Get YouTube Video Transcript in JavaScript

index.js

const fetch = require("node-fetch");
const xml2js = require("xml2js");
const he = require("he");

const TRANSCRIPTION_CHAR_KEY = "transcription";

// Test
(async () => {
  const videoId = "ht14hTTDklA"; // HWPR Pagination video
  // const videoId = "rB9ql0L0cUQ"; // Video with manually added captions
  // const videoId = "YgT6XABqS5Y"; // Video without any captions

  try {
    const transcript = await getTranscript(videoId);
    console.log(JSON.stringify(transcript, null, 2));
  } catch (error) {
    console.log("error was caught!");
    console.error(error.message);
  }
})();

async function getTranscript() {
  const videoId = "ht14hTTDklA"; // HWPR Pagination video
  // const videoId = "rB9ql0L0cUQ"; // Video with manually added captions
  // const videoId = "YgT6XABqS5Y"; // Video without any captions

  try {
    const videoPageHtml = await (
      await fetch(`https://www.youtube.com/watch?v=${videoId}`)
    ).text();
    return await fetchTranscript(videoPageHtml);
  } catch {
    // No transcript available for this video, or we were unable to fetch it.
    return null;
  }
}

async function fetchTranscript(videoPageHtml) {
  const startString = `"captions":`;
  const captionsStartPosition = videoPageHtml.indexOf(startString);
  const wasCaptionStartFound = captionsStartPosition !== -1;
  if (!wasCaptionStartFound) {
    // No error log email is sent here, since videos may be too new to
    // have a transcript yet, or just not have a transcript available.
    throw new Error("Unable to find beginning of captions data.");
  }
  const trimmedHtml = videoPageHtml.substr(
    captionsStartPosition + startString.length
  );
  const endString = `,"videoDetails`;
  const captionsEndPosition = trimmedHtml.indexOf(endString);
  const wasCaptionEndFound = captionsEndPosition !== -1;
  if (!wasCaptionEndFound) {
    throw new Error("Unable to find end of captions data.");
  }
  const captionsDataString = trimmedHtml.substring(0, captionsEndPosition);
  const captionsData = JSON.parse(captionsDataString);
  const englishCaptionTrack =
    captionsData?.playerCaptionsTracklistRenderer?.captionTracks.find(
      (captionTrack) => captionTrack.languageCode === "en"
    );
  if (!englishCaptionTrack) {
    throw new Error("Unable to find english caption track.");
  }
  const captionsUrl = englishCaptionTrack.baseUrl;
  if (!captionsUrl) {
    throw new Error("Unable to extract captions URL.");
  }
  const captionsXml = await (await fetch(captionsUrl)).text();
  const transcriptObject = await xml2js.parseStringPromise(captionsXml, {
    attrkey: "attributes",
    charkey: TRANSCRIPTION_CHAR_KEY,
  });
  const transcript = transcriptObject?.transcript?.text;

  if (!transcript) return null;

  return transcript.map(formatTranscriptText);
}

function formatTranscriptText(transcriptLine) {
  const decodeHtmlEntities = (text) => he.decode(text);
  const removeLineBreaks = (text) => text.replaceAll("\n", " ");

  return {
    ...transcriptLine,
    [TRANSCRIPTION_CHAR_KEY]: removeLineBreaks(
      decodeHtmlEntities(transcriptLine[TRANSCRIPTION_CHAR_KEY])
    ),
  };
}