🃏 Delightful JavaScript Testing
**👩🏻💻 Developer Ready**: A comprehensive JavaScript testing solution. Works out of the box for most JavaScript projects.
**🏃🏽 Instant Feedback**: Fast, interactive watch mode only runs test files related to changed files.
**📸 Snapshot Testing**: Capture snapshots of large objects to simplify testing and to analyze how they change over time.
See more on jestjs.io
## Table of Contents
- [Getting Started](#getting-started)
- [Running from command line](#running-from-command-line)
- [Additional Configuration](#additional-configuration)
- [Generate a basic configuration file](#generate-a-basic-configuration-file)
- [Using Babel](#using-babel)
- [Using Webpack](#using-webpack)
- [Using Parcel](#using-parcel)
- [Using Typescript](#using-typescript)
- [Documentation](#documentation)
- [Badge](#badge)
- [Contributing](#contributing)
- [Code of Conduct](#code-of-conduct)
- [Contributing Guide](#contributing-guide)
- [Good First Issues](#good-first-issues)
- [Credits](#credits)
- [Backers](#backers)
- [Sponsors](#sponsors)
- [License](#license)
## Getting Started
Install Jest using [`yarn`](https://yarnpkg.com/en/package/jest):
```bash
yarn add --dev jest
```
Or [`npm`](https://www.npmjs.com/package/jest):
```bash
npm install --save-dev jest
```
Note: Jest documentation uses `yarn` commands, but `npm` will also work. You can compare `yarn` and `npm` commands in the [yarn docs, here](https://yarnpkg.com/en/docs/migrating-from-npm#toc-cli-commands-comparison).
Let's get started by writing a test for a hypothetical function that adds two numbers. First, create a `sum.js` file:
```javascript
function sum(a, b) {
return a + b;
}
module.exports = sum;
```
Then, create a file named `sum.test.js`. This will contain our actual test:
```javascript
const sum = require('./sum');
test('adds 1 + 2 to equal 3', () => {
expect(sum(1, 2)).toBe(3);
});
```
Add the following section to your `package.json`:
```json
{
"scripts": {
"test": "jest"
}
}
```
Finally, run `yarn test` or `npm test` and Jest will print this message:
```bash
PASS ./sum.test.js
✓ adds 1 + 2 to equal 3 (5ms)
```
**You just successfully wrote your first test using Jest!**
This test used `expect` and `toBe` to test that two values were exactly identical. To learn about the other things that Jest can test, see [Using Matchers](https://jestjs.io/docs/using-matchers).
## Running from command line
You can run Jest directly from the CLI (if it's globally available in your `PATH`, e.g. by `yarn global add jest` or `npm install jest --global`) with a variety of useful options.
Here's how to run Jest on files matching `my-test`, using `config.json` as a configuration file and display a native OS notification after the run:
```bash
jest my-test --notify --config=config.json
```
If you'd like to learn more about running `jest` through the command line, take a look at the [Jest CLI Options](https://jestjs.io/docs/cli) page.
## Additional Configuration
### Generate a basic configuration file
Based on your project, Jest will ask you a few questions and will create a basic configuration file with a short description for each option:
```bash
jest --init
```
### Using Babel
To use [Babel](https://babeljs.io/), install required dependencies via `yarn`:
```bash
yarn add --dev babel-jest @babel/core @babel/preset-env
```
Configure Babel to target your current version of Node by creating a `babel.config.js` file in the root of your project:
```javascript
// babel.config.js
module.exports = {
presets: [['@babel/preset-env', {targets: {node: 'current'}}]],
};
```
_The ideal configuration for Babel will depend on your project._ See [Babel's docs](https://babeljs.io/docs/en/) for more details.
Making your Babel config jest-aware
Jest will set `process.env.NODE_ENV` to `'test'` if it's not set to something else. You can use that in your configuration to conditionally setup only the compilation needed for Jest, e.g.
```javascript
// babel.config.js
module.exports = api => {
const isTest = api.env('test');
// You can use isTest to determine what presets and plugins to use.
return {
// ...
};
};
```
> Note: `babel-jest` is automatically installed when installing Jest and will automatically transform files if a babel configuration exists in your project. To avoid this behavior, you can explicitly reset the `transform` configuration option:
```javascript
// jest.config.js
module.exports = {
transform: {},
};
```
### [Backers](https://opencollective.com/jest#backer)
Thank you to all our backers! 🙏
### [Sponsors](https://opencollective.com/jest#sponsor)
Support this project by becoming a sponsor. Your logo will show up here with a link to your website.
## License
Jest is [MIT licensed](./LICENSE).