Spy.js

//Listener class executes a logging function each time a method is called.
var Spy = new Class({
    __spies: {},
    spyOn: function(mth, fn) {
        this.__spies[mth] = this[mth];
        this[mth] = function() {
            var rtrn = this.__spies[mth].attempt(arguments);
            fn(rtrn, arguments);
            return rtrn;
        };
    },
    spyOff: function(mth) {
        this[mth] = this.__spies[mth];
    }
});
 
//Ridiculous example use...
 
var Adder = new Class({
    addThings: function(var1, var2) { return var1+var2; }
});
 
Adder.implement(Spy);
 
var adder = new Adder();
adder.spyOn('addThings', function(rtrn, args) { 
    console.log("addThings returned "+rtrn);
    console.log(args);
});

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

adder.spyOff('addThings');

adder.addThings(2, 3); //Returns 5