Userscript that reduces the sound of YouTube video ads and can also change their playback speed

YouTube_Mute_Ads.user.js

// ==UserScript==
// @name         YouTube Mute Ads
// @namespace    https://gist.github.com/iRi-E
// @version      0.1
// @description  Reduce the sound of YouTube video ads
// @author       iRi_E
// @license      MIT
// @match        https://www.youtube.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant        none
// ==/UserScript==

const volumeRatio = 0.1; // Ratio of volume of video ads to main video
const playbackSpeed = 0; // If not positive, use the same playback speed as main video

(function() {
    'use strict';

    let player = null;
    let videoAds = null;
    let savedVolume = null;

    function findVideoAds(observer) {
        videoAds = player.querySelector(':scope > .video-ads');
        if (videoAds) {
            observer.observe(videoAds, { childList: true });
            console.log('[YMA] Found video ads');
        }
        return videoAds;
    }

    function onVideoAdsChange() {
        if (videoAds.childNodes.length) {
            if (!Number.isFinite(savedVolume)) {
                savedVolume = player.getVolume();
                player.setVolume(savedVolume * volumeRatio);
                console.log('[YMA] Volume x ' + volumeRatio);
                const speed = playbackSpeed > 0 ? playbackSpeed : player.getPlaybackRate();
                if (speed != 1) {
                    player.querySelector('video').playbackRate = speed;
                    console.log('[YMA] Set playback rate ' + speed);
                }
            }
        } else if (Number.isFinite(savedVolume)) {
            player.setVolume(savedVolume);
            savedVolume = null;
            console.log('[YMA] Restore volume');
        }
    }

    const observer = new MutationObserver(() => {
        if (videoAds || findVideoAds(observer)) {
            onVideoAdsChange();
        }
    });

    const timer = setInterval(() => {
        player = document.getElementById('movie_player');
        if (player) {
            clearInterval(timer);
            if (findVideoAds(observer)) {
                onVideoAdsChange();
            } else {
                observer.observe(player, { childList: true });
            }
            console.log('[YMA] Observer started');
        }
    }, 1000);
    console.log('[YMA] Waiting for player to load...');
})();