I’m using Google Sheets Script and would like to write data to a XLSX but preserve the XLSX file format so other applications can read it. I have tried the following without success. When I open the file with Sheets everything looks good but the file no longer works in Excel or other applications that read Excel files.

function LabelOutput(){
  var select1;
  var selectCount;
  var activeSheet = SpreadsheetApp.getActiveSheet();
  var activeRange = activeSheet.getActiveRange();
  var select1 = activeRange.getRow();
  var selectCount = activeRange.getNumRows();
  var TL = "TL";
  var Loc = "Location";
  var Shape = "Shape";
  var shpColor = "Color";
  var prgName = "Program";
  var Notes = "Notes";
  var outputData = TL + ", " + Loc+ ", " + Shape+ ", " + shpColor + ", " + prgName + ", " + Notes;

  var i;
  var j = selectCount + select1 - 1;

  for (i = select1; i <= j; i = i + 1) {                                       //add selected data
    if(activeSheet.isRowHiddenByFilter(i) == false){
      TL = activeSheet.getRange('T' + i).getValue();
      Loc = activeSheet.getRange('U' + i).getValue();
      Shape = activeSheet.getRange('AN' + i).getValue();
      Shape = Shape.substring(0,1);
      if(Shape == ","){
        Shape = '","';
      }
      shpColor = activeSheet.getRange('AP' + i).getValue();
      prgName = activeSheet.getRange('AM' + i).getValue();
      Notes = activeSheet.getRange('E' + i).getValue();
      outputData += "n" + TL + ", " + Loc+ ", " + Shape + ", " + shpColor + ", " + prgName + ", " + Notes;
    }
  }
  var excelFile = DriveApp.getFileById('1--S0-GLPwVqAj94vbPFxfzGrIT-WOGwz');
  excelFile.setContent(outputData);
}

I’ve also tried to treat the XLSX file like a SpreadSheetApp but it gives a type error.

var testingOutput = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Label Print.XLSX');
testingOutput.getRange("A2").setValue("testing");

I also tried to do something like this and it worked but it keeps making new copies rather than writing over existing copies.

function convertSheetToXLSX() {
  var sheetId = "2SqIXLiic6-gjI2KwQ6OIgb-erbl3xqzohRgE06bfj2c";
  var spreadsheetName = "My Spreadsheet";
  var destination = DriveApp.getFolderById("1vFL98cgKdMHLNLSc542pUt4FMRTthUvL");
  var url = "https://docs.google.com/feeds/download/spreadsheets/Export?key=" + sheetId + "&exportFormat=xlsx";  
  var params = {
    method      : "get",
    headers     : {"Authorization": "Bearer " + ScriptApp.getOAuthToken()},
    muteHttpExceptions: true
  };
  var blob = UrlFetchApp.fetch(url, params).getBlob();
  blob.setName(spreadsheetName + ".xlsx");
  destination.createFile(blob);
}