JavaScript Class模拟基础

Simulating Classes in JavaScript

目前，用js来模拟class是一件非常麻烦的事情，多少有点不优雅。
但是未来的JS肯定会支持class，那么这个语言就是基于class和Prototype的混合继承？到时候会不会越来越混乱呢？笑……


Instance Properties
实例属性

引用

By default, any object property in JavaScript is an instance property.

对象的属性默认状态就是“实例属性”

Instance Methods
实例方法

引用

In JavaScript, an instance method for a class is defined by setting a property in the constructor's prototype object to a function value.

In Java and C++, the scope of instance methods includes the this object.

return width * height;
In JavaScript, however, you've seen that you must explicitly specify the this keyword for these properties:

return this.width * this.height;

If you find it awkward to have to prefix each instance field with this, you can use the with statement (covered in Section 6.18.) in each of your methods.

Rectangle.prototype.area = function( ) {
    with(this) {
        return width*height;
    }
}

在对象的Prototype对象中顶一个包含方法的属性就可以实现“对象的实例方法”

再诸如Java、C++之类的语言中，实例方法的作用域中包含当前对象，而JavaScript不包括。
所以你要在实例方法中引用实例属性的话就要时刻加上this：

return this.width * this.height;

Class Properties
类属性，也就是在JAVA中由static修饰的变量，是通过定义构造函数对象的内置属性来实现的。

Rectangle.UNIT = new Rectangle(1,1);

Class Methods
类方法，与类属性的实现方式一致

Private Members
私有成员

引用

JavaScript can simulate this using closures (an advanced topic, covered in Section 8.8.), but to do so, the accessor methods must be stored on each object instance; they cannot be inherited from the prototype object.

实现私有变量就是重头戏了，要用到闭包，而且必须和JAVA中一样，通过getter和setter访问他们。在JS中getter和setter必须保存到每个对象的实例中，是不能继承的（否则属性值不就乱了么）。

function ImmutableRectangle(w, h) {
    // This constructor does not store the width and height properties
    // in the object it initializes.  Instead, it simply defines
    // accessor methods in the object.  These methods are closures and
    // the width and height values are captured in their scope chains.
    this.getWidth = function( ) { return w; }
    this.getHeight = function( ) { return h; }
}

// Note that the class can have regular methods in the prototype object.
ImmutableRectangle.prototype.area = function( ) {
    return this.getWidth( ) * this.getHeight( );
};

上例中，getter是在构造函数中定义的，所以确保了它的唯一性。闭包使外界无法通过任何手段来访问原来的定义的“私有变量”。例子中，构造函数的引用已经消失，垃圾回收会回收它，而定义的嵌套函数getWidth和getHeight却保留对原构造函数的引用（意味着这些返回的函数的作用链域中包含原构造函数的活动对象，自然可以访问到其参数值,w和h）

而且如果你只定义了getter，和java中一样，那么这个私有变量是只读的了，所以是ImmutableRectangle。