JS_javascript 기술의 터무니없는 valueOf 메소드 소개

彭老湿近期月报里提到了valueOf方法，兴致来了翻了下ECMA5里关于valueOf方法的介绍，如下：

15.2.4.4 Object.prototype.valueOf ( )
When the valueOf method is called, the following steps are taken:
1. Let O be the result of calling ToObject passing the this value as the argument.
2. If O is the result of calling the Object constructor with a host object (15.2.2.1), then
a. Return either O or another value such as the host object originally passed to the constructor. The specific result that is returned is implementation-defined.
3. Return O.
规范里面的对于valueOf的解释很短，大致为：调用ToObject方法（一个抽象方法，后面会讲到），并将this的值作为参数传入。

针对调用ToObject时传入的不同参数（this），返回值分别如下：

1、this为宿主对象时，返回值取决于浏览器的实现，即不同浏览器的返回可能不同（关于宿主对象，可参考http://www.w3school.com.cn/js/pro_js_object_types.asp）
2、this不是宿主对象，则返回ToObject(this)的值 

参数类型	返回结果
Undefined	抛出TypeError异常
Null	抛出TypeError异常
Number	创建一个Number对象，它内部的初始值为传入的参数值
String	创建一个String对象，它内部的初始值为传入的参数值
Boolean	创建一个Boolean对象，它内部的初始值为传入的参数值
Object	返回传入的参数（无转换）

根据Object.prototype.valueOf的定义，以及抽象方法ToObject的描述，可得下表 

obj类型	Object.prototype.valueOf.call(obj)返回结果
Undefined	抛出TypeError异常
Null	抛出TypeError异常
Number	Number类型的对象，值等于obj
String	String类型的对象，值等于obj
Boolean	Boolean类型的对象，值等于obj
Object	obj对象本身

举几个具体的例子：

复制代码 代码如下:

var num = 123;
console.log(num.valueOf()); //输出：123
console.log(num.valueOf()); //输出：'number'

var unde = undefined;
console.log(Object.prototype.valueOf.call(unde)); //输出：'TypeError: Cannot convert null to object'

var obj = {name:'casper'};
var linkObj = obj.valueOf();
linkObj.name = 'change';
console.log(linkObj.name); //输出：'change' ...说明obj.valueOf()返回的是对象自身

实际上，上面没有提到Array、Function对象，根据下面代码可以猜想，当Object.prototype.valueOf调用时，参数为Array、Function类型的对象时，返回的结果也为对象自身：

复制代码 代码如下:

var arr = [1, 2 ,3];
var linkArr = arr.valueOf();
linkArr[0] = ['casper'];
console.log(linkArr); //输出：['casper', 2, 3]

var foo = function(){ return 1; };
var linkFoo = foo.valueOf();
linkFoo.test = 'casper';
console.log(linkFoo.test); //输出：'casper'

看完上面的描述，是不是有种恍然大悟的感觉？如果是的话，恭喜你，可能你跟我一样其实还没完全理解透彻。

简单举个例子，当调用Object.prototype.valueOf的对象为数值类型时，假设该对象是名称为num，num很有可能通过下面两种方式声明：

复制代码 代码如下:

var num = 123; //通过对象字面量声明console.log(typeof num); //输出：'number'
var num = new Number(123); //通过构造方法声明console.log(typeof num); //输出：'object'

更多变态声明方式，可参见《JS中不为人知的五种声明Number的方式》

关于返回值的说明，ECMA5里面原文如下：

Create a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.7 for a description of Number objects.
按照这段文字的说明，似乎num.valueOf()返回的应该是个Number对象（非字面量声明的那种），但实际上：

复制代码 代码如下:

var num = 123;
var tmp = num.valueOf();
console.log(typeof tmp); //输出： 'number'

这是怎么回事呢？于是又仔细翻看了下，似乎有些接近真相了：
5.7.4.4 Number.prototype.valueOf ( )

Returns this Number value.

The valueOf function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
原来Number有属于自身的原型valueOf方法，不是直接从Object.prototype上继承下来，类似的，Boolean、String也有自己的原型valueOf方法，归纳如下：

类型	是否有属于自己的原型valueOf方法
Undefined	无
Null	无
Number	有，Number.prototype.valueOf
String	有，String.prototype.valueOf
Boolean	有，Boolean.prototype.valueOf
Object	-

Apart from this, Array and Function do not have their own prototype valueOf method. See the specification:

NOTE The Array prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the standard built-in Object prototype Object.
The Function prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype Object.
Additional notes: Number.prototype.valueOf The internal conversion rules are a little more complicated than expected and will not be expanded upon here.

After talking about it for a long time, there are still two questions that are confusing :

1. Regarding ToObject, when the parameter is a Function object, the object is returned as How to deal with it seems not to be clearly stated in the specification. At present, it is only based on experimental guesses (it may also be that I have not found it)
2.valueOf usage scenarios, I have not seen any brothers use it in actual development
The last one :

If there are any errors or omissions in the examples in the article, please point them out; if you think the article is useful to you, you can click "Recommend" :)