js 继承

继承(5种方法)

1.使用对象冒充实现继承(该种实现方式可以实现多继承)

实现原理:让父类的构造函数成为子类的方法,然后调用该子类的方法,通过this关键字给所有的属性和方法赋值

    function Parent(firstname)  
    {  
        this.fname=firstname;  
        this.age=40;  
        this.sayAge=function()  
        {  
            console.log(this.age);  
        }  
    }  
    function Child(firstname)  
    {  
        //以下三行就是实现冒充的关键
        this.parent=Parent;  
        this.parent(firstname);  
        delete this.parent;  
        this.saySomeThing=function()  
        {  
            console.log(this.fname);  
            this.sayAge();  
        }  
    }  
    var mychild=new  Child("李四");  
    mychild.saySomeThing();  

2.采用call方法改变函数上下文实现继承(该种方式不能继承原型链,若想继承原型链，则采用5混合模式)

实现原理:改变函数内部的函数上下文this,使它指向传入函数的具体对象

    function Parent(firstname)  
    {  
        this.fname=firstname;  
        this.age=40;  
        this.sayAge=function()  
        {  
            console.log(this.age);  
        }  
    }  
    function Child(firstname)  
    {  
      
        this.saySomeThing=function()  
        {  
            console.log(this.fname);  
            this.sayAge();  
        }  
       this.getName=function()  
       {  
           return firstname;  
       }  
      
    }  
    var child=new Child("张三");  
    Parent.call(child,child.getName());  //call(被继承的对象,参数);
    child.saySomeThing(); 

3.采用Apply方法改变函数上下文实现继承(该种方式不能继承原型链,若想继承原型链，则采用5混合模式)

实现原理:改变函数内部的函数上下文this,使它指向传入函数的具体对象

  function Parent(firstname)  
    {  
        this.fname=firstname;  
        this.age=40;  
        this.sayAge=function()  
        {  
            console.log(this.age);  
        }  
    }  
    function Child(firstname)  
    {  
      
        this.saySomeThing=function()  
        {  
            console.log(this.fname);  
            this.sayAge();  
        }  
        this.getName=function()  
        {  
            return firstname;  
        }  
      
    }  
    var child=new Child("张三");  
    Parent.apply(child,[child.getName()]);  //apply(被继承对象,[参数])**传入参数以数组形式传入;
    child.saySomeThing();  

4.采用原型链的方式实现继承

原型链适合无参数继承

实现原理:使子类原型对象指向父类的实例以实现继承,即重写类的原型,弊端是不能直接实现多继承

 function Parent()  
    {  
      
        this.sayAge=function()  
        {  
            console.log(this.age);  
        }  
    }  
    function Child(firstname)  
    {  
        this.fname=firstname;  
        this.age=40;  
        this.saySomeThing=function()  
        {  
            console.log(this.fname);  
            this.sayAge();  
        }  
    }  
      
    Child.prototype=new  Parent();  
    var child=new Child("张三");  
    child.saySomeThing();  

5.采用混合模式实现继承

    function Parent()  
    {  
      
        this.sayAge=function()  
        {  
            console.log(this.age);  
        }  
    }  
      
    Parent.prototype.sayParent=function()  
    {  
       alert("this is parentmethod!!!");  
    }  
      
    function Child(firstname)  
    {  
        Parent.call(this);  
        this.fname=firstname;  
        this.age=40;  
        this.saySomeThing=function()  
        {  
            console.log(this.fname);  
            this.sayAge();  
        }  
    }  
      
    Child.prototype=new  Parent();  
    var child=new Child("张三");  
    child.saySomeThing();  
    child.sayParent();