Function that adds YouTube video IDs to a playlist using session information from window.ytcfg.data

add-youtube-videos-to-playlist.js

async function addVideosToPlaylist(playlistId, videoIds) {
  const sej = JSON.stringify({
    commandMetadata: {
      webCommandMetadata: {
        url: '/service_ajax',
        sendPost: true,
        apiUrl: '/youtubei/v1/browse/edit_playlist'
      }
    },
    playlistEditEndpoint: {
      playlistId: playlistId,
      actions: videoIds.map(vid => ({ addedVideoId: vid, action: 'ACTION_ADD_VIDEO' }))
    }
  });
  const csn = window.ytcfg.data_['client-screen-nonce'];
  const session_token = window.ytcfg.data_['XSRF_TOKEN'];

  const params = {
    credentials: 'include',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
      'X-YouTube-Client-Name': window.ytcfg.data_['INNERTUBE_CONTEXT_CLIENT_NAME'],
      'X-YouTube-Client-Version': window.ytcfg.data_['INNERTUBE_CONTEXT_CLIENT_VERSION'],
      'X-YouTube-Device': window.ytcfg.data_['DEVICE'],
      'X-Youtube-Identity-Token': window.ytcfg.data_['ID_TOKEN'],
      'X-YouTube-Page-CL': window.ytcfg.data_['PAGE_CL'],
      'X-YouTube-Page-Label': window.ytcfg.data_['PAGE_BUILD_LABEL'],
      'X-YouTube-Variants-Checksum': window.ytcfg.data_['VARIANTS_CHECKSUM'],
    },
    referrer: 'https://www.youtube.com/playlist?list=WL',
    body: `sej=${sej}&csn=${csn}&session_token=${session_token}`,
    method: 'POST',
    mode: 'cors'
  };

  const resp = await fetch('https://www.youtube.com/service_ajax?name=playlistEditEndpoint', params);
  if (resp.status === 200) {
    return await resp.json();
  }
  return false;
}

(async () => {
  try {
    console.log(await addVideosToPlaylist('WL', ['<videoIds>']));
  } catch (err) {
    console.error(err);
  }
})();