Jest setup for Next.js - Mini Project: Build & Apply

Jest setup for Next.js

📖 Scenario: You are building a Next.js app and want to test your components and functions to make sure they work correctly.Testing helps catch bugs early and keeps your app reliable.

🎯 Goal: Set up Jest testing framework in a Next.js project with a simple test example.

📋 What You'll Learn

Create a Jest configuration file named jest.config.js with Next.js preset

Add a test script in package.json to run Jest

Create a simple test file sum.test.js with a test for a sum function

Run Jest tests successfully in the Next.js project

💡 Why This Matters

🌍 Real World

Testing is essential in real projects to catch bugs early and keep apps reliable.

💼 Career

Many companies use Jest with Next.js to ensure their web apps work correctly before release.

1

Create Jest configuration file

Create a file named jest.config.js in the project root. Inside it, write module.exports = { preset: 'next/jest' } exactly as shown.

NextJS

// Your code here

Hint

This file tells Jest to use Next.js settings for testing.

2

Add test script to package.json

Open package.json and add a test script inside the scripts section with the value jest. The line should be exactly "test": "jest".

NextJS

{
  "scripts": {
    // Add the test script here
  }
}

Hint

This script lets you run tests by typing npm test or yarn test.

3

Create a simple test file

Create a file named sum.test.js in the project root. Inside it, write a function sum that returns the sum of two numbers. Then write a Jest test using test and expect to check that sum(1, 2) equals 3.

NextJS

// Your code here

Hint

This test checks if the sum function works correctly.

4

Run Jest tests

Run the Jest tests by executing npm test or yarn test in the terminal. Ensure the test passes without errors.

NextJS

// Run tests in terminal with npm test or yarn test

Hint

Use your terminal to run the tests and see the results.

Practice

1. What is the main purpose of using Jest in a Next.js project?

easy

A. To deploy the app to a server

B. To run automated tests and catch bugs early

C. To style components with CSS

D. To build the Next.js application for production

Solution

1. Step 1: Understand Jest's role

   Jest is a testing framework used to run automated tests on code.

2. Step 2: Identify Jest's purpose in Next.js

   In Next.js, Jest helps find bugs early by running tests on components and functions.

3. Final Answer:

   To run automated tests and catch bugs early -> Option B

4. Quick Check:

   Jest = Testing tool [OK]

Hint: Jest is for testing, not building or styling [OK]

Common Mistakes:

• Confusing Jest with build tools
• Thinking Jest styles components
• Assuming Jest deploys apps

2. Which file is commonly used to configure Jest in a Next.js project?

easy

A. jest.config.js

B. next.config.js

C. package.json

D. babel.config.js

Solution

1. Step 1: Identify Jest configuration file

   Jest uses a dedicated config file named jest.config.js to set options.

2. Step 2: Differentiate from other config files

   next.config.js is for Next.js settings, package.json can hold scripts but not full Jest config, and babel.config.js is for Babel.

3. Final Answer:

   jest.config.js -> Option A

4. Quick Check:

   Jest config = jest.config.js [OK]

Hint: Jest config file always named jest.config.js [OK]

Common Mistakes:

• Choosing next.config.js for Jest config
• Confusing Babel config with Jest config
• Thinking package.json holds full Jest config

3. Given this test code in a Next.js app using Jest and React Testing Library:

import { render, screen } from '@testing-library/react';
import Home from '../pages/index';

test('renders welcome message', () => {
  render(<Home />);
  const message = screen.getByText('Welcome to Next.js!');
  expect(message).toBeInTheDocument();
});

What will happen when this test runs if the Home component renders the text "Welcome to Next.js!"?

medium

A. The test will throw a syntax error

B. The test will fail because the text is missing

C. The test will timeout and never finish

D. The test will pass because the text is found

Solution

1. Step 1: Understand test behavior

   The test renders the Home component and looks for the text "Welcome to Next.js!" on screen.

2. Step 2: Check if text exists in component

   If the Home component actually renders that text, screen.getByText finds it and expect(...).toBeInTheDocument() passes.

3. Final Answer:

   The test will pass because the text is found -> Option D

4. Quick Check:

   Text found = test passes [OK]

Hint: If text exists in component, test passes [OK]

Common Mistakes:

• Assuming test fails if text is present
• Confusing syntax errors with test failures
• Thinking test times out without reason

4. You wrote this Jest test for a Next.js component:

test('shows button', () => {
  render(<MyButton />);
  const btn = screen.getByRole('button');
  expect(btn.textContent).toBe('Click me');
});

But the test fails with error: TypeError: screen.getByRole is not a function. What is the most likely cause?

medium

A. You forgot to import screen from '@testing-library/react'

B. The component does not render a button element

C. You used textContent instead of toHaveTextContent

D. Jest is not installed in the project

Solution

1. Step 1: Analyze the error message

   The error says screen.getByRole is not a function, meaning screen is undefined or missing that method.

2. Step 2: Check imports

   This usually happens if screen was not imported from '@testing-library/react', so it is undefined in the test file.

3. Final Answer:

   You forgot to import screen from '@testing-library/react' -> Option A

4. Quick Check:

   Missing import = screen undefined [OK]

Hint: Always import screen from testing-library/react [OK]

Common Mistakes:

• Assuming component missing button causes this error
• Confusing textContent usage with import errors
• Thinking Jest not installed causes this specific error

5. You want to add Jest testing to a new Next.js project. Which sequence of steps correctly sets up Jest and React Testing Library?

hard

A. Install babel, create babel.config.js, write tests in components folder

B. Install next, create next.config.js, write tests in pages folder

C. Install jest and @testing-library/react, create jest.config.js, write tests in __tests__ folder

D. Install react, create react.config.js, write tests in public folder

Solution

1. Step 1: Install testing dependencies

   You need to install jest and @testing-library/react to test Next.js components.

2. Step 2: Configure Jest

   Create a jest.config.js file to set Jest options for Next.js environment.

3. Step 3: Write tests in proper folder

   Tests are commonly placed in a __tests__ folder or alongside components, not in pages or public folders.

4. Final Answer:

   Install jest and @testing-library/react, create jest.config.js, write tests in __tests__ folder -> Option C

5. Quick Check:

   Correct setup steps = Install jest and @testing-library/react, create jest.config.js, write tests in __tests__ folder [OK]

Hint: Install jest + testing-library, config jest.config.js, write tests in __tests__ [OK]

Common Mistakes:

• Confusing next.config.js with jest.config.js
• Writing tests in public or pages folders
• Installing unrelated packages like babel or react only