function formatNum(str){
var newStr = "";
var count = 0;

if(str.indexOf(".")==-1){
   for(var i=str.length-1;i>=0;i--){
 if(count % 3 == 0 && count != 0){
   newStr = str.charAt(i) + "," + newStr;
 }else{
   newStr = str.charAt(i) + newStr;
 }
 count++;
   }
   str = newStr + ".00"; //自动补小数点后两位
   console.log(str)
}
else
{
   for(var i = str.indexOf(".")-1;i>=0;i--){
 if(count % 3 == 0 && count != 0){
   newStr = str.charAt(i) + "," + newStr;
 }else{
   newStr = str.charAt(i) + newStr; //逐个字符相接起来
 }
 count++;
   }
   str = newStr + (str + "00").substr((str + "00").indexOf("."),3);
   console.log(str)
 }
}

formatNum('13213.24'); //输出13,213.34
formatNum('132134.2');  //输出132,134.20
formatNum('132134');  //输出132,134.00
formatNum('132134.236');  //输出132,134.23