Test fails when mock function returns rejected Promise

🐛 Bug Report

The following test fails because a rejected Promise is set as return value:

test("My test", async () => {
  const myMockFunc = jest.fn().mockReturnValueOnce(Promise.reject("error"))

  setTimeout(() => myMockFunc(), 100)
  await new Promise(resolve => setTimeout(resolve, 200))

  expect(myMockFunc.mock.calls.length).toBe(1)
})

Expected behavior

The test should pass. If e.g. a resolved Promise is returned it passes.

How to reproduce

Execute the test above.

Run npx envinfo --preset jest

System:
    OS: macOS 10.14.3
    CPU: (8) x64 Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz
  Binaries:
    Node: 11.12.0 - /usr/local/bin/node
    npm: 6.7.0 - /usr/local/bin/npm
  npmPackages:
    jest: ^24.5.0 => 24.5.0 

[WeiAnAn]

Because using Promise.reject() will create a rejected promise immediately, a rejected promise without catch will throw an error, so the test fails.

You can return rejected promise like below. It create a rejected promise when you call this function, instead of in the declaration.

jest.fn(() => Promise.reject('error'));
//or
jest.fn().mockImplementationOnce(() => Promise.reject('error'));

The test will look like:

test('My test', async () => {
  const myMockFunc = jest.fn(() => Promise.reject('error'));

  setTimeout(
    () =>
      myMockFunc().catch(e => {
        console.log(e); // error
      }),
    100
  );
  await new Promise(resolve => setTimeout(resolve, 200));

  expect(myMockFunc.mock.calls.length).toBe(1);
});

[xadh00m]

Thank you for your clarification. That cleared things up for me a lot. Thank you for this amazing test framework!