js中如何把9999转换成金额格式，即9,999

Question

//引用梅花雪的代码
//将数字转换成三位逗号分隔的样式
function formatNum(s) 
{
  if(!/^(\+|-)?\d+(\.\d+)?$/.test(num)){alert("wrong!"); return num;}
  var re = new RegExp().compile("(\\d)(\\d{3})(,|\\.|$)");
  num += ""; 
  while(re.test(num))
    num = num.replace(re, "$1,$2$3")
  return num;
}
alert(formatNum(-12345678.123));
alert(formatNum("12345678.123"));
alert(formatNum("10000000000000000000000000000000000000000"));