Fake timers are paired with explicit clock control

Calling vi.useFakeTimers() or jest.useFakeTimers() freezes the JavaScript clock at the moment of the call. Any code that depends on setTimeout, setInterval, or Date.now() then hangs indefinitely — the timer callback never fires because the clock never advances. Tests that use useFakeTimers without a corresponding vi.advanceTimersByTime() or vi.runAllTimers() will either timeout or silently pass without actually executing the time-dependent code path. ISO-25010:2011 testability requires that all branches of code under test are actually exercised.

Always pair vi.useFakeTimers() with an explicit clock advancement, and restore real timers after the test. The test cannot verify timer-dependent behavior unless the clock is manually advanced:

it('debounces rapid input', () => {
  vi.useFakeTimers()
  const callback = vi.fn()
  const debounced = debounce(callback, 100)
  debounced()
  debounced()
  debounced()
  vi.advanceTimersByTime(100) // fires the debounced callback
  expect(callback).toHaveBeenCalledTimes(1)
  vi.useRealTimers()
})