Always return top level domain

get_top_domain.js

/**
 * First domain where cookies can be set by the browser.
 */
function get_top_domain(){
  var i,h,
    weird_cookie='__top_level=cookie',
    hostname = document.location.hostname.split('.');
  for(i=hostname.length-1; i>=0; i--) {
    h = hostname.slice(i).join('.');
    document.cookie = weird_cookie + ';domain=.' + h + ';';
    if(document.cookie.indexOf(weird_cookie)>-1){
      document.cookie = weird_cookie.split('=')[0] + '=;domain=.' + h + ';expires=Thu, 01 Jan 1970 00:00:01 GMT;';
      return h;
    }
  }
  return '';
}

Fails for localhost. In this case you can still set cookies but you need to omit the domain= part.