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 way 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, beforeEach, afterEach
Jest function mocks: jest.fn, jest.spyOn
Inline and external snapshots
Jest cli options: --testNamePattern/-t, --maxWorkers
Jest config options: setupFiles, snapshotSerializers, maxWorkers

Unsupported Jest features

import/require mocks. You can use a custom mocking library such as esmock or proxyquire.
On-the-fly compilation (for example, with Babel or TypeScript). You can use a Node.js module loader, such as ts-node/esm.
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",
};

Using custom Node.js ESM loaders

You can specify custom ESM loaders using Node.js's --loader option. Jest's CLI doesn't allow providing Node.js-specific options, but you can do it in two alternative ways:

Run Jest by explicitly running Node.js:

node --loader ts-node/esm ./node_modules/.bin/jest

Use the NODE_OPTIONS environment variable:
```
NODE_OPTIONS='--loader ts-node/esm' jest
```
or, if you are using cross-env to be able to provide environment variables on multiple OSes:
```
cross-env NODE_OPTIONS='--loader ts-node/esm' jest
```

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 and prettier/prettier repositories, but there are no internal 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.

psychobolt/jest-light-runner