alue: () => []
            },

            /**
             * The caption describing the options group.
             */
            caption: String
          };
        }

        /**
         * Detects whether the given item is checked or not.
         *
         * @param {Object} item the item to compare.
         * @return {boolean} true if the option for given item is checked.
         */
        _isChecked(item) {
          return this.value.some(val => {
            return val === this._getValue(item)