JavaScript[3桁カンマ挿入]

function addcomma (tar) {
　var repstr = tar;
　var newstr = "";

　while(repstr != (newstr = String(repstr).replace(/^([+-]?\d+)(\d\d\d)/,"$1,$2"))) {
 		repstr = newstr;
 	}
  	return newstr;
 }