gas.js

/// パスから末尾の拡張子を取得する
function getFileExtension(path) {
  let splitted= url.split(".");
  return splitted[splitted.length - 1];
}

/// Google Driveのルート直下の指定の名前の
/// フォルダを作成または取得する
function getTargetFolder(folderName) {
  let rootFolder = DriveApp.getRootFolder();
  let targetFolder;
  let iterator = rootFolder.getFoldersByName(folderName);
  while (iterator.hasNext()) {
    let folder = iterator.next();
    if (folder.getName() === folderName) {
      targetFolder = folder;
    }
  }
  if (targetFolder) {
    console.log("Using current folder", folderName);
    return targetFolder
  }
  console.log("Creating a new folder", folderName);
  return rootFolder.createFolder(folderName);
}