Observable AOP

observable.js

function observable(fn, topic) {
  function observed() {
		var result; 
		PubSub.publish(topic + "/before", {obj: this, args: arguments});
		result = fn.apply(this, arguments);
		PubSub.publish(topic + "/after", {obj: this, args: arguments, result: result});
		return result;
	};
	observed.before = function (fn) {
		PubSub.subscribe(topic + "/before", fn);
		return this;
	};
	observed.after = function (fn) {
		PubSub.subscribe(topic + "/after", fn);
		return this;
	};
	return observed;
}

var obj = {
    add: function () {
        this.num += Array.prototype.reduce.call(arguments, function (a, b) {
            return a + b;
        });
        return this.num;
    },
    num: 1
};
obj.add = observable(obj.add, "obj/add");
obj.add.before(function (data, topic) {
    console.log("adding " + Array.apply(null, data.args) + " to num.");
    this.start = this.num;
})
.after(function (data, topic) {
    console.log("num is now " + data.result);
    this.end = this.num;
});
obj.add(1,2,3);