Setting up Jest with Next.js

Jest and React Testing Library are frequently used together for Unit Testing and Snapshot Testing. This guide will show you how to set up Jest with Next.js and write your first tests.

Good to know: Since async Server Components are new to the React ecosystem, Jest currently does not support them. While you can still run unit tests for synchronous Server and Client Components, we recommend using an E2E tests for async components.

Quickstart

You can use create-next-app with the Next.js with-jest example to quickly get started:

Terminal

npx create-next-app@latest --example with-jest with-jest-app

Manual setup

Since the release of Next.js 12, Next.js now has built-in configuration for Jest.

To set up Jest, install jest and the following packages as dev dependencies:

Terminal

npm install -D jest jest-environment-jsdom @testing-library/react @testing-library/dom @testing-library/jest-dom ts-node
# or
yarn add -D jest jest-environment-jsdom @testing-library/react @testing-library/dom @testing-library/jest-dom ts-node
# or
pnpm install -D jest jest-environment-jsdom @testing-library/react @testing-library/dom @testing-library/jest-dom ts-node

Generate a basic Jest configuration file by running the following command:

Terminal

npm init jest@latest
# or
yarn create jest@latest
# or
pnpm create jest@latest

This will take you through a series of prompts to setup Jest for your project, including automatically creating a jest.config.ts|js file.

Update your config file to use next/jest. This transformer has all the necessary configuration options for Jest to work with Next.js:

jest.config.ts

import type { Config } from 'jest'
import nextJest from 'next/jest.js'
 
const createJestConfig = nextJest({
  // Provide the path to your Next.js app to load next.config.js and .env files in your test environment
  dir: './',
})
 
// Add any custom config to be passed to Jest
const config: Config = {
  coverageProvider: 'v8',
  testEnvironment: 'jsdom',
  // Add more setup options before each test is run
  // setupFilesAfterEnv: ['<rootDir>/jest.setup.ts'],
}
 
// createJestConfig is exported this way to ensure that next/jest can load the Next.js config which is async
export default createJestConfig(config)

Under the hood, next/jest is automatically configuring Jest for you, including:

Setting up transform using the Next.js Compiler.
Auto mocking stylesheets (.css, .module.css, and their scss variants), image imports and next/font.
Loading .env (and all variants) into process.env.
Ignoring node_modules from test resolving and transforms.
Ignoring .next from test resolving.
Loading next.config.js for flags that enable SWC transforms.

Good to know: To test environment variables directly, load them manually in a separate setup script or in your jest.config.ts file. For more information, please see Test Environment Variables.

Setting up Jest (with Babel)

If you opt out of the Next.js Compiler and use Babel instead, you will need to manually configure Jest and install babel-jest and identity-obj-proxy in addition to the packages above.

Here are the recommended options to configure Jest for Next.js:

jest.config.js

module.exports = {
  collectCoverage: true,
  // on node 14.x coverage provider v8 offers good speed and more or less good report
  coverageProvider: 'v8',
  collectCoverageFrom: [
    '**/*.{js,jsx,ts,tsx}',
    '!**/*.d.ts',
    '!**/node_modules/**',
    '!<rootDir>/out/**',
    '!<rootDir>/.next/**',
    '!<rootDir>/*.config.js',
    '!<rootDir>/coverage/**',
  ],
  moduleNameMapper: {
    // Handle CSS imports (with CSS modules)
    // https://jestjs.io/docs/webpack#mocking-css-modules
    '^.+\\.module\\.(css|sass|scss)$': 'identity-obj-proxy',
 
    // Handle CSS imports (without CSS modules)
    '^.+\\.(css|sass|scss)$': '<rootDir>/__mocks__/styleMock.js',
 
    // Handle image imports
    // https://jestjs.io/docs/webpack#handling-static-assets
    '^.+\\.(png|jpg|jpeg|gif|webp|avif|ico|bmp|svg)$': `<rootDir>/__mocks__/fileMock.js`,
 
    // Handle module aliases
    '^@/components/(.*)$': '<rootDir>/components/$1',
 
    // Handle @next/font
    '@next/font/(.*)': `<rootDir>/__mocks__/nextFontMock.js`,
    // Handle next/font
    'next/font/(.*)': `<rootDir>/__mocks__/nextFontMock.js`,
    // Disable server-only
    'server-only': `<rootDir>/__mocks__/empty.js`,
  },
  // Add more setup options before each test is run
  // setupFilesAfterEnv: ['<rootDir>/jest.setup.js'],
  testPathIgnorePatterns: ['<rootDir>/node_modules/', '<rootDir>/.next/'],
  testEnvironment: 'jsdom',
  transform: {
    // Use babel-jest to transpile tests with the next/babel preset
    // https://jestjs.io/docs/configuration#transform-objectstring-pathtotransformer--pathtotransformer-object
    '^.+\\.(js|jsx|ts|tsx)$': ['babel-jest', { presets: ['next/babel'] }],
  },
  transformIgnorePatterns: [
    '/node_modules/',
    '^.+\\.module\\.(css|sass|scss)$',
  ],
}

You can learn more about each configuration option in the Jest docs. We also recommend reviewing next/jest configuration to see how Next.js configures Jest.

Handling stylesheets and image imports

Stylesheets and images aren't used in the tests but importing them may cause errors, so they will need to be mocked.

Create the mock files referenced in the configuration above - fileMock.js and styleMock.js - inside a __mocks__ directory:

__mocks__/fileMock.js

module.exports = 'test-file-stub'

__mocks__/styleMock.js

module.exports = {}

For more information on handling static assets, please refer to the Jest Docs.

Handling Fonts

To handle fonts, create the nextFontMock.js file inside the __mocks__ directory, and add the following configuration:

__mocks__/nextFontMock.js

module.exports = new Proxy(
  {},
  {
    get: function getter() {
      return () => ({
        className: 'className',
        variable: 'variable',
        style: { fontFamily: 'fontFamily' },
      })
    },
  }
)

Optional: Handling Absolute Imports and Module Path Aliases

If your project is using Module Path Aliases, you will need to configure Jest to resolve the imports by matching the paths option in the jsconfig.json file with the moduleNameMapper option in the jest.config.js file. For example:

tsconfig.json or jsconfig.json

{
  "compilerOptions": {
    "module": "esnext",
    "moduleResolution": "bundler",
    "baseUrl": "./",
    "paths": {
      "@/components/*": ["components/*"]
    }
  }
}

jest.config.js

moduleNameMapper: {
  // ...
  '^@/components/(.*)$': '<rootDir>/components/$1',
}

Optional: Extend Jest with custom matchers

@testing-library/jest-dom includes a set of convenient custom matchers such as .toBeInTheDocument() making it easier to write tests. You can import the custom matchers for every test by adding the following option to the Jest configuration file:

jest.config.ts

setupFilesAfterEnv: ['<rootDir>/jest.setup.ts']

Then, inside jest.setup, add the following import:

jest.setup.ts

import '@testing-library/jest-dom'

Good to know: extend-expect was removed in v6.0, so if you are using @testing-library/jest-dom before version 6, you will need to import @testing-library/jest-dom/extend-expect instead.

If you need to add more setup options before each test, you can add them to the jest.setup file above.

Add a test script to `package.json`

Finally, add a Jest test script to your package.json file:

package.json

{
  "scripts": {
    "dev": "next dev",
    "build": "next build",
    "start": "next start",
    "test": "jest",
    "test:watch": "jest --watch"
  }
}

jest --watch will re-run tests when a file is changed. For more Jest CLI options, please refer to the Jest Docs.

Creating your first test

Your project is now ready to run tests. Create a folder called __tests__ in your project's root directory.

For example, we can add a test to check if the <Home /> component successfully renders a heading:

export default function Home() {
  return <h1>Home</h1>
}

__tests__/index.test.js

import '@testing-library/jest-dom'
import { render, screen } from '@testing-library/react'
import Home from '../pages/index'
 
describe('Home', () => {
  it('renders a heading', () => {
    render(<Home />)
 
    const heading = screen.getByRole('heading', { level: 1 })
 
    expect(heading).toBeInTheDocument()
  })
})

Optionally, add a snapshot test to keep track of any unexpected changes in your component:

__tests__/snapshot.js

import { render } from '@testing-library/react'
import Home from '../pages/index'
 
it('renders homepage unchanged', () => {
  const { container } = render(<Home />)
  expect(container).toMatchSnapshot()
})

Good to know: Test files should not be included inside the Pages Router because any files inside the Pages Router are considered routes.

Running your tests

Then, run the following command to run your tests:

Terminal

npm run test
# or
yarn test
# or
pnpm test

Additional Resources

For further reading, you may find these resources helpful:

Next.js with Jest example
Jest Docs
React Testing Library Docs
Testing Playground - use good testing practices to match elements.