Under js, separate Arabic numerals every three digits with one comma (for example: 15000000 to 15,000,000)

var cutStr = function(str)
{
  var newStr= new Array(+ parseInt(/3)); 
  newStr[-1]=str[-1]; 
  var currentIndex=-1; 
  for(var i = -1;i >= 0;i--) 
  { 
    if((-i)%4==0) 
    { 
      newStr[i]=","; 
    }
    else
    { 
      newStr[i]=str[currentIndex--]; 
    } 
  } 
  return ("") 
} 
var num="15000000";
(cutStr(num));
function cutStr(str){
    var rmb='';
    var i2=0;
    for(var i=-1; i>=0; i--){

        if(i%3==0&&i!=0){
            rmb+=str[i2]+',';
        }else{
            rmb+=str[i2];
        }
        i2++;
    }
    return rmb;
}
(cutStr(num));
function cutStr(num) {
    var aaa = ();
    var arr1 = ("");
    var arr2 = [];
    for (var i = 0; i < ;) {

        for (j = 0; j < 3; j++) {

            (arr1[i]);
            i++;
            if (j == 2 && i < ) {
                (",");
            }
        }
    }

    alert((""));
}
cutStr(12345678);
'1500000000000'.replace(/\B(?=(\d{3})+$)/g,',')
function formatNumber(value) {
    value = ();
    if ( <= 3) {
        return value;
    } else {
        return formatNumber((0, - 3)) + ',' + ( - 3);
    }
}  
(formatNumber(12345678))
var str = '123123211312.333123'.replace(/(?=(?!^)(?:\d{3})+(?:\.|$))(\d{3}(\.\d+$)?)/g,',$1');
(str)