function createAnimal(name, age) {
  var o = new Object();
  o.name = name;
  o.age = age;
  o.sayName = function() {
    alert(this.name);
  }
  return o;
}
var cat = createAnimal("cat", 12);
var dog = createAnimal("dog", 3);

function Animal(name, age) {
  this.name = name;
  this.age = age;
  this.sayName = function() {
    alert(this.name);
  }
}
var cat = new Animal("cat", 12);
var dog = new Animal("dog", 3);

cat.constructor == Animal // true
cat instanceof Animal // true

function Animal() {}
Animal.prototype.name = "animal";
Animal.prototype.age = 1;
Animal.prototype.sayName = function() {
  alert(this.name);
}
var test1 = new Animal();
test1.sayName(); // "animal"
var test2 = new Animal();
test2.sayName(); // "animal"
alert(test1.sayName === test2.sayName); // true

function Animal() {}
Animal.prototype = {
  constructor: Animal,
  name: "animal",
  age: 1,
  sayName: function() {
    alert(this.name);
  }
};

function Animal() {}
Animal.prototype = {
  constructor: Animal,
  name: "animal",
  age: 1,
  hobbies: ["dance", "sing", "play"],
  sayName: function() {
    alert(this.name);
  }
};
var cat = new Animal();
var dog = new Animal();
cat.hobbies.push("sleep");
alert(cat.hobbies); // "dance", "sing", "play", "sleep"
alert(dog.hobbies); // "dance", "sing", "play", "sleep"
alert(cat.hobbies === dog.hobbies); // true

function Animal(name, age) {
  this.name = "animal";
  this.age = 1;
  this.hobbies = ["dance", "sing", "play"];
}
Animal.prototype = {
  constructor: Animal,
  sayName: function() {
    alert(this.name);
  }
};
var cat = new Animal("cat", 2);
var dog = new Animal("dog", 3);
cat.hobbies.push("sleep");
alert(cat.hobbies); // "dance", "sing", "play", "sleep"
alert(dog.hobbies); // "dance", "sing", "play"
alert(cat.hobbies === dog.hobbies); // false
alert(cat.sayName === dog.sayName); // true

function Animal(name, age) {
  this.name = name;
  this.age = age;
  if(typeof this.sayName != "function") {
    Animal.prototype.sayName = function() {
      alert(this.name);
    }
  }
}
var cat = new Animal("cat", 12);
cat.sayName(); // "cat"

function Animal(name, age) {
  var o = new Object();
  o.name = name;
  o.age = age;
  o.sayName = function() {
    alert(this.name);
  }
  return o;
}
var cat = new Animal("cat", 12);
cat.sayName(); // "cat"

function Animal(name, age) {
  var o = new Object();
  o.name = name;
  o.age = age;
  var msg = "Hello, I'm ";
  o.sayName = function() {
    alert(msg + this.name);
  }
  return o;
}
var cat = new Animal("cat", 12);
cat.sayName(); // "Hello, I'm cat"