对象创建和继承
对象创建指的是自定义对象构造函数，用于批量创建拥有共同属性方法的对象。例如创建一个人对象构造函数，然后实例化出小明，小红
继承指的是两个引用类型对象间属性和方法的继承，原理是利用prototype，因为实例内部保留了对构造函数prototype的指针，prototype保留对该构造函数的指针。所以如果把构造函数的prototype指向另一个对象类型实例的话，那么也就继承了该实例对构造函数prototype的指针以及它对构造函数的指针。
1、最古老的工厂模式，就是在里面新建对象，然后配置属性，返回这个对象，问题是无法判断对象类型（用不了instanceof）：
function createPerson(name, age, job){
var o = new Object();
o.name = name;
o.age = age;
o.job = job;
o.sayName = function(){
alert(this.name);
};
return o;
}
var person1 = createPerson(“Nicholas”, 29, “Software Engineer”);
var person2 = createPerson(“Greg”, 27, “Doctor”);
2、构造函数模式，可以用instanceof来判断了，问题是每个实例都会创建相同的方法，没用公用：
function Person(name, age, job){
this.name = name;
this.age = age;
this.job = job;
this.sayName = function(){
alert(this.name);
};
}
var person1 = new Person(“Nicholas”, 29, “Software Engineer”);
var person2 = new Person(“Greg”, 27, “Doctor”);
当函数被一个 new 调用时，函数的运行方式就改变了， 把 new 看作是一个方法的话，类似下面：
Function.method(‘new‘, function () {
var that = Object.create(this.prototype); // Create a new object that inherits from the constructor‘s prototype.
var other = this.apply(that, arguments); // Invoke the constructor, binding –this- to the new object.
return (typeof other === ‘object‘ && other) || that; // If its return value isn‘t an object, substitute the new object.
});
忘记 new 的话，this 指向的是 全局变量，要避免这个问题，就加多一层检测（scope-safe）：
function Car(sColor) {
if(this instanceof Car ) {
this.color = sColor;
}else {
return new Car(sColor);
}
}
这样做的问题是关闭了可以调用构造函数的上下文：
function Polygon(sides){
if (this instanceof Polygon) {
this.sides = sides;
this.getArea = function(){
return 0;
};
} else {
return new Polygon(sides);
}
}
function Rectangle(width, height){
Polygon.call(this, 2); //这里的调用Polygon返回一个新的Polygon实例，因此this没有被扩张
this.width = width;
this.height = height;
this.getArea = function(){
return this.width * this.height;
};
}
Rectangle.prototype = new Polygon(); //如果没有这句就会得不到2，而是undefined
var rect = new Rectangle(5, 10);
alert(rect.sides); //2
3、prototype 模式，问题是所有属性都公用了，而且还不能传递初始化的值：
function Person(){}
Person.prototype.name = “Nicholas”;
Person.prototype.age = 29;
Person.prototype.job = “Software Engineer”;
Person.prototype.sayName = function(){
alert(this.name);
};
var person1 = new Person();
person1.sayName(); //”Nicholas”
var person2 = new Person();
person2.sayName(); //”Nicholas”
alert(person1.sayName == person2.sayName); //true
4、混合构造函数模式和prototype模式，取各自优点：
function Person(name, age, job){
this.name = name;
this.age = age;
this.job = job;
this.friends = [“Shelby”, “Court”];
}
Person.prototype = {
constructor: Person,
sayName : function () {
alert(this.name);
}
};
5、Dynamic Prototype Pattern：
function Person(name, age, job){
this.name = name;
this.age = age;
this.job = job;
if (typeof this.sayName != “function”) {
Person.prototype.sayName = function() {
alert(this.name);
};
}
}
var friend = new Person(“Nicholas”, 29, “Software Engineer”);
6、Parasitic Constructor Pattern：
function SpecialArray(){
var values = new Array();
values.push.apply(values, arguments);
values.toPipedString = function(){
return this.join(“|”);
};
return values;
}
var colors = new SpecialArray(“red”, “blue”, “green”);
alert(colors.toPipedString()); //”red|blue|green”
7、Durable Constructor Pattern，这种模式实例和构造函数原型没关系，无法使用instanceof：
function Person(name, age, job){
//create the object to return
var o = new Object();
//optional: define private variables/functions here
//attach methods
o.sayName = function(){
alert(name);
};
//return the object
return o;
}
var friend = Person(“Nicholas”, 29, “Software Engineer”);
friend.sayName(); //”Nicholas”