spec.js

describe('My suite', () => {
  it('works with promises', done => {
    multiplyAsync(4, 5)
      .then(r => expect(r).toBe(20))
      .then(done);
  });
  it('works even if promises reject', done => {
    divideAsync(2, 0)
      .catch(e => expect(e.message).toBe('Attempted to divide by zero'))
      .then(done);
  });
});

function multiplyAsync(x, y) {
  return Promise.resolve(x * y);
}

function divideAsync(x, y) {
  if (y === 0) return Promise.reject(new Error('Attempted to divide by zero'));

  return Promise.resolve(x / y);
}