Why Middleware for API routes in NextJS? - Purpose & Use Cases

The Big Idea

Discover how one small piece of code can protect your entire API effortlessly!

The Scenario

Imagine you have many API routes in your Next.js app, and you want to check if a user is logged in before they can access any of them.

You try to add the same login check code inside every single API route handler.

The Problem

Manually repeating login checks in every API route is tiring and easy to forget.

If you miss one, unauthorized users might sneak in.

Also, updating the check means changing many files, which wastes time and causes bugs.

The Solution

Middleware lets you write the login check once and apply it automatically to all or some API routes.

This keeps your code clean, safe, and easy to update.

Before vs After

✗ Before

export default function handler(req, res) {
  if (!req.headers.authorization) {
    return res.status(401).json({ error: 'Unauthorized' });
  }
  // rest of handler
}

✓ After

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

import { NextResponse } from 'next/server';

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

What It Enables

You can protect many API routes with one simple middleware, making your app more secure and maintainable.

Real Life Example

A shopping app uses middleware to check if users are logged in before allowing them to add items to their cart or place orders.

Key Takeaways

Manual checks in every API route cause repeated code and errors.

Middleware centralizes checks for cleaner, safer code.

It saves time and reduces bugs when updating security rules.

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