Jest mock/spy returns undefined even when set up

🐛 Bug Report

Mocking spy return does nothing

I’m about to lose my mind here; it seems the entire mock system is b0rked. I copy-paste the examples from the docs, and they don’t work.

To Reproduce

// __mocks__/someUtil.js
export default {
  m1: jest.fn(() => 42),
}

// someUtil.test.js
import someUtil from './someUtil';

jest.mock('./someUtil')

console.log(someUtil.m1) // expected spy stuff
console.log(someUtil.m1()) // undefined 🤯

Other variations that also do nothing

// __mocks__/someUtil.js

throw; // does throw

export default {
  m1: jest.fn(() => throw), // does NOT throw
}

// ---

export default {
  m1: jest.fn().mockImplementation(() => 42),
}

// ---

export default {
  m1: jest.fn().mockImplementation(() => throw), // does NOT throw
}

// ---

export default {
  m1: jest.fn().mockReturnValue(42),
}

// ---

export default () => ({
  m1: jest.fn().mockImplementation(() => 42),
})

// ---

export default () => ({
  m1: jest.fn().mockReturnValue(42),
})

Expected behavior

As documented: it should return 42

envinfo

jest version: 24.7.1

[dstapleton92]

A little late here, but I was just having this exact issue. I discovered that someone had added resetMocks: true to the jest.config.js file. This means that the implementations of mock functions are reset before each test. So in our case, the mock function was being included in the mocked module at test runtime, but that mock had been reset, so it returned undefined.

Regarding the original issue build environment, it looks like react-scripts does indeed add resetMocks: true into the jest config. (https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/scripts/utils/createJestConfig.js#L69) But you can override it on the jest key of your package.json. (https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/scripts/utils/createJestConfig.js#L74)

[HenryCharlesAnderson]

resetMocks: true was the culprite for me too.

mock implementations can work with resetMocks: true if you setup the mocks in beforeEach, or directly inside the test/it callbacks.

If you set up the mocks at the top level of the module, in the describe callback (but outside of any it/test callback), or in beforeAll, they get overwritten by resetMocks AFAICT