Protected routes with middleware in NextJS - Performance & Optimization

Performance: Protected routes with middleware

MEDIUM IMPACT

This affects the initial page load speed and interaction responsiveness by adding server-side checks before rendering protected pages.

Protecting pages by checking user authentication

NextJS

import { NextResponse } from 'next/server';
export function middleware(request) {
  const token = request.cookies.get('token')?.value;
  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  return NextResponse.next();
}

export const config = { matcher: ['/protected/:path*'] };

Middleware runs before rendering, redirecting unauthenticated users early and avoiding client-side flicker.

📈 Performance GainReduces LCP by 200-300ms by preventing protected page render on unauthenticated access

Protecting pages by checking user authentication

NextJS

export default function Page() {
  useEffect(() => {
    if (!userIsLoggedIn()) {
      router.push('/login');
    }
  }, []);
  return <div>Protected Content</div>;
}

Client-side redirect causes the protected page to load first, increasing LCP and causing flicker.

📉 Performance CostBlocks rendering until redirect triggers, increasing LCP by 200-300ms on slow networks

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Client-side redirect after page load	Full DOM created then discarded	Multiple reflows due to redirect	High paint cost due to flicker	[X] Bad
Middleware redirect before page load	No DOM created for protected page	Zero reflows on redirect	Low paint cost, no flicker	[OK] Good

Rendering Pipeline

Middleware intercepts requests before rendering, allowing redirects or rewrites before HTML is generated.

→Server Request Handling

→HTML Generation

→Client Rendering

⚠️ BottleneckServer Request Handling due to synchronous auth checks

Core Web Vital Affected

LCP

This affects the initial page load speed and interaction responsiveness by adding server-side checks before rendering protected pages.

Optimization Tips

1Use middleware to redirect unauthenticated users before page rendering.

2Avoid client-side redirects for protected routes to reduce flicker and LCP.

3Keep middleware logic simple and cache tokens to optimize server response time.

Performance Quiz - 3 Questions

What is the main performance benefit of using middleware for protected routes in Next.js?

A Loads the entire page before checking authentication

B Increases client-side JavaScript bundle size

C Redirects users before page rendering, reducing LCP

D Triggers multiple reflows during redirect

DevTools: Performance

How to check: Record page load with and without middleware; look for time to first contentful paint and network redirects.

What to look for: Shorter LCP and absence of client-side redirect in waterfall indicate good middleware performance.

Practice

1. What is the main purpose of middleware in Next.js when protecting routes?

easy

A. To check user authentication before allowing access to certain pages

B. To style the pages dynamically based on user preferences

C. To preload images for faster page loading

D. To manage database connections automatically

Solution

1. Step 1: Understand middleware role

   Middleware runs before a page loads to control access or modify requests.

2. Step 2: Identify protection purpose

   In protected routes, middleware checks if a user is authenticated before allowing access.

3. Final Answer:

   To check user authentication before allowing access to certain pages -> Option A

4. Quick Check:

   Middleware protects routes by checking authentication [OK]

Hint: Middleware runs before page load to check user access [OK]

Common Mistakes:

• Thinking middleware styles pages
• Confusing middleware with database management
• Assuming middleware preloads images

2. Which of the following is the correct way to export middleware in Next.js to protect routes?

easy

A. export function middleware() { /* code */ }

B. function middleware() { /* code */ } export middleware

C. export middleware = () => { /* code */ }

D. export default function middleware(req) { /* code */ }

Solution

1. Step 1: Recall Next.js middleware export syntax

   Middleware must be exported as the default export function named middleware.

2. Step 2: Check options for correct syntax

   Only export default function middleware(req) { /* code */ } uses "export default function middleware(req)" which is valid syntax.

3. Final Answer:

   export default function middleware(req) { /* code */ } -> Option D

4. Quick Check:

   Middleware uses default export function [OK]

Hint: Middleware must be default exported as a function named middleware [OK]

Common Mistakes:

• Using named export instead of default
• Assigning middleware to a variable without export default
• Incorrect export statement syntax

3. Given this middleware code snippet, what happens when a user is not authenticated?

import { NextResponse } from 'next/server';

export default function middleware(req) {
  const token = req.cookies.get('token');
  if (!token) {
    return NextResponse.redirect(new URL('/login', req.url));
  }
  return NextResponse.next();
}

medium

A. The user stays on the current page without any change

B. The user is redirected to the /login page

C. The middleware throws an error and stops loading

D. The user is redirected to the homepage

Solution

1. Step 1: Analyze token check in middleware

   The middleware checks if the 'token' cookie exists; if not, it triggers a redirect.

2. Step 2: Understand redirect behavior

   If no token, middleware returns a redirect response to '/login' page.

3. Final Answer:

   The user is redirected to the /login page -> Option B

4. Quick Check:

   No token causes redirect to login [OK]

Hint: No token cookie means redirect to login page [OK]

Common Mistakes:

• Assuming user stays on page without token
• Thinking middleware throws error on missing token
• Confusing redirect target URL

4. Identify the error in this middleware code that aims to protect routes:

import { NextResponse } from 'next/server';

export default function middleware(req) {
  const token = req.cookies.token;
  if (!token) {
    return NextResponse.redirect('/login');
  }
  return NextResponse.next();
}

medium

A. Missing async keyword in middleware function

B. Redirect URL should be absolute, not relative

C. Accessing cookies incorrectly; should use req.cookies.get('token')

D. NextResponse.next() should be replaced with NextResponse.continue()

Solution

1. Step 1: Check cookie access method

   In Next.js middleware, cookies are accessed with req.cookies.get('token'), not req.cookies.token.

2. Step 2: Verify redirect usage

   Redirect can accept a relative path, so that is valid here.

3. Final Answer:

   Accessing cookies incorrectly; should use req.cookies.get('token') -> Option C

4. Quick Check:

   Use req.cookies.get('token') to read cookies [OK]

Hint: Use req.cookies.get('token') to read cookies in middleware [OK]

Common Mistakes:

• Using dot notation for cookies object
• Thinking redirect URL must be absolute
• Confusing NextResponse.next() with continue()

5. You want to protect only the routes starting with /dashboard using middleware. Which is the correct way to apply middleware only to these routes?

hard

A. export const config = { matcher: ['/dashboard/:path*'] };

B. export const config = { matcher: ['/dashboard*'] };

C. export const config = { matcher: ['/dashboard'] };

D. export const config = { matcher: ['/dashboard/**'] };

Solution

1. Step 1: Understand matcher pattern syntax

   The matcher uses path patterns where ':path*' matches all subpaths under /dashboard.

2. Step 2: Compare options for correct pattern

   export const config = { matcher: ['/dashboard/:path*'] }; uses '/dashboard/:path*' which correctly matches /dashboard and all nested routes.

3. Final Answer:

   export const config = { matcher: ['/dashboard/:path*'] }; -> Option A

4. Quick Check:

   Use '/dashboard/:path*' to match dashboard and subpaths [OK]

Hint: Use ':path*' to match all subpaths under a route [OK]

Common Mistakes:

• Using wildcard * without colon for subpaths
• Matching only exact /dashboard without subpaths
• Using invalid glob pattern like /**