Jest Mock Timers

// This has to be called before using fake timers.
jest.useFakeTimers();

it('closes some time after being opened.', (done) => {
  // An automatic door that fires a `closed` event.
  const autoDoor = new AutoDoor();
  autoDoor.on('closed', done);
  autoDoor.open();
  // Runs all pending timers. whether it's a second from now or a year.
  jest.runAllTimers();
});

jest.useFakeTimers();

it('announces it will close before closing.', (done) => {
  const autoDoor = new AutoDoor();
  // The test passes if there's a `closing` event.
  autoDoor.on('closing', done);
  // The test fails if there's a `closed` event.
  autoDoor.on('closed', done.fail);
  autoDoor.open();
  // Only move ahead enough time so that it starts closing, but not enough that it is closed.
  jest.runTimersToTime(autoDoor.TIME_BEFORE_CLOSING);
});