Listener.js

//Listener class executes a logging function each time a method is called.
var Listener = new Class({
    listenStack: {},
    addListener: function(mth, fn) {
        this.listenStack[mth] = this[mth];
        this[mth] = function() {
            var rtrn = this.listenStack[mth].attempt(arguments);
            fn(rtrn, arguments);
            return rtrn;
        };
    }
});

//Ridiculous example use...

var Adder = new Class({
    addThings: function(var1, var2) { return var1+var2; }
});

Adder.implement(Listener);

var adder = new Adder();
adder.addListener('addThings', function(rtrn, args) { 
    console.log("addThings returned "+rtrn);
    console.log(args);
});

adder.addThings(2, 3);
//Logs: addThings returned 5, [2, 3]