Not-found page handling in NextJS - Performance & Optimization

Performance: Not-found page handling

MEDIUM IMPACT

This affects the page load speed and user experience when a user navigates to a non-existent route.

Handling a user visiting a non-existent page

NextJS

import { notFound } from 'next/navigation';

export default function Page() {
  // Next.js built-in to trigger 404
  notFound();
  return null;
}

Using Next.js built-in notFound() triggers server-side 404 response and faster error page display.

📈 Performance GainImproves LCP by serving 404 quickly; reduces unnecessary client JS; better SEO

Handling a user visiting a non-existent page

NextJS

export default function Page() {
  return <div>Page not found</div>;
}

// No special 404 handling, no status code set

This does not set the HTTP 404 status code, causing search engines and browsers to treat it as a normal page, and may delay showing the user the correct error.

📉 Performance CostBlocks rendering until client-side JS loads; no server-side 404 status means slower LCP and SEO issues

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Client-side 404 rendering without status	Moderate (full page DOM)	Multiple (full layout)	High (full paint)	[X] Bad
Server-side 404 with notFound()	Minimal (error page DOM)	Single reflow	Low (simple paint)	[OK] Good

Rendering Pipeline

When a not-found page is handled properly, the server sends a 404 status early, skipping unnecessary data fetching and rendering, so the browser can quickly paint the error page.

→Server Response

→Style Calculation

→Layout

→Paint

⚠️ BottleneckServer Response when notFound is not used, causing delayed error page delivery

Core Web Vital Affected

LCP

This affects the page load speed and user experience when a user navigates to a non-existent route.

Optimization Tips

1Always use Next.js notFound() to trigger server-side 404 status.

2Avoid rendering 404 messages only on the client side without status code.

3Keep the not-found page simple to reduce paint time and improve LCP.

Performance Quiz - 3 Questions

What is the main performance benefit of using Next.js's notFound() for not-found page handling?

A It triggers a server-side 404 status and faster error page rendering

B It loads extra JavaScript for better animations

C It delays the error page to fetch more data

D It disables caching for the page

DevTools: Network

How to check: Open DevTools, go to Network tab, reload a non-existent page, and check the status code of the response.

What to look for: A 404 status code confirms proper not-found handling; absence means bad pattern.

Practice

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

easy

A. To display a custom message when a user visits a page that does not exist.

B. To define the main homepage of the website.

C. To handle user authentication and login.

D. To store global CSS styles for the app.

Solution

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

   The not-found.tsx file is used to show a special page when a user tries to visit a URL that does not match any existing page.

2. Step 2: Compare with other options

   The other options describe unrelated features: defining the main homepage, handling user authentication and login, and storing global CSS styles, which are not handled by not-found.tsx.

3. Final Answer:

   To display a custom message when a user visits a page that does not exist. -> Option A

4. Quick Check:

   Not-found page = Custom message for missing pages [OK]

Hint: Not-found page shows when URL has no matching page [OK]

Common Mistakes:

• Confusing not-found.tsx with homepage or layout files
• Thinking it handles authentication
• Assuming it manages styles

2. Which of the following is the correct location for the not-found.tsx file in a Next.js 14 app using the App Router?

easy

A. Inside the pages directory

B. Inside the components directory

C. Inside the app directory

D. Inside the public directory

Solution

1. Step 1: Recall Next.js 14 App Router structure

   In Next.js 14 with the App Router, special files like not-found.tsx belong inside the app directory to define routes and error pages.

2. Step 2: Eliminate incorrect directories

   The pages directory is for the Pages Router, public is for static assets, and components is for reusable UI parts, not special route files.

3. Final Answer:

   Inside the app directory -> Option C

4. Quick Check:

   App Router special files go in app directory [OK]

Hint: App Router files like not-found.tsx go in app folder [OK]

Common Mistakes:

• Placing not-found.tsx in pages directory (old router)
• Putting it in public or components folders
• Confusing app and pages directories

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

export default function NotFound() {
  return (
    <main>
      <h1>Page Not Found</h1>
      <p>Sorry, we couldn't find that page.</p>
    </main>
  )
}

What will a user see if they visit a non-existent page?

medium

A. A blank page with no content

B. The message: 'Page Not Found' and 'Sorry, we couldn't find that page.'

C. An error message from the server

D. The homepage content

Solution

1. Step 1: Read the JSX returned by NotFound component

   The component returns a main section with a heading 'Page Not Found' and a paragraph apologizing for missing page.

2. Step 2: Understand Next.js behavior on missing pages

   When a user visits a page that does not exist, Next.js renders this not-found.tsx component, showing the messages inside.

3. Final Answer:

   The message: 'Page Not Found' and 'Sorry, we couldn't find that page.' -> Option B

4. Quick Check:

   NotFound component content = shown on missing pages [OK]

Hint: NotFound component content displays on missing pages [OK]

Common Mistakes:

• Expecting server error instead of custom message
• Thinking homepage shows instead
• Assuming blank page appears

4. You wrote a not-found.tsx file but Next.js still shows the default 404 page. What is the most likely mistake?

medium

A. The not-found.tsx file has a syntax error in the JSX.

B. You named the file NotFound.tsx with uppercase letters.

C. You forgot to restart the Next.js development server.

D. The not-found.tsx file is placed inside the components folder instead of app.

Solution

1. Step 1: Check file placement rules for not-found.tsx

   The not-found.tsx file must be inside the app directory to be recognized by Next.js as the custom 404 page.

2. Step 2: Analyze other options

   Syntax errors cause build failures, restarting server is good but not required for recognition, and filename is case sensitive but must be exactly not-found.tsx (lowercase). Placing it in components means Next.js ignores it.

3. Final Answer:

   The not-found.tsx file is placed inside the components folder instead of app. -> Option D

4. Quick Check:

   File location matters for Next.js to detect not-found page [OK]

Hint: Put not-found.tsx inside app folder, not components [OK]

Common Mistakes:

• Placing not-found.tsx outside app directory
• Ignoring case sensitivity of filename
• Assuming server restart fixes detection

5. You want to create a helpful not-found page in Next.js that includes a link back to the homepage. Which code snippet correctly implements this inside not-found.tsx?

hard

A. export default function NotFound() {
return (

Page Not Found
Sorry, we couldn't find that page.
Go back home

)
}

B. export default function NotFound() {
return (

Page Not Found
Sorry, we couldn't find that page.
Go back home

)
}

C. export default function NotFound() {
return (

Page Not Found
Sorry, we couldn't find that page.
Go back home

)
}

D. export default function NotFound() {
return (

Page Not Found
Sorry, we couldn't find that page.
Go back home

)
}

Solution

1. Step 1: Identify correct link usage in Next.js not-found page

   In Next.js, the not-found.tsx page can use a normal <a> tag with href to link back to the homepage. The Link component requires import and is used differently.

2. Step 2: Check each option's link syntax

   The snippet with <a href="/"> is valid JSX/HTML and requires no imports. Snippets with <Link href="/"> reference undefined Link (missing import { Link } from 'next/link';). Snippets with to="/" use invalid attribute ('to' is for React Router; use href).

3. Final Answer:

   Using <a href="/">Go back home</a> -> Option A

4. Quick Check:

   Use <a href="/"> for links in not-found.tsx [OK]

Hint: Use for homepage link in not-found.tsx [OK]

Common Mistakes:

• Using <Link> without import or proper usage
• Using 'to' attribute instead of 'href'
• Forgetting to provide a link back to home