[js高手之路]原型对象(prototype)与原型链相关属性与方法详解

一,instanceof:

instanceof检测左侧的__proto__原型链上,是否存在右侧的prototype原型. 我在之前的两篇文章

[js高手之路]构造函数的基本特性与优缺点

[js高手之路]一步步图解javascript的原型(prototype)对象,原型链

已经分享过了.

 1         function CreateObj(uName) {
 2             this.userName = uName;
 3             this.showUserName = function () {
 4                 return '100';
 5             }
 6         }
 7         CreateObj.prototype.showUserName = function () {
 8             return this.userName;
 9         }
10         var obj1 = new CreateObj('ghostwu');
11         var obj2 = new CreateObj('卫庄');
12 
13         console.log( obj1 instanceof CreateObj ); //true
14         console.log( obj2 instanceof CreateObj ); //true
15         console.log( obj1 instanceof Object ); //true
16         console.log( obj2 instanceof Object ); //true

二、isPrototypeOf:

如果隐式原型__proto__指向调用isPrototypeOf()方法的对象原型( CreateObj ), 那么这个方法就返回true,如:

1         var obj1 = new CreateObj('ghostwu');
2         var obj2 = new CreateObj('卫庄');
3         console.log( CreateObj.prototype.isPrototypeOf( obj1 ) ); //true
4         console.log( CreateObj.prototype.isPrototypeOf( obj2 ) ); //true 

因为obj1,obj2的隐式原型__proto__指向的都是CreateObj.prototype, 有朋友可能会问CreateObj.prototype上面根本就没有isPrototypeOf这个方法,怎么可以

调用呢?

是的,没错,但是CreateObj.prototype的隐式原型__proto__指向了Object.prototype, 而isPrototypeOf存在Object.prototype上,所以就能够调用

三、Object.getPrototypeOf

获取实例的隐式原型(__proto__)的指向,因为obj1,obj2的__proto__都指向CreateObj.prototype

1         var obj1 = new CreateObj('ghostwu');
2         var obj2 = new CreateObj('卫庄');
3         console.log( Object.getPrototypeOf( obj1 ) === CreateObj.prototype ); //true
4         console.log( Object.getPrototypeOf( obj2 ) === CreateObj.prototype ); //true

四,实例访问属性和方法时,遵循就近查找原则

实例先在自己身上查找,有,就停止查找,如果没有,就沿着实例的__proto__继续往上查找,有,就停止查找,如果没有就继续沿着原型链一直往上查找,如果

所有的原型对象上都没有,那就是undefined.

 1         function CreateObj(uName) {
 2             this.userName = uName;
 3         }
 4         CreateObj.prototype.showUserName = function () {
 5             return this.userName;
 6         }
 7         CreateObj.prototype.age = 22;
 8 
 9         var obj1 = new CreateObj('ghostwu');
10         obj1.age = 20;
11         var obj2 = new CreateObj('卫庄');
12 
13         console.log( obj1.age ); //20--->来自实例
14         console.log( obj2.age ); //22--->来自原型对象
15 
16         delete obj1.age;
17         console.log( obj1.age ); //22--->来自原型

五,hasOwnProperty

判断属性是实例上的还是原型对象上的,如果是实例上的,返回true, 原型上的返回false

 1         function CreateObj(uName) {
 2             this.userName = uName;
 3         }
 4         CreateObj.prototype.showUserName = function () {
 5             return this.userName;
 6         }
 7         CreateObj.prototype.age = 22;
 8         var obj1 = new CreateObj('ghostwu');
 9         obj1.age = 20;
10         var obj2 = new CreateObj('卫庄');
11         console.log( obj1.age ); //20--->来自实例
12         console.log( obj1.hasOwnProperty( 'age' ) ); //true
13         console.log( obj2.age ); //22--->来自原型对象
14         console.log( obj2.hasOwnProperty( 'age' ) ); //false
15         delete obj1.age;
16         console.log( obj1.age ); //22--->来自原型
17         console.log( obj1.hasOwnProperty( 'age' ) ); //false

六、in操作符

判断属性是否在实例或者原型对象上,只要一个满足条件,返回值都是true

 1         function CreateObj(uName) {
 2             this.userName = uName;
 3         }
 4         CreateObj.prototype.showUserName = function () {
 5             return this.userName;
 6         }
 7         CreateObj.prototype.age = 22;
 8         var obj1 = new CreateObj('ghostwu');
 9         obj1.age = 20;
10         console.log( 'age' in obj1 ); //true
11         var obj2 = new CreateObj('卫庄');
12         console.log( 'age' in obj2 ); //true
13         delete obj1.age;
14         console.log( 'age' in obj1 ); //true
15         console.log( 'user' in obj1 ); //false
16         console.log( 'user' in obj2 ); //false

七,结合in和hasOwnProperty的用法,可以封装一个函数判断这个属性是否在原型对象上, 返回值为true:在原型对象上, false:不在原型对象上

 1         function CreateObj(uName) {
 2             this.userName = uName;
 3         }
 4         CreateObj.prototype.showUserName = function () {
 5             return this.userName;
 6         }
 7         CreateObj.prototype.age = 20;
 8         function hasPrototypeProperty( obj, name ){
 9             return !obj.hasOwnProperty( name ) && ( name in obj );
10         }
11         var obj1 = new CreateObj('ghostwu');
12         var obj2 = new CreateObj('卫庄');
13         obj1.age = 10;
14         console.log( hasPrototypeProperty( obj1, 'age' ) ); //false
15         console.log( hasPrototypeProperty( obj2, 'age' ) ); //true

八、for...in 可以枚举实例和原型对象上的属性和方法,前提是:该属性和方法是可以枚举

 1          function CreateObj(uName) {
 2             this.userName = uName;
 3         }
 4         CreateObj.prototype.showUserName = function () {
 5             return this.userName;
 6         }
 7         CreateObj.prototype.age = 20;
 8         var obj = new CreateObj( 'ghostwu' );
 9 
10         for( var key in obj ){
11             console.log( key ); //userName,age,showUserName
12         }
13         console.log( Object.prototype );
14         for( var key in Object.prototype ){
15             console.log( key );//枚举不了, Object.prototype上的属性和方法默认不可枚举,枚举属性为false
16         }

 

posted @ 2017-08-26 16:01  ghostwu  阅读(1558)  评论(0编辑  收藏  举报
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