How to test timers (setTimeout, setInterval) in Jest? How to use fake timers?

Jest provides multiple methods for handling timers, used to test code involving setTimeout, setInterval, and other timers:

1. Using Fake Timers:

javascript
jest.useFakeTimers();

test('timer callback', () => {
  const callback = jest.fn();
  setTimeout(callback, 1000);
  
  jest.runAllTimers();
  expect(callback).toHaveBeenCalledTimes(1);
});

2. Run All Timers:

javascript
jest.runAllTimers(); // Run all pending timers

3. Advance to Specific Time:

javascript
jest.advanceTimersByTime(1000); // Advance 1000ms
jest.runOnlyPendingTimers(); // Run only currently pending timers

4. Testing setInterval:

javascript
test('interval timer', () => {
  const callback = jest.fn();
  setInterval(callback, 1000);
  
  jest.advanceTimersByTime(3000);
  expect(callback).toHaveBeenCalledTimes(3);
});

5. Cleanup Timers:

javascript
afterEach(() => {
  jest.useRealTimers(); // Restore real timers
  jest.clearAllTimers(); // Clear all timers
});

6. Testing Timer Clearing:

javascript
test('clear timeout', () => {
  const callback = jest.fn();
  const timeoutId = setTimeout(callback, 1000);
  clearTimeout(timeoutId);
  
  jest.runAllTimers();
  expect(callback).not.toHaveBeenCalled();
});

Best Practices:

• Use jest.useFakeTimers() at the start of test suites
• Restore real timers after testing
• Use advanceTimersByTime to test time-dependent logic
• Ensure cleanup of all timers to avoid memory leaks