js中的加号（+）运算符

一,对于引用类型对象(我指的是String,Date,Object,Array,Function,Boolean)的+运算符运算过程如下! 
1,首先调用此对象的valueOf方法,得到返回数值A 
2,然后把此数值A转换成数字,得到的是最终数值 
function w(s){ 
document.writeln("
"); 
document.writeln(s); 
document.writeln("
-----------------------------"); 
} 
String.prototype.valueOf=function(){return 1;}; 
w(+new String("sss"));//输出1 
String.prototype.valueOf=function(){return "a";}; 
w(+new String("sss"));//输出NaN 
 
 
Date.prototype.valueOf=function(){return 1;}; 
w(+new Date());//输出1 
Date.prototype.valueOf=function(){return "a";}; 
w(+new Date());//输出NaN 
 
Object.prototype.valueOf=function(){return 1;}; 
w(+{});//输出1 
Object.prototype.valueOf=function(){return "a";}; 
w(+{});//输出NaN 
 
Array.prototype.valueOf=function(){return 1;}; 
w(+[]);//输出1 
Array.prototype.valueOf=function(){return "a";}; 
w(+[]);//输出NaN 
 
var s=function(){}; 
Function.prototype.valueOf=function(){return 1;}; 
w(+s);//输出1 
Function.prototype.valueOf=function(){return "a";}; 
w(+s);//输出NaN 
 
Boolean.prototype.valueOf=function(){return 1;}; 
w(+new Boolean());//输出1 
Boolean.prototype.valueOf=function(){return "a";}; 
w(+new Boolean());//输出NaN 

二,对于基本数据数据类型,其值转换成数字

w(+5);//输出5 
w(+true);//输出1 
w(+false);//输出0 
w(+"ss");//输出NaN 
w(+"111");//输出111