Real life examples of EventEmitter in nodejs

The EventEmitter in Node.js is a powerful mechanism that allows objects to emit named events that cause registered listeners to be called. It's widely used for building event-driven architectures, where components communicate asynchronously. Here are three real-life examples where EventEmitter can be applied, along with code examples for each scenario:

1. Notification System

Scenario: Building a notification system where different parts of an application need to react to events such as new messages, user actions, or system alerts.

Example Code:

const EventEmitter = require('events');

// NotificationService emits events for notifications
class NotificationService extends EventEmitter {
  sendNotification(message) {
    console.log('Sending notification:', message);
    this.emit('notification', message);
  }
}

// Usage example
const notificationService = new NotificationService();

// Subscriber 1: Email notification listener
notificationService.on('notification', (message) => {
  console.log('Email notification sent:', message);
});

// Subscriber 2: SMS notification listener
notificationService.on('notification', (message) => {
  console.log('SMS notification sent:', message);
});

// Send notifications
notificationService.sendNotification('New message received');
notificationService.sendNotification('Server down alert');

Explanation:

• NotificationService extends EventEmitter, allowing it to emit 'notification' events.
• Subscribers (listeners) register with .on('notification', callback) to receive notifications.
• sendNotification(message) method emits 'notification' events with the provided message, triggering all registered listeners.

2. Logging System

Scenario: Implementing a logging system where various parts of an application log events, errors, and information to different outputs (e.g., console, files).

Example Code:

const EventEmitter = require('events');
const fs = require('fs');

class Logger extends EventEmitter {
  log(message) {
    console.log('Logging message:', message);
    this.emit('log', message);
  }
}

const logger = new Logger();

// Subscriber 1: Console logging
logger.on('log', (message) => {
  console.log('Console log:', message);
});

// Subscriber 2: File logging
logger.on('log', (message) => {
  fs.appendFile('app.log', `${message}\n`, (err) => {
    if (err) throw err;
    console.log('File log saved');
  });
});

// Log messages
logger.log('User login successful');
logger.log('Database connection failed');

Explanation:

• Logger extends EventEmitter to emit 'log' events with messages.
• Multiple listeners subscribe to 'log' events to perform actions (console logging, file writing).
• log(message) method emits 'log' events, triggering all registered listeners to log messages accordingly.

3. User Authentication and Authorization

Scenario: Implementing a user authentication system where components need to react to events like user login, logout, or access permission changes.

Example Code:

const EventEmitter = require('events');

class AuthSystem extends EventEmitter {
  login(username) {
    console.log('User logged in:', username);
    this.emit('login', username);
  }

  logout(username) {
    console.log('User logged out:', username);
    this.emit('logout', username);
  }
}

const authSystem = new AuthSystem();

// Subscriber 1: Audit log listener
authSystem.on('login', (username) => {
  console.log('Audit log: User logged in -', username);
});

// Subscriber 2: Access control listener
authSystem.on('login', (username) => {
  console.log('Access control: Granting permissions for user -', username);
});

// Usage example
authSystem.login('alice');
authSystem.logout('alice');

Explanation:

• AuthSystem extends EventEmitter to emit 'login' and 'logout' events with usernames.
• Listeners ('login' and 'logout') react to events, performing actions such as audit logging and access control updates.
• Methods like login(username) and logout(username) emit corresponding events, notifying all subscribers.