function copyFolderContents_(source, target) {
  const filesIterator = source.getFiles()
  while (filesIterator.hasNext()) {
    const file = filesIterator.next()
    file.makeCopy(file.getName(), target)
  }
}

function copyFolder_(toCopy, copyInto) {
  const newFolder = copyInto.createFolder(toCopy.getName())
  copyFolderContents_(toCopy, newFolder)
  const foldersIterator = toCopy.getFolders()
  while (foldersIterator.hasNext()) {
    const folder = foldersIterator.next()
    copyFolder_(folder, newFolder)
  }
}

function copyFolder() {
  const toCopy = DriveApp.getFolderById('Folder_ID')
  const copyInto = DriveApp.getFolderById('Folder_ID')
  copyFolder_(toCopy, copyInto)
}