Not-found.tsx customization in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this custom Not-found.tsx component render?

Consider this Next.js 14 custom not-found.tsx component:

export default function NotFound() {
  return (
    <main role="main" aria-label="Page not found">
      <h1>Oops! Page missing.</h1>
      <p>Sorry, we couldn't find what you were looking for.</p>
    </main>
  )
}

What will be the visible output when a user visits a non-existent page?

NextJS

export default function NotFound() {
  return (
    <main role="main" aria-label="Page not found">
      <h1>Oops! Page missing.</h1>
      <p>Sorry, we couldn't find what you were looking for.</p>
    </main>
  )
}

A A blank page with no content rendered.

B A page with heading 'Oops! Page missing.' and a paragraph apologizing for missing content.

C A default Next.js 404 page ignoring this component.

D An error message saying 'Component NotFound not found'.

💡 Hint

Custom not-found.tsx replaces the default 404 page in Next.js.

This component renders the custom message inside a semantic <main> with proper ARIA label. Next.js uses this component for 404 pages.

📝 Syntax

intermediate

Which option correctly exports a NotFound component in Next.js 14?

Choose the correct syntax to export a NotFound component in not-found.tsx for Next.js 14.

A export NotFound = () => <h1>Not Found</h1>

B function NotFound() { return <h1>Not Found</h1> } export NotFound

C export default function NotFound() { return <h1>Not Found</h1> }

D export default NotFound = () => <h1>Not Found</h1>

💡 Hint

Remember to use export default function syntax for components.

Option C uses valid ES module export syntax with a function declaration. Others have syntax errors.

❓ state_output

advanced

What happens if you use useState inside not-found.tsx?

Given this not-found.tsx component in Next.js 14:

import { useState } from 'react';

export default function NotFound() {
  const [count, setCount] = useState(0);
  return (
    

      
Page Not Found
      
    
  )
}

What will be the behavior when this page is shown?

NextJS

import { useState } from 'react';

export default function NotFound() {
  const [count, setCount] = useState(0);
  return (
    <main>
      <h1>Page Not Found</h1>
      <button onClick={() => setCount(count + 1)}>Clicked {count} times</button>
    </main>
  )
}

A The count will always stay 0 because the component is server rendered only.

B The button will increment the count on each click, showing updated count.

C The page will not render at all due to invalid hook usage.

D Next.js will throw a runtime error because useState is not allowed in not-found.tsx.

💡 Hint

Next.js 14 supports client components inside not-found.tsx if you use 'use client'.

By default, components are server components. But if you add 'use client' at the top, hooks like useState work. Without it, this code errors.

🔧 Debug

advanced

Why does this not-found.tsx cause a 500 error?

Review this not-found.tsx code:

export default function NotFound() {
  return (
    <div>
      <h1>Page Not Found</h1>
      <button onClick={() => alert('Oops!')}>Click me</button>
    </div>
  )
}

Visiting a missing page causes a server error 500. Why?

NextJS

export default function NotFound() {
  return (
    <div>
      <h1>Page Not Found</h1>
      <button onClick={() => alert('Oops!')}>Click me</button>
    </div>
  )
}

A Because event handlers like onClick are not allowed in server components without 'use client'.

B Because alert is not defined in Next.js environment.

C Because the component must return a <main> element, not <div>.

D Because the button tag is invalid in Next.js 14.

💡 Hint

Think about client vs server components and event handlers.

By default, components are server components and cannot have event handlers like onClick. Adding 'use client' fixes this.

🧠 Conceptual

expert

How does Next.js 14 decide to render not-found.tsx?

In Next.js 14, when does the framework render the not-found.tsx component?

Choose the most accurate explanation.

A When a route segment or dynamic route returns <code>notFound()</code> from a server component, Next.js renders <code>not-found.tsx</code> for that segment.

B Next.js always renders <code>not-found.tsx</code> on any 404 error regardless of route or data fetching.

C Next.js renders <code>not-found.tsx</code> only if the file is inside the <code>pages</code> directory.

D Next.js renders <code>not-found.tsx</code> only if the user manually navigates to /not-found URL.

💡 Hint

Think about how Next.js 14 handles dynamic routes and server components.

Next.js 14 uses the new App Router. If a server component calls notFound(), the framework renders the nearest not-found.tsx in that route segment.

Practice

1. What is the main purpose of the not-found.tsx file in a Next.js app?

easy

A. To define the main layout of the website

B. To handle user authentication

C. To customize the page shown when a user visits a missing URL

D. To manage API routes

Solution

1. Step 1: Understand the role of not-found.tsx

   This file is specifically for customizing the page shown when a user tries to access a page that does not exist.

2. Step 2: Compare with other options

   Other options like layout, authentication, or API routes are handled by different files or folders in Next.js.

3. Final Answer:

   To customize the page shown when a user visits a missing URL -> Option C

4. Quick Check:

   Missing page display = Custom not-found.tsx [OK]

Hint: Remember: not-found.tsx shows missing page content [OK]

Common Mistakes:

• Confusing not-found.tsx with layout or API files
• Thinking it handles authentication
• Assuming it manages site navigation

2. Which of the following is the correct way to export a default React component in not-found.tsx?

easy

A. default export function NotFound() { return Page Not Found; }

B. function NotFound() { return Page Not Found; } export NotFound;

C. export function NotFound() { return Page Not Found; }

D. export default function NotFound() { return Page Not Found; }

Solution

1. Step 1: Recall correct default export syntax

   In React with Next.js, the correct way is to write export default function ComponentName() { ... }.

2. Step 2: Check each option

   export default function NotFound() { return

   Page Not Found

   ; } matches the correct syntax. function NotFound() { return

   Page Not Found

   ; } export NotFound; has incorrect export syntax. export function NotFound() { return

   Page Not Found

   ; } exports a named function, not default. default export function NotFound() { return

   Page Not Found

   ; } uses invalid syntax.

3. Final Answer:

   export default function NotFound() { return <h1>Page Not Found</h1>; } -> Option D

4. Quick Check:

   Default export = export default function [OK]

Hint: Default export uses 'export default function' syntax [OK]

Common Mistakes:

• Using named export instead of default
• Incorrect export keyword placement
• Missing 'default' keyword

3. Given this not-found.tsx component, what will be displayed when a user visits a missing page?

export default function NotFound() {
  return (
    <main role="main">
      <h1>Oops! Page not found.</h1>
      <p>Try going back or visit the homepage.</p>
      <a href="/">Home</a>
    </main>
  );
}

medium

A. A page with a heading 'Oops! Page not found.', a paragraph, and a link to Home

B. A blank page with no content

C. An error message in the browser console only

D. A redirect to the homepage without showing any message

Solution

1. Step 1: Read the returned JSX

   The component returns a main section with a heading, paragraph, and a link to the homepage.

2. Step 2: Understand what renders on missing page

   When a page is missing, this component renders the content exactly as coded, showing the heading, paragraph, and link.

3. Final Answer:

   A page with a heading 'Oops! Page not found.', a paragraph, and a link to Home -> Option A

4. Quick Check:

   JSX content renders as visible page elements [OK]

Hint: JSX inside return shows on missing page [OK]

Common Mistakes:

• Thinking it redirects automatically
• Assuming no content shows
• Confusing console errors with UI output

4. Identify the error in this not-found.tsx code snippet:

export default function NotFound() {
  return (
    <div>
      <h1>Page Not Found</h1>
      <a href="/">Go Home</a>
    </div>
  )
}

medium

A. Using <div> instead of <main> for accessibility

B. No error; code is valid and works correctly

C. Missing semicolon after return statement

D. Missing parentheses around JSX

Solution

1. Step 1: Check JSX syntax

   The JSX is correctly wrapped and returned; no syntax errors or missing semicolons needed after return.

2. Step 2: Consider accessibility best practices

   Using <main> instead of <div> improves accessibility by defining the main content region.

3. Final Answer:

   Using <div> instead of <main> for accessibility -> Option A

4. Quick Check:

   Use <main> for main content in error pages [OK]

Hint: Use <main> tag for main content, not just <div> [OK]

Common Mistakes:

• Thinking semicolons are required after return
• Ignoring accessibility tags
• Assuming JSX needs extra parentheses

5. You want your not-found.tsx page to show a custom message only if the user is on a mobile device, otherwise show a default message. Which approach fits Next.js best?

hard

A. Create two separate not-found.tsx files, one for mobile and one for desktop

B. Use a React hook like useEffect with window.navigator.userAgent to detect device and conditionally render

C. Use CSS media queries to hide/show different messages in the same component

D. Write server-side code in not-found.tsx to detect device from request headers and render accordingly

Solution

1. Step 1: Understand client vs server rendering

   Device detection using window.navigator.userAgent works only on client side, so use React hooks like useEffect.

2. Step 2: Evaluate other options

   Server-side detection is complex and not typical in not-found.tsx. CSS media queries only style but don't change content. Multiple files for one route is invalid.

3. Final Answer:

   Use a React hook like useEffect with window.navigator.userAgent to detect device and conditionally render -> Option B

4. Quick Check:

   Client detection with hooks = best for device-specific UI [OK]

Hint: Detect device client-side with hooks, not server or multiple files [OK]

Common Mistakes:

• Trying to detect device server-side in this file
• Using multiple not-found.tsx files
• Relying only on CSS for content changes