Node.js provides the events modules, which provides EventEmitter. This modules provides EventListener, an object that can listen for events.

This was heavily inspired by Backbone's listenTo() and stopListening() functions. The idea is to make it easy to de-regster an event listener when you're done with it.

Event Listener Memory Leaks

import EventEmitter from 'events';

class Widget() {
    constructor(emitter) {
        emitter.on('error', err => this.close());
    }
    ...
}

let emitter = new EventEmitter();

let widget = new Widget(emitter);
// Do some stuff with `widget`.  When we're done with it, clear the reference to
// `widget` so the garbage collector can free it.
widget = null;

The code above creates a pretty common memory leak in node.js apps; the problem in the above is that when we call emitter.on('error', ...), we add the function we pass in to the EventEmitter's list of events to call, which means EventEmitter has a reference to the handler function. But, the handler function is an arrow function, which binds this, which means the handler function has a reference to widget. When we set widget = null, the Widget object will never be garbage collected, because emitter still has a reference to it.

One way to solve this problem is to call emitter.removeListener('error', handler), but note that the function we passed to emitter.on(...) was not this.close(), so we can't call emitter.removeListener('error', this.close). We need to keep a reference to the anonymous arrow function we passed in to emitter.on().

EventListener's job is to keep track of these references for you:

import EventEmitter from 'events';
import EventListener from 'events-listener';

class Widget() {
    constructor(emitter) {
        this.listener = new EventListener();
        this.listener.listenTo(emitter, 'error', err => this.close());
    }

    destroy() {
        this.listener.stopListening();
    }
    ...
}

let emitter = new EventEmitter();

let widget = new Widget(emitter);

// Do some stuff with `widget`.

widget.destroy();
widget = null;

API

listenTo(emitter, event, handler)

Similar to calling emitter.on(event, handler).

listenToOnce(emitter, event, handler)

Similar to calling emitter.once(event, handler).

stopListening([emitter,] [event,] [handler]);

Stop listening to some or all events that were registered with calls to listenTo() or listenToOnce().

If all three arguments are passed, this is similar to calling emitter.removeListener(handler).

If no arguments are passed, then this will remove all listeners that have been registered on this listener. If an emitter is passed, this will remove all listeners that have been registered on the specific emitter. If emitter and event are passed, then this will remove all listeners from the specific emitter that were registered for the specific event.