mockResolvedValue / mockRejectedValue should returns a real Promise

🐛 Bug Report

mockResolvedValue and mockRejectedValue should return a real Promise testable with toBeInstanceOf but they don’t.

To Reproduce

    it('should returns a Promise', async () => {
      const fn = jest.fn().mockResolvedValue(123);
      const promise = fn();
      expect(promise).toBeInstanceOf(Promise);
      const result = await promise;
      expect(result).toBe(123);
    });

Expected behavior

It should pass The behaviour should be the same as using

const fn = jest.fn().mockReturnValue(Promise.resolve(123));

Run npx envinfo --preset jest

Paste the results here:

npx: installed 1 in 1.878s

  System:
    OS: macOS High Sierra 10.13.5
    CPU: x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
  Binaries:
    Node: 8.9.3 - ~/.nvm/versions/node/v8.9.3/bin/node
    Yarn: 1.7.0 - /usr/local/bin/yarn
    npm: 5.7.1 - ~/.nvm/versions/node/v8.9.3/bin/npm
  npmPackages:
    @types/jest: ^22.2.0 => 22.2.0 
    jest: ^22.4.3 => 22.4.4 

[yannbriancon]

I agree with @mhombach I struggled with tests for some time to end up discovering this issue.

Would be good to fix it or remove it completely.

[AoDev]

Hello, maybe it’s also related to this issue.

The error I get when using mockResolvedValue is:

TypeError: Cannot read property 'catch' of undefined

I have an async utility to be able to handle individual errors in a Promise.all. It looks like this:

/**
 * Return both failed and successful promises
 * @param {Array.<Promise>} promises
 */
function promiseAllResults (promises) {
  return Promise.all(promises.map((promise) => promise.catch((err) => err)))
}

If I use a “real promise” as a mock, it works. (but then I can not spy)