继承性—javascript面向对象高级

## 继承性 ##
  js不是一门完全是面向对象的语言，它没有专门实现对象继承的机制，但是可以通过语法层面实现对象的继承，要实现继承可以通过以下几种方式实现继承。
  (这里也可以通过ES6中的class语法实现继承，更接近传统语言如java，请见我ES6新特性)

2.通过constructor实现传值。

3.类Class的概念：构造函数

  通过构造函数产生的对象叫做实例。具备构造函数的所有属性
  将公用的方法和属性，提取出来存到原型上


继承的几种方式
 1.通过原型实现继承
      function Father(){
        this.name = 'zhangsan';
        this.age = 12;
        this.gender = 'mail';
        this.sayHello = function(){
          console.log('hello,i m '+this.name +',i m '+this.age+'years old')
         }
       }
         function Child(){};
         
         Child.prototype = new Father();
         var c1 = new Child();
         c1.sayHello()


 2.传参数的原型式继承

    function Father(name,age,gender){
        this.name = name;
        this.age = age;
        this.gender = gender;
     }
    //定义父对象
     Father.prototype.sayHello = function(){
       console.log('hello,i m '+this.name +',i m '+this.age+'years old')
     }

     function Child(name,age,gender){
        this.constructor(name,age,gender)
     // 将参数传给constructor
     };
     
     Child.prototype = new Father();
     var c1 = new Child('zhangsan',40,'femail');
     c1.sayHello()

    **存在的问题：该方法是采用的原型覆盖的方法，会继承父元素的所有属性，但是如果父元素不具备某些子元素特有的原型属性的话。就会丢失该属性。**

 3.对象冒充
   将某一个构造函数当做普通函数调用，执行后会生成指向当前对象
 
     function Father(name,age,gender){
        this.name = name;
        this.age = age;
        this.gender = gender;
     }

     Father.prototype.sayHello = function(){
       console.log('hello,i m '+this.name +',i m '+this.age+'years old')
     }

     function Child(name,age,gender){
        this.newObj = Father;//这句话是重点
        this.newObj(name,age,gender)
        //这里将Father函数当成一个普通函数，执行后会产生一个对象，着就是对象冒充
        delete this.newObj
     };

     var c1 = new Child('lis',40,'femail');
     console.log(c1.name)

   **问题：该方式不能继承Father原型上的属性。 **

 4.通过call和apply手动改变this的指向，原理：(对象冒充)

    function Father(name,age,gender){
        this.name = name;
        this.age = age;
        this.gender = gender;
     }

     Father.prototype.sayHello = function(){
       console.log('hello,i m '+this.name +',i m '+this.age+'years old')
     }

     function Child(name,age,gender){
        Father.call(this,name,age,gender)
        //这句话是关键，将Father里的所有this都指向Child
     };

     var c1 = new Child('zhangsan',40,'femail');
     c1.sayHello()

  ** 问题：因为实质是对象冒充，所以也不能继承Father的原型**
   
 5.组合式继承(混合)

     function Father(name,age,gender){
        this.name = name;
        this.age = age;
        this.gender = gender;
     }

     Father.prototype.sayHello = function(){
       console.log('hello,i m '+this.name +',i m '+this.age+'years old')
     }

     function Child(name,age,gender){
        Father.call(this,name,age,gender)
     };
       //这里不仅用对象冒充用了Father这个构造函数，
       //还继承了Father的prototype

      Child.prototype = Father.prototype;
     var c1 = new Child('lis',40,'femail');
     console.log(c1)
     c1.sayHello()