How to use the rrule.MO function in rrule

if (repeat2freq.hasOwnProperty(val)){
            return repeat2freq[val]
        } else {
            console.error("Unable to convert '" + val + "' to RRule frequency")
        }
    }

    function rRuleFreqToRepeatEvery(val) {
        if (freq2repeat.hasOwnProperty(val)){
            return freq2repeat[val]
        } else {
            console.error("Unable to convert '" + val + "' to repeat type")
        }
    }

    let rRuleWeekDays = [RRule.MO, RRule.TU, RRule.WE, RRule.TH, RRule.FR, RRule.SA, RRule.SU]

    export default {
        name: "RecurrenceSelector",
        components: {
            DateTimePicker
        },
        props: {
            startDate: {
                type: String,
                required: true
            },
            recurrence: {
                type: String,
                default: null
            },
            callback: {

},
    {
      title: 'Recurs yearly on the 10th of the current month',
      color: colors.blue,
      rrule: {
        freq: RRule.YEARLY,
        bymonth: moment().month() + 1,
        bymonthday: 10
      }
    },
    {
      title: 'Recurs weekly on mondays',
      color: colors.red,
      rrule: {
        freq: RRule.WEEKLY,
        byweekday: [RRule.MO]
      }
    }
  ];

  calendarEvents: CalendarEvent[] = [];

  viewPeriod: ViewPeriod;

  constructor(private cdr: ChangeDetectorRef) {}

  updateCalendarEvents(
    viewRender:
      | CalendarMonthViewBeforeRenderEvent
      | CalendarWeekViewBeforeRenderEvent
      | CalendarDayViewBeforeRenderEvent
  ): void {

const doesRecurringEventsOverlap = (startingDate, endingDate, recurringRule) => {
    if (!recurringRule || !startingDate || !endingDate ||
        !('frequency' in recurringRule) || !('interval' in recurringRule)) return false;

    const freqMap = {
        YEARLY: RRule.YEARLY,
        MONTHLY: RRule.MONTHLY,
        WEEKLY: RRule.WEEKLY,
        DAILY: RRule.DAILY,
    };

    const dayMap = {
        MO: RRule.MO,
        TU: RRule.TU,
        WE: RRule.WE,
        TH: RRule.TH,
        FR: RRule.FR,
        SA: RRule.SA,
        SU: RRule.SU,
    };

    const rules = {
        freq: freqMap[recurringRule.frequency],
        interval: parseInt(recurringRule.interval, 10) || 1,
        dtstart: startingDate.toDate(),
        count: 2,
    };

    if ('byday' in recurringRule) {