code: function (len, min, chars) {
        len = len || 10;
        if (min != undefined) {
          len = publicRuleMethods._num(len, min);
        }
        var i;
        var str = '';
        chars = chars || ALPHA_NUMERIC_CHARS;
        var charsCount = chars.length;
        for (i = 0; i < len; i++) {
          str += chars[Math.floor(Math.random() * charsCount)];
        }
        return str;
      }
docs: 获取随机字符串[私有]
  @param {Number} [len] - 字符串长度, 默认为 10, 如果有 min, 则是最大长度
  @param {Number} [min] - 字符串最小长度
  @param {String} [chars] - 字符串源, 默认为 ALPHA_NUMERIC_CHARS