How to use JWT for authentication on Node.js

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Authentication is one of the most important parts of any modern web application.
One of the most popular solutions today is JWT (JSON Web Token).

🤔 What is JWT?

JWT (JSON Web Token) is a compact, URL-safe token used to securely transmit information between parties.

A JWT looks like this: 

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

It consists of three parts: 

HEADER.PAYLOAD.SIGNATURE

🧩 JWT Structure

1️⃣ Header

Contains token type and signing algorithm. 

{
  "alg": "HS256",
  "typ": "JWT"
}

2️⃣ Payload

Contains user data (claims). 

{
  "id": 42,
  "email": "user@example.com"
}

⚠️ Never store passwords or sensitive data in payload

3️⃣ Signature

Used to verify the token wasn’t modified. 

HMACSHA256(base64UrlHeader + "." + base64UrlPayload, secret)

🔄 How JWT Authentication Works

  1. User logs in with email & password
  2. Server verifies credentials
  3. Server generates a JWT
  4. Client stores JWT (usually in memory or cookie)
  5. Client sends JWT in Authorization header
  6. Server verifies JWT on every request

🛠️ Implementing JWT Auth in Node.js (Express)

📦 Install Dependencies

npm install express auth-verify

🔑 Generate (signing) JWT on Login

const AuthVerify = require('auth-verify')
const auth = new AuthVerify({
   jwtSecret: "SUPER_SECRET" // setting secret for jwt
})

// Generating jwt
auth.jwt.sign({userId: 1, user: "John Doe"}, "1h") // 1h expiration time of jwt

🔐 Login Route Example

const express = require('express')
const app = express()
app.use(express.json())
app.use(express.urlencoded({ extended: true }))

const AuthVerify = require('auth-verify')
const auth = new AuthVerify({ jwtSecret: "SUPER_SECRET" })

app.post('/login', async (req, res)=> {
  const { email, password } = req.body

  const user = await findUserByEmail(email)
  if (!user) return res.status(401).json({ message: 'Invalid credentials' })
  const isValid = await auth.crypto.verify(password, user.password)
  if (!isValid) return res.status(401).json({ message: 'Invalid credentials' })

  const token = await auth.jwt.sign({userId: 1, user: "John Doe"}, "1h")
  res.json({ token })
})

🧱 Protecting Routes with JWT Middleware

auth.jwt.protect()

🔒 Protected Route Example

app.get('/profile', auth.jwt.protect(), (req, res)=> {
  res.json({
    message: 'Welcome!',
    user: req.user
  })
})

📤 Sending JWT from Client

Authorization: Bearer YOUR_JWT_TOKEN

⚠️ Common JWT Mistakes

  • ❌ Storing JWT in localStorage (XSS risk)
  • ❌ Putting sensitive data inside payload
  • ❌ No token expiration

  • ❌ Using weak secrets

  • ✅ Use HTTP-only cookies if possible

  • ✅ Always set expiresIn

  • ✅ Rotate secrets in production

🧠 When Should You Use JWT?

JWT is great when:

  • You have stateless APIs
  • You use microservices
  • You need mobile or SPA authentication

JWT is not ideal when:

  • You need instant logout everywhere
  • You need heavy session control

🏁 Conclusion

JWT provides a simple, scalable, and stateless way to handle authentication.
When used correctly, it’s powerful and secure.

If you’re building APIs, SPAs, or mobile apps — JWT is worth mastering.

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