babel-plugin-jest-hoist: The module factory of `jest.mock()` is not allowed to reference any out-of-scope variables.

🐛 Bug Report

Spying on mocked methods as described in docs, throws babel-plugin-jest-hoist: The module factory of jest.mock() is not allowed to reference any out-of-scope variables.:

const mockPlaySoundFile = jest.fn();
jest.mock('./sound-player', () => {
 return jest.fn().mockImplementation(() => {
   return {playSoundFile: mockPlaySoundFile};
   // Now we can track calls to playSoundFile
 });
});

https://jestjs.io/docs/en/es6-class-mocks#spying-on-methods-of-our-class

To Reproduce

Steps to reproduce the behavior:

Follow the docs here https://jestjs.io/docs/en/es6-class-mocks#spying-on-methods-of-our-class

Expected behavior

What is documented should work: https://jestjs.io/docs/en/es6-class-mocks#spying-on-methods-of-our-class

Link to repl or repo (highly encouraged)

Similiar issue https://github.com/facebook/jest/issues/2567

envinfo

npx: installed 1 in 1.27s

  System:
    OS: macOS 10.15.3
    CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  Binaries:
    Node: 10.19.0 - /usr/local/bin/node
    Yarn: 1.12.3 - /usr/local/bin/yarn
    npm: 6.13.4 - /usr/local/bin/npm
  npmPackages:
    jest: ^24.9.0 => 24.9.0

[AndyOGo]

@SimenB Thanks for your answer.

Please check the docs, I think especially this page and the coding example is misleading, in addition it does not mention the mock prefix 🤔 https://jestjs.io/docs/en/es6-class-mocks#spying-on-methods-of-our-class

Also the Readme of babel-plugin-jest-hoist doesn’t mention the mock prefix 🤔 https://github.com/facebook/jest/tree/master/packages/babel-plugin-jest-hoist

But the source code has a comment about it:

// We also allow variables prefixed with mock as an escape-hatch. https://github.com/facebook/jest/blob/master/packages/babel-plugin-jest-hoist/src/index.ts#L14

… and an implementation 🤔 https://github.com/facebook/jest/blob/master/packages/babel-plugin-jest-hoist/src/index.ts#L122

[AndyOGo]

Alright, it seems all mocked functions need to be prefixed with mock*, but if I do that, the value of the mock is undefined 🤔

out-of-scope Error:

const foo = jest.fn();
jest.mock('./foo', () => ({
  // throws: babel-plugin-jest-hoist: The module factory of jest.mock() is not allowed to reference any out-of-scope variables.
  foo,
}))

Prefixing with mock does not throw, but value of mock is undefined:

// prefixing with mock avoids out-of-scope error to be thrown
// but value of foo will be undefined
const mockFoo = jest.fn();
jest.mock('./foo', () => ({
  foo: mockFoo,
}))