No YouTube Volume Normalization.user.js

// ==UserScript==
// @name        No YouTube Volume Normalization
// @namespace   https://gist.github.com/abec2304
// @match       https://www.youtube.com/*
// @grant       none
// @version     2.1
// @author      abec2304
// @description Enjoy YouTube videos at their true volume
// @inject-into content
// @run-at      document-start
// ==/UserScript==

(function(recurse) {
    // try in case script ran later than document-start
    if(!recurse())
        return;

    // fallback to waiting for video element to be added
    const videoObserver = new MutationObserver(function(records) {
        records.forEach(function(mutation) {
            Array.prototype.forEach.call(mutation.addedNodes, function(node) {
                if("VIDEO" === node.tagName) {
                    videoObserver.disconnect();
                    recurse();
                }
            });
        });
    })

    // begin waiting
    videoObserver.observe(document.documentElement, {
        subtree: true,
        childList: true
    });
})(function() {
    // get volume bar element
    const volumeElement = document.querySelector(".ytp-volume-panel");

    // get video element
    const videoElement = document.querySelector(".html5-main-video");

    // if elements weren't found, don't continue
    if(!volumeElement || !videoElement)
        return 1;

    // get volume set function and bind to video element
    const setVolume = Object.getOwnPropertyDescriptor(HTMLMediaElement.prototype, "volume").set.bind(videoElement);

    // define convenience function
    function ytSetVolume(widget) {
        const newVolume = widget.getAttribute("aria-valuenow") / 100;
        setVolume(newVolume);
    }

    // determine index of video element for non-sandboxed world
    const globalIndex = [].slice.call(document.getElementsByTagName("video")).indexOf(videoElement);

    // define function for shadowing the volume field
    const volumeShadow = function(targetIndex) {
      const targetVideo = document.getElementsByTagName("video")[targetIndex];
      Object.defineProperty(targetVideo, "volume", {
          value: 42
      });
    }

    // inject shadowing script (this breaks the volume bar; fixed below)
    const contextScript = document.createElement("script");
    contextScript.id = "ytvolfix2";
    contextScript.textContent = "(" + volumeShadow+ ")(" + globalIndex + ")";
    contextScript.onload = function() {
        // remove script after execution
        this.parentElement.removeChild(this);
    };
    (document.head || document.documentElement).appendChild(contextScript);

    // define observer for volume bar change
    const volumeObserver = new MutationObserver(function(records) {
      ytSetVolume(records[0].target);
    });

    // register observer to make volume bar function
    volumeObserver.observe(volumeElement, {
        attributes: true,
        attributeFilter: ["aria-valuenow"]
    });

    // set volume to current value of bar
    ytSetVolume(volumeElement);
});