JWT token creation in Express - Mini Project: Build & Apply

JWT Token Creation with Express

📖 Scenario: You are building a simple Express server that creates JSON Web Tokens (JWT) for users after they log in. JWTs help keep users logged in securely.

🎯 Goal: Build an Express server that creates a JWT token using a secret key and user data.

📋 What You'll Learn

Create an Express app variable called app

Create a secret key variable called jwtSecret with value 'mysecretkey'

Use the jsonwebtoken library to create a token with user info

Create a POST route /login that returns a JWT token

💡 Why This Matters

🌍 Real World

JWT tokens are widely used in web apps to securely keep users logged in without storing passwords on the client.

💼 Career

Understanding JWT creation is essential for backend developers working on authentication and security in web applications.

1

Setup Express app and user data

Create an Express app variable called app by requiring express and calling it. Also create a user object called user with id 1 and username 'alice'.

Express

const express = require('express');
// Your code here

Hint

Use const app = express() to create the app. Define user as an object with id and username.

2

Add JWT secret key

Create a constant called jwtSecret and set it to the string 'mysecretkey'.

Express

const express = require('express');
const app = express();

const user = { id: 1, username: 'alice' };
// Your code here

Hint

Just create a constant string variable named jwtSecret.

3

Create JWT token in /login route

Require the jsonwebtoken library as jwt. Then create a POST route /login on app. Inside the route, create a token by calling jwt.sign() with the user object and jwtSecret. Send the token as JSON with key token.

Express

const express = require('express');
const app = express();

const user = { id: 1, username: 'alice' };

const jwtSecret = 'mysecretkey';
// Your code here

Hint

Use jwt.sign(user, jwtSecret) to create the token. Send it back with res.json({ token }).

4

Start the Express server

Add code to start the Express server by calling app.listen() on port 3000. Inside the listen callback, log the message 'Server running on port 3000'.

Express

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

const user = { id: 1, username: 'alice' };

const jwtSecret = 'mysecretkey';

app.post('/login', (req, res) => {
    const token = jwt.sign(user, jwtSecret);
    res.json({ token });
});

// Your code here

Hint

Use app.listen(3000, () => { console.log('Server running on port 3000') }) to start the server.

Practice

1. What is the main purpose of creating a JWT token in an Express app?

easy

A. To connect to a database

B. To style the user interface

C. To handle file uploads

D. To securely store user information for authentication

Solution

1. Step 1: Understand JWT token role

   JWT tokens are used to safely store user data for verifying identity.

2. Step 2: Identify correct purpose

   Among the options, only storing user info for authentication matches JWT's role.

3. Final Answer:

   To securely store user information for authentication -> Option D

4. Quick Check:

   JWT purpose = Authentication [OK]

Hint: JWT tokens are for authentication, not UI or database [OK]

Common Mistakes:

• Confusing JWT with UI styling or database connection
• Thinking JWT handles file uploads

2. Which of the following is the correct syntax to create a JWT token using the jsonwebtoken package in Express?

easy

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

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

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

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

Solution

1. Step 1: Recall jsonwebtoken method

   The correct method to create a token is jwt.sign()

2. Step 2: Match syntax with options

   Only jwt.sign(payload, secretKey, { expiresIn: '1h' }) uses jwt.sign() with payload, secretKey, and expiresIn correctly.

3. Final Answer:

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

4. Quick Check:

   Token creation method = sign() [OK]

Hint: Remember: jsonwebtoken uses sign() to create tokens [OK]

Common Mistakes:

• Using incorrect method names like create or generate
• Omitting the expiresIn option or using wrong syntax

3. Given the code snippet:

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

What will be the output when this code runs?

medium

A. 'object'

B. 'string'

C. 'undefined'

D. 'number'

Solution

1. Step 1: Understand jwt.sign output type

   jwt.sign returns a JWT token as a string.

2. Step 2: Check typeof token

   Using typeof on the token returns 'string'.

3. Final Answer:

   'string' -> Option B

4. Quick Check:

   jwt.sign() output type = string [OK]

Hint: jwt.sign() returns a token string, not an object [OK]

Common Mistakes:

• Assuming the token is an object
• Expecting undefined or number type

4. Identify the error in this JWT token creation code:

const jwt = require('jsonwebtoken');
const token = jwt.sign({ id: 1 }, 12345, { expiresIn: '1h' });

medium

A. Secret key should be a string, not a number

B. Payload must be a string, not an object

C. expiresIn option is invalid

D. jwt.sign requires a callback function

Solution

1. Step 1: Check secret key type

   The secret key must be a string for signing the token securely.

2. Step 2: Identify error in code

   The code uses 12345 (a number) as secret key, which is incorrect.

3. Final Answer:

   Secret key should be a string, not a number -> Option A

4. Quick Check:

   Secret key type = string [OK]

Hint: Secret key must always be a string for jwt.sign() [OK]

Common Mistakes:

• Passing number instead of string as secret key
• Thinking payload must be string
• Believing expiresIn is invalid
• Assuming callback is mandatory

5. You want to create a JWT token that expires in 30 minutes and includes the user's email and role. Which code snippet correctly achieves this in Express?

hard

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

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

C. jwt.sign({ email: user.email, role: user.role }, 'mySecret', { expireIn: 1800 })

D. jwt.sign({ email: user.email, role: user.role }, 'mySecret', { expiresIn: 30 })

Solution

1. Step 1: Include correct payload fields

   The payload must include email and role from user object.

2. Step 2: Use correct expiresIn format

   expiresIn accepts string like '30m' for 30 minutes; number means seconds but must be a number type without quotes.

3. Step 3: Identify correct option

   Check each: expiresAt is invalid key; expireIn is misspelled; expiresIn: 30 is only 30 seconds. Only jwt.sign({ email: user.email, role: user.role }, 'mySecret', { expiresIn: '30m' }) is correct.

4. Final Answer:

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

5. Quick Check:

   expiresIn '30m' string format = correct [OK]

Hint: Use expiresIn with string like '30m' for minutes [OK]

Common Mistakes:

• Using expiresAt instead of expiresIn
• Using small numbers like 30 for expiresIn (30 seconds, not minutes)
• Confusing expireIn with expiresIn