Allow mocking `require.resolve`

🚀 Feature Proposal

The require.resolve function is re-created for every file: this is needed so that it starts resolving from the correct location.

However, this makes it impossible to mock it: require.resolve = jest.fn() doesn’t work. I propose adding an API similar to jest.mock specifically for require.resolve’s behavior.

Motivation

We have different tests in @babel/core that test plugins/presets aliasing (ref). For example, we test that when someone uses @babel/env in their config Babel tries to resolve @babel/preset-env.

We currently rely on “fake” node_modules folders, however:

1. We don’t want to test node’s resolution algorithm, we should only test what we are asking node to resolve.
2. Using real FS just to check plugins/presets aliasing makes our tests slower.
3. This approach doesn’t work with Yarn PnP, because it changes the resolution behavior (and it shouldn’t affect our tests).

Example

Maybe something like this?

jest.mockResolution("@babel/env", __dirname + "/fake/@babel/preset-env");

// test

jest.unmockResolution("@babel/env");

Pitch

Mocks are already handled by the Jest core platform 😁

[SimenB]

The feature has been accepted, so no need to try to convince us 🙂 PR very much welcome!

[TheHumbleJester]

Hey guys ! I know that’s not exactly what you’re looking for, but here’s my workaround to be able to “mock” require.resolve. There’s an interesting feature in jest allowing you to define your own module resolver: https://jestjs.io/docs/en/configuration#resolver-string

Having that in mind, here is, basically, what I’ve done:

• I created my own resolver like so:

const glob = require('glob');

let mapping = {};

// Looks for "module-resolution.json" files in all the `__tests__` directories
glob
  .sync(`${__dirname}/../../packages/**/src/**/__tests__/modules-resolution.json`)
  .forEach((file) => {
    // For each of them, merges them in the "mapping" object
    mapping = { ...mapping, ...require(file) };
  });

function resolver(path, options) {
  // If the path corresponds to a key in the mapping object, returns the fakely resolved path
  // otherwise it calls the Jest's default resolver
  return mapping[path] || options.defaultResolver(path, options);
}

module.exports = resolver;

• Then, I configured it in my jest config file:

module.exports = {
  roots: ['<rootDir>'],
  testMatch: ['<rootDir>/packages/**/__tests__/**/*.test.js'],
  collectCoverageFrom: ['packages/**/src/**/*.js', '!packages/**/src/__tests__/**'],
  resolver: '<rootDir>/test-utils/resolver',
};

• Finally, I can create modules-resolution.json files in my test folders that look like this:

{
  "fake-module": "/path/to/fake-module"
}

This do the job for me so far and I think that, starting from this example, we could do something more complex but more developer friendly ! Of course, that would be even better if this feature could be directly included in jest ! Anyway, I hope this will help some of you 😉