function mergeSheets() {
  /* Retrieve the desired folder */
  var myFolder = DriveApp.getFoldersByName(SOURCE).next();

  /* Get all spreadsheets that resided on that folder */
  var spreadSheets = myFolder.getFilesByType('application/vnd.google-apps.spreadsheet');

  /* Create the new spreadsheet that you store other sheets */
  var newSpreadSheet = SpreadsheetApp.create('Merged Sheets');

  /* Iterate over the spreadsheets over the folder */
  while (spreadSheets.hasNext()) {
    var sheet = spreadSheets.next();

    /* Open the spreadsheet */
    var spreadSheet = SpreadsheetApp.openById(sheet.getId());

    /* Get all its sheets */
    for (var y in spreadSheet.getSheets()) {
      /* Copy the sheet to the new merged Spread Sheet */
      spreadSheet.getSheets()[y].copyTo(newSpreadSheet);
    }
  }
}