Edit in JSFiddle

var Sith = function(name) {
	this.name = name;
	this.getName = function() {
	 return "I am " + this.name;
	};
};

var vader = new Sith("Darth Vader");
console.log(vader.getName());

var maul = new Sith("Darth Maul");
console.log(maul.getName());

maul.getName = function(greet) {
	return greet + " from " + this.name;
}

console.log(maul.getName("Hello"));

delete(maul.getName);

Sith.prototype.getName = function() {
	return "I am " + this.name;
};

console.log(maul.getName());
console.log(vader.getName());