Guide · Open source

How to use Jest: install, write your first test and run it

Updated 2026-10-29 · written with AI from the project's own documentation and checked against our catalogue data

Jest is an open-source testing tool for JavaScript code. You write small test files that call your functions and check the results against expected values, and Jest runs them and reports which pass or fail. This guide is for developers new to Jest, and by the end you will be able to install it, write and run a test, and set up a basic configuration.

Jestjestjs/jest · ★ 45.5k · MIT Install options & downloads

Quick start

1Check prerequisites. Make sure Node.js and npm or yarn are installed on your machine.
2Install Jest. Run npm install --save-dev jest or yarn add --dev jest in your project.
3Write the code to test. Create a file such as sum.js with the function you want to check.
4Write a test file. Create sum.test.js, call your function and use expect with a matcher such as toBe.
5Add a test script. Set "test": "jest" under scripts in package.json.
6Run the tests. Run yarn test or npm test and read the PASS or FAIL output.

Install Jest with npm or yarn

Jest is installed as a dev dependency in your project. The README shows two options: npm or yarn. Both work; the Jest documentation uses yarn commands, but npm works the same way.

You need Node.js, plus npm or yarn, installed first.

With npm: install it as a dev dependency
With yarn: add it as a dev dependency

npm install --save-dev jest

yarn add --dev jest

Write your first test

Start with a small function and a test file for it. Create sum.js with a function that adds two numbers and exports it.

Then create sum.test.js. This file calls the function and uses expect and toBe to check that the result is exactly 3.

sum.js holds the code you want to test
sum.test.js holds the test that checks it
toBe checks that two values are exactly identical

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

const sum = require('./sum');

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

Add a test script and run Jest

Add a test script to the scripts section of your package.json that runs jest. Then run your test script with yarn or npm. Jest prints each test file with PASS or FAIL and lists each test result.

To learn about the other things Jest can check, read the Using Matchers page on jestjs.io.

Add "test": "jest" under scripts in package.json
Run yarn test or npm test

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

Run Jest from the command line

You can run Jest directly with the jest command if it is available in your PATH, for example after a global install. You can pass options, such as a file name pattern, a configuration file, and a native OS notification after the run.

The Jest CLI Options page on jestjs.io lists the full set of options.

--notify displays a native OS notification after the run
--config points to a configuration file

jest my-test --notify --config=config.json

Generate a basic configuration file

If you want a configuration file, Jest can create one for you. It asks a few questions about your project and writes a basic configuration file with a short description for each option.

Jest also has a dedicated configuration reference on jestjs.io for changing options later.

Run the create command once in your project

yarn create jest

Use Jest with Babel, TypeScript or a bundler

Jest works with Babel for transforming code. The README lists install steps for Babel with babel-jest, @babel/core and @babel/preset-env, and suggests a babel.config.js targeting the current Node version. Jest sets process.env.NODE_ENV to 'test' when it is not set, which you can use in your Babel config.

For TypeScript, Jest supports it via Babel using @babel/preset-typescript. Be aware that Babel does not type-check your tests as they run. If you want type checking, the README suggests ts-jest or running the TypeScript compiler tsc separately.

The README also links guides for webpack, Parcel and Vite. It notes that Jest is not fully supported by Vite, so check the limitations before you adopt that setup.

Babel: install babel-jest, @babel/core and @babel/preset-env
TypeScript: add @babel/preset-typescript, or use ts-jest or tsc for type checking
webpack and Parcel: see the linked guides
Vite: check the stated limitations first

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

What people use it for

Checking that a utility function such as sum returns the expected value
Adding automated tests to an existing Node-based or front-end project
Re-running only the tests related to files you changed while editing, using watch mode
Saving snapshots of large objects or outputs to see when they change over time
Running tests on a TypeScript project through Babel, with type checking handled separately

Frequently asked questions

Is Jest free to use?

Yes. The repository is released under the MIT licence, as shown in the README badge and the LICENSE link.

What do I need to install Jest?

The README lists Node.js, plus npm or yarn as the package manager. You install Jest as a dev dependency in your project.

Can Jest test TypeScript?

Yes, through Babel with @babel/preset-typescript. The README notes that this does not type-check tests, so use ts-jest or tsc if you need type checking.

Does Jest do browser end-to-end testing?

The README describes Jest as a unit-style test runner and does not mention browser automation. For end-to-end browser tests, a different tool is needed.

What is the difference between Jest and Vitest?

Vitest is listed as a comparable project. This guide does not cover a detailed comparison, so check each project's documentation for differences that matter to your project.

How do I create a Jest config file?

Run yarn create jest. Jest asks a few questions based on your project and creates a basic configuration file with a description for each option.

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Our verdict

Jest suits JavaScript and TypeScript developers who want unit-style tests that work with little configuration, especially if they want watch mode or snapshots. If you need full browser end-to-end testing or type checking inside tests without extra tools, pick a different setup or add ts-jest or tsc alongside it.

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