30 Next.js Interview Questions: Your Ultimate Guide to Landing Your Dream Job

Are you ready to take your web development career to the next level? If you're eyeing a position that involves Next.js, you're in the right place. Next.js has revolutionized the React ecosystem, offering a powerful framework that combines the best of server-side rendering and static site generation. In this comprehensive guide, we'll dive deep into 30 essential Next.js interview questions, covering everything from the basics to advanced concepts. By the time you finish reading, you'll be well-prepared to ace your interview and land that dream job.

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Beginner Level Questions: Building Your Foundation

1. What is Next.js and how does it differ from React?

Next.js is a React-based framework that extends React's capabilities, providing a robust solution for building modern web applications. While React focuses primarily on creating user interfaces, Next.js adds a layer of server-side functionality and optimization. This combination results in faster initial page loads, improved search engine optimization (SEO), and a more streamlined development experience.

One of the key differences lies in how pages are rendered. React typically renders content on the client-side, which can lead to slower initial loads and potential SEO issues. Next.js, on the other hand, offers server-side rendering out of the box, ensuring that content is ready to be displayed as soon as the page loads. This not only improves performance but also makes your applications more search engine friendly.

2. What are the key advantages of using Next.js?

Next.js brings a plethora of benefits to the table, making it a go-to choice for many developers and companies. Here are some of the standout advantages:

Server-side rendering (SSR) is perhaps the most prominent feature. It allows for faster initial page loads and improved SEO, as search engines can easily crawl and index your content. This is particularly crucial for content-heavy websites or applications where search visibility is a priority.

Automatic code splitting is another significant advantage. Next.js intelligently splits your code into smaller chunks, ensuring that only the necessary JavaScript is loaded for each page. This results in faster load times and a more efficient use of resources, especially on mobile devices or in areas with slower internet connections.

The file-system based routing in Next.js simplifies the process of creating and managing routes in your application. By simply creating files in the pages directory, you automatically generate corresponding routes, eliminating the need for complex route configurations.

API routes are a powerful feature that allows you to build backend functionality directly within your Next.js application. This is particularly useful for creating serverless functions or handling data requests without the need for a separate backend server.

Static site generation (SSG) capabilities in Next.js allow you to pre-render pages at build time, resulting in lightning-fast static websites. This is ideal for content that doesn't change frequently, such as marketing pages or blogs.

Lastly, the built-in CSS support and optimization in Next.js streamline the styling process. Features like CSS Modules and automatic CSS inlining help maintain a clean and efficient codebase while ensuring optimal performance.

3. How do you create a new Next.js application?

Creating a new Next.js application is a straightforward process that gets you up and running quickly. To start, open your terminal and run the following command:

npx create-next-app my-next-app

This command utilizes npx, which comes bundled with npm 5.2+ and higher, to execute the create-next-app package without installing it globally. The my-next-app argument specifies the name of your project folder.

Once you run this command, you'll be prompted to make a few choices about your project setup. You can select whether to use TypeScript, ESLint, and other configuration options. After making your selections, Next.js will set up a new project with a basic file structure and necessary dependencies.

This process creates a fully functional Next.js application with a sensible default configuration, including:

A pages directory for your routes
A public directory for static assets
A styles directory for your CSS files
A basic package.json with essential scripts and dependencies

After the installation is complete, you can navigate into your project directory and start the development server:

cd my-next-app
npm run dev

This will start your Next.js application on http://localhost:3000, where you can see your new app in action and begin development.

4. Explain server-side rendering (SSR) in Next.js

Server-side rendering is a cornerstone feature of Next.js, addressing many of the limitations associated with client-side rendering in traditional React applications. In SSR, the initial HTML content is generated on the server for each request, rather than in the browser.

When a user requests a page, the server executes the JavaScript code, fetches any necessary data, and renders the React components to HTML. This fully rendered HTML is then sent to the client, along with the JavaScript needed to make the page interactive. The result is a faster initial page load and improved SEO, as search engines can easily crawl and index the content.

In Next.js, SSR is implemented automatically for pages that use the getServerSideProps function. This function runs on every request and allows you to fetch data and pass it as props to your page component. Here's a simple example:

export async function getServerSideProps(context) {
  const res = await fetch('https://api.example.com/data')
  const data = await res.json()

  return {
    props: { data }, // will be passed to the page component as props
  }
}

function Page({ data }) {
  // Render data...
}

export default Page

In this example, getServerSideProps fetches data from an API on every request. The fetched data is then passed to the Page component as props, ensuring that the content is always up-to-date.

SSR is particularly beneficial for dynamic content that changes frequently or requires user-specific data. However, it's important to note that while SSR improves initial load times and SEO, it can increase the load on your server and potentially slow down subsequent page navigations if not optimized correctly.

5. What is static site generation (SSG) in Next.js?

Static site generation is another powerful feature of Next.js that allows you to pre-render pages at build time, resulting in blazing-fast static websites. Unlike server-side rendering, which generates content on each request, SSG creates HTML files during the build process, which can then be served directly from a Content Delivery Network (CDN).

SSG is ideal for content that doesn't change frequently, such as marketing pages, documentation sites, or blogs. It offers several advantages:

Extremely fast load times, as pages are pre-rendered and can be cached at the edge
Reduced server load, as pages don't need to be generated on each request
Improved security, as there's no server-side code execution for static pages
Lower hosting costs, as static files are cheap and easy to distribute

In Next.js, you can implement SSG using the getStaticProps function. This function runs at build time and provides props to your page component. Here's an example:

export async function getStaticProps() {
  const res = await fetch('https://api.example.com/posts')
  const posts = await res.json()

  return {
    props: {
      posts,
    },
  }
}

function Blog({ posts }) {
  // Render posts...
}

export default Blog

In this example, getStaticProps fetches blog posts at build time. The resulting HTML file includes the pre-rendered content, which can be served statically.

For pages with dynamic routes, you'll also need to use getStaticPaths to specify which paths should be pre-rendered. This allows you to generate static pages for dynamic content, such as individual blog posts or product pages.

Next.js also offers Incremental Static Regeneration (ISR), which allows you to update static content without rebuilding your entire site. With ISR, you can specify a revalidation time, after which Next.js will regenerate the page in the background, ensuring your content stays fresh while maintaining the benefits of static generation.

6. How does routing work in Next.js?

Routing in Next.js is one of its most intuitive and powerful features, leveraging a file-system based approach that simplifies the process of creating and managing routes in your application. This system is built on the principle of "convention over configuration," meaning that the file structure of your project directly maps to your application's routes.

Here's how it works:

Pages Directory: The core of Next.js routing is the pages directory. Each file inside this directory automatically becomes a route in your application.
File-to-Route Mapping: The file name directly corresponds to the route path. For example:
- pages/index.js maps to the root route /
- pages/about.js maps to /about
- pages/blog/post.js maps to /blog/post
Dynamic Routes: Next.js supports dynamic routes using square brackets. For instance:
- pages/posts/[id].js creates a dynamic route for /posts/1, /posts/2, etc.
- You can access the id parameter in your component using the useRouter hook.
Nested Routes: You can create nested routes simply by nesting folders within the pages directory. For example, pages/blog/[slug].js would create routes like /blog/my-first-post.
Catch-all Routes: Using [...param] in your file name allows you to catch all routes under a certain path. For example, pages/posts/[...slug].js would match /posts/2020/01/first-post.
Link Component: Next.js provides a Link component for client-side navigation between routes. This preserves the single-page application experience while allowing for optimized prefetching of pages.

Here's a simple example of how you might use the Link component:

import Link from 'next/link'

function NavBar() {
  return (
    <nav>
      <Link href="/">
        <a>Home</a>
      </Link>
      <Link href="/about">
        <a>About</a>
      </Link>
    </nav>
  )
}

Programmatic Navigation: For cases where you need to navigate programmatically, you can use the useRouter hook:

import { useRouter } from 'next/router'

function MyComponent() {
  const router = useRouter()

  const handleClick = () => {
    router.push('/about')
  }

  return <button onClick={handleClick}>Go to About</button>
}

This routing system not only simplifies development but also contributes to better performance. Next.js automatically code-splits your application by route, ensuring that only the necessary JavaScript is loaded for each page.

7. What is the purpose of the `getStaticProps` function?

The getStaticProps function is a key feature in Next.js that enables static site generation (SSG). Its primary purpose is to fetch data at build time and provide that data as props to your page components. This allows you to create statically generated pages that include dynamic content, combining the benefits of static sites (fast load times, improved SEO) with the flexibility of data-driven pages.

Here's a breakdown of how getStaticProps works and why it's so useful:

Build-Time Execution: getStaticProps runs at build time in production, and in the background when using next dev in development mode. This means the data is fetched and the page is rendered before the user makes a request.
Data Fetching: You can use getStaticProps to fetch data from any source – an external API, a database, or even local files. This data is then used to render the page.
Props Injection: The function returns an object with a props key. These props are then passed to the page component.
Static Generation: Once getStaticProps has run and the page has been rendered, the resulting HTML is saved and can be reused on each request, improving performance.
TypeScript Support: If you're using TypeScript, you can strongly type the props returned by getStaticProps.

Here's an example of how you might use getStaticProps:

export async function getStaticProps() {
  const res = await fetch('https://api.example.com/products')
  const products = await res.json()

  return {
    props: {
      products,
    },
    // Next.js will attempt to re-generate the page:
    // - When a request comes in
    // - At most once every 10 seconds
    revalidate: 10, // In seconds
  }
}

function ProductList({ products }) {
  return (
    <ul>
      {products.map((product) => (
        <li key={product.id}>{product.name}</li>
      ))}
    </ul>
  )
}

export default ProductList

In this example, getStaticProps fetches a list of products from an API. This data is then passed to the ProductList component as props. The revalidate option enables Incremental Static Regeneration, allowing the page to be regenerated in the background after a certain period.

Use cases for getStaticProps include:

Marketing pages
Blog posts and portfolios
E-commerce product listings
Help and documentation

It's important to note that getStaticProps only runs on the server-side, so you can safely use direct database queries or access the filesystem without exposing sensitive information to the client.

By leveraging getStaticProps, you can create fast, SEO-friendly pages that still incorporate dynamic data, offering the best of both static and dynamic web applications.

8. How can you pass data between pages in Next.js?

Passing data between pages in Next.js is a common requirement in many applications. There are several methods to achieve this, each with its own use cases and trade-offs. Here are some of the most common approaches:

Query Parameters:
One of the simplest ways to pass data between pages is through URL query parameters. You can add data to the URL when navigating and then retrieve it on the destination page.

// On the source page
import { useRouter } from 'next/router'

function SourcePage() {
  const router = useRouter()
  const handleClick = () => {
    router.push('/destination?data=Hello')
  }
  return <button onClick={handleClick}>Go to Destination</button>
}

// On the destination page
function DestinationPage() {
  const router = useRouter()
  const { data } = router.query
  return <div>Received data: {data}</div>
}

This method is suitable for small amounts of data and when you want the data to be visible in the URL.

State Management Libraries:
For more complex data sharing, especially in larger applications, you might want to use a state management library like Redux, MobX, or Recoil. These libraries allow you to maintain a global state that can be accessed from any component.

// Using Redux as an example
import { createStore } from 'redux'
import { Provider, useSelector } from 'react-redux'

const store = createStore(rootReducer)

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

// In any component
function MyComponent() {
  const data = useSelector(state => state.someData)
  // Use data...
}

This approach is beneficial for managing complex application state and when data needs to be shared across many components.

Context API:
React's Context API, which is fully supported in Next.js, provides a way to pass data through the component tree without having to pass props down manually at every level.

import { createContext, useContext, useState } from 'react'

const DataContext = createContext()

function DataProvider({ children }) {
  const [data, setData] = useState(null)
  return (
    <DataContext.Provider value={{ data, setData }}>
      {children}
    </DataContext.Provider>
  )
}

// In _app.js
function MyApp({ Component, pageProps }) {
  return (
    <DataProvider>
      <Component {...pageProps} />
    </DataProvider>
  )
}

// In any component
function MyComponent() {
  const { data } = useContext(DataContext)
  // Use data...
}

This method is useful for sharing data that many components need, like user authentication status or theme settings.

Local Storage or Session Storage:
For persisting data across page reloads or browser sessions, you can use the browser's storage mechanisms.
```
// Saving data
localStorage.setItem('myData', JSON.stringify(data))

// Retrieving data
const data = JSON.parse(localStorage.getItem('myData'))
```
This approach is useful for storing user preferences or temporary data that should persist across page reloads.
Server-Side Data Fetching: