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docs: rephrase the readme
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kettanaito committed Mar 18, 2024
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<h1 align="center">jest-fixed-jsdom</h1>
<p align="center">This library helps re-attach missing NodeJS globals definitions in JSDOM that Jest strips out</p>
<p align="center">A superset of the JSDOM environment for Jest that respects Node.js globals.</p>

## Motivation

Jest strips out a number of NodeJS globals that are used in tests and libraries involving JSDOM, such as structuredClone, ReadableStream, and so on. This library patches these globals back in - there is no polyfilling or mocking involved, it simply re-attaches the missing globals to the JSDOM environment. If you've ever come across errors such as `ReferenceError: ReadableStream is not defined` or `ReferenceError: structuredClone is not defined`, this library is for you. If you were previously using (undici)[https://www.npmjs.com/package/undici] purely to solve this, you will no longer need it.
When you use Jest with JSDOM you are getting a broken test environment. Some Node.js globals cease to exist (e.g. `Request`, `Response`, `TextEncoder`, `TextDecoder`, `ReadableStream`<sup><a href="https://github.com/mswjs/msw/issues/1916">1</a></sup>), while others stop behaving correctly (e.g. `Event`, `MessageEvent`<sup><a href="https://github.com/nodejs/undici/issues/2663">2</a></sup>, `structuredClone()`<sup><a href="https://github.com/mswjs/msw/issues/1931">3</a></sup>). That is caused by `jest-environment-jsdom` and JSDOM relying on polyfills to implement standard APIs that have been available globally both in the browser and in Node.js for years.

## Installation
Here's a piece of valid JavaScript that works in both the browser and Node.js but fails in Jest/JSDOM:

`npm install jest-fixed-jsdom -D`
```js
new TextEncoder().encode('hello')
```

```
ReferenceError: TextEncoder is not defined
```

**We strongly believe that a valid JavaScript code must compile regardless of what test environment you are using**. In fact, having a proper test environment is crucial to get any kind of value from your tests. Jest/JSDOM take that already working environment away from you.

We've built this project aims to fix that problem, restoring the global APIs that are present in both environments, providing better interoperability, stability, and consistent runtime behavior.

## Changes

## Configuring your jest environment to use jest-fixed-jsdom
This project "fixes" the following global APIs, overriding whichever polyfills they have with respective Node.js globals:

You will need to add/modify two properties in your jest configuration to use jest-fixed-jsdom.
- `EventTarget`
- `Event`
- `MessageEvent`
- `fetch()`
- `Blob`
- `FormData`
- `Headers`
- `Request`
- `Response`
- `ReadableStream`
- `TextEncoder`
- `TextDecoder`
- `structuredClone()`

## Getting started

### Install

```sh
npm i jest-fixed-jsdom --save-dev
```

### Configure Jest

In your `jest.config.js`, set the `testEnvironment` option to `jest-fixed-jsdom`:

```json
// jest.config.js
{
"testEnvironment": "jest-fixed-jsdom"
}
```

Setting `testEnvironment` to `jest-fixed-jsdom` will tell Jest to use jest-fixed-jsdom as the test environment. Setting `customExportConditions` to an empty array will stop JSDOM from using the browser environment to load exports.

> You can use any other `testEnvironmentOptions` you need. Those will be forwarded to the underlying `jest-environment-jsdom`.

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