Why is my async Jest test not failing when it should?

Question

I have some asynchronous actions that I need to test with Jest. My test is currently passing when it should fail.

describe('Asynchronous Code', () => {
  it('should execute promise', () => {
    console.log(1);
    someFunctionThatReturnsAPromise()
      .then(() => {
        console.log(2);
        expect(true).toBeFalsy();
        console.log(3);
      });
    console.log(4);
  });
});

When I run npm test, I get the following output:

PASS  __tests__/Async.test.js
 ● Console

   console.log __tests__/Async.test.js:3
     1
   console.log static-content-test/react/actions/DashboardActions.test.js:6
     2
   console.log static-content-test/react/actions/DashboardActions.test.js:10
     4

As you can see, the test is passing, but console.log(3) is never executed because true is not falsy, and the expectation fails.

How can I get Jest to recognize my expectations inside async callbacks?

Answer 1

When testing asynchronous code, you need to return the promise from the test. Change the test body to:

return someFunctionThatReturnsAPromise()
  .then(() => {
    expect(true).toBeFalsy();
  });

With that, the test fails as expected:

FAIL  __tests__/Async.test.js
 ● Asynchronous Code › should execute promise

   expect(received).toBeFalsy()

   Expected value to be falsy, instead received
     true

This is the pattern facebook uses for testing async code with jest.