n=1233456;
toQfw(n);
function toQfw(){
  var arr_n=n.toString().split("");
  var result=[];
  var cs=0;
  for(var i=arr_n.length-1;i>=0;i--){
    cs++;
    result.unshift(arr_n[i]);
    if(!(cs%3)&&i!=0){ //i!=0即取到第1位的时候，前面不用加逗号
        result.unshift(",");
    }
    alert(result.join(""))
  }
};

n=1233456;
toQfw(n);
function toQfw(){
  var str_n=n.toString();
  var result="";
  var cs=0;
  for(var i=str_n.length-1;i>=0;i--){
    cs++;
    result=str_n.charAt(i)+result;
    if(!(cs%3)&&i!=0){
        result=","+result;
    }
    alert(result)
  }
};

n=1233456;
toQfw(n);
function toQfw(){
  var str_n=n.toString();
  var result="";
  while(str_n.length>3){
    result=","+str_n.slice(-3)+result;
    str_n=str_n.slice(0,str_n.length-3)
  }
  if(str_n){
    alert(str_n+result)
  }
};

n=1233456;
toQfw(n);
function toQfw(){
  var str_n=n.toString();
  var result="";
  var re=/\d{3}$/;
  while(re.test(str_n)){
    result=RegExp.lastMatch+result;
    if(str_n!=RegExp.lastMatch){
      result=","+result;
      str_n=RegExp.leftContext; //匹配成功，返回匹配内容左侧的字符信息
    }
    else{
      num="";
    }
  }
  if(str_n){
    alert(str_n+result);
  }
}

var str="123598752";
var re=/(?=(?!(\b))(\d{3})+$)/g;
str=str.replace(re,",");
alert(str);