Hold alt for fast scrolling

FastScroll.js

// ==UserScript==
// @name Fast Scroll with Alt
// @namespace http://tampermonkey.net/
// @version 1.0
// @description Hold alt for fast scrolling
// @author Mariven
// @match *://*/*
// @grant none
// ==/UserScript==

(function() {
	'use strict';
	const scrollMultiplier = 4;
	let altKeyPressed = false;
	let didScroll = false;

	document.addEventListener('keydown', function(event) {
		if (event.key === 'Alt')
			altKeyPressed = true;
	});

	document.addEventListener('keyup', function(event) {
		if (event.key === 'Alt') {
			altKeyPressed = false;
			if (didScroll) {
				// stop alt from toggling the menu bar iff alt was used to scroll
				event.preventDefault();
				didScroll = false;
			}
		}
	});

	window.addEventListener('blur', function() {
		// handle window blur to prevent stuck alt state
		altKeyPressed = false;
	});

	document.addEventListener('wheel', function(event) {
		if (altKeyPressed) {
			event.preventDefault();
			didScroll = true;
			const deltaX = event.deltaX * scrollMultiplier;
			const deltaY = event.deltaY * scrollMultiplier;
			if (event.deltaY !== 0)
				window.scrollBy({
					top: deltaY,
					left: 0,
				});
			if (event.deltaX !== 0)
				window.scrollBy({
					top: 0,
					left: deltaX,
				});
		}
	}, { passive: false });
})();