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https://github.com/nicolo-ribaudo/jest-light-runner

A Jest runner that runs tests directly in bare Node.js, without virtualizing the environment.
https://github.com/nicolo-ribaudo/jest-light-runner

esm javascript jest node test testing

Last synced: 8 days ago
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A Jest runner that runs tests directly in bare Node.js, without virtualizing the environment.

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# 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](https://github.com/babel/babel/pull/13966#pullrequestreview-819765720) the speed of [Babel](https://github.com/babel/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](https://github.com/nicolo-ribaudo/jest-light-runner/discussions/new) 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`, `jest.clearAllMocks`, `jest.resetAllMocks`

- Jest timer mocks: `jest.useFakeTimers`, `jest.useRealTimers`, `jest.setSystemTime`, `jest.advanceTimersByTime`

- Inline and external snapshots

- Jest cli options: `--testNamePattern`/`-t`, `--maxWorkers`, `--runInBand`

- Jest config options: `setupFiles`, `setupFilesAfterEnv`, `snapshotSerializers`, `maxWorkers`, `snapshotFormat`, `snapshotResolver`



### Unsupported Jest features



- `import`/`require` mocks. You can use a custom mocking library such as [`esmock`](https://github.com/iambumblehead/esmock) or [`proxyquire`](https://github.com/thlorenz/proxyquire).

- On-the-fly compilation (for example, with Babel or TypeScript). You can use a Node.js module loader, such as [`ts-node/esm`](https://github.com/TypeStrong/ts-node).

- 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`](https://nodejs.org/api/cli.html#--frozen-intrinsics) to prevent such modifications.

- `import.meta.jest`. You can use the `jest` global instead.



### 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`).

- Coverage reporting. This runner wires coverage data generated by [`babel-plugin-istanbul`](https://github.com/istanbuljs/babel-plugin-istanbul) (Jest's [default coverage provider](https://jestjs.io/docs/configuration#coverageprovider-string)). However, you have to manually run that plugin:

  - If you are already using Babel to compile your project and you are running Jest on the compiled files, you can add that plugin to your Babel configuration when compiling for the tests

  - If you are not using Babel yet, you can add a `babel.config.json` file to your project with these contents:

    ```json

    {

      "plugins": ["babel-plugin-istanbul"]

    }

    ```

    and you can run Babel in Jest using a Node.js ESM loader such as [`babel-register-esm`](https://github.com/giltayar/babel-register-esm).



## Usage



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



In `package.json`:



```json

{

  "jest": {

    "runner": "jest-light-runner"

  }

}

```



or in `jest.config.js`:



```js

module.exports = {

  runner: "jest-light-runner",

};

```



### Using custom Node.js ESM loaders



You can specify custom ESM loaders using Node.js's [`--loader`](https://nodejs.org/api/cli.html#--loadermodule) option. Jest's CLI doesn't allow providing Node.js-specific options, but you can do it by using the [`NODE_OPTIONS`](https://nodejs.org/docs/latest-v17.x/api/cli.html#node_optionsoptions) environment variable:



```bash

NODE_OPTIONS="--loader ts-node/esm" jest

```



Or, if you are using [`cross-env`](https://www.npmjs.com/package/cross-env) to be able to provide environment variables on multiple OSes:



```bash

cross-env NODE_OPTIONS="--loader ts-node/esm" jest

```



**Don't** run Node.js directly:



```bash

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

```



This will result in `ERR_UNKNOWN_FILE_EXTENSION`, due to the loader argument not being passed to the sub-processes. This is a [known limitation](https://github.com/TypeStrong/ts-node/issues/1062#issuecomment-1143518446), and ts-node documentation [recommends using NODE_OPTIONS](https://typestrong.org/ts-node/docs/usage/#node-flags-and-other-tools).



### Source maps support



If you are running transpiled code and you want to load their source maps to map errors to the original code, you can install the `source-map-support` package and add the following to your Jest configuration:



```js

setupFiles: ["source-map-support/register"];

```



## 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`](https://github.com/babel/babel/) and [`prettier/prettier`](https://github.com/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](https://github.com/sponsors/nicolo-ribaudo)! You can also donate to Jest on their [OpenCollective page](https://opencollective.com/jest).