mockResolvedValue / mockRejectedValue should returns a real Promise

[jbdemonte]

🐛 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 

[SimenB]

I wonder if this is due to #2549?

[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)

[idler8]

facebook/create-react-app#10571 (comment)
@AoDev

[github-actions]

This issue is stale because it has been open for 1 year with no activity. Remove stale label or comment or this will be closed in 30 days.

[mhombach]

This issue isn't fixed for 4 years (?!) and it's still causing problems like this (#13206)? Please fix asap or remove that sugar-function.

[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.

[SimenB]

https://github.com/facebook/jest/releases/tag/v29.2.2

[mhombach]

@SimenB hey :) I tested the v29.2.2 but for me it still shows a difference in instanceof:

This is tested with a brandnew angular project using Jest 29.2.2.

I also tested with without the fakeAsync, same result:

Any idea?

[SimenB]

@mhombach that's probably a separate issue - the reproduction from the OP passes now. Could you open up a new issue with a minimal reproduction?

[mhombach]

@SimenB I created a repro-repo (https://github.com/mhombach/jest_repro_6645) can you have a look? The thing is that if I test

const resolvedPromise = jest.fn().mockResolvedValue('hello')();
const rejectedPromise = jest.fn().mockRejectedValue('hello')();

expect(resolvedPromise).toBeInstanceOf(Promise);
expect(rejectedPromise).toBeInstanceOf(Promise);

the unit test seems to not fail hard but just be "not passing" in some way:

Please use npm ci when installing the packages to make sure you have the exact same pacakges as I do.
If you verify to me after testing that this is some other issue, I will ofc open a new issue. But just now my tests are somewhat "silently failing" when I use exactly the test lines you added in the jest fix.

PS: Use the file app.component.spec.ts for testing, it contains the tests, partially commented out.

[SimenB]

You need to await at least the rejected promise, or it's an unhandled rejection.

---

Can you open up a new issue? This one is resolved and easy to lose track of 🙂

[mhombach]

@SimenB I feel there is a misunderstanding here.
I am using the exactly same code you are using in the official Jest unit test that is added in the new release to make sure the change works properly. This code "silently fails" the unit test, as it's preventing the test from exiting with some error code. This means, that the unit test, which was added in the "fix", is not properly testing the new implementation... Or am I missing something?
Again: If I would now open a new issue, the issue would contain the same you added to the jest lib and then I would claim that this code does not work properly, which can be reproduced by my linked repo... That sounds conflicting to you saying that it's not related to this issue here and that the fix you implemented works properly.

Just to make it even more precise:

This is the test that you added (if I got it right) to make sure the change in your fix works as expected.

const resolvedPromise = jest.fn().mockResolvedValue('hello')();
const rejectedPromise = jest.fn().mockRejectedValue('hello')();

expect(resolvedPromise).toBeInstanceOf(Promise);
expect(rejectedPromise).toBeInstanceOf(Promise);

The same code is what I am testing locally. And on my machine it's silently failing. Which makes me believe, that it's also failing silently on Github-CI. Which means, the test is flawed and you cannot ensure that the functionality of your fix is working as expected.

Again, if this is some huge misunderstanding, correct me and I will gladly open a new issue :)

[SimenB]

You're missing the await expect(rejectedPromise).rejects.toBe('hello') part at the bottom (and the resolved promise, but that doesn't matter in this context as it resolves) which means the rejected promise is unhandled, and fails the test run

[mhombach]

@SimenB

This is 100% the code you added in the fix (except the one commented line which I added.
This test should be failing (obviously) because of the expect(true).toBeFalsy();, but it isn't.
The code silently fails, which means it would never catch one of the other failing expects, right?

[SimenB]

Same thing - expect throws which means you never attach a rejection handler to the rejected promise, leading to node throwing an unhandled rejection error.

Put expect(true).toBeFalsy() below await expect(rejectedPromise).rejects.toBe('hello'); and it'll fail like you expect 🙂

[mhombach]

@SimenB This means if I negate the 2 tests you have there by adding a not...

your 2 expects are also not failing, which makes them irrelevant/wrong (?):

Adding 2 console-logs there you can see that the testing of the "instance of" is not working at all:

... Which was the hole purpose of this mockRejectedValue should returns a real Promise PR, right? :/

[SimenB]

Please open up a new issue with a minimal reproduction