javascript - 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.

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.