Events & the EventEmitter

🌿 Node.js Lesson 4 Beginner

Node.js is fundamentally event-driven. The EventEmitter class is the backbone of Node's event system — almost every built-in module (HTTP, Streams, File System) extends it to emit and listen for named events.

1 Creating & Using EventEmitter

JavaScript — EventEmitter Basics

const { EventEmitter } = require('events');

// Create an emitter instance
const emitter = new EventEmitter();

// Register a listener for 'data' event
emitter.on('data', (payload) => {
  console.log('Data received:', payload);
});

// One-time listener (fires only once, then auto-removed)
emitter.once('connect', (host) => {
  console.log('Connected to:', host);
});

// Emit events with payloads
emitter.emit('data', { id: 1, value: 42 });
emitter.emit('connect', 'database.server.com');
emitter.emit('connect', 'second.server.com'); // ignored — once() already fired

// Remove a specific listener
const handler = (msg) => console.log(msg);
emitter.on('message', handler);
emitter.off('message', handler); // removes it

2 Custom Event Classes

Best practice is to extend EventEmitter to create typed event sources:

JavaScript — Custom EventEmitter class

const { EventEmitter } = require('events');

class TaskQueue extends EventEmitter {
  constructor() {
    super();
    this.tasks = [];
  }

  addTask(task) {
    this.tasks.push(task);
    this.emit('taskAdded', task);
  }

  processNext() {
    if (this.tasks.length === 0) {
      this.emit('empty');
      return;
    }
    const task = this.tasks.shift();
    this.emit('processing', task);
    // ... process task
    this.emit('done', task);
  }
}

const queue = new TaskQueue();

queue.on('taskAdded', t => console.log('Added task:', t.name));
queue.on('done', t => console.log('Completed:', t.name));
queue.on('empty', () => console.log('Queue is empty.'));

queue.addTask({ name: 'Send Email', priority: 1 });
queue.processNext();

3 Code Challenge

Challenge: Build a Logger class extending EventEmitter that emits 'info', 'warn', and 'error' events. Register listeners that format and print each log level with a timestamp prefix.