Node.js — Events

Import Events Module

const EventEmitter = require('events');

Basic Usage

const emitter = new EventEmitter();

// Listen for event
emitter.on('greet', (name) => {
    console.log(`Hello, ${name}!`);
});

// Emit event
emitter.emit('greet', 'John');

EventEmitter Methods

Method	Description
on(event, listener)	Add listener
once(event, listener)	Add one-time listener
emit(event, ...args)	Trigger event
off(event, listener)	Remove listener
removeAllListeners()	Remove all listeners

Multiple Listeners

emitter.on('data', (chunk) => {
    console.log('Listener 1:', chunk);
});

emitter.on('data', (chunk) => {
    console.log('Listener 2:', chunk);
});

emitter.emit('data', 'Hello');

Once Listener

emitter.once('connect', () => {
    console.log('Connected!');
});

emitter.emit('connect'); // Logs
emitter.emit('connect'); // Does nothing

Extending EventEmitter

class Server extends EventEmitter {
    start() {
        this.emit('started');
    }
}

const server = new Server();
server.on('started', () => console.log('Server started'));
server.start();

Error Events

emitter.on('error', (err) => {
    console.error('Error:', err.message);
});

emitter.emit('error', new Error('Something went wrong'));

Mini Practice

1. Create an EventEmitter
2. Add and remove listeners
3. Use once for one-time events
4. Extend EventEmitter in a class

Up Next

Continue with HTTP — building web servers.