const playlistId = '';

let playlist = await youtube.getPlaylist(playlistId);
let videos = playlist.items;

while (playlist.has_continuation) {
  playlist = await playlist.getContinuation();
  videos = videos.concat(playlist.items);
}

for (const video of videos) {
  const videoInfo = await youtube.getInfo(video.id);
  if (videoInfo.basic_info.is_liked) {
    console.log(`Video already liked: ${videoInfo.basic_info.title}`);
  } else {
    console.log(`Liking video: ${videoInfo.basic_info.title}`);
    await videoInfo.like();
  }
}