JWT token verification middleware in Express - Step-by-Step Execution

Concept Flow - JWT token verification middleware

Incoming HTTP Request

↓

Check for Authorization Header

Yes / No↓

Extract JWT

↓

Verify JWT Signature

↓

Attach User Info

↓

Call Next Middleware/Route Handler

The middleware checks the request for a JWT token, verifies it, and either attaches user info and continues or rejects the request.

Execution Sample

Express

function verifyToken(req, res, next) {
  const token = req.headers['authorization']?.split(' ')[1];
  if (!token) return res.status(401).send('No token');
  jwt.verify(token, secret, (err, user) => {
    if (err) return res.status(401).send('Invalid token');
    req.user = user;
    next();
  });
}

This middleware extracts a JWT from the Authorization header, verifies it, and either rejects or passes control with user info.

Execution Table

Step	Action	Token Extracted	Verification Result	Request User Attached	Response Sent	Next Called
1	Check Authorization header	Bearer abc.def.ghi	N/A	No	No	No
2	Extract token from header	abc.def.ghi	N/A	No	No	No
3	Verify token signature	abc.def.ghi	Valid	No	No	No
4	Attach user info to req	abc.def.ghi	Valid	Yes	No	No
5	Call next middleware	abc.def.ghi	Valid	Yes	No	Yes
6	Request proceeds to next handler	abc.def.ghi	Valid	Yes	No	Yes

💡 Middleware ends after calling next() if token is valid; otherwise, sends 401 response.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
req.headers['authorization']	Bearer abc.def.ghi	Bearer abc.def.ghi	Bearer abc.def.ghi	Bearer abc.def.ghi	Bearer abc.def.ghi	Bearer abc.def.ghi
token	undefined	undefined	abc.def.ghi	abc.def.ghi	abc.def.ghi	abc.def.ghi
verification error (err)	undefined	undefined	undefined	null	null	null
user (decoded token)	undefined	undefined	undefined	{id:123}	{id:123}	{id:123}
req.user	undefined	undefined	undefined	undefined	{id:123}	{id:123}

Key Moments - 3 Insights

Why does the middleware send a 401 response if the token is missing?

Because if no token is extracted, the middleware immediately sends a 401 response to reject unauthorized access.

What happens if the token verification fails?

If verification returns an error, the middleware sends a 401 response and does not call next(), stopping further processing.

How does the middleware pass user info to later handlers?

In step 4, the middleware attaches the decoded user info to req.user, so subsequent middleware or routes can access it.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'token' after Step 2?

A undefined

B "Bearer abc.def.ghi"

C "abc.def.ghi"

D null

💡 Hint

Check the 'Token Extracted' column in row for Step 2.

At which step does the middleware attach user info to the request object?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Request User Attached' column in execution_table.

If the token is invalid, what will the middleware do according to the execution flow?

A Attach user info and call next()

B Send 401 Unauthorized response

C Ignore and call next()

D Send 200 OK response

💡 Hint

Refer to the concept_flow and execution_table step 3 for invalid verification.

Concept Snapshot

JWT Verification Middleware in Express:
- Extract token from Authorization header
- If missing, send 401 Unauthorized
- Verify token signature with secret
- If invalid, send 401 Unauthorized
- If valid, attach decoded user info to req.user
- Call next() to continue processing

Full Transcript

This middleware runs on each incoming HTTP request. It first looks for the Authorization header. If the header is missing or does not contain a token, it immediately sends a 401 Unauthorized response. If a token is found, it verifies the token's signature using a secret key. If verification fails, it sends a 401 Unauthorized response. If verification succeeds, it attaches the decoded user information to the request object as req.user. Finally, it calls next() to pass control to the next middleware or route handler. This ensures only requests with valid tokens proceed.

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