原型继承的几种方式

原型继承

父级：

	function Parent(name){
		this.name=name;
	}
	Parent.prototype.sayHello=function(){
		console.log("Hello,"+this.name);
	}

1. 原型链继承

    function Kid(){};
    Kid.prototype=new Parent("who");
   
    var k=new Kid();
    console.log(k.name); //who
    console.log(k.sayHello()); //Hello,who
   

   弊端：创建实例时无法向父级传参

2. 构造继承

    function Kid(name){
    	Parent.call(this,name);
    };
   
    var k=new Kid("who");
    console.log(k.name); //who
    console.log(k.sayHello()); //error
   

   弊端：无法获取父级原型链属性

3. 实例继承

    function Kid(name){
    	var p=new Parent(name);
    	return p;
    };
   
    var k=new Kid("who");
    console.log(k.name); //who
    console.log(k.sayHello()); //Hello,who
   

   弊端：实例是父级的实例

4. 拷贝继承

    function Kid(name){
    	var p=new Parent(name);
    	for(var item in p){
    		Kid.prototype[item]=p[item];
    	}
    }
   
    var k=new Kid("who");
    console.log(k.name); //who
    console.log(k.sayHello()); //Hello,who
   

   弊端：太占用内存

5. 组合继承

    function Kid(name){
    	Parent.call(this,name);
    }
    Kid.prototype=new Parent();
   
    var k=new Kid("who");
    console.log(k.name); //who
    console.log(k.sayHello()); //Hello,who
   

   弊端：调用了两次父类构造函数

6. 寄生组合继承

    function Kid(name){
    	Parent.call(this,name);
    }
    (function(){
    	var p=function(){};
    	p.prototype=Parent.prototype;
    	Kid.prototype=new p();
    })()
   

   弊端：写法比较繁琐