javascript中原型继承（prototype inherit）的理解

1.在javascript对象中，都有一个prototype属性，该属性指向一个prototype对象，在该prototype对象中，又有一个constructor属性，该属性指向这个对象本身，可以大致用一下
过程语言描述：
object={
        prototype:{
                constructor:object       
                   }
}
prototype所指的这个对象即为原型对象；
既然constructor所指的是该对象本身，那么说明在用构造函数创建对象如：new cat();
其实也就是用原型对象的构造函数创建对象，也就是说cat.prototype.constructor==cat
当然也可以重写这个原型对象，在重写的原型对象中不添加constructor属性，那么这个条
件就不成立了。
2.javascript是就是基于对象中的原型对象构造原型对象链(prototype chain)来实现继承的。
3.实例说明：

			//Animal构造函数
			function Animal(name) {
				this.name = name;
			}

			//重写Animal的prototype对象
			Animal.prototype = {
				sex : "famale",
				say : function() {
					alert("animal");
				}
			}
			function people(name) {
				this.name = name;
			}


			people.prototype = new Animal("animal");
			people.prototype.say = function() {
				alert("peopel")
			};
			//一定要创建新的对象，不要直接：people.say()或people.sex;因为people还没有实例化
			var zhangsan = new people("zhangsan");
			alert(zhangsan.sex);//famale
			zhangsan.say();//people
			
			//没有重写prototype，此时默认的是继承object的prototype object
			function cat() {

			}
			
			alert( function() {} instanceof Object);//true
			alert( typeof Animal.prototype);//object
			alert( typeof Animal.constructor);//function
			alert(Animal.prototype.constructor == Animal);//false
			alert(cat.prototype.constructor == cat);//true

在上例中，演示了people集成animal的属性过程，从下面的alert语句的结果中，可以验证 用

object={
        prototype:{
                constructor:this        
                   }
}

描述的正确性，这只是大致内部结构的描述，有力于理解原型继承。

举一反三，那么可以继续构造其他种类对象，使用原型对象指向people，从而形成继承链。

比如people的sex属性，程序先查找people自身的sex属性，如果没有，那么就去查找prototype所指向的对象的sex，如果有，正好，如果没有，那么就继续查找该对象的prototype所指向的对象，顺着链一直找到object为止，如果还没有返回undefine

以下是Javascript:The Definitive Guide中关于Prototype的原文，以供参考：

Every JavaScript object has a second JavaScript object (or null,but this is rare) associated with it.

This second object is known as a prototype, and thefirst object inherits properties from the prototype.
          All objects created by object literals have the same prototype object, and we can refer
to this prototype object in JavaScript code as Object.prototype. Objects created using
the new keyword and a constructor invocation use the value of the prototype property
of the constructor function as their prototype. So the object created by new Object()
inherits from Object.prototype just as the object created by {} does. Similarly, the
object created by new Array() uses Array.prototype as its prototype, and the object
created by new Date() uses Date.prototype as its prototype.
          Object.prototype is one of the rare objects that has no prototype: it does not inherit
any properties. Other prototype objects are normal objects that do have a prototype.
All of the built-in constructors (and most user-defined constructors) have a prototype
that inherits from Object.prototype. For example, Date.prototype inherits properties
from Object.prototype, so a Date object created by new Date() inherits properties from
both Date.prototype and Object.prototype. This linked series of prototype objects is
known as a prototype chain.