jest-light-runner

A Jest runner that runs tests directly in bare Node.js, without virtualizing the environment.

Comparison with the default Jest runner

This approach is wasy faster than the default Jest runner (it more than doubled the speed of Babel‘s tests suite) and has complete support for the Node.js ESM implementation. However, it doesn’t provide support for most of Jest’s advanced features.

The lists below are not comprehensive: feel free to start a discussion regarding any other missing Jest feature!

Supported Jest features

  • Jest globals: expect, test, it, describe, beforeAll, afterAll, beoreEach, afterEach
  • Jest function mocks: jest.fn, jest.spyOn
  • Inline and external snapshots
  • --testNamePattern/-t, to only run some specific tests

Unsupported Jest features

  • import/require mocks. You can use a custom mocking library such as esmock or proxyquire.
  • Tests isolation. Jest runs every test file in its own global environment, meaning that modification to built-ins done in one test file don’t affect other test files. This is not supported, but you can use the Node.js option --frozen-intrinsics to prevent such modifications.

Partially supported features

  • process.chdir. This runner uses Node.js workers, that don’t support process.chdir(). It provides a simple polyfill so that process.chdir() calls still affect the process.cwd() result, but they won’t affect all the other Node.js API (such as fs.* or path.resolve).

Usage

After installing jest and jest-light-runner, add it to your Jest config.

In package.json:

{
  "jest": {
    "runner": "jest-light-runner"
  }
}

or in jest.config.js:

module.exports = {
    runner: "jest-light-runner",
};

Stability

This project follows semver, and it’s currently in the 0.x release line.

It is used to run tests in the babel/babel repository, but there are no tests for the runner itself. I would gladly accept a pull requests adding a test infrastructure!

Donations

If you use this package and it has helped with your tests, please consider sponsoring me on GitHub! You can also donate to Jest on their OpenCollective page

GitHub

View Github