Converts YouTube video transcriptions (accessed via Innertube API (which has incredibly laxed rate-limiting) instead of /timedtext) to basic VTT (since afaik transcriptions don't contain any styling info, and is extremely spammy on certain custom YTT/SRV3 subtitled videos)

yt-transcript-to-vtt.js

// non-atob base64 encoder from https://gist.github.com/jonleighton/958841
/*
MIT LICENSE
Copyright 2011 Jon Leighton
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
function toBase64(arrayBuffer) {
  var base64    = ''
  var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

  var bytes         = new Uint8Array(arrayBuffer)
  var byteLength    = bytes.byteLength
  var byteRemainder = byteLength % 3
  var mainLength    = byteLength - byteRemainder

  var a, b, c, d
  var chunk

  // Main loop deals with bytes in chunks of 3
  for (var i = 0; i < mainLength; i = i + 3) {
    // Combine the three bytes into a single integer
    chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2]

    // Use bitmasks to extract 6-bit segments from the triplet
    a = (chunk & 16515072) >> 18 // 16515072 = (2^6 - 1) << 18
    b = (chunk & 258048)   >> 12 // 258048   = (2^6 - 1) << 12
    c = (chunk & 4032)     >>  6 // 4032     = (2^6 - 1) << 6
    d = chunk & 63               // 63       = 2^6 - 1

    // Convert the raw binary segments to the appropriate ASCII encoding
    base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d]
  }

  // Deal with the remaining bytes and padding
  if (byteRemainder == 1) {
    chunk = bytes[mainLength]

    a = (chunk & 252) >> 2 // 252 = (2^6 - 1) << 2

    // Set the 4 least significant bits to zero
    b = (chunk & 3)   << 4 // 3   = 2^2 - 1

    base64 += encodings[a] + encodings[b] + '=='
  } else if (byteRemainder == 2) {
    chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1]

    a = (chunk & 64512) >> 10 // 64512 = (2^6 - 1) << 10
    b = (chunk & 1008)  >>  4 // 1008  = (2^6 - 1) << 4

    // Set the 2 least significant bits to zero
    c = (chunk & 15)    <<  2 // 15    = 2^4 - 1

    base64 += encodings[a] + encodings[b] + encodings[c] + '='
  }
  
  return base64
}

const encoder = new TextEncoder();
function generateLangParams(lang, sub_type = '', sub_variant = '') {
    return encodeURIComponent(toBase64(new Uint8Array([
        0x0a, sub_type.length, ...encoder.encode(sub_type),
        0x12, lang.length, ...encoder.encode(lang),
        0x1a, sub_variant.length, ...encoder.encode(sub_variant),
    ])))
}
function generateTranscriptParams(video_id, lang_params) {
    return typeof lang_params === 'undefined' ? encodeURIComponent(toBase64(new Uint8Array([
        0x0a, 0x0b, ...encoder.encode(video_id)
    ]))) : encodeURIComponent(toBase64(new Uint8Array([
        0x0a, 0x0b, ...encoder.encode(video_id),
        0x12, lang_params.length, ...encoder.encode(lang_params)
    ])))
}
function fetchTranscription(params) {
    return fetch('https://www.youtube.com/youtubei/v1/get_transcript?key=AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8', {
        method: 'POST',
        body: JSON.stringify({
            'context': {
                'client': {
                    'hl': 'en',
                    'gl': 'US',
                    'clientName': 'WEB',
                    'clientVersion': '2.20210101',
                }
            },
            'params': params,
        })
    }).then(res => res.json())
}
function videoTranscriptionToVtt(transcript_json) {
    const transcript_renderer = transcript_json.actions.find(action => action.updateEngagementPanelAction?.targetId === 'engagement-panel-transcript').updateEngagementPanelAction.content.transcriptRenderer
    const transcript_lang = transcript_renderer.footer?.transcriptFooterRenderer?.languageMenu?.sortFilterSubMenuRenderer?.subMenuItems?.find(item => item.selected)?.title
    const transcript_cue_groups = transcript_renderer.body.transcriptBodyRenderer.cueGroups
    let vtt = `WEBVTT - Transcription ${transcript_lang ? ` in ${transcript_lang}` : ''}\n\n`
    transcript_cue_groups.forEach(cue_group => {
        cue_group.transcriptCueGroupRenderer.cues.forEach(cue => {
            const cue_renderer = cue.transcriptCueRenderer
            const start_ts = parseInt(cue_renderer.startOffsetMs)
            const end_ts = start_ts + parseInt(cue_renderer.durationMs)
            const text = cue_renderer.cue.simpleText
            if (!text) return; // Hacky fix for hidden transcripts with mismatched timestamps
            vtt += `${new Date(start_ts).toISOString().slice(11, -1)} --> ${new Date(end_ts).toISOString().slice(11, -1)}\n${text}\n\n`
        })
    })
    return vtt
}
function listTranscriptionLanguageContinuations(transcript_json) {
    const transcript_renderer = transcript_json.actions.find(action => action.updateEngagementPanelAction?.targetId === 'engagement-panel-transcript').updateEngagementPanelAction.content.transcriptRenderer
    const transcript_langs = transcript_renderer.footer.transcriptFooterRenderer.languageMenu.sortFilterSubMenuRenderer.subMenuItems
    const returned_langs = {}
    for (const lang of transcript_langs) {
        returned_langs[lang.title] = lang.continuation.reloadContinuationData.continuation
    }
    return returned_langs
}

// Examples
// Get default language transcription
fetchTranscription(generateTranscriptParams('oImkbkxytn8')).then(res => console.log(videoTranscriptionToVtt(res)))

// List continuation params for each listed language
fetchTranscription(generateTranscriptParams('qY1IFnjM8-Y')).then(res => console.log(listTranscriptionLanguageContinuations(res)))

// Get transcription of different language variants
fetchTranscription(generateTranscriptParams('qY1IFnjM8-Y', generateLangParams('id'))).then(res => console.log(videoTranscriptionToVtt(res)))
fetchTranscription(generateTranscriptParams('qY1IFnjM8-Y', generateLangParams('es', 'asr'))).then(res => console.log(videoTranscriptionToVtt(res)))
fetchTranscription(generateTranscriptParams('qY1IFnjM8-Y', generateLangParams('id', '', 'BAHASA MANADO'))).then(res => console.log(videoTranscriptionToVtt(res)))