// the original Animal class and sayName method
function Animal(name, numLegs) {
    this.name = name;
    this.numLegs = numLegs;
}
Animal.prototype.sayName = function() {
    console.log("Hi my name is " + this.name);
};

// define a Penguin class
function Penguin() {
    this.name = "penguin";
    this.numLegs = 2;
}

// set its prototype to be a new instance of Animal
var penguin = new Animal("Penguin", 2);

penguin.sayName();