JWT Authentication: How It Works and How to Implement It

JWT Authentication: How It Works and How to Implement It

JSON Web Tokens (JWT) are the most widely used authentication mechanism for REST APIs. Unlike server-side sessions, JWTs are stateless — the server does not store session data. Instead, the token itself contains the user's identity and is signed by the server to prevent tampering.

JWT Structure

A JWT consists of three Base64URL-encoded parts separated by dots: header.payload.signature. The header specifies the algorithm (typically HS256 or RS256). The payload contains claims — the user's ID, email, role, and expiration time. The signature is created by hashing the header and payload with a secret key, preventing forgery.

{
  "id": "507f1f77bcf86cd799439011",
  "email": "alice@example.com",
  "role": "admin",
  "iat": 1705316200,
  "exp": 1705402600
}

Implementing JWT in Express

npm install jsonwebtoken bcryptjs

const jwt = require('jsonwebtoken');
const bcrypt = require('bcryptjs');

const login = async (req, res) => {
  const { email, password } = req.body;
  const user = await User.findOne({ email }).select('+password');

  if (!user || !(await bcrypt.compare(password, user.password))) {
    return res.status(401).json({
      success: false,
      error: { code: 'INVALID_CREDENTIALS', message: 'Invalid email or password' }
    });
  }

  const token = jwt.sign(
    { id: user._id, email: user.email, role: user.role },
    process.env.JWT_SECRET,
    { expiresIn: '1d' }
  );

  res.status(200).json({ success: true, data: { token } });
};

JWT Verification Middleware

const protect = async (req, res, next) => {
  const authHeader = req.headers.authorization;
  if (!authHeader || !authHeader.startsWith('Bearer ')) {
    return res.status(401).json({ success: false, error: { code: 'UNAUTHORIZED' } });
  }
  try {
    const token = authHeader.split(' ')[1];
    const decoded = jwt.verify(token, process.env.JWT_SECRET);
    req.user = await User.findById(decoded.id).select('-password');
    next();
  } catch (err) {
    res.status(401).json({ success: false, error: { code: 'TOKEN_INVALID' } });
  }
};

Access and Refresh Tokens

Access tokens should have short expiry (15 minutes to 1 hour) to limit exposure if compromised. Refresh tokens have long expiry (7-30 days), are stored securely in httpOnly cookies, and are used to obtain new access tokens without re-login.