我們先來(lái)一個(gè)簡(jiǎn)單的構(gòu)造函數(shù)+原型對(duì)象的小程序

function CreateObj( uName, uAge ) {      this.userName = uName;      this.userAge = uAge;    }    CreateObj.prototype.showUserName = function () {      return this.userName;    }    CreateObj.prototype.showUserAge = function () {      return this.userAge;    }

這個(gè)程序，沒有什么問(wèn)題，但是非常的冗余，每次擴(kuò)展一個(gè)方法，都要寫一次原型對(duì)象，我們可以把原型對(duì)象prototype當(dāng)做一個(gè)字面量對(duì)象，所有的方法都在字面量對(duì)象中擴(kuò)展，可以達(dá)到同樣的效果：

CreateObj.prototype = {      showUserAge : function(){        return this.userAge;      },      showUserName : function(){        return this.userName;      },    }    var obj1 = new CreateObj( 'ghostwu', 22 );    var obj2 = new CreateObj( '衛(wèi)莊', 24 );    console.log( obj1.showUserName(), obj1.showUserAge() ); //ghostwu 22    console.log( obj2.showUserName(), obj2.showUserAge() ); //衛(wèi)莊 24

但是這種原型(prototype)對(duì)象的寫法，屬于重寫了CreateObj的默認(rèn)原型對(duì)象，造成的第一個(gè)問(wèn)題就是constructor不再指向CreateObj.

沒有重寫之前，constructor指向CreateObj

function CreateObj( uName, uAge ) {      this.userName = uName;      this.userAge = uAge;    }    CreateObj.prototype.showUserName = function () {      return this.userName;    }    CreateObj.prototype.showUserAge = function () {      return this.userAge;    }    console.log( CreateObj.prototype.constructor === CreateObj ); //true

重寫之后,constructor指向Object

CreateObj.prototype = {      showUserAge : function(){        return this.userAge;      },      showUserName : function(){        return this.userName;      },    }    console.log( CreateObj.prototype.constructor === CreateObj ); //false    console.log( CreateObj.prototype.constructor === Object ); //true

所以說(shuō)，constructor不能準(zhǔn)確的標(biāo)識(shí)對(duì)象，因?yàn)樗麜?huì)被修改

我們之前寫的程序，基本都是在原型對(duì)象(prototype)上擴(kuò)展完了方法之后，再實(shí)例化對(duì)象，我們看下，先實(shí)例化對(duì)象，再在原型對(duì)象(prototype)上擴(kuò)展函數(shù)，

實(shí)例對(duì)象是否能正常的調(diào)用到擴(kuò)展的方法？

function CreateObj( uName, uAge ) {      this.userName = uName;      this.userAge = uAge;    }    var obj1 = new CreateObj( 'ghostwu', 22 );    CreateObj.prototype.showUserName = function(){      return this.userName;    }    console.log( obj1.showUserName() ); //ghostwu

可以正常調(diào)用，但是，如果原型對(duì)象是重寫的，就調(diào)用不到了

function CreateObj( uName, uAge ) {      this.userName = uName;      this.userAge = uAge;    }    var obj1 = new CreateObj( 'ghostwu', 22 );    CreateObj.prototype = {      showUserName : function(){        return this.userName;      }    }    console.log( obj1.showUserName() ); //報(bào)錯(cuò)