How to implement real-time notifications in Node.js

const WebSocket = require('ws')
const http = require('http')

class NotificationService {
  constructor(server) {
    this.wss = new WebSocket.Server({ server })
    this.userConnections = new Map() // userId -> Set of WebSocket connections

    this.wss.on('connection', (ws, req) => {
      this.handleConnection(ws, req)
    })
  }

  handleConnection(ws, req) {
    let userId = null

    ws.on('message', (data) => {
      try {
        const message = JSON.parse(data)

        // Authenticate user
        if (message.type === 'auth') {
          userId = message.userId // In production, validate token
          this.registerUser(userId, ws)

          ws.send(JSON.stringify({
            type: 'auth_success',
            message: 'Connected to notification service'
          }))
        }
      } catch (error) {
        console.error('Error handling message:', error)
      }
    })

    ws.on('close', () => {
      if (userId) {
        this.unregisterUser(userId, ws)
      }
    })
  }

  registerUser(userId, ws) {
    if (!this.userConnections.has(userId)) {
      this.userConnections.set(userId, new Set())
    }
    this.userConnections.get(userId).add(ws)
    console.log(`User ${userId} connected. Total connections: ${this.userConnections.get(userId).size}`)
  }

  unregisterUser(userId, ws) {
    const connections = this.userConnections.get(userId)
    if (connections) {
      connections.delete(ws)
      if (connections.size === 0) {
        this.userConnections.delete(userId)
      }
    }
  }

  sendToUser(userId, notification) {
    const connections = this.userConnections.get(userId)
    if (!connections) return

    const message = JSON.stringify({
      type: 'notification',
      ...notification,
      timestamp: new Date().toISOString()
    })

    connections.forEach(ws => {
      if (ws.readyState === WebSocket.OPEN) {
        ws.send(message)
      }
    })
  }

  sendToMultipleUsers(userIds, notification) {
    userIds.forEach(userId => {
      this.sendToUser(userId, notification)
    })
  }

  broadcast(notification) {
    const message = JSON.stringify({
      type: 'notification',
      ...notification,
      timestamp: new Date().toISOString()
    })

    this.userConnections.forEach((connections) => {
      connections.forEach(ws => {
        if (ws.readyState === WebSocket.OPEN) {
          ws.send(message)
        }
      })
    })
  }
}

// Usage example
const server = http.createServer()
const notificationService = new NotificationService(server)

// Simulate sending notifications
setInterval(() => {
  notificationService.sendToUser('user123', {
    title: 'New Message',
    body: 'You have received a new message',
    priority: 'high'
  })
}, 10000)

// Broadcast to all users
notificationService.broadcast({
  title: 'System Maintenance',
  body: 'Scheduled maintenance in 1 hour',
  priority: 'medium'
})

server.listen(3000, () => {
  console.log('Notification service running on port 3000')
})

const ws = new WebSocket('ws://localhost:3000')

ws.onopen = () => {
  // Authenticate
  ws.send(JSON.stringify({
    type: 'auth',
    userId: 'user123',
    token: 'your-auth-token'
  }))
}

ws.onmessage = (event) => {
  const data = JSON.parse(event.data)

  if (data.type === 'notification') {
    showNotification(data.title, data.body, data.priority)
  }
}

function showNotification(title, body, priority) {
  // Display notification in UI
  console.log(`[${priority}] ${title}: ${body}`)
}