Mastering Jest in Next.js: A Complete Guide for App Router and TypeScript 🚀

Dive into the world of modern web testing with this comprehensive guide on integrating Jest into your Next.js project, and learn how to set up a robust testing environment for your App Router-based application using TypeScript.
We'll walk you through installing essential dependencies, configuring Jest for Next.js, and writing your first test.

Perfect for developers looking to enhance their Next.js applications with reliable, type-safe testing practices. Boost your code quality and confidence with Jest and React Testing Library in your Next.js TypeScript projects.

To start working with Jest in a Next.js project you need to follow these steps:

1. Install dependencies:

npm install --save-dev jest @types/jest @testing-library/react @testing-library/jest-dom jest-environment-jsdom

Let's break down this npm installation command and explain each package:

• --save-dev: This flag saves these packages as development dependencies in your package.json. It means these packages are only needed for development and testing, not for production.

• jest: This is the core Jest library, a popular JavaScript testing framework developed by Facebook. It provides the test runner and assertion library.

• @types/jest: This package provides TypeScript type definitions for Jest. It enables better autocomplete and type-checking when writing tests in TypeScript.

• @testing-library/react: This is a set of helpers that allow you to test React components without relying on their implementation details, it encourages better testing practices.

• @testing-library/jest-dom: This package extends Jest with custom matchers for asserting on DOM nodes, making it easier to test the state of the DOM in your tests.

• jest-environment-jsdom: This package provides a simulated DOM environment for Jest tests. It's necessary when testing React components that interact with the DOM, as Next.js components often do.

2. Create a Jest configuration file (jest.config.js) in your project root:

const nextJest = require('next/jest')

const createJestConfig = nextJest({
  dir: './',
})

const customJestConfig = {
  setupFilesAfterEnv: ['<rootDir>/jest.setup.js'],
  testEnvironment: 'jest-environment-jsdom',
}

module.exports = createJestConfig(customJestConfig)

This code configures Jest to work with Next.js. Let's break it down line by line:

• const nextJest = require('next/jest'): This imports the Next.js Jest configuration function. Next.js provides this to help set up Jest in a way that's compatible with Next.js projects.

• const createJestConfig = nextJest({ dir: './', }): This creates a Jest configuration function specific to your Next.js project, and dir: './' option tells Next.js that the root directory of your project is the current directory.

• const customJestConfig = { ... }: This object contains custom Jest configuration options.

• setupFilesAfterEnv: ['<rootDir>/jest.setup.js']: This specifies files to run after the Jest environment is set up but before tests are run, and it points to jest.setup.js file in your root directory, where you can add global setup code for your tests.

• testEnvironment: 'jest-environment-jsdom': This sets the test environment to jsdom, which simulates a DOM environment in Node.js, and it's necessary for testing React components that interact with the DOM.

• module.exports = createJestConfig(customJestConfig): This exports the final Jest configuration, it calls createJestConfig with your custom config, which merges your settings with Next.js's default Jest settings.

3. Create a jest.setup.js file in your project root:

import '@testing-library/jest-dom'

4. Update your package.json to include Jest scripts:

"scripts": {
  "test": "jest",
  "test:watch": "jest --watch"
}

These scripts provide two different ways to run your tests:

• The test script is for running all tests once, which is ideal for CI/CD or before committing changes.

• The test:watch script is for development, allowing you to see test results update in real-time as you code.

5. Create a __tests__ folder in your project root or alongside your components

6. Write your first test (e.g., Home.test.tsx):

import '@testing-library/jest-dom'
import { render, screen } from '@testing-library/react'
import Home from '../app/page'

describe('Home', () => {
    it('renders a heading', () => {
        render(<Home />)
        const heading = screen.getAllByRole('heading', { level: 2 })
        expect(heading).toHaveLength(4);
    })
})

7. Run your tests:

npm test

This setup should get you started on using Jest in a Next.js project.

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Incorporating Jest into your Next.js project with TypeScript marks a significant step towards robust, maintainable code. By following this guide, you've laid the groundwork for a comprehensive testing strategy that will serve you well as your application grows.

By prioritizing testing in your development workflow, you're not just catching bugs early—you're also documenting your code's behavior and making it easier for your team to collaborate and refactor with confidence.

Happy testing 😀😀