JSFiddle - React, Tailwind, and code Playground

JavaScript

// ==UserScript==
// @name         DeGiro Kernselectie
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Show whether the ISIN is in the kernselectie
// @match        https://trader.degiro.nl/*
// @grant        none
// @run-at document-end
// ==/UserScript==

(function() {
    'use strict';

    //Re-check the page whenever the url changes
    window.onhashchange = function() {
        setTimeout(highlightKernselectie, 1000);
    }

    function highlightKernselectie() {
        let pageType = determinePageType();

        switch (pageType) {
            case 'FAVOURITES':
                highlightKernselectieForFavouritesPage();
                break;
            case 'INSTRUMENT_PAGE':
                highlightKernselectieForInstrumentPage();
                break;
            case 'UNKNOWN':
            default:
                console.log(`Unable to determine page type for ${window.location.hash}`);
                break;
        }
    };

    function highlightKernselectieForFavouritesPage() {
        document.querySelectorAll('.favourites__table tbody tr').forEach((el)=>{
            let isin = el.querySelector('[data-field=isin]').textContent;
            if(isKernSelectie(isin)) {
                el.querySelector('[data-field=isin]').parentElement.style.color = deGiroBlue;
                el.querySelector('[data-field=isin]').parentElement.title = kernSelectieTitle;
            }
        });
    };

    function highlightKernselectieForInstrumentPage() {
        let el = document.querySelectorAll('.product-details_buysell table tr')[1].querySelectorAll('td')[1];
        let isin = el.textContent;
        if(isKernSelectie(isin)) {
            el.style.color = deGiroBlue;
            el.title = kernSelectieTitle;
        } else {
            el.style.color = null;
            el.title = null;
        };
    };

    function determinePageType() {
        if (window.location.hash === '#/favourites') {
            return...