Userscript that remembers and applies video playback speeds on YouTube per channel

YouTube Channel Playback Speed.user.js

// ==UserScript==
// @name         YouTube Channel Playback Speed
// @namespace    https://gist.github.com/neon-dev
// @version      0.1
// @description  Remembers playback speeds by channel. Change the speed on one video and it automatically applies to all videos of the same channel.
// @author       Neon
// @match        https://www.youtube.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @updateURL    https://gist.github.com/neon-dev/f641eb74a91a01d0e4c7434ffe0fb5ef/raw
// @downloadURL  https://gist.github.com/neon-dev/f641eb74a91a01d0e4c7434ffe0fb5ef/raw
// @grant        GM.info
// @grant        GM.setValue
// @grant        GM.getValue
// @grant        GM.deleteValue
// ==/UserScript==

(function() {
    'use strict';

    let video, navigated;
    document.addEventListener('yt-navigate-finish', (event) => {
        navigated = true;
        video?.removeEventListener('ratechange', speedChangeListener);
    });
    document.body.addEventListener('yt-page-data-updated', (event) => {
        if (navigated) {
            navigated = false;
            const channelHandle = getChannelHandle();
            const videoContainer = document.getElementById('movie_player');
            const isLiveStream = videoContainer?.querySelector('.ytp-live') || false;
            video = videoContainer?.querySelector('video');
            if (channelHandle && video && !isLiveStream) {
                applySavedSpeed(channelHandle, video);
                setTimeout(() => video.addEventListener('ratechange', speedChangeListener), 250); // slight delay since youtube applies the speed from the previous video
            }
        }
    });

    function speedChangeListener(event) {
        const channelHandle = getChannelHandle();
        if (channelHandle) {
            saveOrDeleteSpeed(channelHandle, event.target.playbackRate);
        } else {
            log('Could not find channel handle, current speed cannot be saved');
        }
    };

    async function saveOrDeleteSpeed(channelHandle, speed) {
        if (speed == 1) {
            GM.deleteValue(channelHandle);
            log(`Deleted playback speed for ${channelHandle}`);
        } else if (speed != await GM.getValue(channelHandle)) {
            GM.setValue(channelHandle, speed);
            log(`Saved playback speed ${speed}x for ${channelHandle}`);
        };
    }

    async function applySavedSpeed(channelHandle, video) {
        const channelSpeed = await GM.getValue(channelHandle);
        if (channelSpeed) {
            video.playbackRate = channelSpeed;
            log(`Restored saved speed ${channelSpeed}x for ${channelHandle}`);
        }
    }

    function getChannelHandle() {
        const channelUrl = document.querySelector('#owner a')?.href;
        return channelUrl ? channelUrl.substring(channelUrl.lastIndexOf('/') + 1) : null;
    }

    function log() {
        console.debug(`[${GM.info.script.name}]`, ...arguments);
    }
})();