Writing Unit Tests with Jest

What Jest feature enables testing time-dependent code?

jest.useFakeTimers();

test('delayed operation', () => {
  const callback = jest.fn();
  setTimeout(callback, 1000);
  
  jest.runAllTimers();
  expect(callback).toHaveBeenCalled();
});

Real timers
Fake timers
Fake timers enable time-dependent testing: 1) Replaces real timers with controlled versions, 2) Allows immediate execution of delayed operations, 3) Prevents tests from actual waiting, 4) Enables testing timing-dependent code, 5) Improves test speed and reliability, 6) Provides precise timing control, 7) Essential for testing animations or delays, 8) Makes time-based tests deterministic.
Time mocking
Delay simulation

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