Learning Redux

.resolves/.rejects

Since Jest Version 20, special matchers for promises are provided. Instead of returning a promise and using .then to check the result, you can use .resolves, as follows:

test('fetchPosts returns array of posts', () => {  expect.assertions(1)  return expect(fetchPosts()).resolves.toContain({ title: 'test' })})

Make sure to return the assertion; otherwise, the test will complete early (but still fail, because we expected one assertion).

The same also works for failing promises, via .rejects:

test('logging in without username/password should fail', () => {  expect.assertions(1)  return expect(login()).rejects.toMatch('could not login')})