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A Complete Guide to Quality: Testing, Linting & Monitoring

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Quality: Testing, Linting & Monitoring in Next.js

Prerequisites

Before diving into this tutorial on testing, linting, and monitoring your Next.js applications, ensure you have the following prerequisites:

Basic understanding of Next.js: Familiarity with the concepts covered in the previous parts of this series, especially Parts 1-16.
Node.js and npm installed: You need Node.js and npm to run your Next.js applications and install necessary packages.
A Next.js application: You should have a Next.js project set up. If you haven't created one yet, you can use the command below to set it up quickly:

bash

   npx create-next-app@latest my-next-app
   cd my-next-app

In this tutorial, we'll cover quality assurance across several dimensions: unit tests, end-to-end (E2E) tests, linting, logging, and error tracking. By the end, you'll have a robust understanding of how to maintain quality in your Next.js app.

1. Understanding Unit Tests

Unit tests are essential for verifying that individual components of your application work as expected. They allow you to catch bugs early in the development process.

1.1 Setting Up Jest for Unit Testing

Install Jest:

To get started, install Jest and its TypeScript support if you're using TypeScript:

bash

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

Configure Jest:

Create a jest.config.js file in the root directory of your project:

javascript

   module.exports = {
     testEnvironment: 'jsdom',
     transform: {
       '^.+\\.tsx?$': 'ts-jest',
     },
     moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json', 'node'],
   };

Create a Sample Component:

Create a simple component for testing. In components/Hello.tsx:

tsx

   const Hello = ({ name }: { name: string }) => {
     return <h1>Hello, {name}!</h1>;
   };

   export default Hello;

Write a Test for the Component:

Create a test file components/Hello.test.tsx:

tsx

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

   test('renders Hello component', () => {
     render(<Hello name="World" />);
     const heading = screen.getByText(/Hello, World!/i);
     expect(heading).toBeInTheDocument();
   });

Run the Tests:

Execute your tests using the following command:

bash

   npx jest

Expected Output:

plaintext

   PASS  ./Hello.test.tsx
     ✓ renders Hello component (XX ms)

1.2 Troubleshooting Common Issues

Test Not Found: Ensure your test files are named correctly (e.g., *.test.tsx) and are located in the correct directory.
Module Not Found: Verify that you have installed all necessary libraries, especially @testing-library/react.

2. End-to-End (E2E) Testing

While unit tests verify individual components, E2E tests check the entire application flow, ensuring all parts work together.

2.1 Setting Up Playwright for E2E Testing

Install Playwright:

Install Playwright for E2E testing:

bash

   npm install --save-dev @playwright/test

Configure Playwright:

Create a playwright.config.ts file:

typescript

   import { defineConfig } from '@playwright/test';

   export default defineConfig({
     use: {
       headless: true, // Run tests in headless mode
     },
   });

Write a Sample E2E Test:

Create a test file e2e/home.test.ts:

typescript

   import { test, expect } from '@playwright/test';

   test('homepage has the correct title', async ({ page }) => {
     await page.goto('http://localhost:3000');
     await expect(page).toHaveTitle(/Next.js/);
   });

Run the E2E Tests:

Start your Next.js application and run the tests in a separate terminal:

bash

   npm run dev
   npx playwright test

Expected Output:

plaintext

   Running 1 test using 1 worker

     ✓ homepage has the correct title (XX ms)

   1 passed (XX ms)

2.2 Troubleshooting Common Issues

Server Not Running: Ensure your Next.js application is running before executing the E2E tests.
Incorrect URL: Double-check the URL in your tests matches your local server's URL.

3. Linting Your Next.js Application

Linting is crucial for maintaining code quality and ensuring consistency across your codebase.

3.1 Setting Up ESLint

Install ESLint:

Install ESLint and related plugins:

bash

   npm install --save-dev eslint eslint-plugin-react eslint-plugin-next

Configure ESLint:

Create an .eslintrc.json file in your project root:

json

   {
     "extends": ["next/core-web-vitals"],
     "rules": {
       "react/react-in-jsx-scope": "off"
     }
   }

Run ESLint:

Lint your codebase by running:

bash

   npx eslint .

Expected Output:

plaintext

   /path/to/file.js
     1:1  error  Unexpected console statement  no-console

3.2 Troubleshooting Common Issues

Configuration Errors: Ensure the ESLint configuration file is correctly formatted.
Missing Plugins: Verify that all necessary ESLint plugins are installed.

4. Logging in Next.js

Effective logging helps you monitor the behavior of your application and troubleshoot issues.

4.1 Setting Up Logging

Install a Logging Library:

For this tutorial, we will use winston, a popular logging library:

bash

   npm install winston

Create a Logger:

Create a new file lib/logger.ts:

typescript

   import winston from 'winston';

   const logger = winston.createLogger({
     level: 'info',
     format: winston.format.json(),
     transports: [
       new winston.transports.Console(),
     ],
   });

   export default logger;

Use the Logger:

In your API routes or components, import and use the logger:

typescript

   import logger from '../lib/logger';

   export default function handler(req, res) {
     logger.info('API called');
     res.status(200).json({ message: 'Hello World' });
   }

4.2 Expected Output

When you call the API, you should see the log message in the console:

plaintext

{"level":"info","message":"API called","timestamp":"..."}

4.3 Troubleshooting Common Issues

Log Not Appearing: Ensure that your logger is correctly configured and you are calling it in the right places.

5. Error Tracking

To maintain high-quality applications, implementing error tracking is essential for identifying issues in production.

5.1 Integrating Sentry for Error Tracking

Install Sentry:

Install the Sentry SDK:

bash

   npm install @sentry/nextjs

Configure Sentry:

Create a Sentry configuration file sentry.server.config.js:

javascript

   import * as Sentry from '@sentry/nextjs';

   Sentry.init({
     dsn: 'YOUR_SENTRY_DSN',
     tracesSampleRate: 1.0,
   });

Wrap Your Application:

In pages/_app.tsx, wrap your application with Sentry's error boundary:

typescript

   import * as Sentry from '@sentry/nextjs';

   function MyApp({ Component, pageProps }) {
     return (
       <Sentry.ErrorBoundary>
         <Component {...pageProps} />
       </Sentry.ErrorBoundary>
     );
   }

   export default MyApp;

5.2 Expected Output

When an error occurs in your application, it will automatically be sent to Sentry, and you can view it in your Sentry dashboard.

5.3 Troubleshooting Common Issues

No Errors Reported: Ensure your DSN is correct and that Sentry is initialized in your application.

Conclusion

In this tutorial, we explored various aspects of maintaining quality in your Next.js applications, covering unit tests, E2E tests with Playwright, linting with ESLint, logging with Winston, and error tracking utilizing Sentry. As we discussed in previous parts of this series, quality assurance is essential for robust application development.

By implementing these practices, you’ll ensure that your Next.js app not only functions correctly but also remains maintainable and reliable as it grows. As we move on to the next part of the series, we will explore advanced performance optimization techniques.

If you found this tutorial helpful, please share your thoughts or questions in the comments below and stay tuned for the next installment of the "Next.js A to Z: Complete Mastery Series for 2026"!

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A Complete Guide to Quality: Testing, Linting & Monitoring

Quality: Testing, Linting & Monitoring in Next.js

Prerequisites

1. Understanding Unit Tests

1.1 Setting Up Jest for Unit Testing

1.2 Troubleshooting Common Issues

2. End-to-End (E2E) Testing

2.1 Setting Up Playwright for E2E Testing

2.2 Troubleshooting Common Issues

3. Linting Your Next.js Application

3.1 Setting Up ESLint

3.2 Troubleshooting Common Issues

4. Logging in Next.js

4.1 Setting Up Logging

4.2 Expected Output

4.3 Troubleshooting Common Issues

5. Error Tracking

5.1 Integrating Sentry for Error Tracking

5.2 Expected Output

5.3 Troubleshooting Common Issues

Conclusion

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