oo.js

function Animal (name, numLegs) {
  this.name = name;
  this.numLegs = numLegs;
}

Animal.prototype.greet = function () {
  console.log("Hi! My name is " + this.name);
}

function Horse (name) {
  this.name = name;
  this.numLegs = 4;
}

Horse.prototype = Animal.prototype;

var tom = new Animal('tom', 8);

tom.greet(); // Hi! My name is tom

var nelly = new Horse('nelly');

nelly.greet(); // Hi! My name is nelly