Basic JavaScript PubSub implementation.

PubSub.js

function PubSub() {
	this.events = [];
}

PubSub.prototype.on = function(namespace, fn) {
	this.events[namespace] = this.events[namespace] || [];
	this.events[namespace].push(fn);
}

PubSub.prototype.off = function(namespace, fn) {
	if(this.events.hasOwnProperty(namespace)) {
		for(var i=0;i<this.events[namespace].length;i++) {
			if(''+this.events[namespace][i] === ''+fn) {
				this.events[namespace].splice(i, 1);
				break;
			}
		}
	}
}

PubSub.prototype.emit = function(namespace, args)  {
	if(this.events.hasOwnProperty(namespace)) {
		this.events[namespace].forEach(function(fn) {
			fn(args);
		});
	}
}

Basic PubSub implementation
This implementation converts functions into strings for comparison. The reason for this decision is that otherwise you would need to store a function in a variable for it to be evaluated to true when comparison occurs in the off method (explained in greater detail here). I've also chosen to use prototypes instead of defining methods in the constructor because performance is better with the former as well as preventing inheritance issues (both explained in greater detail here).

Usage

var pubsub = new PubSub();

pubsub.on('test', data => {
    console.log(data);
});

pubsub.emit('test', 'Hello world!');// outputs 'Hello world!'.

/////////////////////////////////////////////////////////////////////////////////////

pubsub.off('test', data => {
    console.log(data);
});

pubsub.emit('test', 'Hello world!');// event was removed, so nothing happens.