// Tutorial //

Asynchronous Tests in Jest

Published on June 6, 2017
    Default avatar
    By Matthew Garcia
    Developer and author at DigitalOcean.
    Asynchronous Tests in Jest

    Not everything executes synchronously, so not everything should be tested synchronously. Jest offers a few options for asynchronous testing.

    Using a Callback

    If your function accepts a parameter, Jest will give it a done callback; the test will run until done or done.fail are called or it times out. This can be useful for testing observables:

    // Let's say you're testing a cup that emits an event when it is full or spills.
    it('fills without spilling.', (done) => {
      const smartCup = new SmartCup();
      // The event happened, so call `done`.
      smartCup.on('full', () => done());
      // If a failing condition happens, you can call `done.fail`.
      smartCup.on('spilled', () => done.fail());

    Jest will wait for done if the function accepts a parameter. So something like:

    it('works synchronously.', (done) => {

    …will run until it times out, since done is never called.

    Returning a Promise

    You can also make a test asynchronous by returning a promise:

    it('returns a number.', () => {
      return getId()
        // After `getId` resolves, you can test it against your expectations.
        .then(id => expect(typeof id).toEqual('number'));

    Since async functions return promises, you can use those, too:

    // Same as above.
    it('returns a number.', async () => {
      const id = await getId()
      expect(typeof id).toEqual('number');

    Want to learn more? Join the DigitalOcean Community!

    Join our DigitalOcean community of over a million developers for free! Get help and share knowledge in our Questions & Answers section, find tutorials and tools that will help you grow as a developer and scale your project or business, and subscribe to topics of interest.

    Sign up
    About the authors
    Default avatar
    Developer and author at DigitalOcean.

    Still looking for an answer?

    Was this helpful?
    Leave a comment