testing.md

description
Learn how to set up Next.js with three commonly used testing tools — Cypress, Jest, and React Testing Library.

Testing

Examples

• Next.js with Cypress
• Next.js with Jest and React Testing Library

Learn how to set up Next.js with three commonly used testing tools: Cypress, Jest, and React Testing Library.

Cypress

Cypress is a test runner used for End-to-End (E2E) and Integration Testing.

Quickstart

You can use create-next-app with the with-cypress example to quickly get started.

npx create-next-app --example with-cypress with-cypress-app

Manual setup

To get started with Cypress, install the cypress package:

npm install --save-dev cypress

Add Cypress to the package.json scripts field:

"scripts": {
  "dev": "next dev",
  "build": "next build",
  "start": "next start",
  "cypress": "cypress open",
}

Run Cypress for the first time to generate examples that use their recommended folder structure:

npm run cypress

You can look through the generated examples and the Writing Your First Test section of the Cypress Documentation to help you get familiar with Cypress.

Creating your first Cypress integration test

Assuming the following two Next.js pages:

// pages/index.js
import Link from 'next/link'

export default function Home() {
  return (
    <nav>
      <Link href="/about">
        <a>About</a>
      </Link>
    </nav>
  )
}

// pages/about.js
export default function About() {
  return (
    <div>
      <h1>About Page</h1>
    </div>
  )
}

Add a test to check your navigation is working correctly:

// cypress/integration/app.spec.js

describe('Navigation', () => {
  it('should navigate to the about page', () => {
    // Start from the index page
    cy.visit('http://localhost:3000/')

    // Find a link with an href attribute containing "about" and click it
    cy.get('a[href*="about"]').click()

    // The new url should include "/about"
    cy.url().should('include', '/about')

    // The new page should contain an h1 with "About page"
    cy.get('h1').contains('About Page')
  })
})

You can use cy.visit("/") instead of cy.visit("http://localhost:3000/") if you add "baseUrl": "http://localhost:3000" to the cypress.json configuration file.

Running your Cypress tests

Since Cypress is testing a real Next.js application, it requires the Next.js server to be running prior to starting Cypress. We recommend running your tests against your production code to more closely resemble how your application will behave.

Run npm run build and npm run start, then run npm run cypress in another terminal window to start Cypress.

Note: Alternatively, you can install the start-server-and-test package and add it to the package.json scripts field: "test": "start-server-and-test start http://localhost:3000 cypress" to start the Next.js production server in conjuction with Cypress. Remember to rebuild your application after new changes.

Getting ready for Continuous Integration (CI)

You will have noticed that running Cypress so far has opened an interactive browser which is not ideal for CI environments. You can also run Cypress headlessly using the cypress run command:

// package.json

"scripts": {
  //...
  "cypress": "cypress open",
  "cypress:headless": "cypress run",
  "e2e": "start-server-and-test start http://localhost:3000 cypress",
  "e2e:headless": "start-server-and-test start http://localhost:3000 cypress:headless"
}

You can learn more about Cypress and Continuous Integration from these resources:

• Cypress Continuous Integration Docs
• Cypress GitHub Actions Guide
• Official Cypress Github Action

Jest and React Testing Library

Jest and React Testing Library are frequently used together for Unit Testing.

Quickstart

You can use create-next-app with the with-jest example to quickly get started with Jest and React Testing Library:

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

Manual setup

To manually set up Jest and React Testing Library, install jest , @testing-library/react, @testing-library/jest-dom as well as some supporting packages:

npm install --save-dev jest babel-jest @testing-library/react @testing-library/jest-dom identity-obj-proxy react-test-renderer

Configuring Jest

Create a jest.config.js file in your project's root directory and add the following configuration options:

// jest.config.js

module.exports = {
  collectCoverageFrom: [
    '**/*.{js,jsx,ts,tsx}',
    '!**/*.d.ts',
    '!**/node_modules/**',
  ],
  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 */
    '^.+\\.(jpg|jpeg|png|gif|webp|svg)$': '<rootDir>/__mocks__/fileMock.js',
  },
  testPathIgnorePatterns: ['<rootDir>/node_modules/', '<rootDir>/.next/'],
  testEnvironment: 'jsdom',
  transform: {
    /* Use babel-jest to transpile tests with the next/babel preset
    https://jestjs.io/docs/configuration#transform-objectstring-pathtotransformer--pathtotransformer-object */
    '^.+\\.(js|jsx|ts|tsx)$': ['babel-jest', { presets: ['next/babel'] }],
  },
  transformIgnorePatterns: [
    '/node_modules/',
    '^.+\\.module\\.(css|sass|scss)$',
  ],
}

You can learn more about each option above in the Jest docs.

Handling stylesheets and image imports

These files aren't useful in tests but importing them may cause errors, so we will need to mock them. Create the mock files we 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.

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.js

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

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

// jest.setup.js

import '@testing-library/jest-dom/extend-expect'

If you need to add more setup options before each test, it's common to add them to the jest.setup.js file above.

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": {
    "baseUrl": ".",
    "paths": {
      "@/components/*": ["components/*"]
    }
  }
}

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

Add a test script to package.json

Add the Jest executable in watch mode to the package.json scripts:

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

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

Create your first tests

Your project is now ready to run tests. Follow Jests convention by adding tests to the __tests__ folder in your project's root directory.

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

// __tests__/index.test.jsx

/**
 * @jest-environment jsdom
 */

import React from 'react'
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', {
      name: /welcome to next\.js!/i,
    })

    expect(heading).toBeInTheDocument()
  })
})

Note: The @jest-environment jsdom comment above configures the testing environment as jsdom inside the test file because React Testing Library uses DOM elements like document.body which will not work in Jest's default node testing environment. Alternatively, you can also set the jsdom environment globally by adding the Jest configuration option: "testEnvironment": "jsdom" in jest.config.js.

Optionally, add a snapshot test to keep track of any unexpected changes to your <Index /> component:

// __tests__/snapshot.js
import React from 'react'
import renderer from 'react-test-renderer'
import Index from '../pages/index'

it('renders homepage unchanged', () => {
  const tree = renderer.create(<Index />).toJSON()
  expect(tree).toMatchSnapshot()
})

Note: Test files should not be included inside the pages directory because any files inside the pages directory are considered routes.

Running your test suite

Run npm run jest to run your test suite. After your tests pass or fail, you will notice a list of interactive Jest commands that will be helpful as you add more tests.

For further reading, you may find these resources helpful:

• Jest Docs
• React Testing Library Docs
• Testing Playground - use good testing practices to match elements.

Community Packages and Examples

The Next.js community has created packages and articles you may find helpful:

• next-page-tester for DOM Integration Testing.
• next-router-mock for Storybook.
• Test Preview Vercel Deploys with Cypress by Gleb Bahmutov.

For more information on what to read next, we recommend:

Test Environment Variables Learn more about the test environment variables.