Best Way to Test Promises in Jest

问题:

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));
    });            
});

回答1:

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);
  });
});

回答2:

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.