Adds an extra input box to youtube to set the real volume of the video

extra-volume-changer.user.js

// ==UserScript==
// @name         Extra volume changer for YouTube
// @namespace    http://tampermonkey.net/
// @version      0.2
// @description  try to take over the world!
// @author       Marcsello
// @match        https://www.youtube.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const volumeChangerID = "volume_changer_Mah7phop"

    function pullVolumeFromVideo(i, video) {
        i.value = video.volume * 100.0
    }

    function createVolumeChanger(video) {

        let i = document.createElement('input')
        i.id = volumeChangerID
        i.type = 'number'
        i.style.width = '50px'
        i.style.margin = '0 1em'
        i.max = 100
        i.min = 0
        i.step = 0.25

        // box -> video
        i.onchange = e => {
            const vol = e.target.value / 100.0
            video.volume = vol
        }

        // video -> box
        const volUpdateFn = e => {
            pullVolumeFromVideo(i, e.target);
        }
        video.onvolumechange = volUpdateFn;
        video.onchange = volUpdateFn;
        video.onchange = volUpdateFn;
        video.onplay = volUpdateFn;
        video.oncanplay = volUpdateFn;
        video.ondurationchange = volUpdateFn;
        video.ondurationchange = volUpdateFn;

        pullVolumeFromVideo(i, video); // set initial value

        // add it next to the subscribe button
        return i
    }

    function checkVolumeChangerPresence() {
        const elm = document.getElementById(volumeChangerID);
        if (elm === null) {
            return false;
        }
        const sb = document.getElementById('subscribe-button')
        if (sb === null) {
            return false;
        }
        if (sb.parentElement === null) {
            return false;
        }
        let found = false
        sb.parentElement.childNodes.forEach(e => {
            if (e === elm) {
                found = true;
            }
        });
        return found;
    }


    const initialInitWaiterInterval = setInterval(function () {
        // check url
        const path = window.location.pathname
        if (!path.startsWith("/watch")) { // not a video
            return // don't add it (yet)
        }

        // check if the video is loaded (youtube is re-using the video tag)
        const videos = document.getElementsByTagName('video');

        if (videos.length !== 1) {
            return // don't add it (yet)
        }

        // check if the subscribe button is there
        const sb = document.getElementById('subscribe-button')
        if (sb === null) { // subscribe button not there
            return // don't add it (yet)
        }

        clearInterval(initialInitWaiterInterval); // <- player is kindof ready

        // start monitoring and adding volume changer in case it's removed
        setInterval(function() {
            if (!checkVolumeChangerPresence()) {

                const videos = document.getElementsByTagName('video');
                if (videos.length !== 1) {
                    return; // no video, ignore
                }
                const video = videos[0];

                let i = document.getElementById(volumeChangerID);
                if (i === null) {
                    i = createVolumeChanger(video);
                } else {
                    pullVolumeFromVideo(i, video);
                }
                const sb = document.getElementById('subscribe-button')
                sb.parentElement.appendChild(i)
                console.log("Extra volume changer added!")
            }
        }, 1500)
    }, 500)

})();