Combine multiple Google spreadsheets into one file

combinator.gs

//Setup:
// 1. Files must be of type spreadhsheet - not text/csv
// 2. All spreadsheet files must sit in the same G Drive folder
// 3. Get the folder by ID and loop through the files
// 4. Concatenate the results of all files in a separate spreadsheet (outside of the folder)

function myFunction() {
  // get the folder ID
  var folder = DriveApp.getFolderById("insert-your-folder-id-here")
  var filesIterator = folder.getFiles()
  var file;
  var fileType; 
  var combinedData = [];
  var data;

  while(filesIterator.hasNext()){
    file = filesIterator.next()
    fileType = file.getMimeType();
    if(fileType == "application/vnd.google-apps.spreadsheet") {
      ssID = file.getId();
      data = getDataFromSpreadsheet(ssID);
      combinedData = combinedData.concat(data)
    }
  }

// getActiveSpreadsheet only works when we initialize this script from the spreadhseet file
  ws = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("combined");
  ws.getRange(2, 1, combinedData.length, combinedData[0].length).setValues(combinedData)
}

function getDataFromSpreadsheet(ssID) {
  var ss = SpreadsheetApp.openById(ssID);
  var ws = ss.getSheets()[0];
  var data = ws.getRange("A2:T" + ws.getLastRow()).getValues();
  return data
}