Testing Your Next.js Application: A Step-by-Step Guide

As a developer, you understand the importance of testing your application to ensure it works as expected. In this article, we’ll walk you through the process of setting up and writing tests for your Next.js application using Jest.

Prerequisites

• Working knowledge of React and Next.js
• Familiarity with testing applications
• Node.js installed on your machine
• A code editor (such as Visual Studio Code)

Setting Up Your Next.js Application

To get started, create a new Next.js application by running the following command in your terminal:


npx create-next-app jest-tutorial


Once the installation is complete, open the project in your code editor and start the development server with:


npm run dev


Building a Calculator Application

For the purpose of this tutorial, we’ll build a simple calculator application. We won’t go into the details of the calculator’s logic, but we’ll provide the code for the key components.

Create a new file called index.js in the pages directory and add the following code:

“`jsx
import React from ‘react’;

const Home = () => {
return (

);
};

export default Home;
“`

Setting Up Jest

To set up Jest, install the required packages by running:


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


Create a new file called jest.config.js in the root of your project and add the following configuration:

js
module.exports = {
preset: 'next',
collectCoverage: true,
};


Writing Tests

Create a new file called index.test.js in the root of your project and add the following test:

“`jsx
import React from ‘react’;
import { render, fireEvent } from ‘@testing-library/react’;
import Home from ‘./pages/index’;

describe(‘Home page’, () => {
it(‘renders correctly’, () => {
const { getByTestId } = render();
expect(getByTestId(‘num1’)).toBeInTheDocument();
expect(getByTestId(‘num2’)).toBeInTheDocument();
expect(getByTestId(‘add’)).toBeInTheDocument();
expect(getByTestId(‘result’)).toBeInTheDocument();
});
});
“`

Running Tests

Run your tests with the following command:


npm run test


Testing Calculator Operations

Add more tests to check if the calculator operations are working correctly:

jsx
it('adds numbers correctly', () => {
const { getByTestId } = render(<Home />);
fireEvent.change(getByTestId('num1'), { target: { value: '2' } });
fireEvent.change(getByTestId('num2'), { target: { value: '3' } });
fireEvent.click(getByTestId('add'));
expect(getByTestId('result')).toHaveTextContent('5');
});


Simulating an Error

Simulate an error in your application by changing the add function to subtract instead of add:

jsx
const add = (a, b) => a - b;


Run your tests again and see how Jest reports the error.

Conclusion

In this article, we’ve shown you how to set up and write tests for your Next.js application using Jest. We’ve also covered how to test calculator operations and simulate errors. By following these steps, you can ensure your application works as expected and catch any bugs before they reach production.

Additional Tips

• Mocking external modules and APIs can help isolate your code and make your tests more reliable.
• Testing React Hooks requires rendering the component that uses the hook and testing its behavior.
• Integration testing allows you to test the core business logic and workflow of your application by testing pages and their interaction with API routes and external dependencies.