How some different companies load their third-party JS async style

async.html

<!-- Google Analytics -->
<script type="text/javascript">

  var _gaq = _gaq || [];
  _gaq.push(['_setAccount', 'UA-XXXXX-X']);
  _gaq.push(['_trackPageview']);

  (function() {
    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
  })();

</script>



<!-- BuySellAds -->
<script type="text/javascript">
(function(){
  var bsa = document.createElement('script');
     bsa.type = 'text/javascript';
     bsa.async = true;
     bsa.src = '//s3.buysellads.com/ac/bsa.js';
  (document.getElementsByTagName('head')[0]||document.getElementsByTagName('body')[0]).appendChild(bsa);
})();
</script>



<!-- Typekit -->
<script type="text/javascript">
  TypekitConfig = {
    kitId: 'abc1def',
    scriptTimeout: 3000
  };
  (function() {
    var h = document.getElementsByTagName('html')[0];
    h.className += ' wf-loading';
    var t = setTimeout(function() {
      h.className = h.className.replace(/(\s|^)wf-loading(\s|$)/g, '');
      h.className += ' wf-inactive';
    }, TypekitConfig.scriptTimeout);
    var tk = document.createElement('script');
    tk.src = '//use.typekit.com/' + TypekitConfig.kitId + '.js';
    tk.type = 'text/javascript';
    tk.async = 'true';
    tk.onload = tk.onreadystatechange = function() {
      var rs = this.readyState;
      if (rs && rs != 'complete' && rs != 'loaded') return;
      clearTimeout(t);
      try { Typekit.load(TypekitConfig); } catch (e) {}
    };
    var s = document.getElementsByTagName('script')[0];
    s.parentNode.insertBefore(tk, s);
  })();
</script>