extractHostname & extractRootHostname extracts the domain name and the root domain name too for common URLs.

extractHostname

//See also my answer on StackOverflow: https://stackoverflow.com/a/23945027/1675237
const extractHostname = url => {
  //find & remove protocol (http, ftp, etc.) and get hostname
  let hostname = url.split('/')[url.indexOf("//") > -1 ? 2 : 0];
  //find & remove port number
  hostname = hostname.split(':')[0];
  //find & remove "?"
  return hostname.split('?')[0];
};

const extractRootHostname = url => {
  let domain = extractHostname(url),
      splitArr = domain.split('.'),
      arrLen = splitArr.length
  ;
  //extracting the root domain here
  //if there is a subdomain 
  if (arrLen > 2) {
    domain = splitArr[arrLen - 2] + '.' + splitArr[arrLen - 1];
    //check to see if it's using a Country Code Top Level Domain (ccTLD) (i.e. ".me.uk")
    if (splitArr[arrLen - 2].length == 2 && splitArr[arrLen - 1].length == 2) {
      //this is using a ccTLD
      domain = splitArr[arrLen - 3] + '.' + domain;
    }
  }
  return domain;
};