download-youtube-transcript.js

let $segments = document.querySelector('.ytd-transcript-search-panel-renderer #segments-container')
let sections = []
let parts = []

for (let $el of $segments.children) {
  if ($el.tagName == 'YTD-TRANSCRIPT-SECTION-HEADER-RENDERER') {
    if (parts.length > 0) {
			sections.push(parts.join(' '))
      parts = []
    }
    sections.push($el.querySelector('#title').innerText)
  } else {
    parts.push($el.querySelector('.segment-text').innerText.trim())
  }
}
if (parts.length > 0) {
  sections.push(parts.join(' '))
}

let $link = document.createElement('a')
let url = URL.createObjectURL(new Blob([sections.join('\n\n')], {type: "text/plain"}))
let title = document.querySelector('#above-the-fold #title')?.innerText ?? 'transcript'
$link.setAttribute('href', url)
$link.setAttribute('download', `${title}.txt`)
$link.click()
URL.revokeObjectURL(url)