// Tutorial //

Asynchronous Tests in Jest

Published on June 6, 2017
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    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());
      smartCup.fill();
    });
    

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

    it('works synchronously.', (done) => {
      expect(1).toBeGreaterThan(0);
    });
    

    …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');
    });
    

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    About the authors
    Default avatar
    Developer and author at DigitalOcean.

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