Why is a mocked jest promise not rejected with allSettled?

Question

I want to test a method which returns the result of Promise.allSettled() and calls another function which returns promises.

I reduced the problem to the following test code:

  describe('Promise tests', () => {
    it('should reject directly', async () => {
      const f = jest.fn().mockRejectedValue(new Error('foo'));
      const p = async () => await f();

      // works
      await expect(p).rejects.toThrow('foo');
    });

    it('should reject with allSettled', async () => {
      const f = jest.fn().mockRejectedValue(new Error('foo'));
      const p = async () => await f();

      const results = await Promise.allSettled([p]);
      expect(results[0].status).toBe('rejected'); // fulfilled - but why?
      expect(results[0].reason).toBe('foo');
    });
  });

Why is the second case not receiving a rejected promise?

• node.js v14.3.0
• jest v25.4.0

Answer 1

You are almost there. Promise.allSettled expects to receive an array of Promises not an array of functions returning a promise, which in fact is what your constant p does.

By simply calling the p() you solve your issue:

  describe('Promise tests', () => {
    it('should reject directly', async () => {
      const f = jest.fn().mockRejectedValue(new Error('foo'));
      const p = async () => await f();

      // works
      await expect(p()).rejects.toThrow('foo');
    });

    it('should reject with allSettled', async () => {
      const f = jest.fn().mockRejectedValue(new Error('foo'));
      const p = async () => await f();

      const results = await Promise.allSettled([p()]);
      expect(results[0].status).toBe('rejected'); // fulfilled - but why?
      expect(results[0].reason).toBe('foo');
    });
  });

By the way: My linter complains about unnecessary awaits :-)