Install Jest usingyarn:

yarn add --dev jest

Ornpm:

npm install --save-dev jest

Note: Jest documentation usesyarn commands, butnpm will also work. You can compareyarn andnpm commands in theyarn docs, here.

Let's get started by writing a test for a hypothetical function that adds two numbers. First, create asum.js file:

functionsum(a, b){return a + b;}module.exports = sum;

Then, create a file namedsum.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 yourpackage.json:

{"scripts": {"test":"jest"  }}

Finally, runyarn test ornpm run test and Jest will print this message:

PASS  ./sum.test.js✓ adds 1 + 2 to equal 3 (5ms)

You just successfully wrote your first test using Jest!

This test usedexpect andtoBe to test that two values were exactly identical. To learn about the other things that Jest can test, seeUsing Matchers.

Running from command line

You can run Jest directly from the CLI (if it's globally available in yourPATH, e.g. byyarn global add jest ornpm install jest --global) with a variety of useful options.

Here's how to run Jest on files matchingmy-test, usingconfig.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 runningjest through the command line, take a look at theJest 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:

jest --init

Using Babel

To useBabel, install required dependencies viayarn:

yarn add --dev babel-jest @babel/core @babel/preset-env

Configure Babel to target your current version of Node by creating ababel.config.js file in the root of your project:

// babel.config.jsmodule.exports = {presets: [['@babel/preset-env', {targets: {node:'current'}}]],};

The ideal configuration for Babel will depend on your project. SeeBabel's docs for more details.

// babel.config.jsmodule.exports =api => {const isTest = api.env('test');// You can use isTest to determine what presets and plugins to use.return {// ...  };};

// jest.config.jsmodule.exports = {transform: {},};

"dependencies": {"babel-core":"^6.26.3","babel-jest":"^23.6.0","babel-preset-env":"^1.7.0","jest":"^24.0.0"}

Using webpack

Jest can be used in projects that usewebpack to manage assets, styles, and compilation. webpack does offer some unique challenges over other tools. Refer to thewebpack guide to get started.

Using parcel

Jest can be used in projects that useparcel-bundler to manage assets, styles, and compilation similar to webpack. Parcel requires zero configuration. Refer to the officialdocs to get started.

Using TypeScript

Jest supports TypeScript, via Babel. First, make sure you followed the instructions onusing Babel above. Next, install the@babel/preset-typescript viayarn:

yarn add --dev @babel/preset-typescript

Then add@babel/preset-typescript to the list of presets in yourbabel.config.js.

// babel.config.jsmodule.exports = {  presets: [    ['@babel/preset-env', {targets: {node: 'current'}}],+    '@babel/preset-typescript',  ],};

However, there are somecaveats to using TypeScript with Babel. Because TypeScript support in Babel is purely transpilation, Jest will not type-check your tests as they are run. If you want that, you can usets-jest instead, or just run the TypeScript compilertsc separately (or as part of your build process).

You may also want to install the@types/jest module for the version of Jest you're using. This will help provide full typing when writing your tests with TypeScript.

For@types/* modules it's recommended to try to match the version of the associated module. For example, if you are using26.4.0 ofjest then using26.4.x of@types/jest is ideal. In general, try to match the major (26) and minor (4) version as closely as possible.

yarn add --dev @types/jest