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 

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