code: function(res) {
        // handle thumbnail
        var picurl = '';
        // TODO: further process to thumbnail according to its size
        if (!res.thumbnail) {
          // if no thumbnail exists, a default pic should be returned.
          picurl = this.conf.CONST.PIC_PLACEHOLDER;
        } else {
          picurl = res.thumbnail.source;
        }
        res.extract = _.trunc(res.extract, {
          'length': this.conf.CONST.EXTRACT_REPLY_LIMIT, 
          'separator': /(，|。|\n)/
        });
        return {
          title: res.title,
          description: res.extract,
          url: this._page_url(res.title),
          picurl: picurl
        };
      }
docs: /*
  Form wechat single message according to the result returned by API
  
  *res* represents a single record of data to be replied.