Grab Notifier

notifier.js

class Notifier 
{
	constructor() {
		this.events = [];
	}

	on(eventName, callback) {
		var self = this;

		this.events.push(callback);
		return {
			off: function() {
				if (self.events) {
					let index = self.events.indexOf(callback);
					if (index > -1) {
						self.events.splice(index, 1);
					}
				}
			}
		};
	}

	trigger(eventName, ...parameters) {
		if (this.events) {
			this.events.forEach((event) => {
				event(...parameters);
			});
		}
	}

}
/*
Write a Notifier class that supports the following operations:

1. Constructor.
*/
var notifier = new Notifier();
/*
2. Listening to events.
*/
var listenerJohn = notifier.on('MY_EVENT', function (action, item) {
  console.log(`John is ${action} ${item}`);
});
var listenerJane = notifier.on('MY_EVENT', function (action, item) {
  console.log(`Jane is ${action} ${item}`);
});
/*
3. Triggering of events.

This particular example should lead to both callbacks
above being invoked with 'eating' and 'a burger' as parameters:
*/

console.log('-------');

notifier.trigger('MY_EVENT', 'eating', 'a burger');
// Should see the following in the console:
// John is eating a burger
// Jane is eating a burger
/*
4. Unsubscribing a listener from existing events.
Note that `off` is not a method no `Notifier`.

`listenerJohn` is the reference returned by `on` above.
*/

listenerJohn.off();
notifier.trigger('MY_EVENT', 'eating', 'a burger');

// Only Jane's callback is invoked.
// Jane is eating a burger