Next JS
Basic Next.js Interview Questions and Answers
1. What is Next.js, and why is it popular?
Next.js is an open-source React framework used for building server-side rendering (SSR) and static web applications. It simplifies the development of modern web applications by offering features like routing, API building, and image optimization out of the box.
Its popularity stems from its ability to improve SEO performance through SSR, deliver faster page loads, and provide a seamless developer experience with its file-based routing system and built-in support for TypeScript. Companies like Netflix, Hulu, and Twitch leverage Next.js to create robust, scalable applications.
2. How does Server-Side Rendering (SSR) work in Next.js?
In Next.js, SSR generates HTML pages dynamically on the server at request time. When a user visits a page, the server fetches the necessary data, processes it, and serves the fully rendered HTML to the browser.
This approach improves SEO by ensuring search engine crawlers can index complete pages and enhances performance by reducing client-side rendering time. Developers can enable SSR using the getServerSideProps function in Next.js.
3. What are the main advantages of using Next.js?
Next.js offers several benefits that make it superior to traditional React applications:
- Improved SEO: Since SSR and SSG ensure pre-rendered content, search engines can easily index pages, leading to better rankings.
- Performance Optimization: Features like automatic static optimization, image optimization, and lazy loading reduce page load time.
- Built-in API Routes: Next.js eliminates the need for a separate backend, allowing developers to build serverless functions inside the pages/api directory.
- Flexible Data Fetching: Developers can use SSG, SSR, ISR (Incremental Static Regeneration), and CSR, depending on the nature of the content.
- Automatic Code Splitting: It loads only the necessary JavaScript required for a particular page, reducing the initial bundle size and improving speed.
- Enhanced Routing System: Next.js simplifies navigation with a file-based routing system, eliminating the need for third-party routing libraries like React Router.
4. What is the difference between SSR and CSR in Next.js?
Server-Side Rendering (SSR) and Client-Side Rendering (CSR) are two different approaches to rendering content in a web application.
- SSR (Server-Side Rendering): In this approach, the server processes a request, fetches the required data, and sends a fully-rendered HTML page to the user. This improves SEO and provides faster initial load times. However, subsequent interactions may still require client-side JavaScript execution.
- CSR (Client-Side Rendering): In CSR, the browser initially loads a blank HTML page, and JavaScript then fetches and renders content dynamically. While this reduces server load, it leads to slower first-page load times, making it less SEO-friendly.
Next.js allows developers to choose between SSR, CSR, SSG, and ISR, depending on the use case.
5. How does Static Site Generation (SSG) work in Next.js?
Static Site Generation (SSG) is a method where Next.js pre-generates pages at build time and serves them as static HTML files. This approach significantly improves performance by eliminating the need for real-time data fetching.
For example, a blog website with thousands of articles can be pre-built during deployment, ensuring that pages load instantly for users. Developers use the getStaticProps function to fetch data before build time, ensuring optimized content delivery. Since SSG content is already generated, it is extremely fast, reliable, and cost-efficient for production applications.
6. What is Incremental Static Regeneration (ISR), and how does it help?
Incremental Static Regeneration (ISR) allows Next.js to update pre-rendered static pages after deployment without requiring a full site rebuild. Unlike traditional SSG, where pages remain unchanged unless manually rebuilt, ISR enables developers to define a refresh interval for automatic content updates.
For example, an e-commerce website with frequently changing product prices can use ISR to revalidate pages dynamically. By setting a revalidate interval in getStaticProps, Next.js regenerates pages in the background, ensuring users receive fresh content without impacting page speed.
7. What are API Routes in Next.js, and how do they work?
Next.js provides a built-in backend solution through API Routes, allowing developers to create server-side functions within the pages/api directory. Each file inside this directory acts as an independent serverless function, making it easy to handle data processing, authentication, form submissions, and database interactions.
For instance, a file named pages/api/user.js can be accessed via /api/user, returning JSON responses or executing backend logic. These API routes eliminate the need for setting up an external backend framework like Express.js.
8. How does Next.js handle routing?
Next.js uses a file-based routing system, where the folder and file structure within the pages directory automatically defines URL paths.
- Basic Routing: A file pages/about.js corresponds to /about.
- Dynamic Routing: A file like pages/product/[id].js dynamically generates routes like /product/1 and /product/2.
- Nested Routing: Developers can organize files in subdirectories (pages/blog/post.js → /blog/post).
Unlike React Router, Next.js requires no additional setup, making routing more intuitive and efficient.
9. What is the purpose of getStaticProps in Next.js?
getStaticProps is a special function in Next.js that enables Static Site Generation (SSG) by fetching data at build time and pre-rendering pages as static HTML files. This function is particularly useful for pages that don’t require real-time data updates, making them fast, efficient, and SEO-friendly.
When a page uses getStaticProps, Next.js fetches all required data during the build process, generates static pages, and serves them without additional server processing. This results in near-instant loading speeds since users receive pre-rendered content directly from a CDN.
For example, an e-commerce website displaying product listings can use getStaticProps to fetch product details before deployment, ensuring that users don’t experience delays due to API calls.
10. Explain getServerSideProps and when to use it.
getServerSideProps is another data fetching method in Next.js, specifically used for Server-Side Rendering (SSR). Unlike getStaticProps, which fetches data at build time, getServerSideProps runs on every request, ensuring that users always receive the latest, real-time data.
This function is ideal for applications where frequent data updates are necessary, such as:
- Live dashboards displaying stock market prices, analytics, or weather reports.
- E-commerce platforms with dynamic pricing and product availability changes.
- User-specific content, such as personalized news feeds or logged-in user profiles.
Since getServerSideProps executes on the server before rendering the page, it provides accurate, up-to-date content, but at the cost of slightly longer response times compared to static pages. Developers must carefully optimize server-side logic to prevent performance bottlenecks.
11. What is _app.js in Next.js, and why is it important?
_app.js is a custom root component in Next.js that wraps all pages in the application, allowing developers to manage global state, persistent layouts, and shared functionalities across different pages.
By default, Next.js automatically renders each page independently, but with _app.js, developers can:
- Maintain global styles by importing CSS/SCSS files.
- Persist state across page navigations (e.g., using Context API or Redux).
- Customize error handling by integrating global error boundaries.
- Inject third-party providers such as authentication systems, analytics, and UI libraries.
For example, if an application uses a common header, footer, or sidebar on all pages, instead of duplicating the code on every page, it can be placed inside _app.js, ensuring a consistent user experience.
12. What is _document.js, and how is it different from _app.js?
While _app.js controls the main application logic, _document.js is a lower-level file that allows developers to modify the default HTML structure of a Next.js application.
_document.js primarily affects the <html> and <body> tags, making it useful for:
- Adding meta tags for SEO, viewport settings, and Open Graph integration.
- Including custom fonts from Google Fonts or external stylesheets.
- Injecting third-party scripts for analytics, performance monitoring, or security policies.
13. How does Next.js handle image optimization?
Next.js includes a built-in <Image> component that provides automatic image optimization, significantly improving performance, SEO, and user experience.
Traditional <img> tags load images at full resolution, leading to unnecessary bandwidth usage and slow page loads. In contrast, Next.js <Image>:
- Resizes and compresses images dynamically, reducing file sizes without losing quality.
- Supports modern formats like WebP, which offer better compression than JPEG/PNG.
- Lazy loads images, ensuring that images load only when they enter the viewport, reducing initial page load time.
- Optimizes images for different screen sizes, making it perfect for responsive design.
14. How does Next.js improve SEO?
One of the biggest advantages of Next.js over traditional React applications is its SEO optimization capabilities. By default, React relies on Client-Side Rendering (CSR), which means search engines may struggle to index content since pages are rendered dynamically in the browser.
Next.js improves SEO through:
- Server-Side Rendering (SSR) – Ensures pages are pre-rendered on the server, making them search engine-friendly.
- Static Site Generation (SSG) – Generates pre-built HTML pages, improving both performance and indexability.
- Custom Meta Tags – The <Head> component in Next.js allows developers to define title, description, and Open Graph tags, optimizing search rankings.
- Faster Load Times – Next.js reduces JavaScript bundle sizes, implements code splitting, and optimizes images, all of which contribute to better SEO rankings.
15. What is the difference between Static Site Generation (SSG) and Server-Side Rendering (SSR) in Next.js?
Static Site Generation (SSG) and Server-Side Rendering (SSR) are two key data-fetching strategies in Next.js, each serving different use cases.
- SSG (getStaticProps) generates HTML at build time, making it ideal for static content that doesn’t change frequently. This leads to faster load times and better SEO since the pre-rendered pages can be cached by a CDN.
- SSR (getServerSideProps) generates HTML at request time, meaning the page is dynamically rendered each time a user visits it. This is useful for pages requiring real-time data, such as live stock prices, personalized dashboards, or user-specific content.
16. What is Client-Side Rendering (CSR) in Next.js, and when should it be used?
Client-Side Rendering (CSR) in Next.js is a technique where JavaScript fetches data after the page loads and updates the UI dynamically. Unlike SSG and SSR, where HTML is generated before reaching the client, CSR relies on the browser to handle rendering.
CSR is best suited for:
- User-interactive applications such as dashboards, chat applications, and social media feeds.
- Highly dynamic pages that require real-time updates, like cryptocurrency price trackers or live sports scores.
- Single-page applications (SPAs) where content updates frequently without needing a full page reload.
However, since CSR relies on JavaScript execution, SEO may suffer as search engines struggle to index dynamically loaded content. To improve SEO and initial page load times, developers can combine CSR with SSR or SSG for a hybrid approach.
17. What is Incremental Static Regeneration (ISR) in Next.js?
Incremental Static Regeneration (ISR) is a powerful Next.js feature that allows static pages to be dynamically updated without requiring a full site rebuild.
Traditionally, SSG pages remain unchanged after deployment unless manually rebuilt. ISR enables developers to update static content on-demand, making it ideal for:
- E-commerce platforms where product details and prices frequently change.
- News websites that need to refresh articles periodically.
- Marketing websites that require content updates without redeploying the app.
With ISR, Next.js rebuilds only specific pages in the background at a set interval using the revalidate property. This allows users to see fresh content while keeping the performance benefits of static pages.
18. What are Middleware functions in Next.js, and how do they work?
Middleware in Next.js is a powerful feature that enables custom logic execution before completing a request. It allows developers to modify requests, protect routes, and handle authentication at the edge (server-side) level before serving a page.
Middleware functions are created inside middleware.js in the root directory and can perform tasks like:
- User authentication and role-based access control.
- Logging and monitoring user activity.
- URL redirection or A/B testing before rendering content.
19. What is the difference between _app.js and _document.js?
Although both _app.js and _document.js are used to customize a Next.js application, they serve different purposes:
- _app.js: Controls global logic, such as persistent layouts, authentication, and global styles. It is executed on both client and server and affects how pages are rendered.
- _document.js: Customizes the HTML structure, including <html>, <head>, and <body>. It runs only on the server, making it ideal for injecting third-party scripts, setting meta tags, and adding fonts.
Both files help developers streamline their applications, but _app.js focuses on application-level logic, while _document.js is specific to HTML modifications.
20. How does Next.js handle code splitting and optimization?
Next.js automatically optimizes JavaScript bundles by splitting code into smaller, efficient chunks to reduce load times.
Key optimization techniques include:
- Automatic Code Splitting: Only the required JavaScript for the current page is loaded, preventing unnecessary scripts from slowing down performance.
- Lazy Loading: Components and assets are loaded on demand instead of all at once.
- Tree Shaking: Unused code is removed, keeping bundle sizes minimal.
- Static Optimization: Pages that don’t require data fetching are automatically converted into static files.
These optimizations enhance performance, improve SEO, and reduce bandwidth usage, making Next.js a great choice for high-performance applications.
21. What are the advantages of using Next.js over React?
While React is a powerful frontend library, Next.js enhances it by providing server-side capabilities, routing, and optimizations.
Key advantages of Next.js over React include:
- Pre-rendering for SEO: Next.js offers SSG and SSR, making content more discoverable by search engines.
- Faster Performance: Automatic code splitting, image optimization, and lazy loading result in better user experience.
- Built-in API Routes: Eliminates the need for a separate backend for handling requests.
- Hybrid Rendering Support: Developers can mix static, server-rendered, and client-side pages based on needs.
While React is ideal for building SPAs, Next.js is a better choice for SEO-focused, high-performance applications.
22. What is the difference between getInitialProps and getServerSideProps in Next.js?
Both getInitialProps and getServerSideProps are used to fetch data on the server, but they differ in execution and performance.
- getInitialProps (deprecated in pages): Used in older versions of Next.js for both server-side and client-side data fetching. It blocks page rendering until data is retrieved, which may affect performance.
- getServerSideProps: Runs only on the server at request time, ensuring faster load times for SEO-critical pages. It prevents extra JavaScript from being sent to the client, making it more efficient.
For example, a user dashboard with real-time updates should use getServerSideProps to fetch the latest user data on every request. Meanwhile, getInitialProps is no longer recommended due to its performance drawbacks.
23. What is Next.js App Router, and how does it differ from the Pages Router?
Next.js App Router (introduced in Next.js 13) introduces a new file-based routing system using React Server Components (RSC), providing better performance and scalability compared to the traditional Pages Router.
Key differences:
- Pages Router (pages/): Uses the older getServerSideProps, getStaticProps, and getInitialProps for data fetching.
- App Router (app/): Uses server components, layouts, and streaming to enhance performance.
For example, with the App Router, developers can use loading states and Suspense boundaries for a more optimized user experience. It is the future of Next.js, offering faster rendering and improved developer experience.
24. What is Edge Computing in Next.js, and how does it improve performance?
Edge Computing in Next.js allows code execution closer to the user, reducing latency and improving performance by serving content from nearby servers instead of a central data center.
Key benefits:
- Faster API responses due to execution at the network edge.
- Improved security by reducing exposure to central servers.
- Lower server load, leading to cost savings.
For example, an e-commerce site with personalized recommendations can process requests at the edge, ensuring users see relevant products instantly without waiting for data from a remote server. Next.js enables Edge Computing through platforms like Vercel Edge Functions.
25. How does Next.js support Internationalization (i18n)?
Next.js provides built-in support for Internationalization (i18n), making it easy to build multi-language applications without requiring third-party libraries.
Features include:
- Automatic locale detection, allowing users to see content in their preferred language.
- Localized routing (/en, /fr, /es etc.) to serve different languages based on URL structure.
- Integration with translation libraries like next-intl or react-i18next for handling dynamic text translations.
For example, an e-learning platform targeting global users can dynamically switch languages based on user preferences, making content more accessible.
