Control YouTube playback rate

bookmarklet.js

/*
 * JAVASCRIPT BOOKMARKLET
 *
 * In order to have this function always at hand and use it in
 * any website, just add the code below to a bookmark in your favorite browser.
 * Every time you click it, the browser will prompt you for a new playback
 * rate for first video in the page.
 */

javascript:(function() {
  const rate = prompt('Set the new playback rate', 2.5);
  if (rate != null) {
    const video = document.getElementsByTagName('video')[0];
    video.playbackRate = parseFloat(rate);
  }
})();

playbackrate.js

/*
 * CONTROL YOUTUBE PLAYBACK RATE
 *
 * YouTube videos (along with all modern video implementations on the web)
 * uses the HTML5 video element (https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video).
 * This new media tag implements the HTMLMediaElement API (https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement),
 * which gives a plenty of media-related methods common to audio and video.
 *
 * The standard YouTube player just allow us to increase video speed up to 2x, but, if
 * you want to increase it even more? Well, there's a solution for that: just set the
 * playback rate to whatever number you want!
 * 
 * The function below receives one argument with the playback rate, selects the first
 * <video> element in the page and applies the new playback rate.
 */

function setVideoPlaybackRate(rate = 1) {
    document.getElementsByTagName('video')[0].playbackRate = rate;
}