Middleware.ts file convention in NextJS - Performance & Optimization

Performance: Middleware.ts file convention

MEDIUM IMPACT

Middleware.ts affects the initial request handling and routing speed, impacting how fast the server responds and the page starts loading.

Handling authentication and redirects in middleware

NextJS

import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  if (!request.cookies.get('token')) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  return NextResponse.next();
}

Removes heavy computation, allowing middleware to run quickly and pass control to next handler faster.

📈 Performance GainReduces server blocking to under 5 ms, improving LCP significantly

Handling authentication and redirects in middleware

NextJS

import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  // Heavy synchronous computation
  for (let i = 0; i < 1000000000; i++) {}
  if (!request.cookies.get('token')) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  return NextResponse.next();
}

Blocking synchronous code delays middleware completion, increasing server response time and delaying page load.

📉 Performance CostBlocks server response for 200+ ms depending on CPU, increasing LCP

Performance Comparison

Pattern	Server Blocking Time	Middleware Complexity	Impact on LCP	Verdict
Heavy synchronous logic in middleware	200+ ms	High	Delays LCP significantly	[X] Bad
Minimal logic with quick checks	Under 5 ms	Low	Improves LCP	[OK] Good

Rendering Pipeline

Middleware.ts runs before the page renders, intercepting requests to modify or redirect them. It affects the server response time, which impacts the browser's ability to start rendering the page.

→Server Request Handling

→Routing

→Initial Response

⚠️ BottleneckServer Request Handling when middleware is slow or does heavy work

Core Web Vital Affected

LCP

Middleware.ts affects the initial request handling and routing speed, impacting how fast the server responds and the page starts loading.

Optimization Tips

1Avoid heavy synchronous computations in middleware.ts.

2Keep middleware logic minimal and fast to reduce server blocking.

3Use middleware only for essential routing or authentication checks.

Performance Quiz - 3 Questions

What is the main performance risk of adding heavy synchronous code in middleware.ts?

A It increases CSS paint time

B It blocks the server response, delaying page load

C It causes layout shifts on the page

D It reduces JavaScript bundle size

DevTools: Performance

How to check: Record a server-side profile or use Next.js telemetry to measure middleware execution time during request handling.

What to look for: Look for long blocking times in middleware functions that delay server response and increase LCP.

Practice

1. What is the primary purpose of the middleware.ts file in a Next.js project?

easy

A. To run code before requests reach pages or API routes

B. To define React components for UI rendering

C. To store global CSS styles

D. To configure database connections

Solution

1. Step 1: Understand middleware role

   Middleware runs before the request reaches pages or APIs, allowing pre-processing.

2. Step 2: Compare with other file roles

   React components handle UI, CSS files style, and database configs are separate; middleware is for request handling.

3. Final Answer:

   To run code before requests reach pages or API routes -> Option A

4. Quick Check:

   Middleware = pre-request code [OK]

Hint: Middleware runs before pages or APIs handle requests [OK]

Common Mistakes:

• Confusing middleware with UI components
• Thinking middleware manages styles or database
• Assuming middleware runs after page rendering

2. Which of the following is the correct way to export a middleware function in middleware.ts?

easy

A. export default function middleware(req) { return NextResponse.next(); }

B. export function middleware(req) { NextResponse.next(); }

C. export const middleware = (req) => NextResponse.next();

D. module.exports = function middleware(req) { return NextResponse.next(); }

Solution

1. Step 1: Identify modern export syntax

   Next.js middleware uses named export with const arrow function for clarity and modern style.

2. Step 2: Check syntax validity

   export const middleware = (req) => NextResponse.next(); uses export const middleware = (req) => NextResponse.next(); which is valid and recommended.

3. Final Answer:

   export const middleware = (req) => NextResponse.next(); -> Option C

4. Quick Check:

   Use named const export for middleware [OK]

Hint: Use named const arrow function export for middleware [OK]

Common Mistakes:

• Using CommonJS module.exports instead of ES module export
• Using default export instead of named export
• Declaring middleware as a regular function without export

3. Given this middleware.ts snippet, what will happen when a request to /dashboard is made?

import { NextResponse } from 'next/server';

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

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

medium

A. The user is redirected to /login if no token cookie is present

B. The request proceeds to /dashboard regardless of cookies

C. The middleware throws a runtime error due to missing cookie method

D. The middleware blocks all requests to /dashboard

Solution

1. Step 1: Analyze matcher and cookie check

   The middleware runs only on /dashboard and checks if 'token' cookie exists.

2. Step 2: Determine behavior based on cookie presence

   If no token cookie, it redirects to /login; otherwise, it allows the request.

3. Final Answer:

   The user is redirected to /login if no token cookie is present -> Option A

4. Quick Check:

   Missing token cookie triggers redirect [OK]

Hint: Check cookie presence to decide redirect or continue [OK]

Common Mistakes:

• Assuming middleware runs on all routes
• Thinking request proceeds without token cookie
• Confusing redirect URL construction

4. Identify the error in this middleware.ts code snippet:

import { NextResponse } from 'next/server';

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

medium

A. Missing import of NextRequest from 'next/server'

B. Accessing cookies directly as an object instead of using req.cookies.get()

C. Using arrow function instead of regular function

D. Not exporting config.matcher for route matching

Solution

1. Step 1: Check cookie access method

   In Next.js middleware, cookies are accessed via req.cookies.get('name'), not as object properties.

2. Step 2: Identify error cause

   Using req.cookies.token will be undefined or cause error; correct is req.cookies.get('token').

3. Final Answer:

   Accessing cookies directly as an object instead of using req.cookies.get() -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Accessing cookies as properties instead of using get() method
• Forgetting to import NextResponse
• Assuming arrow functions are invalid in middleware

5. You want your middleware.ts to run only on API routes starting with /api/private and redirect users without a valid auth cookie to /api/auth/unauthorized. Which config and middleware code correctly implements this?

hard

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

export const middleware = (req) => {
if (!req.cookies.get('auth')) {
return NextResponse.rewrite(new URL('/api/auth/unauthorized', req.url));
}
return NextResponse.next();
};

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

export const middleware = (req) => {
if (req.cookies.auth === undefined) {
return NextResponse.redirect('/api/auth/unauthorized');
}
return NextResponse.next();
};

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

export default function middleware(req) {
if (!req.cookies.get('auth')) {
return NextResponse.redirect('/api/auth/unauthorized');
}
return NextResponse.next();
};

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

export const middleware = (req) => {
if (!req.cookies.get('auth')) {
return NextResponse.redirect(new URL('/api/auth/unauthorized', req.url));
}
return NextResponse.next();
};

Solution

1. Step 1: Verify matcher pattern for API routes

   The pattern /api/private/:path* correctly matches all routes under /api/private.

2. Step 2: Check cookie access and redirect method

   Using req.cookies.get('auth') is correct. Redirect uses NextResponse.redirect(new URL(...)) with full URL.

3. Step 3: Compare options for correctness

   export const config = { matcher: ['/api/private/:path*'] }; export const middleware = (req) => { if (!req.cookies.get('auth')) { return NextResponse.redirect(new URL('/api/auth/unauthorized', req.url)); } return NextResponse.next(); }; uses correct matcher, cookie access, and redirect syntax. Others have errors like wrong cookie access, missing URL object, or rewrite instead of redirect.

4. Final Answer:

   export const config = { matcher: ['/api/private/:path*'] }; export const middleware = (req) => { if (!req.cookies.get('auth')) { return NextResponse.redirect(new URL('/api/auth/unauthorized', req.url)); } return NextResponse.next(); }; -> Option D

5. Quick Check:

   Use matcher with :path*, get() for cookies, and redirect with URL [OK]

Hint: Use :path* matcher and req.cookies.get() with NextResponse.redirect(URL) [OK]

Common Mistakes:

• Using wildcard * instead of :path* in matcher
• Accessing cookies as properties instead of get()
• Using rewrite instead of redirect for unauthorized access
• Not wrapping redirect URL in new URL()