background.js

// React when a browser action's icon is clicked.
chrome.browserAction.onClicked.addListener(function(tab) {
  var whois_host = "https://who.is/whois/";
  var current_host = url_domain(tab.url);
  window.location = whois_host + current_host;
});

function url_domain(data) {
  var a = document.createElement('a');
  a.href = data;
  return a.hostname;
}