The Node.js EventEmitter: A Core Concept for Asynchronous Events

The `EventEmitter` class is at the heart of Node.js's asynchronous, event-driven architecture. Many of Node's built-in modules, like HTTP servers and streams, inherit from `EventEmitter`. It allows you to create, listen for, and emit custom events.

Core Methods

• `.on(eventName, listener)`: Registers a listener function for a specific event.
• `.emit(eventName, [args...])`: Triggers an event, calling all registered listeners for it.
• `.once(eventName, listener)`: Registers a listener that will be called only once for an event.

Creating a Custom Emitter

Let's create a simple logger that emits events for different log levels.

const EventEmitter = require('events');

class Logger extends EventEmitter {
  log(level, message) {
    const timestamp = new Date().toISOString();
    this.emit('message', { level, message, timestamp });
  }
}

const logger = new Logger();

// Register a listener
logger.on('message', (data) => {
  console.log(`[${data.level.toUpperCase()}] ${data.timestamp}: ${data.message}`);
});

// Trigger events
logger.log('info', 'User logged in.');
logger.log('error', 'Database connection failed.');

The EventEmitter pattern is a powerful way to decouple different parts of your application. Instead of one component calling another directly, it can simply emit an event, and any interested components can listen for it.