load.js

function loadScript(path, fn) {
  var el = document.createElement('script'),
      loaded = 0,
      onreadystatechange = 'onreadystatechange',
      readyState = 'readyState';

  el.onload = el.onerror = el[onreadystatechange] = function () {
    if (loaded || (el[readyState] && !(/^c|loade/.test(el[readyState])))) return;
    el.onload = el.onerror = el[onreadystatechange] = null;
    loaded = 1;
    fn();
  };

  el.async = 1;
  el.src = path;
  document.getElementsByTagName('head')[0].appendChild(el);
}