Объект JavaScript OPTIONS не изменяется

/**
     * @type {object}
     */
    Object.defineProperty(DateTimePicker.prototype,
        "OPTIONS",
        {
            enumerable: true,
            get: function () {
                return {
                    useStrict: false,
                    format: this.FORMAT,
                    maxDate: this.MAX_DATE,
                    minDate: this.MIN_DATE,
                    defaultDate: null,
                    viewDate: null,
                    icons: this.ICONS
                };
            }
        });

let value = services.Date.momentFromDateString(val).format(this.FORMAT);
                        this.OPTIONS.defaultDate = value;
                        this.OPTIONS.viewDate = value;