loadScript.js

function loadScript(src, fn) {
  var script, done, head;

  script = document.createElement('script');
  script.src = src;
  script.async = true;

  script.onload = script.onreadystatechange = function() {
    if (!done && (!this.readyState || this.readyState === 'loaded' || this.readyState === 'complete')) {
      done = true;

      script.onload = script.onreadystatechange = null;

      if (fn) {
        fn();
      }

      if (script && script.parentNode) {
        script.parentNode.removeChild(script);
      }
    }
  };

  if (!head) {
    head = document.getElementsByTagName('head')[0];
  }

  head.appendChild(script);
}