Async javascript loading

async.js

var async = function(url, callback) {
  var script = document.createElement("script");
  script.src = url;

  if (callback != null) {
    callback.done = false;

    script.onreadystatechange = s.onload = function() {
      var state = script.readyState;

      if (!callback.done && (!state || (state === "loaded" || state === "complete"))) {
        callback.done = true;
        script.onreadystatechange = script.onload = null;

        callback();
      }
    }
  }

  (document.body || document.getElementsByTagName("head")[0]).append(script);
}

window.async = async;