loadScript.js

var jQuery = jQuery || undefined;

(function($) {
  tdFun($);

  function tdFun($) {
    if ($) {
      setEvent();
    } else {
      // 載入jQ
      loadScript('https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js', function () {
        tdFun(jQuery);
      });
    }

    // 設置事件
    function setEvent() {
      console.log('設置事件');
      console.log('$ = ', $);
    }

    function loadScript(url, callback) {
      var script = document.createElement('script');
      script.type = 'text/javascript';
      script.async = 'async';
      script.src = url;
      document.body.appendChild(script);
      if (script.readyState) { //IE
        script.onreadystatechange = function () {
          if (script.readyState == 'complete' || script.readyState == 'loaded') {
            script.onreadystatechange = null;
            callback();
          }
        }
      } else { //非IE
        script.onload = function () {
          callback();
        }
      }
    }
  }
})(jQuery)