JS的千分位算法实现思路

function commafy() { 
var num = document.getElementById("NumA").value; 
//1.先去除空格,判断是否空值和非数 
num = num + ""; 
num = num.replace(/[ ]/g, ""); 
if (num == "") { 
alert("空值，结束"); 
return; 
} 
if (isNaN(num)) { 
alert("非数，结束"); 
return; 
} 
//2.针对是否有小数点，分情况处理 
var index = num.indexOf("."); 
if (index==-1) {//无小数点 
var reg = /(-?\d+)(\d{3})/; 
while (reg.test(num)) { 
num = num.replace(reg, "$1,$2"); 
} 
} else { 
var intPart = num.substring(0, index); 
var pointPart = num.substring(index + 1, num.length); 
var reg = /(-?\d+)(\d{3})/; 
while (reg.test(intPart)) { 
intPart = intPart.replace(reg, "$1,$2"); 
} 
num = intPart +"."+ pointPart; 
} 
return alert(num); 
}