firefox addon adding a button which shows whether the current url has been added into Delicious.com

main.js

var buttons = require('sdk/ui/button/action');
var tabs = require("sdk/tabs");
var prefs = require('sdk/simple-prefs');
var Request = require("sdk/request").Request;

var button = buttons.ActionButton({
  id: "delicious-check",
  label: "delicious-check",
  icon: "./tagOff.png",
  onClick: check
});

function check(state) {
  var username = prefs.prefs['username'];
  var password = prefs.prefs['password'];
  var querystring = 'http://' + username + ':' + password + '@api.del.icio.us/v1/posts/get?url=' + encodeURIComponent(tabs.activeTab.url);
  var latestTweetRequest = Request({
    url: querystring,
      onComplete: function (response) {
        if(response.text.indexOf("no bookmarks") == -1)
          button.state("tab", { icon: "./tagOn.png" });
        else
          button.state("tab", { icon: "./tagOff.png" });
      }
  }).get();
}

tabs.on('ready', function(tab) {
  check(null);
});