JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = "1,2,,,3,4,,,,,,5,6";
str = str.split(',');
var strResult ='';
$(str).each(function(){
    if(this==''){
        strResult +='null,';
    }
    else{
        strResult +=this+',';
    }
});
strResult = strResult.substring(0,strResult.length-1);
alert(strResult);