Google Script createEvent() function suddenly stopped working

Question

Finally I was able to create function that will automatically update event in a calendar. I tested it and it worked perfectly; it will add new event in the calendar, and when user edits it, it will delete previous event and create new event along with new ID.

The next day however, when I tested it again, it doesn't work, I checked all functions in the code.gs, only this function has problem.

function updateCalendar(request) {
var sheet = SpreadsheetApp.getActiveSheet();
var lastRow = sheet.getLastRow(); 
var range = sheet.getRange(2,1,lastRow,13);
var values = range.getDisplayValues(); 
var calendar = CalendarApp.getCalendarById('XXXXXXXXXXXXXXXXXXX@group.calendar.google.com');
var numValues = 0;
for (var i = 0; i < values.length; i++) {    
    if (values[i][0].length > 0) {      
        if (values[i][12] == undefined) {
            var newEvent = calendar.createEvent("booked", request.date, request.endTime);
            var newEventId = newEvent.getId().split('@')[0];
            //mark as entered, enter ID
            sheet.getRange(i+2,13).setValue('y');
            sheet.getRange(i+2,12).setValue(newEventId);
        } 
        else if (values[i][12] == 'y') {
            var eventEditId = calendar.getEventSeriesById(values[i][11]);
            eventEditId.deleteEventSeries();
            var newEvent = calendar.createEvent("booked", request.date, request.endTime);
            var newEventId = newEvent.getId().split('@')[0];;
            //mark as entered, enter ID
            sheet.getRange(i+2,13).setValue('y');
            sheet.getRange(i+2,12).setValue(newEventId);
        } 
    }
    numValues++;
}
};

I also checked the calendar settings and stuffs, and made sure that I was logged into the admin account. Any help is greatly appreciated, thanks in advance.

Answer 1

I simply changed "undefined" to simply a null string ' ', it works again. But again, for some reason i am curious why it worked last time with "undefined" command