Install Jest using your favorite package manager:

npminstall --save-dev jest

yarnadd--dev jest

pnpmadd --save-dev jest

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 test and Jest will print this message:

PASS  ./sum.test.js
✓ adds1 +2 to equal3(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:

npm init jest@latest

yarn create jest

pnpm create jest

Using Babel​

To useBabel, install required dependencies:

npminstall --save-dev babel-jest @babel/core @babel/preset-env

yarnadd--dev babel-jest @babel/core @babel/preset-env

pnpmadd --save-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:

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

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

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

return{
// ...
};
};

module.exports={
transform:{},
};

Using with bundlers​

Most of the time you do not need to do anything special to work with different bundlers - the exception is if you have some plugin or configuration which generates files or have custom file resolution rules.

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 Vite​

Jest is not supported by Vite due to incompatibilities with the Viteplugin system.

There are examples for Jest integration with Vite in thevite-jest library. However, this library is not compatible with versions of Vite later than 2.4.2.

One alternative isVitest which has an API compatible Jest.

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​

Viababel​

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

npminstall --save-dev @babel/preset-typescript

yarnadd--dev @babel/preset-typescript

pnpmadd --save-dev @babel/preset-typescript

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

module.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).

Viats-jest​

ts-jest is a TypeScript preprocessor with source map support for Jest that lets you use Jest to test projects written in TypeScript.

npminstall --save-dev ts-jest

yarnadd--dev ts-jest

pnpmadd --save-dev ts-jest

In order for Jest to transpile TypeScript withts-jest, you may also need to create aconfiguration file.

Type definitions​

There are two ways to haveJest global APIs typed for test files written in TypeScript.

You can use type definitions which ships with Jest and will update each time you update Jest. Install the@jest/globals package:

npminstall --save-dev @jest/globals

yarnadd--dev @jest/globals

pnpmadd --save-dev @jest/globals

And import the APIs from it:

import{describe, expect, test}from'@jest/globals';
import{sum}from'./sum';

describe('sum module',()=>{
test('adds 1 + 2 to equal 3',()=>{
expect(sum(1,2)).toBe(3);
});
});

Or you may choose to install the@types/jest package. It provides types for Jest globals without a need to import them.

npminstall --save-dev @types/jest

yarnadd--dev @types/jest

pnpmadd --save-dev @types/jest

Using ESLint​

Jest can be used with ESLint without any further configuration as long as you import theJest global helpers (describe,it, etc.) from@jest/globals before using them in your test file. This is necessary to avoidno-undef errors from ESLint, which doesn't know about the Jest globals.

If you'd like to avoid these imports, you can configure yourESLint environment to support these globals by adding thejest environment:

import{defineConfig}from'eslint/config';
importglobalsfrom'globals';

exportdefaultdefineConfig([
{
files:['**/*.js'],
languageOptions:{
globals:{
...globals.jest,
},
},
rules:{
'no-unused-vars':'warn',
'no-undef':'warn',
},
},
]);

Or useeslint-plugin-jest, which has a similar effect:

{
"overrides":[
{
"files":["tests/**/*"],
"plugins":["jest"],
"env":{
"jest/globals":true
}
}
]
}