function getDriveFiles(folder, path) {
  // If Drive folder is not specified, start from the root folder
  if (folder == null && path == null) {
    return getDriveFiles(DriveApp.getRootFolder(), '');
  }

  var files = [];
  path = path + '/' + folder.getName();

  // Specify the MimeType of files you wish to search
  var fileIt = folder.getFilesByType(MimeType.GOOGLE_SHEETS);
  while (fileIt.hasNext()) {
    var f = fileIt.next();
    files.push({ id: f.getId(), path: path + '/' + f.getName() });
  }

  // Get all the sub-folders and iterate
  var folderIt = folder.getFolders();
  while (folderIt.hasNext()) {
    fs = getDriveFiles(folderIt.next(), path);
    for (var i = 0; i < fs.length; i++) {
      files.push(fs[i]);
    }
  }

  return files;
}