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 thebabel-jest andregenerator-runtime packages:

yarn add --dev babel-jest babel-core regenerator-runtime

Note: If you are using Babel version 7 then you need to installbabel-jest,babel-core@^7.0.0-bridge.0 and@babel/core with the following command:

yarn add --dev babel-jest babel-core@^7.0.0-bridge.0 @babel/core regenerator-runtime

You will need to usebabel.config.js in order to transpilenode_modules. Seehttps://babeljs.io/docs/en/next/config-files for more information.

You can also see the example in the Jest repository:https://github.com/facebook/jest/tree/54f4d4ebd3d1a11d65962169f493ce41efdd784f/examples/babel-7

Note: Explicitly installingregenerator-runtime is not needed if you usenpm 3 or 4 or Yarn

Don't forget to add a.babelrc file in your project's root folder. For example, if you are using ES6 andReact.js with thebabel-preset-env andbabel-preset-react presets:

{"presets": ["env","react"]}

You are now set up to use all ES6 features and React specific syntax.

Note: If you are using a more complicated Babel configuration, using Babel'senv option, keep in mind that Jest will automatically defineNODE_ENV astest. It will not usedevelopment section like Babel does by default when noNODE_ENV is set.

Note: If you've turned off transpilation of ES6 modules with the option{ "modules": false }, you have to make sure to turn this on in your test environment.

{"presets": [["env", {"modules":false}],"react"],"env": {"test": {"presets": [["env"],"react"]    }  }}

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 thetransform configuration option:

// package.json{"jest": {"transform": {}  }}

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

To use TypeScript in your tests you can usets-jest.

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