JWT token verification middleware in Express - Mini Project: Build & Apply

JWT Token Verification Middleware in Express

📖 Scenario: You are building a simple Express server that needs to protect certain routes by verifying JWT tokens sent by clients.This helps ensure only users with valid tokens can access protected data.

🎯 Goal: Create a JWT verification middleware function in Express that checks the token from the request headers and allows or denies access accordingly.

📋 What You'll Learn

Create a variable called jwt that requires the jsonwebtoken package

Create a middleware function called verifyToken that reads the token from req.headers['authorization']

Check if the token exists and starts with 'Bearer '

Verify the token using jwt.verify with the secret key 'mysecretkey'

If verification passes, call next() to continue; otherwise, respond with status 401 and message 'Unauthorized'

Export the verifyToken middleware function

💡 Why This Matters

🌍 Real World

JWT token verification middleware is used in real web servers to protect routes and ensure only authenticated users can access certain resources.

💼 Career

Understanding how to implement middleware for JWT verification is a common requirement for backend developers working with Node.js and Express in secure web applications.

1

Setup JWT package import

Create a variable called jwt that requires the jsonwebtoken package.

Express

# Your code here

Hint

Use require('jsonwebtoken') to import the JWT library.

2

Create the verifyToken middleware function

Create a middleware function called verifyToken that reads the token from req.headers['authorization'] and stores it in a variable called authHeader.

Express

const jwt = require('jsonwebtoken');

// Your code here

Hint

Middleware functions take req, res, and next as parameters.

Read the token from req.headers['authorization'].

3

Check token presence and verify it

Inside verifyToken, check if authHeader exists and starts with 'Bearer '. Extract the token part after 'Bearer ' into a variable called token. Then verify the token using jwt.verify(token, 'mysecretkey', callback). If verification fails, respond with status 401 and message 'Unauthorized'. If it passes, call next().

Express

const jwt = require('jsonwebtoken');

function verifyToken(req, res, next) {
    const authHeader = req.headers['authorization'];
    // Your code here
}

Hint

Check if authHeader exists and starts with 'Bearer '.

Use slice(7) to get the token part.

Use jwt.verify with a callback to handle success or failure.

4

Export the verifyToken middleware

Export the verifyToken middleware function using module.exports = verifyToken;.

Express

const jwt = require('jsonwebtoken');

function verifyToken(req, res, next) {
    const authHeader = req.headers['authorization'];
    if (authHeader && authHeader.startsWith('Bearer ')) {
        const token = authHeader.slice(7);
        jwt.verify(token, 'mysecretkey', (err, user) => {
            if (err) {
                return res.status(401).json({ message: 'Unauthorized' });
            }
            req.user = user;
            next();
        });
    } else {
        return res.status(401).json({ message: 'Unauthorized' });
    }
}

// Your code here

Hint

Use module.exports to export the middleware function.

Practice

1. What is the main purpose of JWT token verification middleware in an Express app?

easy

A. To check if the incoming request has a valid JWT token before allowing access

B. To store user sessions on the server

C. To encrypt the user's password before saving

D. To serve static files like images and CSS

Solution

1. Step 1: Understand JWT middleware role

   JWT middleware checks the token sent by the client to confirm identity.

2. Step 2: Compare options with JWT purpose

   Only "To check if the incoming request has a valid JWT token before allowing access" describes verifying a token before access, which is the middleware's job.

3. Final Answer:

   To check if the incoming request has a valid JWT token before allowing access -> Option A

4. Quick Check:

   JWT middleware verifies token [OK]

Hint: JWT middleware always verifies token validity before access [OK]

Common Mistakes:

• Confusing JWT with session storage
• Thinking JWT middleware encrypts passwords
• Assuming middleware serves static files

2. Which of the following is the correct way to extract the JWT token from the Authorization header in Express middleware?

easy

A. const token = req.headers.authorization.split(' ')[1];

B. const token = req.body.token;

C. const token = req.query.token;

D. const token = req.cookies.token;

Solution

1. Step 1: Identify standard JWT token location

   JWT tokens are usually sent in the Authorization header as 'Bearer token'.

2. Step 2: Extract token correctly

   Splitting the header string by space and taking the second part gets the token.

3. Final Answer:

   const token = req.headers.authorization.split(' ')[1]; -> Option A

4. Quick Check:

   Authorization header split [OK]

Hint: JWT token is after 'Bearer ' in Authorization header [OK]

Common Mistakes:

• Trying to get token from body or query instead of header
• Not splitting the header string
• Assuming token is in cookies by default

3. Given this Express JWT middleware snippet, what happens if the token is invalid?

const jwt = require('jsonwebtoken');
function verifyToken(req, res, next) {
  const token = req.headers.authorization?.split(' ')[1];
  if (!token) return res.status(401).send('Access denied');
  try {
    const verified = jwt.verify(token, 'secretkey');
    req.user = verified;
    next();
  } catch (err) {
    res.status(400).send('Invalid token');
  }
}

medium

A. The middleware calls next() and allows access

B. The middleware crashes with an unhandled exception

C. The middleware sends a 401 status with 'Access denied' message

D. The middleware sends a 400 status with 'Invalid token' message

Solution

1. Step 1: Check token verification flow

   If token is invalid, jwt.verify throws an error caught by catch block.

2. Step 2: Observe catch block response

   Catch block sends status 400 with message 'Invalid token'.

3. Final Answer:

   The middleware sends a 400 status with 'Invalid token' message -> Option D

4. Quick Check:

   Invalid token triggers 400 response [OK]

Hint: Invalid token triggers catch block sending 400 error [OK]

Common Mistakes:

• Confusing 401 and 400 status codes
• Assuming next() is called on invalid token
• Thinking middleware crashes on invalid token

4. Identify the error in this JWT verification middleware code:

const jwt = require('jsonwebtoken');
function verifyToken(req, res, next) {
  const token = req.headers.authorization.split(' ')[1];
  if (!token) res.status(401).send('Access denied');
  try {
    const verified = jwt.verify(token, 'secretkey');
    req.user = verified;
    next();
  } catch (err) {
    res.status(400).send('Invalid token');
  }
}

medium

A. jwt.verify is called with wrong secret key

B. Missing return after sending 401 response causes jwt.verify to run anyway

C. Token is extracted incorrectly from headers

D. next() is called inside catch block instead of try block

Solution

1. Step 1: Check handling when token is missing

   If token is missing, res.status(401).send() is called but no return statement stops execution.

2. Step 2: Understand consequence of missing return

   Without return, code continues and jwt.verify runs with undefined token, causing errors or unexpected behavior.

3. Final Answer:

   Missing return after sending 401 response causes jwt.verify to run anyway -> Option B

4. Quick Check:

   Return needed after 401 response [OK]

Hint: Always return after sending response to stop middleware [OK]

Common Mistakes:

• Forgetting to return after res.send()
• Assuming jwt.verify secret is wrong here
• Misreading token extraction line

5. You want to protect multiple routes with JWT verification but also allow public access to some routes. Which is the best way to apply JWT middleware in Express?

hard

A. Apply JWT middleware after route handlers to catch errors

B. Apply JWT middleware globally to all routes and skip it conditionally inside middleware

C. Apply JWT middleware only to protected routes using router.use or route-specific middleware

D. Apply JWT middleware only once in app.listen callback

Solution

1. Step 1: Understand middleware scope

   Applying middleware globally affects all routes, including public ones, which is not ideal.

2. Step 2: Use route-specific middleware for protection

   Applying JWT middleware only on protected routes keeps public routes accessible without token.

3. Final Answer:

   Apply JWT middleware only to protected routes using router.use or route-specific middleware -> Option C

4. Quick Check:

   Protect routes selectively with middleware [OK]

Hint: Use middleware only on routes needing protection [OK]

Common Mistakes:

• Applying middleware globally and skipping inside code
• Applying middleware after route handlers
• Trying to apply middleware in app.listen