[divinus]

Number.prototype.factorial = function() {
	if (isNaN(this) == false && this>=0 && this-Math.round(this) == 0) {
		trace("");
		var factor = 1;
		for (var i = 1; i<=this; i++) {
			factor *= i;
		}
		return factor;
	} else {
		return Nan;
	}
};
var a = 3;
trace(a.factorial());

попробуй так