Middleware.ts file convention in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the NextResponse object from the correct module.

NextJS

import { [1] } from 'next/server';

Drag options to blanks, or click blank then click option'

ANextMiddleware

BNextRequest

CNextResponse

DNextHandler

Attempts:

3 left

💡 Hint

Common Mistakes

Importing NextRequest instead of NextResponse

Using incorrect module names

Confusing middleware handler names

The NextResponse object is imported from next/server to create responses in middleware.

2fill in blank

medium

Complete the code to define a middleware function that receives a request.

NextJS

export function middleware([1]) {
  return NextResponse.next();
}

Drag options to blanks, or click blank then click option'

Arequest

Bcontext

Cresponse

Devent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'response' as the parameter instead of 'request'

Using unrelated parameter names

The middleware function receives a request object representing the incoming request.

3fill in blank

hard

Fix the error in the middleware export syntax.

NextJS

export function [1](request) {
  return NextResponse.next();
}

Drag options to blanks, or click blank then click option'

Amiddleware

BMiddleware

ChandleMiddleware

DnextMiddleware

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase or different function names

Exporting the function as default

The middleware function must be named exactly middleware for Next.js to recognize it.

4fill in blank

hard

Fill both blanks to create a matcher that applies middleware only to API routes.

NextJS

export const config = {
  matcher: [1] 
};

// Matches paths starting with [2]

Drag options to blanks, or click blank then click option'

A["/api/:path*"]

B"/api/*"

C["/app/:path*"]

D"/app/*"

Attempts:

3 left

💡 Hint

Common Mistakes

Using string instead of array for matcher

Using wrong path prefixes

The matcher uses an array with the pattern "/api/:path*" to apply middleware to all API routes.

5fill in blank

hard

Fill all three blanks to create middleware that redirects users from '/' to '/home'.

NextJS

import { NextResponse } from 'next/server';

export function middleware([1]) {
  if (request.nextUrl.pathname === [2]) {
    return NextResponse.redirect(new URL([3], request.url));
  }
  return NextResponse.next();
}

Drag options to blanks, or click blank then click option'

Arequest

B"/"

C"/home"

Dresponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names

Incorrect path strings

Not using NextResponse.redirect properly

The middleware checks if the path is '/' and redirects to '/home' using NextResponse.redirect.

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()