Authentication in middleware in NextJS - Mini Project: Build & Apply

Authentication in middleware

📖 Scenario: You are building a Next.js app that needs to protect certain pages so only logged-in users can access them.Middleware is a special place where you can check if a user is logged in before they see the page.

🎯 Goal: Create a Next.js middleware that checks if a user has a valid token cookie. If not, redirect them to the login page.

📋 What You'll Learn

Create a middleware function in middleware.ts

Check for a cookie named token

If token cookie is missing, redirect to /login

Allow access if token cookie exists

💡 Why This Matters

🌍 Real World

Middleware is used in real apps to protect pages and APIs by checking if users are logged in before showing content.

💼 Career

Understanding authentication middleware is important for building secure web apps and is a common task for frontend and full-stack developers.

1

Create the middleware file

Create a file named middleware.ts and export a function named middleware that takes a request parameter.

NextJS

export function middleware(request) {
  // Your code here
}

Hint

Middleware must be exported from middleware.ts and receive a request object.

2

Check for the token cookie

Inside the middleware function, create a variable named token that gets the token cookie from request.cookies.

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const token = request.cookies.get('token');
  // Your code here
}

Hint

Use request.cookies.get('token') to read the cookie named token.

3

Redirect if token is missing

Add an if statement that checks if token is undefined. If so, return NextResponse.redirect(new URL('/login', request.url)).

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const token = request.cookies.get('token');
  if (!token) {
    // Your code here
  }
}

Hint

If the token cookie is missing, redirect the user to the login page.

4

Allow access if token exists

At the end of the middleware function, return NextResponse.next() to allow the request to continue if the token exists.

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));
  }
  // Allow access
}

Hint

Use NextResponse.next() to continue the request when the user is authenticated.

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