Jest Mock Timers

Matthew Garcia

One of the biggest hurdles to testing timeouts is waiting for them to time out. Jest provides a way around this.

runAllTimers

Let’s say you’re testing a program that emits an event after some time, but you don’t want to wait however long for that event to actually be emitted. Jest gives you the option of instantly running callbacks set with setTimeout through the jest.runAllTimers function.

// 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();
});

runTimersToTime

But what if you don’t want to run all timers? What if you only want to simulate some time passing? runTimersToTime is here for you.

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);
});
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