Best Way to Test Promises in Jest

Question

Unless I'm misunderstanding something, the resolves and rejects (https://facebook.github.io/jest/docs/expect.html#resolves) won't be available until vNext. What is the recommended way now/in the meantime to test promises with Jest? Is it just putting expects in the thens and catches?

For example:

describe('Fetching', () => {
    const filters = {
        startDate: '2015-09-01'
    };
    const api = new TestApiTransport();

    it('should reject if no startdate is given', () => {
        MyService.fetch().catch(e => expect(e).toBeTruthy()); // see rejects/resolves in v20+
    });            

    it('should return expected data', () => {
        MyService.fetch(filters, null, api).then(serviceObjects => {
            expect(serviceObjects).toHaveLength(2);
        }).catch(e => console.log(e));
    });            
});

UPDATE 15 June 2019: Not too long after I posted this question, Jest started supporting this out of the box. I changed the accepted answer below to reflect the currently best way to do this.

UPDATE 8 Dec 2021: At some point Jest started supporting async/await. So while other methods noted work, I've taken to simply (for most cases) using something like:

it('should do something', async () => {
    const expected = true; 
    expect(await funcToTest()).toEqual(expected);
});

As with most cases, async/await is much more readable than alternatives. The only case I use resolves or rejects now is for simple cases like:

it('should not throw when doing something', async () => {
    await expect(funcToTest()).resolves.not.toThrow();
});

it('should throw when something is wrong', async () => {
    await expect(funcToTest()).rejects.toThrow();
});

Answer 1

Nowadays you can write it in this way as well: docs

describe('Fetching', () => {
    const filters = {
        startDate: '2015-09-01'
    };
    const api = new TestApiTransport(); 

 it('should reject if no startdate is given', () => {
   expect.assertions(1);
   return expect(MyService.fetch()).rejects.toEqual({
     error: 'Your code message',
   });
 });          


 it('should return expected data', () => {
   expect.assertions(1);
   return expect(MyService.fetch(filters, null, api)).resolves.toEqual(extectedObjectFromApi);
 });            
});

Update (06.01.2019)

Agree that the accepted answer doesn't work correctly as line expect.assertions(1); does all the magic. Link to docs

expect.assertions(number) verifies that a certain number of assertions are called during a test. This is often useful when testing asynchronous code, in order to make sure that assertions in a callback actually got called.

So putting this line at the top will control that the specific number of assertions are made by the time when the test is run.

Answer 2

Either return a promise and expect in the resolve or catch

describe('Fetching', () = > {
  const filters = {
    startDate: '2015-09-01'
  };
  const api = new TestApiTransport();
  it('should reject if no startdate is given', () = > {
    return MyService.fetch()
      .catch (e => expect(e).toBeTruthy()); // see rejects/resolves in v20+
  });
  it('should return expected data', () = > {
    return MyService.fetch(filters, null, api)
      .then(serviceObjects => {
        expect(serviceObjects).toHaveLength(2);
      })
  });
});

or using async/await

describe('Fetching', () = > {
  const filters = {
    startDate: '2015-09-01'
  };
  const api = new TestApiTransport();
  it('should reject if no startdate is given', async() = > {
    try {
      const r = await MyService.fetch()
    } catch (e) {
      expect(e).toBeTruthy()
    }
  });
  it('should return expected data', async() = > {
    const serviceObjects = await MyService.fetch(filters, null, api)
    expect(serviceObjects).toHaveLength(2);
  });
});

Answer 3

I was able to test JEST with AXIOS for HTTP REST calls like this.

it('has an API worth testing', async () => {
  let httpResult = null;
  await callThefunctionThatReturnsPromiseToMakeTheAxiosApiCall()
    .then(function(result) {httpResult=result;})
    .catch(function(err) {httpResult=err;});
  expect(httpResult.data.myData).toBe("myExpectedValue");
});

or

it('has an API worth testing', async () => {
  let httpResult = await callThefunctionThatReturnsPromiseToMakeTheAxiosApiCall();
  expect(httpResult.data.myData).toBe("myExpectedValue");
});

Answer 4

For additional Jest matchers maintained by the Jest Community check out jest-extended.

^{https://jestjs.io/docs/expect}

Using jest-extended you can expect your promise toResolve() or toReject(). Then you can expect the result or the error to match something. For example:

test('OK status', async () => {
  const request = fetch(...)
  await expect(request).toResolve() // First, make sure it resolves
  const data = await request
  expect(data).toEqual(...) // Then test the result
})
test('ERROR status', async () => {
  const request = fetch(...)
  await expect(request).toReject() // First, make sure it rejects
  await request.catch((error) => expect(error).toBe('...')) // Then test the error
})