Bring Your Own Favicon

byof.user.js

// ==UserScript==
// @name         Bring Your Own Favicon
// @namespace    https://github.com/LenAnderson
// @version      1.0.0
// @description  Use your own favicon
// @author       LenAnderson
// @match        *://*/*
// @grant        GM_registerMenuCommand
// @grant        GM_getValue
// @grant        GM_setValue
// ==/UserScript==

(function() {
    'use strict';

	const get = ()=>{
		return JSON.parse(GM_getValue('byof', '[]'));
	};
	const set = (value)=>{
		GM_setValue('byof', JSON.stringify(value));
	};

	const updateFavicon = ()=>{
		const urls = get().filter(it=>location.href.startsWith(it.start)).toSorted((a,b)=>b.start.length-a.start.length);
		if (urls.length == 0) return;
		const url = urls[0];
		let link = document.querySelector("link[rel~='icon']");
		if (!link) {
			link = document.createElement('link'); {
				link.rel = 'icon';
				document.head.append(link);
			}
		}
		link.href = url.favicon;
	};
	updateFavicon();
    GM_registerMenuCommand('Set Favicon', ()=>{
		const urls = get();
		const start = prompt(`You have custom favicons set URLs starting with:\n\n${urls.map(it=>it.start).join('\n')}\n\nAdd a new favicon for URLs starting with:`, location.href);
		if (start) {
			const favicon = prompt(`You are setting a custom favicon for all URLs starting with:\n${start}\n\nEnter a data URI:`);
			if (favicon) {
				const old = urls.find(it=>it.start.toLowerCase() == start.toLowerCase());
				if (old) {
					old.favicon = favicon;
				} else {
					urls.push({start, favicon});
				}
				set(urls);
				updateFavicon();
			}
		}
	});
    GM_registerMenuCommand('Remove Favicon', ()=>{
		const urls = get();
		const idx = prompt(`You have custom favicons set URLs starting with:\n\n${urls.map((it,idx)=>`${idx}: ${it.start}`).join('\n')}\n\nEnter the number of the favicon you want to remove:`);
		if (idx !== null) {
			urls.splice(idx, 1);
			set(urls);
			updateFavicon();
		}
	});
})();