JWT token verification middleware in Express

Introduction

JWT token verification middleware checks if a user is allowed to access certain parts of a web app by confirming their token is valid.

When you want to protect routes so only logged-in users can access them.

When you need to check user identity before allowing actions like posting or deleting data.

When you want to make sure the token sent by the user is not expired or tampered with.

When building APIs that require secure access control.

When you want to add a simple way to verify users without storing session data on the server.

Syntax

Express

const jwt = require('jsonwebtoken');

function verifyToken(req, res, next) {
  const authHeader = req.headers['authorization'];
  if (!authHeader) {
    return res.status(401).send('Access denied. No token provided.');
  }
  const token = authHeader.split(' ')[1];
  if (!token) {
    return res.status(401).send('Access denied. Token missing.');
  }
  try {
    const decoded = jwt.verify(token, 'your-secret-key');
    req.user = decoded;
    next();
  } catch (err) {
    res.status(400).send('Invalid token.');
  }
}

The middleware reads the token from the Authorization header.

Use jwt.verify() to check if the token is valid and not expired.

Examples

Get the token from the request headers.

Express

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

Verify the token using your secret key and get the decoded user info.

Express

const decoded = jwt.verify(token, 'your-secret-key');

Attach the decoded data to the request and call next() to continue.

Express

req.user = decoded;
next();

Sample Program

This Express app has three routes:

• /public: anyone can access.
• /login: simulates login and returns a JWT token.
• /protected: only accessible if a valid JWT token is sent in the Authorization header.

The verifyToken middleware checks the token before allowing access to the protected route.

Express

const express = require('express');
const jwt = require('jsonwebtoken');

const app = express();
const PORT = 3000;
const SECRET_KEY = 'mysecretkey';

// Middleware to verify JWT token
function verifyToken(req, res, next) {
  const authHeader = req.headers['authorization'];
  if (!authHeader) {
    return res.status(401).send('Access denied. No token provided.');
  }
  const token = authHeader.split(' ')[1]; // Expecting 'Bearer <token>'
  if (!token) {
    return res.status(401).send('Access denied. Token missing.');
  }
  try {
    const decoded = jwt.verify(token, SECRET_KEY);
    req.user = decoded;
    next();
  } catch (err) {
    res.status(400).send('Invalid token.');
  }
}

// Public route
app.get('/public', (req, res) => {
  res.send('This is a public route.');
});

// Protected route
app.get('/protected', verifyToken, (req, res) => {
  res.send(`Welcome ${req.user.name}, you accessed a protected route!`);
});

// Route to get a token (simulate login)
app.get('/login', (req, res) => {
  // Normally, you'd check user credentials here
  const user = { id: 1, name: 'Alice' };
  const token = jwt.sign(user, SECRET_KEY, { expiresIn: '1h' });
  res.json({ token });
});

app.listen(PORT, () => {
  console.log(`Server running on http://localhost:${PORT}`);
});

Important Notes

Always keep your secret key safe and never share it publicly.

The token is usually sent in the header as Authorization: Bearer <token>.

Use HTTPS to keep tokens secure during transmission.

Summary

JWT middleware checks if a token is valid before allowing access.

It protects routes by verifying user identity without sessions.

Tokens are sent in the Authorization header and verified with a secret key.

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