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; //碰到3的倍数则加上“,”号
   }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132,134.236

function regexNum(str){
 var regex = /(\d)(?=(\d\d\d)+(?!\d))/g;

 if(str.indexOf(".") == -1){

  str= str.replace(regex,',') + '.00';
  console.log(str)

 }else{
  var newStr = str.split('.');
  var str_2 = newStr[0].replace(regex,',');

  if(newStr[1].length <= 1){ 
   //小数点后只有一位时
   str_2 = str_2 + '.' + newStr[1] +'0';
   console.log(str_2)

  }else if(newStr[1].length > 1){ 
   //小数点后两位以上时
   var decimals = newStr[1].substr(0,2);
   var srt_3 = str_2 + '.' + decimals;
   console.log(srt_3)
  }
 }
};
regexNum('23424224'); //输出2,42,224.00 
regexNum('23424224.2'); //输出2,42,224.20
regexNum('23424224.22'); //输出2,42,224.22
regexNum('23424224.233'); //输出2,42,224.23