How to dynamically load external javascript code or css styles with JS functions

load.js

function loadScript(src) {
    return new Promise((resolve, reject) => {
        const script = document.createElement("script");
        script.type = "text/javascript";
        script.onload = resolve;
        script.onerror = reject;
        script.src = src;
        document.getElementsByTagName("head")[0].appendChild(script);
    });
}

function loadLink(href) {
    return new Promise((resolve, reject) => {
        const link = document.createElement("link");
        link.rel = "stylesheet";
        link.onload = resolve;
        link.onerror = reject;
        link.href = href;
        document.getElementsByTagName("head")[0].appendChild(link);
    });
}