code: function () {
        var ran = Math.floor(Math.random() * 3);
        if (ran == 0) {
          return this._getStr();
        }
        if (ran == 1) {
          return this._getNum();
        }
        if (ran == 2) {
          return this._getBool();
        }
      }
docs: 当为可变类型时，随机返回一种类型