Authentication in middleware in NextJS

Introduction

Authentication in middleware helps check who a user is before they see a page. It keeps parts of your app safe by stopping strangers from getting in.

When you want to protect private pages like user profiles or settings.

When you want to check if a user is logged in before showing content.

When you want to redirect users to login if they are not signed in.

When you want to run checks on every request to your app.

When you want to add security without repeating code in every page.

Syntax

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  // Check user authentication here
  const token = request.cookies.get('token');

  if (!token) {
    // Redirect to login if no token
    return NextResponse.redirect(new URL('/login', request.url));
  }

  // Allow request if authenticated
  return NextResponse.next();
}

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

The middleware function runs before the page loads.

Use NextResponse.redirect() to send users to login if not authenticated.

Examples

Basic check for a token cookie. Redirects to login if missing.

NextJS

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

Protects multiple routes by specifying them in matcher.

NextJS

export function middleware(request) {
  const token = request.cookies.get('token');
  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  // You can add more checks here
  return NextResponse.next();
}

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

Sample Program

This middleware checks if a user has a 'token' cookie when they try to access any page under '/profile'. If no token is found, it sends them to the login page. Otherwise, it lets them continue.

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const token = request.cookies.get('token');

  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url));
  }

  return NextResponse.next();
}

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

Important Notes

Middleware runs on the server before the page loads.

Cookies are a common way to store authentication tokens.

Use the matcher config to limit middleware to specific routes for better performance.

Summary

Middleware checks user identity before showing pages.

Redirect users to login if they are not authenticated.

Use matcher to protect only certain routes.

Practice

1. What is the main purpose of using middleware for authentication in Next.js?

easy

A. To fetch data from an external API before rendering

B. To check if a user is logged in before allowing access to certain pages

C. To style the pages dynamically based on user preferences

D. To optimize images for faster loading

Solution

1. Step 1: Understand middleware role

   Middleware runs before page rendering to control access.

2. Step 2: Identify authentication use

   Middleware checks if user is logged in to allow or block access.

3. Final Answer:

   To check if a user is logged in before allowing access to certain pages -> Option B

4. Quick Check:

   Middleware controls access = C [OK]

Hint: Middleware runs before pages to check login [OK]

Common Mistakes:

• Thinking middleware styles pages
• Confusing middleware with data fetching
• Assuming middleware optimizes images

2. Which of the following is the correct way to import middleware in Next.js 14+ for authentication?

easy

A. import { NextResponse } from 'next/server';

B. import { middleware } from 'next/auth';

C. import middleware from 'next/middleware';

D. import { useMiddleware } from 'next/hooks';

Solution

1. Step 1: Check Next.js middleware import

   Next.js middleware uses 'next/server' for NextResponse and request handling.

2. Step 2: Identify correct import

   Only 'import { NextResponse } from "next/server";' is valid for middleware response.

3. Final Answer:

   import { NextResponse } from 'next/server'; -> Option A

4. Quick Check:

   Middleware uses NextResponse from next/server [OK]

Hint: Middleware uses NextResponse from 'next/server' [OK]

Common Mistakes:

• Importing middleware from 'next/auth' which doesn't exist
• Using default import from 'next/middleware' which is invalid
• Trying to import hooks for middleware

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

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

medium

A. The middleware throws an error

B. The user stays on the current page without changes

C. The user is redirected to the /login page

D. The user is redirected to the homepage

Solution

1. Step 1: Check token presence

   The code checks if 'token' cookie exists; if not, it redirects.

2. Step 2: Understand redirect behavior

   Without token, middleware returns redirect to '/login' URL.

3. Final Answer:

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

4. Quick Check:

   No token means redirect to /login [OK]

Hint: No token cookie triggers redirect to /login [OK]

Common Mistakes:

• Assuming user stays on page without token
• Thinking middleware throws error on missing token
• Confusing redirect to homepage instead of /login

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

import { NextResponse } from 'next/server';
export function middleware(request) {
  const token = request.cookies.token;
  if (!token) {
    return NextResponse.redirect('/login');
  }
  return NextResponse.next();
}

medium

A. Accessing cookies should use request.cookies.get('token') instead of request.cookies.token

B. NextResponse.redirect requires a full URL, not just '/login'

C. Middleware function must be async

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 request.cookies.get('token'), not as a property.

2. Step 2: Verify redirect argument

   NextResponse.redirect accepts a URL object or string, but string '/login' is allowed; full URL preferred but not mandatory.

3. Final Answer:

   Accessing cookies should use request.cookies.get('token') instead of request.cookies.token -> Option A

4. Quick Check:

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

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

Common Mistakes:

• Accessing cookies as properties instead of using get()
• Thinking redirect needs full URL always
• Assuming middleware must be async
• Confusing NextResponse.next() with continue()

5. You want to protect only the /dashboard and /profile pages using middleware authentication. Which matcher configuration correctly applies middleware only to these paths?

export const config = {
  matcher: ???
};

hard

A. ['/dashboard', '/profile']

B. '/dashboard|/profile'

C. '/dashboard/*,/profile/*'

D. ['/dashboard*', '/profile*']

Solution

1. Step 1: Understand matcher syntax

   Matcher accepts array of path patterns; '*' matches subpaths.

2. Step 2: Choose correct pattern for pages

   Using ['/dashboard*', '/profile*'] matches both exact and nested routes under these paths.

3. Final Answer:

   ['/dashboard*', '/profile*'] -> Option D

4. Quick Check:

   Use array with wildcard for matcher [OK]

Hint: Use array with '*' wildcard for matcher paths [OK]

Common Mistakes:

• Using string with pipe '|' instead of array
• Omitting '*' wildcard to match subpaths
• Using comma-separated string instead of array