code: function loadJS(url, callback, fail, charset) {
          var node = DOC.createElement('script');
          var args = _arrSlice.call(arguments, 0);
          if($.isString(fail) && args.length === 3) {
              //如果fail为字符串，认为是charset
              charset = fail;
          } else if(args.length === 4 && $.isString(charset)) {
  
          } else {
              charset = config.charset;
          }
          $.isFunction(callback) && jsCallback(node, callback, fail);
  
          node.charset = charset;
          node.async = true;
          node.src = url;
          HEAD.insertBefore(node, BASEELEMENT);
          return $;
      }
docs: 加载js
  @param  {[type]}   url      [description]
  @param  {Function} callback [description]
  @param  {[type]}   fail     [description]
  @param  {[type]}   charset  [description]
  @return {[type]}            [description]