What is eventemitter in Node.js?

Node.js EventEmitter

EventEmitter is the foundation of Node.js's event-driven architecture. It's a built-in class (events module) that implements the observer pattern — allowing objects to emit named events and register listeners.

Basic Usage

const EventEmitter = require('events');

const emitter = new EventEmitter();

// Register a listener
emitter.on('greet', (name) => {
  console.log(`Hello, ${name}!`);
});

// Emit the event
emitter.emit('greet', 'Node.js'); // → Hello, Node.js!

Core Methods

Method	Description
`emitter.on(event, listener)`	Add a listener (alias: `addListener`)
`emitter.once(event, listener)`	Add one-time listener
`emitter.off(event, listener)`	Remove a listener (alias: `removeListener`)
`emitter.emit(event, ...args)`	Emit an event
`emitter.removeAllListeners(event)`	Remove all listeners
`emitter.listeners(event)`	List all listeners
`emitter.listenerCount(event)`	Count listeners

once() — Fire Once

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

emitter.emit('connect'); // fires
emitter.emit('connect'); // ignored

Custom EventEmitter Class

const EventEmitter = require('events');

class Database extends EventEmitter {
  connect() {
    // simulate async connection
    setTimeout(() => {
      this.emit('connected', { host: 'localhost' });
    }, 1000);
  }

  query(sql) {
    // simulate query
    setTimeout(() => {
      this.emit('data', [{ id: 1, name: 'Alice' }]);
    }, 500);
  }
}

const db = new Database();

db.on('connected', ({ host }) => {
  console.log(`Connected to ${host}`);
  db.query('SELECT * FROM users');
});

db.on('data', rows => {
  console.log('Got rows:', rows);
});

db.connect();

Error Events

'error' is a special event — if emitted with no listener, Node.js throws an uncaught exception:

// Always listen for 'error'!
emitter.on('error', (err) => {
  console.error('Caught error:', err.message);
});

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

Max Listeners Warning

Node.js warns when you add more than 10 listeners to a single event (memory leak protection):

emitter.setMaxListeners(20); // increase limit
// or
emitter.setMaxListeners(0);  // unlimited

EventEmitter in Node.js Core

Many built-in Node.js objects extend EventEmitter:

http.Server → 'request', 'close', 'error'
net.Socket → 'data', 'end', 'error'
fs.ReadStream → 'data', 'end', 'error'
process → 'exit', 'uncaughtException', 'SIGTERM'

Summary

EventEmitter enables loose coupling and event-driven architecture in Node.js. It's the backbone of streams, HTTP servers, and most core Node.js modules. Use it to build custom event-driven systems in your applications.

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