youtube-music/plugins/precise-volume/front.js

module.exports = () => {
	setPlaybarOnwheel();
	setObserver();
	setFirstTooltip();
};

function setFirstTooltip() {
	const videoStream = document.querySelector(".video-stream");
	if (videoStream?.volume) {
		setTooltip(toPercent(videoStream.volume));
	} else {
		setTimeout(setFirstTooltip, 500); // Try again in 500 milliseconds
	}
}

function setPlaybarOnwheel() {
    // Add onwheel event to play bar
    document.querySelector("ytmusic-player-bar").onwheel = event => {
        event.preventDefault();
        // Event.deltaY < 0 => wheel up
        changeVolume(event.deltaY < 0);
    };
}

// The last volume set by changeVolume() is stored here
let newVolume; // Used to determine if volume-slider was manually moved

function changeVolume(increase) {
	// Need to change both the slider and the actual volume
	const videoStream = document.querySelector(".video-stream");
	const slider = document.querySelector("#volume-slider");

	// Apply volume change if valid
	videoStream.volume = increase ?
		Math.min(videoStream.volume + 0.01, 1) :
		Math.max(videoStream.volume - 0.01, 0);

	// Save the new volume
	newVolume = toPercent(videoStream.volume);
	// Slider value automatically rounds to multiples of 5
	slider.value = newVolume;
	// Finally change tooltip to new value
	setTooltip(newVolume);
}

// Update the volume tooltip when volume-slider is manually changed
function setObserver() {
	const observer = new MutationObserver(mutations => {
		for (const mutation of mutations) {
			// This checks that volume-slider was manually set
			if (mutation.oldValue !== mutation.target.value &&
                (!newVolume || Math.abs(newVolume - mutation.target.value) > 4)) {
				// Diff>4 means it was manually set, so update tooltip accordingly
				setTooltip(mutation.target.value);
			}
		}
	});

	// Observing only changes in 'value' of volume-slider
	observer.observe(document.querySelector("#volume-slider"), {
		attributeFilter: ["value"],
		attributeOldValue: true
	});
}

// Set new volume as tooltip for volume slider and icon + expanding slider (appears when window size is small)
const tooltipTargets = [
	"#volume-slider",
	"tp-yt-paper-icon-button.volume.style-scope.ytmusic-player-bar",
	"#expand-volume-slider",
	"#expand-volume"
];

function setTooltip(newValue) {
	newValue += "%";
	for (target of tooltipTargets) {
		document.querySelector(target).title = newValue;
	}
}

function toPercent(volume) {
	return Math.round(Number.parseFloat(volume) * 100);
}