How do you subscribe to an event?

Node.jsJunior~1 min read

1. Subscribing via `.on(eventName, listener)`

.on() registers a handler (listener) for a given event. When the event fires via .emit(), every handler subscribed through .on() gets called.

Example:

javascript

const EventEmitter = require('events');
const emitter = new EventEmitter();

// Subscribe to the 'message' event
emitter.on('message', (data) => {
  console.log('Message received:', data);
});

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

Output:

javascript

Message received: Hello, Node.js!

2. Subscribing via `.once(eventName, listener)`

.once() is like .on(), but the handler fires only once, then removes itself automatically.

Example:

javascript

emitter.once('connect', () => {
  console.log('Connection established (once only)');
});

emitter.emit('connect'); // fires
emitter.emit('connect'); // won't fire again

3. Several subscribers on the same event

Several handlers can subscribe to the same event, they fire in registration order:

javascript

emitter.on('update', () => console.log('First handler'));
emitter.on('update', () => console.log('Second handler'));

emitter.emit('update');

Output:

javascript

First handler
Second handler

4. Removing a subscription

To unsubscribe from an event, use .off() or .removeListener():

javascript

function handler() {
  console.log('An event happened');
}

emitter.on('event', handler);
emitter.emit('event'); // fires

emitter.off('event', handler);
emitter.emit('event'); // doesn't fire

5. Where subscribing to events is used in Node.js

Many built-in objects are EventEmitters, with the same .on() and .once() methods:

javascript

const fs = require('fs');

const stream = fs.createReadStream('file.txt');

// Subscribing to system events
stream.on('data', (chunk) => console.log('Got a chunk of data'));
stream.on('end', () => console.log('Reading finished'));
stream.on('error', (err) => console.error('Error:', err));

The same works with:

http.Server (on('request', handler))
net.Socket (on('connect', handler))
process (on('exit', handler))

Summary

To subscribe to an event in Node.js, use the .on(eventName, listener) method of an EventEmitter object.

.on(), a permanent subscription

.once(), a one-time subscription

.off() / .removeListener(), unsubscribing

This mechanism is used everywhere: in streams, HTTP, the filesystem, processes, and custom events.

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