JWT token generation and verification in Node.js

Introduction

JWT tokens help safely share information between a user and a server. They prove who you are without sending your password every time.

When a user logs in and you want to remember them without storing their password.

When you want to protect parts of a website so only logged-in users can see them.

When you need to send user info securely between different parts of your app.

When building APIs that require users to prove their identity.

When you want a simple way to check if a user's session is still valid.

Syntax

Node.js

const jwt = require('jsonwebtoken');

// To create a token
const token = jwt.sign(payload, secretKey, options);

// To check a token
const verifiedData = jwt.verify(token, secretKey);

payload is the data you want to include, like user ID.

secretKey is a secret string only your server knows.

Examples

Creates a token with user ID 123 using 'mySecret' as the secret key.

Node.js

const token = jwt.sign({ userId: 123 }, 'mySecret');

Checks the token using the same secret key and gets the original data back.

Node.js

const data = jwt.verify(token, 'mySecret');

Creates a token that expires in 1 hour.

Node.js

const token = jwt.sign({ userId: 123 }, 'mySecret', { expiresIn: '1h' });

Sample Program

This code creates a JWT token for user 'alice' that lasts 2 hours. Then it checks the token and prints the user info if valid.

Node.js

const jwt = require('jsonwebtoken');

const secretKey = 'superSecret123';

// Create a token with user info
const token = jwt.sign({ username: 'alice' }, secretKey, { expiresIn: '2h' });

console.log('Generated Token:', token);

// Later, verify the token
try {
  const data = jwt.verify(token, secretKey);
  console.log('Verified Data:', data);
} catch (error) {
  console.log('Token is invalid or expired');
}

Important Notes

Keep your secret key safe and never share it publicly.

Tokens can expire, so always handle expired tokens gracefully.

Use HTTPS to keep tokens safe when sent over the internet.

Summary

JWT tokens let you prove who you are without sending passwords repeatedly.

You create tokens with jwt.sign() and check them with jwt.verify().

Always protect your secret key and handle token expiration.

Practice

1. What is the main purpose of a JWT token in a Node.js application?

easy

A. To securely transmit user identity without sending passwords repeatedly

B. To store user passwords in the database

C. To encrypt all user data on the server

D. To create a permanent session without expiration

Solution

1. Step 1: Understand JWT token purpose

   JWT tokens are used to prove user identity securely without sending passwords every time.

2. Step 2: Compare options

   Only To securely transmit user identity without sending passwords repeatedly correctly describes this purpose; others describe unrelated or incorrect uses.

3. Final Answer:

   To securely transmit user identity without sending passwords repeatedly -> Option A

4. Quick Check:

   JWT purpose = Secure identity proof [OK]

Hint: JWT = identity proof without password resend [OK]

Common Mistakes:

• Thinking JWT stores passwords
• Confusing JWT with encryption of all data
• Assuming JWT creates permanent sessions

2. Which of the following is the correct syntax to generate a JWT token using the jsonwebtoken library in Node.js?

easy

A. jwt.generate(payload, secretKey, { expiresAt: '1h' })

B. jwt.create(payload, secretKey, { expiresIn: '1h' })

C. jwt.sign(payload, secretKey, { expiresIn: '1h' })

D. jwt.make(payload, secretKey, { expire: '1h' })

Solution

1. Step 1: Recall jsonwebtoken method for token creation

   The correct method to create a token is jwt.sign with payload, secret, and options.

2. Step 2: Check option syntax

   Only jwt.sign(payload, secretKey, { expiresIn: '1h' }) uses jwt.sign and correct option key expiresIn; others use invalid method names or option keys.

3. Final Answer:

   jwt.sign(payload, secretKey, { expiresIn: '1h' }) -> Option C

4. Quick Check:

   Token creation method = jwt.sign [OK]

Hint: Use jwt.sign() with expiresIn option for token creation [OK]

Common Mistakes:

• Using wrong method names like create or generate
• Using expiresAt or expire instead of expiresIn
• Missing the secret key argument

3. Given the code snippet:

const jwt = require('jsonwebtoken');
const token = jwt.sign({ userId: 123 }, 'secret123', { expiresIn: '2h' });
const decoded = jwt.verify(token, 'secret123');
console.log(decoded.userId);

What will be the output when this code runs?

medium

A. 123

B. undefined

C. Error: invalid token

D. null

Solution

1. Step 1: Understand token creation and verification

   The token is created with payload { userId: 123 } and secret 'secret123', then verified with the same secret.

2. Step 2: Check decoded payload output

   jwt.verify returns the original payload if token is valid, so decoded.userId is 123.

3. Final Answer:

   123 -> Option A

4. Quick Check:

   Decoded userId = 123 [OK]

Hint: Verify with same secret returns original payload [OK]

Common Mistakes:

• Expecting undefined or null instead of payload
• Using different secret causing invalid token error
• Not understanding jwt.verify returns payload

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

const jwt = require('jsonwebtoken');
const token = jwt.sign({ id: 1 }, 'mysecret');
const decoded = jwt.verify(token, 'wrongsecret');
console.log(decoded.id);

medium

A. Token is expired

B. jwt.sign method is used incorrectly

C. Payload is missing id property

D. Secret key used for verification is incorrect

Solution

1. Step 1: Check token creation and verification secrets

   Token is signed with 'mysecret' but verified with 'wrongsecret', causing verification failure.

2. Step 2: Understand jwt.verify behavior on secret mismatch

   jwt.verify throws an error if secret does not match, so decoded.id is not accessible.

3. Final Answer:

   Secret key used for verification is incorrect -> Option D

4. Quick Check:

   Secret mismatch = verification error [OK]

Hint: Verify with same secret as sign to avoid errors [OK]

Common Mistakes:

• Assuming token expired without expiration set
• Thinking payload lacks id property
• Misusing jwt.sign method syntax

5. You want to generate a JWT token that expires in 30 minutes and includes the user's email and role. Which code snippet correctly does this using the jsonwebtoken library?

hard

A. jwt.sign({ email: user.email, role: user.role }, 'secretKey', { expire: 1800 })

B. jwt.sign({ email: user.email, role: user.role }, 'secretKey', { expiresIn: '30m' })

C. jwt.sign({ email: user.email, role: user.role }, 'secretKey', { expiresAt: '30min' })

D. jwt.sign({ email: user.email, role: user.role }, 'secretKey', { timeout: '30m' })

Solution

1. Step 1: Identify correct payload and expiration option

   The payload includes email and role, and expiration uses expiresIn with '30m' for 30 minutes.

2. Step 2: Validate option keys and values

   Only jwt.sign({ email: user.email, role: user.role }, 'secretKey', { expiresIn: '30m' }) uses correct option key expiresIn and valid time format '30m'; others use invalid keys or formats.

3. Final Answer:

   jwt.sign({ email: user.email, role: user.role }, 'secretKey', { expiresIn: '30m' }) -> Option B

4. Quick Check:

   expiresIn '30m' = 30 minutes expiry [OK]

Hint: Use expiresIn: '30m' for 30-minute token expiry [OK]

Common Mistakes:

• Using expire or expiresAt instead of expiresIn
• Wrong time format like '30min'
• Using timeout option which is invalid