Middleware for API routes in NextJS

Introduction

Middleware lets you run code before your API route handles a request. It helps you check or change requests easily.

Check if a user is logged in before giving access to data.

Log details about each API request for debugging.

Add security headers to API responses.

Limit how many times a user can call an API in a short time.

Modify request data before it reaches the main API handler.

Syntax

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  // Your code here
  return NextResponse.next();
}

export const config = {
  matcher: '/api/:path*',
};

The middleware function runs before your API route.

Use NextResponse.next() to continue to the API handler.

Examples

This logs every API call URL before continuing.

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  console.log('API called:', request.url);
  return NextResponse.next();
}

This checks for an authorization token and blocks requests without it.

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const token = request.headers.get('authorization');
  if (!token) {
    return new NextResponse('Unauthorized', { status: 401 });
  }
  return NextResponse.next();
}

Sample Program

This middleware blocks requests from user agents that include the word 'bot'. Others continue to the API route.

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const userAgent = request.headers.get('user-agent') || '';
  if (userAgent.toLowerCase().includes('bot')) {
    return new NextResponse('Bots are not allowed', { status: 403 });
  }
  return NextResponse.next();
}

export const config = {
  matcher: '/api/:path*',
};

Important Notes

Middleware runs on the edge, so keep it fast and simple.

Use matcher to limit middleware to API routes only.

You can modify requests or responses inside middleware before they reach your API.

Summary

Middleware runs code before your API route handles a request.

Use it to check, block, or modify requests easily.

Remember to use NextResponse.next() to continue to the API handler.

Practice

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

easy

A. To run code before the API route handles a request

B. To replace the API route handler completely

C. To style the API response

D. To store data permanently on the server

Solution

1. Step 1: Understand middleware role

   Middleware runs before the API route handler to process requests.

2. Step 2: Identify correct purpose

   It can check, block, or modify requests but does not replace handlers or style responses.

3. Final Answer:

   To run code before the API route handles a request -> Option A

4. Quick Check:

   Middleware runs before API handler [OK]

Hint: Middleware runs before API handler to control requests [OK]

Common Mistakes:

• Thinking middleware replaces the API handler
• Confusing middleware with styling or storage
• Assuming middleware runs after the API handler

2. Which of the following is the correct way to continue to the API route handler inside Next.js middleware?

easy

A. return NextResponse.stop()

B. return NextResponse.redirect()

C. return NextResponse.next()

D. return NextResponse.error()

Solution

1. Step 1: Recall Next.js middleware continuation method

   To continue processing the request, middleware must call NextResponse.next().

2. Step 2: Match correct method

   NextResponse.stop() halts, redirect() sends elsewhere, error() signals failure.

3. Final Answer:

   return NextResponse.next() -> Option C

4. Quick Check:

   Use NextResponse.next() to continue [OK]

Hint: Use NextResponse.next() to proceed to API handler [OK]

Common Mistakes:

• Using NextResponse.stop() which blocks the request
• Confusing redirect() with continuing
• Forgetting to return NextResponse.next()

3. Given this middleware code, what will happen when a request with header x-auth: secret is sent?

import { NextResponse } from 'next/server';

export function middleware(request) {
  if (request.headers.get('x-auth') !== 'secret') {
    return NextResponse.redirect(new URL('/unauthorized', request.url));
  }
  return NextResponse.next();
}

medium

A. The request is redirected to /unauthorized

B. The request continues to the API route handler

C. The middleware throws an error

D. The request is blocked with no response

Solution

1. Step 1: Check header condition

   The middleware checks if 'x-auth' header equals 'secret'. If yes, it continues.

2. Step 2: Analyze given header

   The request has 'x-auth: secret', so condition is false and middleware returns NextResponse.next().

3. Final Answer:

   The request continues to the API route handler -> Option B

4. Quick Check:

   Header matches 'secret' so continue [OK]

Hint: Check header value to decide redirect or continue [OK]

Common Mistakes:

• Assuming redirect happens even if header matches
• Thinking middleware throws error on mismatch
• Ignoring header case sensitivity

4. Identify the error in this Next.js middleware code:

import { NextResponse } from 'next/server';

export function middleware(request) {
  if (!request.headers.get('authorization')) {
    NextResponse.redirect('/login');
  }
  return NextResponse.next();
}

medium

A. Using 'authorization' header instead of 'auth'

B. Middleware function must be async

C. NextResponse.next() should be inside the if block

D. Missing return before NextResponse.redirect

Solution

1. Step 1: Check redirect usage

   NextResponse.redirect must be returned to stop further processing.

2. Step 2: Identify missing return

   The code calls NextResponse.redirect but does not return it, so middleware continues incorrectly.

3. Final Answer:

   Missing return before NextResponse.redirect -> Option D

4. Quick Check:

   Always return redirect response [OK]

Hint: Always return redirect to stop middleware flow [OK]

Common Mistakes:

• Not returning redirect response
• Thinking middleware must be async
• Misplacing NextResponse.next() inside if block

5. You want to create middleware that blocks requests to API routes if the query parameter token is missing or empty. Which code correctly implements this behavior?

hard

A. export function middleware(request) {
const url = new URL(request.url);
if (!url.searchParams.get('token')) {
return NextResponse.redirect(new URL('/error', request.url));
}
return NextResponse.next();
}

B. export function middleware(request) {
if (!request.query.token) {
return NextResponse.redirect('/error');
}
return NextResponse.next();
}

C. export function middleware(request) {
if (request.url.token === '') {
return NextResponse.next();
}
return NextResponse.redirect('/error');
}

D. export function middleware(request) {
const token = request.headers.get('token');
if (!token) {
return NextResponse.next();
}
return NextResponse.redirect('/error');
}

Solution

1. Step 1: Access query parameters correctly

   Use new URL(request.url) and url.searchParams.get('token') to read query params.

2. Step 2: Check token presence and redirect if missing

   If token is missing or empty, redirect to /error; otherwise continue with NextResponse.next().

3. Final Answer:

   Code that checks query param and redirects if missing -> Option A

4. Quick Check:

   Use URL and searchParams for query checks [OK]

Hint: Use URL and searchParams to check query tokens [OK]

Common Mistakes:

• Trying to access query params directly on request
• Checking headers instead of query parameters
• Reversing redirect and continue logic