What does once() do?

Node.jsJunior~1 min read

1. What `once()` does

The .once(eventName, listener) method registers an event handler that will run only once, then remove itself automatically.

After the event fires the first time, the listener won't be called again, even if an event with the same name fires again.

Example:

javascript

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

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

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

Output:

javascript

Connection established!

2. What `on()` does

The .on(eventName, listener) method registers a permanent event handler. It will be called every time the event fires.

Example:

javascript

emitter.on('data', () => {
  console.log('Data received');
});

emitter.emit('data'); // fires
emitter.emit('data'); // fires again

Output:

javascript

Data received
Data received

3. The key difference between `once()` and `on()`

Characteristic	`on()`	`once()`
Number of firings	Unlimited (every `emit()`)	Only once
Manual listener removal needed?	Yes, if needed (`off()`)	No, removed automatically
Typical use cases	Repeating events (e.g. `data`, `message`)	One-time events (`connect`, `ready`, `close`, `finish`)
Memory impact	Listener stays around	Listener is removed after the call

4. Why `once()` is useful

Avoids memory leaks, the listener is removed automatically.
Convenient for initialization, e.g. a server's ready event or a stream's open event.
Simplifies logic, no need to call .off() manually.

5. A real-world example

For instance, on an HTTP server:

javascript

const http = require('http');
const server = http.createServer();

server.once('listening', () => {
  console.log('Server started!');
});

server.listen(3000);

The 'listening' event should happen once, so .once() is a perfect fit.

Summary

on() adds a permanent listener that's called every time emit() fires. once() adds a one-time listener that's removed after its first call.

Recommended usage:

use on() for repeating events (data, message, update);

use once() for one-time events (connect, ready, close, init).

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