Grab top domain name based on cookie write testing

get_root_domain.js

var getRootDomain = function(domain=document.location.host){
      var rootDomain = null;
      if (domain.substring(0,4)=="www.") {
       domain=domain.substring(4,domain.length);
      }
      var domParts = domain.split('.');
      for(var i=1;i<=domParts.length;i++)
      {   
        document.cookie="testcookie=1; path=/; domain="+domParts.slice(i*-1).join('.');
        if(document.cookie.indexOf("testcookie") != -1){
           var rootDomain = domParts.slice(i*-1).join('.');
           document.cookie="testcookie=1; expires=Thu, 01 Jan 1970 00:00:01 GMT; path=/; domain="+domParts.slice(i*-1).join('.');
           break; 
        }      
      }
      return rootDomain;    
}