gistfile1.js

function Animal(name, age){
  var animal = this
  // animal is the object
  animal.name = name
  animal.age = age
  return animal
}

Animal.prototype.addYears = addYears
function addYears(years){
  var animal = this
  animal.age += years
}

function Dog(name, age){
  return Animal.apply(this, arguments) // $super 
}

Dog.prototype = Object.create(Animal.prototype) // inherits
Dog.prototype.constructor = Dog // prevent it from being Animal

Dog.prototype.sayHello = sayHello
function sayHello(){
  return "woof, I'm " + this.name
}

// animals won't have .sayHello, unless they're dogs

var zebra = new Animal("foo", 3)
  , dog = new Dog("bar", 2)
  
typeof dog.sayHello // "function"
typeof zebra.sayHello // "undefined"

dog.sayHello() // "woof, I'm bar"

/* prototype is shared */
Dog.prototype.sayHello = null
dog.sayHello // null

/* owned properties aren't */
dog.sayHello = sayHello
typeof dog.sayHello // "function"

var otherDog = new Dog("baz", 2)
otherDog.sayHello // null

Object.prototype.hasOwnProperty.call(otherDog, "sayHello") // false (only in [[Prototype]])
Object.prototype.hasOwnProperty.call(dog, "sayHello") // true