Getting Started
Install Jest using yarn
:
yarn add --dev jest
Or npm
:
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.
Let’s get started by writing a test for a hypothetical function that adds two numbers. First, create a sum.js
file:
function sum(a, b) {
return a + b;
}
module.exports = sum;
Then, create a file named sum.test.js
. This will contain our actual test:
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
:
{
"scripts": {
"test": "jest"
}
}
Finally, run yarn test
or npm test
and Jest will print this message:
PASS ./sum.test.js
ā 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.
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:
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 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:
yarn create jest
Using Babel
To use Babel, install required dependencies via yarn
:
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:
// 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 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: {}, }; ```Using webpack
Jest can be used in projects that use webpack to manage assets, styles, and compilation. webpack does offer some unique challenges over other tools. Refer to the webpack guide to get started.
Using Parcel
Jest can be used in projects that use parcel-bundler to manage assets, styles, and compilation similar to webpack. Parcel requires zero configuration. Refer to the official docs to get started.
Documentation
Learn more about using Jest on the official site!
Badge
Show the world you’re using Jest ā
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[![jest tested](https://img.shields.io/badge/Jest-tested-eee.svg?logo=jest&labelColor=99424f)](https://github.com/jestjs/jest)
[![jest](https://jestjs.io/img/jest-badge.svg)](https://github.com/jestjs/jest)
Code of Conduct
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.
Good First Issues
To help you get your feet wet and get you familiar with our contribution process, we have a list of good first issues that contain bugs which have a relatively limited scope. This is a great place to get started.
Backers
Thank you to all our backers! š
Copyright
Copyright Contributors to the Jest project.