Hacking-toString-and-valueOf.js

(function() { // Make numbers look like strings
    var oldToString = Number.prototype.toString;
    Number.prototype.toString = function() {
        if(this == 4) { return "four"; }
        else { return oldToString.call(this); }
    };
})();

var four = new Number(4);
alert(1 + four); // shows 5
alert(four); // shows "four"

(function() { // Make strings look like numbers
    var oldValueOf = String.prototype.valueOf;
    String.prototype.valueOf = function() {
        if(this.toLowerCase() == "five") { return 5; }
        else { return oldValueOf.call(this); }
    };
})();

var five = new String("five");
alert(1 + five); // shows 6
alert(five); // shows "five"