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);
}