Google Spreedsheets script merging and setting border to week days

merge weeks.gs

function numberToLetter(number){
  var temp = "" 
  var letter = "";
  while (number > 0){
    temp = (number - 1) % 26;
    letter = String.fromCharCode(temp + 65) + letter;
    number = (number - temp - 1) / 26;
  }
  return letter;
}

function getFirstMonday(colData) {
  const MONDAY_LETTER = 'M';
  for(i = 0; i < colData.length; i++) {
    if(colData[i] == MONDAY_LETTER)
      return i+1;
  }
  return 1; // no Monday found
}

function mergeWeeks() {
  var ss = SpreadsheetApp.getActiveSheet();

  // from Columns B to H (B = 2, NB = 366)
  var loadingWeekDays = {
    colStart: 2,
    numColumns: 365,
    row: 4,
    numRows: 1
  };
  
  var mergeArr = []; // days that are starts of merge ranges
  var colData = ss.getRange(loadingWeekDays.row, loadingWeekDays.colStart, loadingWeekDays.numRows, loadingWeekDays.numColumns).getValues().toString().split(","); 

  mergeArr.push(1);

  var firstMonday = getFirstMonday(colData);

  var currentDay = firstMonday;
  if(currentDay != 1)
    mergeArr.push(currentDay);

  const DAYS_IN_WEEK = 7;
  while(currentDay-1 < colData.length) {
    currentDay += DAYS_IN_WEEK;
    mergeArr.push(currentDay);
  }

  var weekNumber = 1;   
  var mergeRow = 3;
  for (i = 1; i < mergeArr.length; i++){
    var mergeStart = mergeArr[i-1] + 1;      
    var mergeEnd = mergeArr[i]; // -1 + 1 = 0

    if (mergeEnd - mergeStart >= 1){
      // merge week cells
      var weekRange = ss.getRange(numberToLetter(mergeStart) + mergeRow + ':' + numberToLetter(mergeEnd) + mergeRow);
      weekRange.merge().setValue('WEEK ' + weekNumber); 

      // set borders
      var borderRowRange = {
        start: 3,
        end: 100
      }
      var weekRangeForWholeSpreadSheet = ss.getRange(numberToLetter(mergeStart) + borderRowRange.start + ':' + numberToLetter(mergeEnd) + borderRowRange.end);
      weekRangeForWholeSpreadSheet.setBorder(null, true, null, true, null, null, 'blue', SpreadsheetApp.BorderStyle.DASHED);
    }

    weekNumber += 1;
  }     
}