Get Started with JEST

Get Started:

Install Jest using your favorite package manager:

Yarn:

yarn add --dev jest

npm:

npm install --save-dev jest

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
✓ 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 to run Jest on files matching my-test, using config.json as a configuration file and display a native OS notification afther the run:

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

If you'd like to lean more about running jestthrough 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 fwe questions and will create a basic configuration file with a short description for each option:

jest --init

Using Babel

To use Babel, install required dependencies:

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

npm install --save-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:

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 proccess.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.
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:
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 challengers over other tools. Refer to the Using Jest with webpack to get started.

Using Vite

Jest can be used in projects that use vite to serves source code over native ESM to provide some frontend tooling, vite is an opinionated tool and does offer some out-of-the box workflows. Jest does not fully supported by vite due to how the plugin system from vite works, but there is some working examples for first-class jest integration using the vite-jest, since this is not fully supported, you might as well read the limitation of the vite-jest. Refer to the vite guide to get started. Alternatively you can use[vitest](https://github.com/vitest-dev/vitest.

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 parcel-docs to get started.

Using TypeScript

Via `babel`

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

yarn add --dev @babel/preset-typescript

npm install --save-dev @babel/preset-typescript

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

babel.config.json

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

However, there are some caveats 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 use ts-jest instead, or just run the TypeScript compiler tsc separately (or as part of your build process).

Via `ts-jest`

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

yarn add --dev ts-jest

npm install --save-dev ts-jest

Type definitions

There are two ways to have Jest 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. Simply import the APIs from @jest/globals package.

sum.test.ts

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)
    })
})

TIP

See the additional usage documentation of describe.each/test.each and mock functions

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

INFO

@types/jest is a third party library maintained at DefinitelyTyped, hence the latest Jest features or versions may not be covered yet. Try to match versions of Jest and @types/jest as closely as possible. For example, if you are using Jest 27.4.0 then installing 27.4.x of @types/jest is ideal.

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