Async Jest

By default, Jest tests complete once they reach the end of their execution. Therefore, async tests will not work by default

Callbacks

This code doesn't work, because fetchData returns before callback

// Don't do this!
test('the data is peanut butter', () => {
  function callback(data) {
    expect(data).toBe('peanut butter')
  }

  fetchData(callback);
})

We can solve this with the done arg

test('the data is peanut butter', done => {
  function callback(data) {
    try {
      expect(data).toBe('peanut butter')
      done()
    } catch (error) {
      done(error)
    }
  }

  fetchData(callback)
})

Now, Jest will wait until the done callback is called before finishing the test.

• if never called, it will fail with a timeout error, which is the desired way of handling this.
• If we want to see in the test log why it failed, we have to wrap expect in a try block (as shown above)