npm init -y npm install express bcryptjs jsonwebtoken body-parser mongoose 

const mongoose = require('mongoose');  mongoose.connect('mongodb://localhost:27017/auth_demo', {   useNewUrlParser: true,   useUnifiedTopology: true,   useCreateIndex: true });  const db = mongoose.connection; db.on('error', console.error.bind(console, 'connection error:')); db.once('open', () => {   console.log('Connected to MongoDB'); });  module.exports = mongoose; 

const mongoose = require('mongoose'); const bcrypt = require('bcryptjs');  const userSchema = new mongoose.Schema({   username: { type: String, required: true, unique: true },   password: { type: String, required: true } });  userSchema.pre('save', async function (next) {   if (this.isModified('password') || this.isNew) {     const salt = await bcrypt.genSalt(10);     this.password = await bcrypt.hash(this.password, salt);   }   next(); });  userSchema.methods.comparePassword = async function (password) {   return await bcrypt.compare(password, this.password); };  const User = mongoose.model('User', userSchema);  module.exports = User; 

const express = require('express'); const bodyParser = require('body-parser'); const jwt = require('jsonwebtoken'); const User = require('./userModel'); const mongoose = require('./database');  const app = express(); app.use(bodyParser.json());  const JWT_SECRET = 'your_jwt_secret';  // User Registration app.post('/register', async (req, res) => {   try {     const { username, password } = req.body;     const user = new User({ username, password });     await user.save();     res.status(201).send('User registered successfully');   } catch (err) {     res.status(400).send('Error registering user');   } });  // User Login app.post('/login', async (req, res) => {   try {     const { username, password } = req.body;     const user = await User.findOne({ username });     if (!user || !(await user.comparePassword(password))) {       return res.status(401).send('Invalid username or password');     }     const token = jwt.sign({ userId: user._id }, JWT_SECRET, { expiresIn: '1h' });     res.send({ token });   } catch (err) {     res.status(400).send('Error logging in');   } });  // Middleware to Protect Routes const authMiddleware = (req, res, next) => {   const token = req.headers['authorization'];   if (!token) {     return res.status(401).send('Access denied. No token provided.');   }   try {     const decoded = jwt.verify(token, JWT_SECRET);     req.user = decoded;     next();   } catch (err) {     res.status(401).send('Invalid token');   } };  // Protected Route app.get('/protected', authMiddleware, (req, res) => {   res.send('This is a protected route'); });  const PORT = 3000; app.listen(PORT, () => {   console.log(`Server running at http://localhost:${PORT}/`); });