feat(auth): implement user authentication system

src/app.js

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+const express = require('express');
+const cors = require('cors');
+const helmet = require('helmet');
+const authRoutes = require('./routes/auth');
+const { authenticateToken } = require('./middleware/auth');
+
+const app = express();
+
+// Middleware
+app.use(helmet());
+app.use(cors());
+app.use(express.json());
+
+// Routes
+app.use('/api/auth', authRoutes);
+
+// Protected route example
+app.get('/api/protected', authenticateToken, (req, res) => {
+  res.json({ message: 'This is a protected route', user: req.user });
+});
+
+// Health check
+app.get('/health', (req, res) => {
+  res.json({ status: 'OK', timestamp: new Date().toISOString() });
+});
+
+module.exports = app;

src/config/index.js

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+// Configuration file
+require('dotenv').config();
+
+const config = {
+  JWT_SECRET: process.env.JWT_SECRET || 'default-secret-key-for-development',
+  PORT: process.env.PORT || 3000,
+};
+
+module.exports = config;

src/middleware/auth.js

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+const jwt = require('jsonwebtoken');
+const { JWT_SECRET } = require('../config');
+
+const authenticateToken = (req, res, next) => {
+  const authHeader = req.headers['authorization'];
+  const token = authHeader && authHeader.split(' ')[1];
+
+  if (!token) {
+    return res.status(401).json({ error: 'Access token required' });
+  }
+
+  jwt.verify(token, JWT_SECRET, (err, user) => {
+    if (err) {
+      return res.status(403).json({ error: 'Invalid or expired token' });
+    }
+    req.user = user;
+    next();
+  });
+};
+
+module.exports = { authenticateToken };

src/routes/auth.js

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+const express = require('express');
+const jwt = require('jsonwebtoken');
+const bcrypt = require('bcrypt');
+const { JWT_SECRET } = require('../config');
+
+const router = express.Router();
+
+// Mock user database (in production, this would be a real database)
+const users = [];
+
+// Register endpoint
+router.post('/register', async (req, res) => {
+  try {
+    const { username, email, password } = req.body;
+    
+    // Check if user already exists
+    const existingUser = users.find(u => u.email === email);
+    if (existingUser) {
+      return res.status(400).json({ error: 'User already exists' });
+    }
+    
+    // Hash password
+    const hashedPassword = await bcrypt.hash(password, 10);
+    
+    // Create user
+    const newUser = {
+      id: users.length + 1,
+      username,
+      email,
+      password: hashedPassword
+    };
+    
+    users.push(newUser);
+    
+    // Generate token
+    const token = jwt.sign(
+      { id: newUser.id, email: newUser.email },
+      JWT_SECRET,
+      { expiresIn: '24h' }
+    );
+    
+    res.status(201).json({ 
+      message: 'User registered successfully',
+      token,
+      user: { id: newUser.id, username, email }
+    });
+  } catch (error) {
+    res.status(500).json({ error: 'Registration failed' });
+  }
+});
+
+// Login endpoint
+router.post('/login', async (req, res) => {
+  try {
+    const { email, password } = req.body;
+    
+    // Find user
+    const user = users.find(u => u.email === email);
+    if (!user) {
+      return res.status(400).json({ error: 'Invalid credentials' });
+    }
+    
+    // Check password
+    const isValidPassword = await bcrypt.compare(password, user.password);
+    if (!isValidPassword) {
+      return res.status(400).json({ error: 'Invalid credentials' });
+    }
+    
+    // Generate token
+    const token = jwt.sign(
+      { id: user.id, email: user.email },
+      JWT_SECRET,
+      { expiresIn: '24h' }
+    );
+    
+    res.json({ 
+      message: 'Login successful',
+      token,
+      user: { id: user.id, username: user.username, email: user.email }
+    });
+  } catch (error) {
+    res.status(500).json({ error: 'Login failed' });
+  }
+});
+
+module.exports = router;

src/server.js

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+const app = require('./app');
+const { PORT } = require('./config');
+
+const server = app.listen(PORT, () => {
+  console.log(`Server running on port ${PORT}`);
+});
+
+module.exports = server;