Userscript to allow students of CTU to open multiple tabs of KOS

KOS_multiple_tabs.user.js

// ==UserScript==
// @name        KOS multiple tabs
// @namespace   util
// @description Allows you to use KOS in mutiple tabs.
// @include     https://www.kos.cvut.cz/kos/*
// @version     1
// @grant       none
// ==/UserScript==
// Name of local storage entry where page id is stored
const DATA_NAME = "KOS_PAGE_ID";

console.info("Multiple tab KOS init, page code is: ", pageCode);
/**
 * This is called on DOM load. It will broadcast MD5 hash page names to all other open tabs **/

function code_broadcast()
{
    var data = {};
    data.pageCode = pageCode;
    console.info("Broadcasting code: ", pageCode);
    localStorage.setItem(DATA_NAME,JSON.stringify(data));
}

function code_receive(event)
{
    console.log(event);
    if (event.key != DATA_NAME)
        return;
    var message;
    try {    
        message = JSON.parse(event.newValue);
        if(typeof message!="object" || typeof message.pageCode!="string")
            throw new Error("pageCode is missing from data.");
        console.log("New code received: ", message);
    }
    catch(e) {
         console.error("Invalid data for KOS page id in local storage. This really shouldn't happen normally, so if it happened to you, you probably deserve it.");
         throw e;
    }
    // Only replace if different
    if(pageCode!=message.pageCode) {
        console.info("Replacing code", pageCode, "with",message.pageCode);
        // Replace page code in URL, if applicable
        var url = window.location.href;
        url = url.replace(/page=([a-f0-9]{20,32})/i, "page="+message.pageCode);
        if(url!=window.location.href)
            window.history.replaceState("", "", url);
        // Change the idiotic pageCode global variable
        pageCode = message.pageCode;
    }
    else {
        console.info("Code did not change: ", pageCode);
    }
}

// This condition checks if page is in iframe. Only non-iframe 
// pages are handled by this script
if(window.self == window.top) {
    code_broadcast();
    window.addEventListener("storage", code_receive);
}