Next.js Interview: Top 50 Questions and Answers 2023

In this fast-moving web development field, it’s very important to stay updated and ahead. Next.js is one such technology which is gaining much attention in this era and revolves around the vast ecosystem of React.

Next.js is a powerful Javascript-based framework which is built over React. The main catch is it comprises of powerful server-side rendering features which other Javascript frameworks or libraries lag. Other than that it provides Static site generation, optimises the site for SEO, and API Routing which makes an application fully SEO compatible and is in addition to the latest tech features.

Due to the rising demand for Next.js, there is also an increasing demand for professionals who carries the skills to build up workers. A solid understanding of Nextjs concepts can boost your marketability in the tech job market, even if you are a beginner or an experienced developer.

In this post, we have put together the top 50 Next.js interview questions with complete detailed answers and code examples. These interview questions will range from fundamentals to advanced features of Next.js to ensure a holistic preparation for your upcoming interviews.

So, let’s get started and dive into the world of Next.js!

Q1 ) What is Next.js?

Next.js is a React-based framework for building server-side rendered and static websites. It’s designed to provide a better development experience, better performance, and several built-in features like routing, prefetching, and code splitting.

Q2 ) How do you create a new Next.js project?

You can create a new Next.js project using Create Next App, which sets up everything automatically for you. To create a project, you can use the following command in your terminal:

npx create-next-app@latest my-app

Q3 ) What is the difference between getStaticProps and getServerSideProps in Next.js?

getStaticProps is used for data fetching at build time for static rendering, which means the data is fetched once during the build and used for all requests. On the other hand, getServerSideProps fetches data on each request at runtime.

Q4 ) Can you explain the file-based routing in Next.js?

In Next.js, the routing is based on the file system. It’s automatic and you don’t need to do any additional configurations. For example, if you create a React component inside a file at pages/about.js, it will be accessible at the /about URL.

Q5 ) How can you create a dynamic route in Next.js?

You can create a dynamic route by adding square brackets [] around the file name or a part of it. For example, pages/posts/[id].js will match routes like /posts/1, /posts/2 and so on. The value between the brackets will be accessible through the query property of the router object.

Q6 ) How can you handle 404 errors in Next.js?

You can create a custom 404 page by adding a 404.js file inside the pages directory. The page will automatically be rendered when a route is not found.

Q7 ) How can you use CSS in Next.js?

Next.js supports CSS and Sass out of the box. You can import them directly in your JavaScript files. For example: import '../styles/globals.css'. Additionally, you can use CSS Modules which generate unique class names to avoid naming collisions.

Q8 ) What is Image Optimization in Next.js?

Image Optimization is a feature in Next.js that allows you to optimize images on-the-fly as they’re requested by the client. You can use the built-in Image component from next/image to use this feature. The component also supports lazy loading by default.

Q9 ) What are Next.js API Routes?

Next.js has API Routes support out of the box. By creating a file inside the pages/api directory, you can create an API endpoint, which can be used for building a backend application along with your frontend.

Q10 ) What is next/link and how does it work?

next/link is a built-in component in Next.js that allows you to do client-side navigation to a different page in the application. It works by wrapping the component with <Link> tag. For example:

import Link from 'next/link'

function Navigation() {
  return (
    <nav>
      <ul>
        <li>
          <Link href="/">
            <a>Home</a>
          </Link>
        </li>
        <li>
          <Link href="/about">
            <a>About</a>
          </Link>
        </li>
        <li>
          <Link href="/contact">
            <a>Contact</a>
          </Link>
        </li>
      </ul>
    </nav>
  )
}

export default Navigation

Q11 ) What is the use of _app.js in Next.js?

_app.js is a custom App component in Next.js. You can override it to control the page initialization. This allows you to do amazing things like persisting layout between page changes, keeping state when navigating pages, custom error handling, injecting additional data into pages, and more.

Q12 ) What is the use of _document.js in Next.js?

_document.js is a custom Document component in Next.js. This is useful for modifying the <html> and <body> tags that are sent to the client. For example, you might need to add some global CSS or set the language for your site from here.

Q13 ) How to add a global CSS file in Next.js?

You can add a global CSS file by importing it in your _app.js file. For example:

import '../styles/globals.css'

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

export default MyApp

Q14 ) What are the advantages of using Next.js?

• Next.js offers many benefits, such as:
• Server-Side Rendering (SSR) and Static Site Generation (SSG) for better performance and SEO.
• Automatic Routing based on the file system.
• Built-in API routes to build your API.
• Hot Code Reloading: changes in the code get reflected in the app immediately without refreshing the page.
• Automatic code splitting for faster page loads.
• Support for CSS and SASS with zero configuration.

Q15 ) How do you use environment variables in Next.js?

You can create a .env.local file at the root of your project and add environment variables there. Then, you can use process.env.YOUR_VARIABLE to access the variable. If you want to use a variable in the browser, you need to prefix the variable name with NEXT_PUBLIC_.

Q16 ) How to do client-side navigation in Next.js?

You can use the next/router package for client-side navigation. It provides a useRouter hook, which gives you access to the router object. You can use router.push('/path') to navigate to a different page.

Q17 ) What is next.config.js in Next.js?

next.config.js is a configuration file for Next.js. You can use it to customize the behavior of your Next.js application. For example, you can add a custom webpack configuration, set up a proxy, define environment variables, and more.

Q18 ) How to add a custom webpack configuration in Next.js?

You can add a custom webpack configuration by exporting a function from next.config.js that modifies the config object. Here’s an example that adds a new alias:

module.exports = {
  webpack: (config, { buildId, dev, isServer, defaultLoaders, webpack }) => {
    config.resolve.alias['~'] = path.resolve(__dirname);
    return config;
  },
};

Q19 ) How to use SASS in Next.js?

Next.js supports SASS out of the box. You can create a .scss file and import it into your component. For example:

First, you’ll need to install the sass package. You can install it by running the following command:

npm install sass

Once that’s installed, you can create a .scss file. Let’s say you create a file named styles.scss with the following contents:

// styles.scss
$background-color: #f9f9f9;

.container {
  background-color: $background-color;
  padding: 20px;
  border-radius: 5px;
  box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1);
}

Then, you can import this .scss file in your React component like this:

// MyComponent.js
import React from 'react'
import './styles.scss'

function MyComponent() {
  return <div className="container">Hello, World!</div>
}

export default MyComponent

Q20 ) What is Incremental Static Regeneration (ISR) in Next.js?

ISR is a feature in Next.js that allows you to update static pages after they have been built, without needing to rebuild the entire site. You can specify a revalidate property in the getStaticProps function to define the time in seconds after which a page re-generation can occur.

Q21 ) How can you prefetch a page in Next.js?

Next.js automatically prefetches pages in the background as soon as they appear in the viewport. You don’t need to do anything to enable this feature. However, you can disable it by passing prefetch={false} to the Link component.

Q22 ) How to use TypeScript in Next.js?

Next.js supports TypeScript out of the box. You just need to create an empty tsconfig.json file and run npm run dev. Next.js will automatically configure TypeScript for you.

Q23 ) How to add a custom server in Next.js?

You can create a custom server by creating a server.js file at the root of your project. Here’s an example of a custom server with Express:

const express = require('express')
const next = require('next')

const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })
const handle = app.getRequestHandler()

app.prepare().then(() => {
  const server = express()
  
  server.get('*', (req, res) => {
    return handle(req, res)
  })
  
  server.listen(3000, (err) => {
    if (err) throw err
    console.log('> Ready on http://localhost:3000')
  })
})

Note: Using a custom server will disable some Next.js features like Automatic Static Optimization and Serverless Functions.

Q24 ) What are Next.js Serverless Functions and how do you use them?

Serverless Functions are a way to write backend code that runs whenever a certain API route is hit. You can create a serverless function by creating a file inside the pages/api directory. Here’s an example of a serverless function that returns a list of users:

export default function handler(req, res) {
  const users = [{ id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }];
  res.status(200).json(users);
}

Q25 ) What is SWR in Next.js and how do you use it?

SWR is a React Hooks library for remote data fetching created by the team behind Next.js. The name “SWR” is derived from stale-while-revalidate, a cache invalidation strategy. Here’s a simple example of how to use it:

import useSWR from 'swr'

function Profile() {
  const { data, error } = useSWR('/api/user', fetcher)
  
  if (error) return <div>Failed to load user</div>
  if (!data) return <div>Loading...</div>
  
  return <div>Hello {data.name}!</div>
}

Q26 ) How do you deploy a Next.js application?

Next.js can be deployed on any hosting provider that supports Node.js. However, Vercel, the company behind Next.js, provides a zero-configuration solution for deploying Next.js apps. You can also deploy on platforms like Netlify or Heroku.

Q27 ) What is the difference between Next.js and Create React App?

While both are used to build applications with React, Next.js comes with features such as server-side rendering and static site generation, automatic routing, and API routes. Create React App is more of a basic setup for a client-side rendered React application.

Q28 ) How do you fetch data in Next.js?

You can fetch data in Next.js using the built-in data fetching methods like getStaticProps, getServerSideProps, or getInitialProps. You can also fetch data on the client side using React hooks and libraries like SWR or axios.

Q29 ) How do you manage state in Next.js?

Managing state in Next.js is similar to how you’d do it in a regular React application. You can use React’s local state and context API, or use state management libraries like Redux, MobX, Zustand, or others.

Q30 ) What is Next.js’s automatic static optimization?

Automatic Static Optimization is a feature where Next.js will automatically determine that a page is static (has no server-side data dependencies) and serve the page as a static HTML file. This feature is automatically enabled and requires no configuration.

Q31 ) What is the difference between next/link and next/router?

next/link is a component that allows you to do client-side navigation to a different page in the application. next/router, on the other hand, gives you access to the router object, which can be used for imperatively navigating to different routes or accessing route parameters.

Q32 ) How do you use a layout component in Next.js?

A layout component can be used to wrap your page components and include common elements like headers, footers, or sidebars. You can import the layout component in your pages and wrap the page component with it. For example:

import Layout from '../components/Layout'

function HomePage() {
  return (
    <Layout>
      <h1>Welcome to our website</h1>
    </Layout>
  )
}

export default HomePage

Q33 ) How to handle form submission in Next.js?

Form submission in Next.js can be handled like in any other React app. You can use controlled components and handle the onSubmit event of the form. If you want to submit the form to a server, you can use fetch or axios to send a POST request.

Q34 ) How do you handle authentication in Next.js?

There are many ways to handle authentication in Next.js, depending on your needs. You can use Next.js’s API routes to create your authentication endpoints and use cookies or local storage to store the authentication state. You can also use third-party libraries like NextAuth.js or Passport.js for more complex authentication flows.

Q35 ) How do you test Next.js applications?

You can test Next.js applications using testing libraries like Jest and React Testing Library. You can test your components, custom hooks, and API routes. For end-to-end testing, you can use tools like Cypress or Playwright.

Q36 ) What is ISR and when should you use it?

ISR stands for Incremental Static Regeneration. It’s a feature in Next.js that allows you to update static content after it has been built without rebuilding the entire site. You can use ISR when you want to have static pages for SEO and performance, but your data updates frequently.

Q37 ) What is the difference between getStaticProps and getServerSideProps?

getStaticProps generates static pages at build time and revalidates them at runtime if specified. getServerSideProps generates pages on every request at runtime. Use getStaticProps for static pages and getServerSideProps for pages that need server-side computations or data on every request.

Q38 ) How do you style components in Next.js?

You can style components in Next.js using CSS modules, styled-jsx (comes with Next.js), or any other CSS-in-JS library like styled-components or emotion. You can also use regular CSS or SASS files.

Q39 ) How do you handle errors in Next.js?

You can handle errors in Next.js by using error boundary components for client-side errors and try-catch blocks for server-side errors. Next.js also provides a custom error page (pages/_error.js) that you can customize to handle all errors.

Q40 ) What is a dynamic route in Next.js?

A dynamic route is a page that gets created dynamically based on data. In Next.js, you can create dynamic routes by adding square brackets [] to a page name in the pages directory. For example, pages/posts/[id].js will match any route like /posts/1 or /posts/abc.

Q41 ) How do you use a dynamic route in Next.js?

To use a dynamic route, you can use the next/link or next/router packages to navigate to the route. You can also use the getStaticPaths function to specify which paths for the dynamic route should be pre-rendered at build time.

Q42 ) How do you deploy a Next.js app on Vercel?

To deploy a Next.js app on Vercel, you need to push your code to a Git provider (like GitHub, GitLab, or Bitbucket), then import your project into Vercel. Vercel will automatically detect that it’s a Next.js app and choose the right build settings for you.

Q43 ) What is the public folder in Next.js?

The public folder in Next.js is used to serve static files. Any file you put in the public folder will be accessible at the root of your domain. For example, if you put an image at public/my-image.png, you can access it in your app at /my-image.png.

Q44 ) How do you use images in Next.js?

You can put your images in the public folder and use them in your JSX like this: <img src="/my-image.png" alt="My Image" />. Next.js also has an Image component from the next/image package that provides optimizations like lazy-loading and resizing.

Q45 ) What is the Next.js Image component and how do you use it?

The Image component is a built-in component in Next.js that provides image optimizations like lazy-loading, resizing, and more. You can use it like this:

import Image from 'next/image'

function MyComponent() {
  return <Image src="/my-image.png" alt="My Image" width={500} height={300} />
}

Q46 ) What are the built-in data fetching methods in Next.js?

Next.js provides three built-in methods for data fetching:

1. getStaticProps: This function runs at build time and fetches data that will be used to generate a static page.
2. getServerSideProps: This function runs on each request at runtime and fetches data that is needed to render the page.
3. getInitialProps: This is a legacy method that can be used for either static generation or server-side rendering but is not recommended for new projects. It runs either on the server (for the initial request) or in the browser (for navigation after the initial page load).

Q47 ) How do you add a global stylesheet in Next.js?

You can add a global stylesheet by importing it in your custom _app.js file. The stylesheet must be a CSS or SASS file and cannot be a CSS Module. Here’s an example of how to import a global CSS file:

import '../styles/global.css'

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

export default MyApp

Q48 ) What is Next.js’s API Middleware?

Next.js’s API Middleware allows you to add middleware to your API routes for handling things like cookies, parsing request bodies, and more. The API Middleware is powered by the micro library, and you can use any middleware that works with micro.

Q49 ) What is the Next.js’s next/head component?

next/head is a built-in component in Next.js that allows you to modify the <head> of your document. You can use it to add or update meta tags, title, styles, and more. Here’s an example of how to use it:

import Head from 'next/head'

function MyPage() {
  return (
    <div>
      <Head>
        <title>My New Title</title>
      </Head>
      <h1>Hello world</h1>
    </div>
  )
}

export default MyPage

Q50 ) How do you prefetch data in Next.js?

You can prefetch data in Next.js by using the getStaticProps or getServerSideProps functions. These functions run at build time or request time (respectively) and fetch the data that is needed for rendering the page. The data is then passed as props to your page component.