1. Testing Strategies

a. Unit Testing with Jest
Set up Jest for testing individual functions and components:

1. Install dependencies:

   npm install --save-dev jest babel-jest @babel/preset-env @babel/preset-react

2. Configure Babel (babel.config.js):

   module.exports = {
     presets: ['next/babel']
   };

3. Add tests (__tests__/sum.test.js):

   import sum from '../utils/sum';
   
   test('adds 1 + 2 to equal 3', () => {
     expect(sum(1, 2)).toBe(3);
   });

4. Run tests:

   npx jest

b. Component Testing with React Testing Library
Test React components in isolation:

1. Install:

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

2. Example (__tests__/button.test.js):

   import { render, screen, fireEvent } from '@testing-library/react';
   import Button from '../components/Button';
   
   test('renders button and handles click', () => {
     const handleClick = jest.fn();
     render(<Button onClick={handleClick}>Click Me</Button>);
     const btn = screen.getByText(/Click Me/i);
     fireEvent.click(btn);
     expect(handleClick).toHaveBeenCalledTimes(1);
   });

c. End-to-End Testing with Cypress
Simulate user flows in a real browser:

1. Install:

   npm install --save-dev cypress

2. Configure: add cypress.json:

   {
     "baseUrl": "http://localhost:3000"
   }

3. Write a test (cypress/integration/home.spec.js):

   describe('Home Page', () => {
     it('loads and displays welcome message', () => {
       cy.visit('/');
       cy.contains('Welcome to Next.js');
     });
   });

4. Run:

   npx cypress open

2. Performance Optimization

a. Automatic Code Splitting
Every page only loads necessary JS:

• Dynamic Imports for components:

  import dynamic from 'next/dynamic';
  const Chart = dynamic(() => import('../components/Chart'), {
    loading: () => <p>Loading chart...</p>,
    ssr: false,
  });

b. Image Optimization
Use the built-in <Image> component:

import Image from 'next/image';

export default function Avatar() {
  return <Image src="/me.png" width={200} height={200} alt="My Avatar" />;
}

c. Font Optimization
Host fonts locally or use next/font:

import { Inter } from 'next/font/google';
const inter = Inter({ subsets: ['latin'] });

export default function Home() {
  return <div className={inter.className}>Hello with Inter</div>;
}

d. Prefetching and Link Optimization
Next.js prefetches pages linked with <Link> when in viewport:

import Link from 'next/link';

<Link href="/about" prefetch={false}>About</Link>

e. Bundle Analysis
Visualize and reduce bundle size:

1. Install:

   npm install --save-dev @next/bundle-analyzer

2. Configure (next.config.js):

   const withBundleAnalyzer = require('@next/bundle-analyzer')({
     enabled: process.env.ANALYZE === 'true',
   });
   module.exports = withBundleAnalyzer({});

3. Run:

   ANALYZE=true npm run build

f. Lighthouse Audits
Use Chrome DevTools or CLI:

npm install -g lighthouse
lighthouse https://your-deployment-url.com --view

g. Caching and CDN
Deploy on Vercel or use a CDN (e.g., Cloudflare) for static assets caching and edge functions.

h. Static Export (Sitemap & Robots)
Generate sitemap and robots.txt to improve SEO:

npm install --save-dev next-sitemap

Add next-sitemap.js:

module.exports = {
  siteUrl: process.env.SITE_URL || 'https://example.com',
  generateRobotsTxt: true,
};

With solid testing in place and these optimizations, your Next.js application will be both reliable and lightning-fast. In Next Article, we’ll explore internationalization (i18n) and localization.