50 Essential Next.js Interview Questions and Answers 🚀

Next.js is a powerful React framework that enables server-side rendering (SSR), static site generation (SSG), and much more, making it a critical skill for modern frontend developers. This guide covers 50 essential Next.js interview questions, ranging from basic concepts to advanced architecture and performance.

💡 Fundamentals and Core Concepts

What is Next.js and why would you use it over a standard React application?
- Answer: Next.js is a full-stack React framework that provides out-of-the-box solutions for rendering, routing, and styling. Its primary advantages are Server-Side Rendering (SSR) and Static Site Generation (SSG), which improve performance, SEO, and developer experience compared to a purely Client-Side Rendered (CSR) React app created with something like Vite or Create React App.
Explain the different rendering methods available in Next.js.
- Answer: The main methods are:
  - Static Site Generation (SSG): Pre-renders at build time. Fast and highly cacheable.
  - Server-Side Rendering (SSR): Renders on the server per request. Good for dynamic, frequently changing data.
  - Incremental Static Regeneration (ISR): SSG with a mechanism to re-generate pages in the background after deployment.
  - Client-Side Rendering (CSR): Initial shell is rendered, data fetching and rendering happen in the browser after hydration (similar to a standard React app).
What is the purpose of the pages directory in a Next.js application?
- Answer: The pages directory defines the routes (pages) of the application using the file-system based router. Each file in this directory (e.g., pages/contact.js) automatically becomes a route (/contact).
How does Next.js handle routing?
- Answer: Next.js uses a file-system based router. Dynamic routes are handled using brackets, e.g., pages/posts/[slug].js handles routes like /posts/my-first-post. It also includes a Link component for client-side transitions and useRouter hook for programmatic navigation.
What is the role of the _app.js file?
- Answer: It's used to initialize pages. It allows you to inject global CSS, maintain state across page transitions, and apply global layouts (like navigation bars or footers).
What is the role of the _document.js file?
- Answer: It's used to extend the default HTML document (<html>, <body>). It's necessary for modifying the initial server-rendered markup, often for setting up custom lang attributes, injecting external scripts, or configuring CSS/style libraries like styled-components. It runs only on the server.
What is Static Site Generation (SSG) and how do you implement it?
- Answer: SSG pre-renders the page at build time. The resulting HTML is served immediately, making it extremely fast. You implement it by exporting an async function called getStaticProps from a page file.
What is Server-Side Rendering (SSR) and how do you implement it?
- Answer: SSR renders the page on the server at request time. This ensures the page is always up-to-date with the latest data. You implement it by exporting an async function called getServerSideProps from a page file.
Differentiate between getStaticProps and getServerSideProps.
- Answer: getStaticProps runs at build time (or based on revalidate interval for ISR), and the result is cached and served globally. getServerSideProps runs on every incoming request and is not cached by default, ensuring the latest data.
What is Incremental Static Regeneration (ISR)?
- Answer: ISR is an extension of SSG that allows you to update static pages after the site has been built without needing a full rebuild/redeploy. You implement it by adding a revalidate property (e.g., revalidate: 60) to the object returned by getStaticProps, which tells Next.js to re-generate the page in the background after 60 seconds if a request comes in.

🌐 Data Fetching and API Routes

How do you fetch external data in a Next.js page component?
- Answer: For pre-rendering, use getStaticProps (SSG) or getServerSideProps (SSR). For client-side data, you can use SWR or React Query (or standard useEffect/fetch) inside the component, but CSR will hurt initial load performance.
When should you use getStaticPaths?
- Answer: You use getStaticPaths on dynamic pages (e.g., [id].js) that are using SSG (getStaticProps). It tells Next.js which paths (IDs, slugs, etc.) need to be pre-rendered at build time.
Explain the fallback option in getStaticPaths.
- Answer: The fallback option determines how Next.js handles paths not returned by getStaticPaths:
  - fallback: false: Any path not pre-rendered results in a 404.
  - fallback: true: For uncached paths, the page is rendered on the server for the first time (SSR), then cached for subsequent requests (becomes SSG).
  - fallback: 'blocking': Similar to true, but the server waits for the data to be fetched and the page to be rendered before sending the response (no loading state on the client).
What are API Routes in Next.js?
- Answer: API Routes are files inside the pages/api directory that are used to build your own backend API endpoints. They run on the server-side and allow you to securely handle database interaction, external API calls, and authentication.
How do you handle different HTTP methods (GET, POST, etc.) in an API Route?
- Answer: You typically use a switch statement or a series of if statements on the req.method property inside the handler function to execute different logic based on the request method.
How can you share data between different getStaticProps calls or API routes without fetching again?
- Answer: You can use an in-memory cache (like node-cache) outside the handler function (at the module level) to store data. For API routes, you can also use a faster internal connection instead of an external HTTP request, or leverage a database connection pool.
Where does getStaticProps/getServerSideProps execute (client or server)?
- Answer: They only execute on the server (or at build time). This means you can write Node.js code and safely access sensitive data like file systems or database credentials without exposing them to the client.
How do you secure API routes?
- Answer: By validating the request's origin and method, using middleware for authentication/authorization (e.g., checking for a valid JWT in the request headers), and ensuring sensitive operations are only accessible via POST, PUT, or DELETE methods and are protected.
What are the limitations of using fetch directly inside getStaticProps or getServerSideProps in a production deployment?
- Answer: If you use fetch to call an external public URL of your own Next.js API Routes (e.g., fetch('https://my-app.com/api/data')), the server makes an unnecessary external network call. It's better to import the API logic directly or use a relative internal path (fetch('http://localhost:3000/api/data') or just /api/data) to save network latency.
How do you access the request object in getServerSideProps?
- Answer: The getServerSideProps function receives a single object argument called context. This object contains the req (request) and res (response) objects, allowing access to headers, cookies, query parameters, etc.

🎨 Styling and Assets

How do you include global CSS in a Next.js application?
- Answer: Global CSS must be imported inside the pages/_app.js file (or a custom App Component in the App Router). Importing global CSS elsewhere will result in an error.
What are CSS Modules and how are they used in Next.js?
- Answer: CSS Modules are a solution to scope CSS locally to a component, preventing style conflicts. In Next.js, files named [name].module.css are automatically treated as CSS Modules. You import them and reference class names as properties of the imported object.
How do you use the built-in Image component and what are its benefits?
- Answer: The next/image component replaces the standard <img> tag. Its benefits include automatic image optimization (resizing, quality compression) on demand, support for modern formats like WebP, and lazy loading of images by default for improved Core Web Vitals.
How does the Head component work?
- Answer: The next/head component allows you to inject elements into the <head> of the page for SEO and metadata purposes. Any tags you place inside it (like <title>, <meta>, <link>) are automatically merged and deduplicated with the default <head> content.
How can you prefetch resources in Next.js?
- Answer: The next/link component automatically prefetches the JavaScript bundle for the linked page when the link appears in the viewport (or on hover). You can also use the next/script component to strategically load external scripts.

⚙️ Performance and Optimization

What is Automatic Code Splitting in Next.js?
- Answer: Next.js automatically splits the JavaScript code into small chunks for each page. When a user requests a page, only the necessary code for that page is loaded, improving initial load time.
How does the Link component optimize navigation?
- Answer: The next/link component performs client-side routing (no full page reload) and automatically prefetches the page components and data for the destination route when the link is visible on the viewport, making navigation instant.
What is Fast Refresh?
- Answer: Fast Refresh is a Next.js development feature that provides instant feedback on edits. It attempts to preserve the component state when refreshing code changes, avoiding a full page reload and maintaining development flow.
How do you optimize fonts in Next.js?
- Answer: Next.js provides the next/font module which automatically handles font optimization, including self-hosting Google Fonts, inlining CSS (removing external network requests), and ensuring fonts are loaded with minimal layout shift.
What is the significance of the next/script component?
- Answer: It allows you to strategically load third-party scripts (like analytics or chat widgets) to avoid blocking the main thread and ensure better performance. It uses a strategy prop (beforeInteractive, afterInteractive, lazyOnload) to control when the script loads.

🛠️ Advanced Features and Deployment

Explain the concept of Custom Server in Next.js and when you might need one.
- Answer: A Custom Server is a custom Node.js server (e.g., using Express) that programmatically handles requests instead of the built-in Next.js server. It's rarely needed but might be used to integrate with an existing non-Node.js backend, apply very specific caching policies, or use a custom routing logic.
What are Environment Variables and how do you expose them to the client-side?
- Answer: Environment variables (.env.* files) store configuration values. By default, they are only available on the server. To expose them to the client, the variable name must be prefixed with NEXT_PUBLIC_.
How do you implement internationalization (i18n) in Next.js?
- Answer: Next.js has built-in support for i18n routing. You configure locales and domains in next.config.js. The framework automatically handles routing based on the locale (e.g., /en/about, /fr/a-propos).
What are Middleware in Next.js?
- Answer: Middleware (in the middleware.js or _middleware.js file) allows you to run code before a request is completed on the server. It's used for things like authentication checks, redirects, URL rewriting, or A/B testing, and runs across all routes.
How do you handle redirects and rewrites in Next.js?
- Answer: They are configured in the next.config.js file.
  - Redirects change the URL and fetch content from the new location (client sees the new URL).
  - Rewrites mask the destination path, allowing you to proxy requests while the user stays on the original URL.
Explain Absolute Imports and Module Path Aliases.
- Answer: Next.js supports importing modules using absolute paths (e.g., import Component from 'components/MyComponent') instead of relative paths (e.g., import Component from '../../components/MyComponent'). This is configured in jsconfig.json or tsconfig.json using the baseUrl or paths properties, making imports cleaner.
What is the public directory used for?
- Answer: The public directory is for static assets that need to be served directly, like images, manifest files, and robots.txt. Files placed here are available at the root URL (e.g., /image.png).
How can you handle errors (e.g., 404, 500) in a Next.js application?
- Answer: Next.js uses specific page files:
  - 404: Create a custom pages/404.js.
  - 500/General Errors: Create a custom pages/500.js or use the built-in pages/_error.js to handle all unhandled server-side errors.
Describe the Next.js production build process.
- Answer: Running next build first compiles the application. Then, it pre-renders all static pages (SSG) determined by getStaticProps and generates a list of routes to be handled by the server (SSR or API Routes). It generates an optimized output of HTML, CSS, and JS chunks ready for deployment.
How can you use TypeScript with Next.js?
- Answer: You can start with TypeScript by adding a tsconfig.json file to your project root. When you run next dev, Next.js automatically configures TypeScript and suggests adding necessary dependencies. Files are named .ts or .tsx.

🧩 App Router (Next.js 13+)

What is the App Router and how does it differ from the Pages Router?
- Answer: The App Router (in the app directory) is a newer, React Server Components-based routing system. It differs by focusing on shared layouts, nested routing, and Server Components by default, whereas the Pages Router uses files for routes and is primarily Client Components.
What are Server Components and Client Components?
- Answer: Server Components (RSC) are the default in the App Router. They run only on the server (or at build time), don't have access to browser APIs (window, localStorage), and don't re-render/hydrate. Client Components must use the 'use client' directive, run on both the server (for SSR) and the client (for hydration), and allow for state, effects, and interactivity.
How do you define layouts and templates in the App Router?
- Answer: A layout.js file defines UI shared across a segment and its children (it persists state on navigation). A template.js defines UI that is not shared, causing the component tree to be re-mounted on navigation (good for applying entry/exit animations).
What is the default rendering mechanism in the App Router?
- Answer: The default rendering mechanism for the App Router is a form of Static Rendering (similar to SSG) combined with Server Components, leading to high performance and minimal client-side JS.
How is data fetching handled within Server Components?
- Answer: Data fetching is done directly within the component using await fetch(...) (or a database query). Since it runs on the server, you can use async/await and don't need dedicated data-fetching functions like getStaticProps.
How do you manage client-side interactivity in the App Router?
- Answer: Any component that needs state, effects (useEffect), or event handlers must be a Client Component. You separate the interactive part of the UI into a file marked with 'use client' and import it into the parent Server Component.
What is the purpose of the loading.js file?
- Answer: The loading.js file defines UI (like a skeleton or spinner) that is displayed immediately while the content of a segment is being fetched and rendered on the server.
How do you handle dynamic segments in the App Router?
- Answer: Dynamic segments are defined using a folder named in brackets, e.g., app/blog/[slug]/page.js. You access the parameter (e.g., slug) within the page or layout component using the params prop.
Explain the role of metadata in the App Router.
- Answer: The App Router simplifies SEO by allowing you to export a static or dynamic metadata object (or an async generateMetadata function) from a layout.js or page.js file. Next.js automatically generates the correct <head> tags.
How do Route Handlers (equivalent to API Routes) work in the App Router?
- Answer: Route Handlers replace API Routes in the App Router. They are files named route.js inside an app directory segment. They export functions corresponding to HTTP methods (e.g., GET, POST) and receive Request and Response objects (based on the Web Standard API).

Tip: When answering these questions, always try to relate the concept to a real-world performance or developer experience benefit. For example, when discussing SSG, mention better performance and lower hosting costs.

50 Essential Next.js Interview Questions and Answers