Middleware.ts file convention in NextJS - Mini Project: Build & Apply

Middleware.ts File Convention in Next.js

📖 Scenario: You are building a Next.js app that needs to run some code before every page loads. This is useful for things like checking if a user is logged in or redirecting them to a special page.

🎯 Goal: Create a middleware.ts file in a Next.js app that runs on every request and redirects users from the homepage to a welcome page.

📋 What You'll Learn

Create a middleware.ts file with the correct export

Use the Next.js NextResponse to handle redirects

Check the request URL pathname to decide when to redirect

Export a config object to specify which paths the middleware runs on

💡 Why This Matters

🌍 Real World

Middleware in Next.js apps helps run code before pages load, like redirects or authentication checks.

💼 Career

Understanding middleware is important for building secure and user-friendly web apps with Next.js.

1

Create the middleware function

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

NextJS

// Your code here

Hint

Start by importing NextResponse from next/server and create the middleware function.

2

Check the request URL pathname

Inside the middleware function, create a variable called pathname that gets the pathname from request.nextUrl.

NextJS

import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  const pathname = request.nextUrl.pathname;
  // Your code here
}

Hint

Use request.nextUrl.pathname to get the current path the user is visiting.

3

Redirect from homepage to /welcome

Inside the middleware function, add an if statement that checks if pathname is exactly "/". If yes, return NextResponse.redirect(new URL('/welcome', request.url)).

NextJS

import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  const pathname = request.nextUrl.pathname;
  if (pathname === '/') {
    // Your code here
  }
}

Hint

Use NextResponse.redirect with a new URL to send users from the homepage to /welcome.

4

Add middleware config for matching paths

After the middleware function, export a constant called config with a matcher array that includes "/" and "/welcome".

NextJS

import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  const pathname = request.nextUrl.pathname;
  if (pathname === '/') {
    return NextResponse.redirect(new URL('/welcome', request.url));
  }
}

export const config = {
  matcher: [
    // Your code here
  ]
};

Hint

The config export tells Next.js which paths to run the middleware on. Use matcher with an array of paths.

Practice

1. What is the primary purpose of the middleware.ts file in a Next.js project?

easy

A. To run code before requests reach pages or API routes

B. To define React components for UI rendering

C. To store global CSS styles

D. To configure database connections

Solution

1. Step 1: Understand middleware role

   Middleware runs before the request reaches pages or APIs, allowing pre-processing.

2. Step 2: Compare with other file roles

   React components handle UI, CSS files style, and database configs are separate; middleware is for request handling.

3. Final Answer:

   To run code before requests reach pages or API routes -> Option A

4. Quick Check:

   Middleware = pre-request code [OK]

Hint: Middleware runs before pages or APIs handle requests [OK]

Common Mistakes:

• Confusing middleware with UI components
• Thinking middleware manages styles or database
• Assuming middleware runs after page rendering

2. Which of the following is the correct way to export a middleware function in middleware.ts?

easy

A. export default function middleware(req) { return NextResponse.next(); }

B. export function middleware(req) { NextResponse.next(); }

C. export const middleware = (req) => NextResponse.next();

D. module.exports = function middleware(req) { return NextResponse.next(); }

Solution

1. Step 1: Identify modern export syntax

   Next.js middleware uses named export with const arrow function for clarity and modern style.

2. Step 2: Check syntax validity

   export const middleware = (req) => NextResponse.next(); uses export const middleware = (req) => NextResponse.next(); which is valid and recommended.

3. Final Answer:

   export const middleware = (req) => NextResponse.next(); -> Option C

4. Quick Check:

   Use named const export for middleware [OK]

Hint: Use named const arrow function export for middleware [OK]

Common Mistakes:

• Using CommonJS module.exports instead of ES module export
• Using default export instead of named export
• Declaring middleware as a regular function without export

3. Given this middleware.ts snippet, what will happen when a request to /dashboard is made?

import { NextResponse } from 'next/server';

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

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

medium

A. The user is redirected to /login if no token cookie is present

B. The request proceeds to /dashboard regardless of cookies

C. The middleware throws a runtime error due to missing cookie method

D. The middleware blocks all requests to /dashboard

Solution

1. Step 1: Analyze matcher and cookie check

   The middleware runs only on /dashboard and checks if 'token' cookie exists.

2. Step 2: Determine behavior based on cookie presence

   If no token cookie, it redirects to /login; otherwise, it allows the request.

3. Final Answer:

   The user is redirected to /login if no token cookie is present -> Option A

4. Quick Check:

   Missing token cookie triggers redirect [OK]

Hint: Check cookie presence to decide redirect or continue [OK]

Common Mistakes:

• Assuming middleware runs on all routes
• Thinking request proceeds without token cookie
• Confusing redirect URL construction

4. Identify the error in this middleware.ts code snippet:

import { NextResponse } from 'next/server';

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

medium

A. Missing import of NextRequest from 'next/server'

B. Accessing cookies directly as an object instead of using req.cookies.get()

C. Using arrow function instead of regular function

D. Not exporting config.matcher for route matching

Solution

1. Step 1: Check cookie access method

   In Next.js middleware, cookies are accessed via req.cookies.get('name'), not as object properties.

2. Step 2: Identify error cause

   Using req.cookies.token will be undefined or cause error; correct is req.cookies.get('token').

3. Final Answer:

   Accessing cookies directly as an object instead of using req.cookies.get() -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Accessing cookies as properties instead of using get() method
• Forgetting to import NextResponse
• Assuming arrow functions are invalid in middleware

5. You want your middleware.ts to run only on API routes starting with /api/private and redirect users without a valid auth cookie to /api/auth/unauthorized. Which config and middleware code correctly implements this?

hard

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

export const middleware = (req) => {
if (!req.cookies.get('auth')) {
return NextResponse.rewrite(new URL('/api/auth/unauthorized', req.url));
}
return NextResponse.next();
};

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

export const middleware = (req) => {
if (req.cookies.auth === undefined) {
return NextResponse.redirect('/api/auth/unauthorized');
}
return NextResponse.next();
};

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

export default function middleware(req) {
if (!req.cookies.get('auth')) {
return NextResponse.redirect('/api/auth/unauthorized');
}
return NextResponse.next();
};

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

export const middleware = (req) => {
if (!req.cookies.get('auth')) {
return NextResponse.redirect(new URL('/api/auth/unauthorized', req.url));
}
return NextResponse.next();
};

Solution

1. Step 1: Verify matcher pattern for API routes

   The pattern /api/private/:path* correctly matches all routes under /api/private.

2. Step 2: Check cookie access and redirect method

   Using req.cookies.get('auth') is correct. Redirect uses NextResponse.redirect(new URL(...)) with full URL.

3. Step 3: Compare options for correctness

   export const config = { matcher: ['/api/private/:path*'] }; export const middleware = (req) => { if (!req.cookies.get('auth')) { return NextResponse.redirect(new URL('/api/auth/unauthorized', req.url)); } return NextResponse.next(); }; uses correct matcher, cookie access, and redirect syntax. Others have errors like wrong cookie access, missing URL object, or rewrite instead of redirect.

4. Final Answer:

   export const config = { matcher: ['/api/private/:path*'] }; export const middleware = (req) => { if (!req.cookies.get('auth')) { return NextResponse.redirect(new URL('/api/auth/unauthorized', req.url)); } return NextResponse.next(); }; -> Option D

5. Quick Check:

   Use matcher with :path*, get() for cookies, and redirect with URL [OK]

Hint: Use :path* matcher and req.cookies.get() with NextResponse.redirect(URL) [OK]

Common Mistakes:

• Using wildcard * instead of :path* in matcher
• Accessing cookies as properties instead of get()
• Using rewrite instead of redirect for unauthorized access
• Not wrapping redirect URL in new URL()