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?

Luke Willis · Accepted Answer · 2016-10-31T20:21:15.137

12

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.

edited Oct 31 '16 at 20:21

answered Oct 31 '16 at 19:53

Luke Willis

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Is there a way to test multiple such promises wihtin one test in Jest or do I have to make one test per promise? (async / await does not work in this case) – Sergej Jan 27 '18 at 07:27
@Sergej That depends on a few different things. If you're trying to verify the relationship between the promises, you might be able to [use `Promise.all`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all). If they're distinct, though, you probably want them in separate tests. – Luke Willis Jan 29 '18 at 17:53

score 3 · Answer 2 · edited May 23 '17 at 12:17

3

Alternatively, you can follow the done pattern as described here:

it('should execute promise', (done) => {
  someFunctionThatReturnsAPromise()
    .then(() => {
      expect(true).toBeFalsy();
      done();
    });
});

This will work with Jest, but is more commonly used with Jasmine and Mocha.

edited May 23 '17 at 12:17

Community

1
1

answered Oct 31 '16 at 20:19

Luke Willis

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score 1 · Answer 3 · edited Jul 11 '20 at 06:06

Here is the alternate solution.

Jest will be terminated once it reaches the end of context. So you need to return the promise from the callback to tell it to wait for the promise to get resolved and tested.

Lets say there is a promise

const promise=fetch("blah.com/api")

test("should return valid data",()=>{
   return expect(promise).resolves.toBeTruthy() 
})

.resolves waits for the promise to resolve, and you apply appropriate matchers as your wish.

And also you can use .rejects when you are checking for the error cases.

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

3 Answers3