Protected routes with middleware in NextJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of middleware in Next.js when protecting routes?

Middleware in Next.js runs before a request reaches a route. It can check if a user is authenticated and redirect them if not, helping protect private pages.

intermediate

How do you check if a user is authenticated inside Next.js middleware?

You typically check for a valid token or session cookie in the request headers or cookies. If missing or invalid, you redirect the user to a login page.

beginner

What is the role of the NextResponse object in middleware?

NextResponse allows you to modify the response, like redirecting users or rewriting URLs before the request reaches the page or API route.

intermediate

Why is middleware a better choice for protecting routes than client-side checks?

Middleware runs on the server before the page loads, so it prevents unauthorized users from even loading protected content, improving security and user experience.

beginner

Give an example of a simple middleware redirecting unauthenticated users to '/login'.

Inside middleware, check if the request has a valid auth token. If not, return NextResponse.redirect(new URL('/login', request.url)). Otherwise, allow the request to continue.

What does Next.js middleware run before?

A After the page is rendered

B Only after user logs in

C Only on client-side navigation

D Before the request reaches a route or API

Middleware runs on the server before the request reaches the route or API handler.

Which object do you use to redirect users inside Next.js middleware?

A NextResponse

B NextRequest

C Router

D Response

NextResponse lets you modify the response, including redirects.

What is a common way to identify an authenticated user in middleware?

A Check the user's IP address

B Check for a session cookie or token

C Check the browser's local storage

D Check the page URL

Authentication is usually verified by checking cookies or tokens sent with the request.

Why is server-side middleware protection more secure than client-side checks?

A Because it runs after the page loads

B Because it uses JavaScript on the client

C Because it prevents unauthorized content from loading at all

D Because it only runs on mobile devices

Middleware blocks unauthorized requests before the page loads, improving security.

Where do you place the middleware file in a Next.js project?

A In the root directory as middleware.js or middleware.ts

B Inside /public folder

C Inside /components folder

D In the /pages directory

Middleware is placed at the root as middleware.js or middleware.ts to run on all routes or specific paths.

Explain how you would protect a Next.js route using middleware.

Think about checking user identity before the page loads.

Describe the benefits of using middleware for route protection compared to client-side checks.

Consider timing and security differences.

Practice

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

easy

A. To check user authentication before allowing access to certain pages

B. To style the pages dynamically based on user preferences

C. To preload images for faster page loading

D. To manage database connections automatically

Solution

1. Step 1: Understand middleware role

   Middleware runs before a page loads to control access or modify requests.

2. Step 2: Identify protection purpose

   In protected routes, middleware checks if a user is authenticated before allowing access.

3. Final Answer:

   To check user authentication before allowing access to certain pages -> Option A

4. Quick Check:

   Middleware protects routes by checking authentication [OK]

Hint: Middleware runs before page load to check user access [OK]

Common Mistakes:

• Thinking middleware styles pages
• Confusing middleware with database management
• Assuming middleware preloads images

2. Which of the following is the correct way to export middleware in Next.js to protect routes?

easy

A. export function middleware() { /* code */ }

B. function middleware() { /* code */ } export middleware

C. export middleware = () => { /* code */ }

D. export default function middleware(req) { /* code */ }

Solution

1. Step 1: Recall Next.js middleware export syntax

   Middleware must be exported as the default export function named middleware.

2. Step 2: Check options for correct syntax

   Only export default function middleware(req) { /* code */ } uses "export default function middleware(req)" which is valid syntax.

3. Final Answer:

   export default function middleware(req) { /* code */ } -> Option D

4. Quick Check:

   Middleware uses default export function [OK]

Hint: Middleware must be default exported as a function named middleware [OK]

Common Mistakes:

• Using named export instead of default
• Assigning middleware to a variable without export default
• Incorrect export statement syntax

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

import { NextResponse } from 'next/server';

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

medium

A. The user stays on the current page without any change

B. The user is redirected to the /login page

C. The middleware throws an error and stops loading

D. The user is redirected to the homepage

Solution

1. Step 1: Analyze token check in middleware

   The middleware checks if the 'token' cookie exists; if not, it triggers a redirect.

2. Step 2: Understand redirect behavior

   If no token, middleware returns a redirect response to '/login' page.

3. Final Answer:

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

4. Quick Check:

   No token causes redirect to login [OK]

Hint: No token cookie means redirect to login page [OK]

Common Mistakes:

• Assuming user stays on page without token
• Thinking middleware throws error on missing token
• Confusing redirect target URL

4. Identify the error in this middleware code that aims to protect routes:

import { NextResponse } from 'next/server';

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

medium

A. Missing async keyword in middleware function

B. Redirect URL should be absolute, not relative

C. Accessing cookies incorrectly; should use req.cookies.get('token')

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 req.cookies.get('token'), not req.cookies.token.

2. Step 2: Verify redirect usage

   Redirect can accept a relative path, so that is valid here.

3. Final Answer:

   Accessing cookies incorrectly; should use req.cookies.get('token') -> Option C

4. Quick Check:

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

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

Common Mistakes:

• Using dot notation for cookies object
• Thinking redirect URL must be absolute
• Confusing NextResponse.next() with continue()

5. You want to protect only the routes starting with /dashboard using middleware. Which is the correct way to apply middleware only to these routes?

hard

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

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

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

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

Solution

1. Step 1: Understand matcher pattern syntax

   The matcher uses path patterns where ':path*' matches all subpaths under /dashboard.

2. Step 2: Compare options for correct pattern

   export const config = { matcher: ['/dashboard/:path*'] }; uses '/dashboard/:path*' which correctly matches /dashboard and all nested routes.

3. Final Answer:

   export const config = { matcher: ['/dashboard/:path*'] }; -> Option A

4. Quick Check:

   Use '/dashboard/:path*' to match dashboard and subpaths [OK]

Hint: Use ':path*' to match all subpaths under a route [OK]

Common Mistakes:

• Using wildcard * without colon for subpaths
• Matching only exact /dashboard without subpaths
• Using invalid glob pattern like /**