server.js

const express = require('express');
const { MongoClient, ObjectId } = require('mongodb');
require('dotenv').config();

const app = express();
const PORT = process.env.PORT || 3000;
const MONGODB_URI = process.env.MONGODB_URI || 'mongodb://localhost:27017';
const DB_NAME = process.env.DB_NAME || 'userdb';

let db;

// Connect to MongoDB
async function connectToMongoDB() {
  try {
    const client = new MongoClient(MONGODB_URI);
    await client.connect();
    db = client.db(DB_NAME);
    console.log('Connected to MongoDB successfully');
  } catch (error) {
    console.error('Failed to connect to MongoDB:', error);
    process.exit(1);
  }
}

// Middleware
app.use(express.json());

// GET endpoint for retrieving user by ID
app.get('/users/:id', async (req, res) => {
  try {
    const { id } = req.params;
    
    // Validate ObjectId format
    if (!ObjectId.isValid(id)) {
      return res.status(400).json({ 
        error: 'Invalid user ID format' 
      });
    }

    const usersCollection = db.collection('users');
    
    // Query for user with matching _id and age > 21
    const user = await usersCollection.findOne({
      _id: new ObjectId(id),
      age: { $gt: 21 }
    });

    if (!user) {
      return res.status(404).json({ 
        error: 'User not found or user is 21 or younger' 
      });
    }

    res.json(user);
  } catch (error) {
    console.error('Error retrieving user:', error);
    res.status(500).json({ 
      error: 'Internal server error' 
    });
  }
});

// Health check endpoint
app.get('/health', (req, res) => {
  res.json({ status: 'OK', message: 'Server is running' });
});

// Start server
async function startServer() {
  await connectToMongoDB();
  app.listen(PORT, () => {
    console.log(`Server is running on port ${PORT}`);
    console.log(`Health check: http://localhost:${PORT}/health`);
    console.log(`User endpoint: http://localhost:${PORT}/users/:id`);
  });
}

startServer();