Protecting routes with auth middleware in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Express module.

Express

const express = require([1]);

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

A"express"

B"http"

C"fs"

D"path"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong module name like "http" or "fs".

Forgetting the quotes around the module name.

The Express module is imported by requiring the string "express".

2fill in blank

medium

Complete the code to create an Express app instance.

Express

const app = [1]();

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

Arouter

Brequire

Cexpress

Dmiddleware

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call require() instead of express().

Using undefined variables like router().

Calling express() creates a new Express application instance.

3fill in blank

hard

Fix the error in the middleware function to correctly call next.

Express

function authMiddleware(req, res, next) {
  if (req.user) {
    [1]();
  } else {
    res.status(401).send('Unauthorized');
  }
}

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

Ares.next

Breq.next

Cnext

Dnext()

Attempts:

3 left

💡 Hint

Common Mistakes

Writing next without parentheses.

Trying to call res.next() which does not exist.

The middleware must call next() as a function to pass control to the next middleware or route handler.

4fill in blank

hard

Fill both blanks to protect the route with the auth middleware.

Express

app.[1]('/dashboard', [2], (req, res) => {
  res.send('Welcome to your dashboard');
});

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

Aget

Bpost

CauthMiddleware

Dexpress.json()

Attempts:

3 left

💡 Hint

Common Mistakes

Using POST instead of GET for a page route.

Using unrelated middleware like express.json() instead of auth middleware.

The route uses app.get to handle GET requests, and the authMiddleware is added before the route handler to protect it.

5fill in blank

hard

Fill all three blanks to create and use auth middleware that checks for a token header.

Express

function [1](req, res, next) {
  const token = req.headers['[2]'];
  if (token === 'secret') {
    next();
  } else {
    res.status(403).send('Forbidden');
  }
}

app.[3]('/protected', [1], (req, res) => {
  res.send('Protected content');
});

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

AauthMiddleware

Bauthorization

Cget

Dx-access-token

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong header names like 'authorization' when the code expects 'x-access-token'.

Using POST instead of GET for a simple protected page.

The middleware is named authMiddleware. It checks the 'x-access-token' header for the token. The route uses app.get to protect the '/protected' path.

Practice

1. What is the main purpose of auth middleware in an Express app?

easy

A. To check if a user is allowed to access a route

B. To format the response data before sending

C. To log every request made to the server

D. To serve static files like images and CSS

Solution

1. Step 1: Understand middleware role

   Middleware runs before route handlers to process requests.

2. Step 2: Identify auth middleware function

   Auth middleware specifically checks user permissions to allow or deny access.

3. Final Answer:

   To check if a user is allowed to access a route -> Option A

4. Quick Check:

   Auth middleware = Access control [OK]

Hint: Auth middleware controls access to routes [OK]

Common Mistakes:

• Confusing auth middleware with logging middleware
• Thinking middleware serves static files
• Assuming middleware formats response data

2. Which of the following is the correct way to use auth middleware for a route in Express?

easy

A. app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); });

B. app.get(authMiddleware, '/profile', (req, res) => { res.send('Profile'); });

C. app.get('/profile', (req, res) => { authMiddleware(); res.send('Profile'); });

D. app.get('/profile', (req, res) => { res.send('Profile'); }, authMiddleware);

Solution

1. Step 1: Recall Express route syntax

   Middleware functions come before the final route handler in the argument list.

2. Step 2: Check each option's order

   Only app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); }); places authMiddleware correctly before the handler function.

3. Final Answer:

   app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); }); -> Option A

4. Quick Check:

   Middleware before handler = app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); }); [OK]

Hint: Middleware goes before the route handler function [OK]

Common Mistakes:

• Placing middleware after the handler
• Passing middleware as the first argument instead of path
• Calling middleware inside the handler instead of passing it

3. Given this auth middleware, what will happen when a request without a valid token hits the protected route?

function authMiddleware(req, res, next) {
  if (req.headers.authorization === 'valid-token') {
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
}

app.get('/dashboard', authMiddleware, (req, res) => {
  res.send('Welcome to dashboard');
});

medium

A. The user sees 'Welcome to dashboard' regardless of token

B. The server crashes due to missing next() call

C. The user gets a 401 Unauthorized response if token is missing or invalid

D. The user gets a 404 Not Found error

Solution

1. Step 1: Analyze authMiddleware logic

   If the authorization header equals 'valid-token', next() is called to continue.

2. Step 2: Check behavior when token is missing or invalid

   Else block sends 401 Unauthorized response and does not call next(), blocking access.

3. Final Answer:

   The user gets a 401 Unauthorized response if token is missing or invalid -> Option C

4. Quick Check:

   Invalid token = 401 Unauthorized [OK]

Hint: Middleware sends 401 if token invalid, else calls next() [OK]

Common Mistakes:

• Assuming next() is always called
• Thinking user always sees dashboard
• Confusing 401 with 404 errors

4. Identify the error in this auth middleware code:

function authMiddleware(req, res, next) {
  if (!req.user) {
    res.status(403).send('Forbidden');
  }
  next();
}

medium

A. Missing call to next() inside the if block

B. next() is called even after sending a response, causing an error

C. Status code 403 is incorrect for unauthorized access

D. req.user should be checked with req.auth instead

Solution

1. Step 1: Understand middleware flow

   If !req.user is true, response is sent with status 403.

2. Step 2: Check what happens after sending response

   next() is called unconditionally after the if block, so it runs even after response sent, causing errors.

3. Final Answer:

   next() is called even after sending a response, causing an error -> Option B

4. Quick Check:

   Call next() only if no response sent [OK]

Hint: Do not call next() after sending a response [OK]

Common Mistakes:

• Calling next() after res.send()
• Not stopping middleware after response
• Using wrong status codes for auth errors

5. You want to protect multiple routes with the same auth middleware and also log the user ID if authenticated. Which is the best way to do this?

function authMiddleware(req, res, next) {
  if (!req.headers.authorization) {
    return res.status(401).send('Unauthorized');
  }
  req.userId = req.headers.authorization;
  next();
}

// How to apply this middleware and log userId for routes '/profile' and '/settings'?

hard

A. Apply authMiddleware after route handlers to log userId

B. Add authMiddleware only to '/profile' route and log userId in '/settings' without middleware

C. Call authMiddleware inside each route handler manually before logging userId

D. Use app.use(authMiddleware) before defining both routes, then log req.userId inside each route handler

Solution

1. Step 1: Understand middleware application

   app.use(authMiddleware) applies middleware to all routes defined after it, protecting multiple routes easily.

2. Step 2: Logging userId in route handlers

   Since authMiddleware sets req.userId, route handlers can access and log it safely after middleware runs.

3. Final Answer:

   Use app.use(authMiddleware) before defining both routes, then log req.userId inside each route handler -> Option D

4. Quick Check:

   Use app.use for shared middleware [OK]

Hint: Use app.use(authMiddleware) to protect many routes [OK]

Common Mistakes:

• Applying middleware only to some routes inconsistently
• Calling middleware inside handlers manually
• Applying middleware after route handlers