jest.advanceTimersByTime` for async chains	Microtasks don't drain	Use `Async` variant
Mix real fetch with fake timers	Fetch resolves at real time; tests race	Mock fetch
Hardcoded ms count for "5 minutes"	Brittle; magic numbers	Use named constants
Sleep loops in test body	Real time still passes when fake timers are off	Mock everything time-related
Skip setSystemTime, then call Date	Date returns real time	Always setSystemTime
Tests assume fake-timer state persists across files	Per-test or per-file; doesn't	Re-enable per file

Limitations

Modern fake timers are the default since Jest 27. Older projects may still use legacy.
doNotFake is fragile. Some functions internally use Date.now() - may behave unexpectedly.
DST + TZ interaction is Node-runtime-dependent. ICU data ships with Node.
advanceTimersByTime doesn't process Promises. Use the Async variant for promise-chain testing.

References

Jest timer mocks: jestjs.io/docs/timer-mocks.
@sinonjs/fake-timers (underlying): github.com/sinonjs/fake-timers.
Companion catalog: dst-transition-reference.
Sibling library (non-Jest test runners): sinon-fake-timers-js.
Cross-language: freezegun-python, timecop-ruby, mockclock-jvm, libfaketime-c.
Test matrix: timezone-test-matrix-builder.

.tessl-plugin

references

SKILL.md

tile.json