js面试题--js的继承

  js是门灵活的语言，实现一种功能往往有多种做法,ECMAScript没有明白的继承机制。而是通过模仿实现的。依据js语言的本身的特性,js实现继承有下面通用的几种方式
1.使用对象冒充实现继承(该种实现方式能够实现多继承)
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thiskeyword给全部的属性和方法赋值

function Parent(firstname)
{
    this.fname=firstname;
    this.age=40;
    this.sayAge=function()
    {
        console.log(this.age);
    }
}
function Child(firstname)
{
    this.parent=Parent;
    this.parent(firstname);
    delete this.parent;
    this.saySomeThing=function()
    {
        console.log(this.fname);
        this.sayAge();
    }
}
var mychild=new  Child("李");
mychild.saySomeThing();

 

2.採用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，则採用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详细对象

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");
Parent.call(child,child.getName());
child.saySomeThing();

 

3.採用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。则採用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详细对象

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");
Parent.apply(child,[child.getName()]);
child.saySomeThing();

 

4.採用原型链的方式实现继承
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");
child.saySomeThing();

 

5.採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");
child.saySomeThing();
child.sayParent();