native code to include a javascript / css file dynamically in a html document

include.js

function include(src){
    return new Promise((resolve, reject) => {
        var ext = src.split(/[\#\?]/)[0].split(".").pop().toLowerCase();
        var inc;
        if (ext == "js") {
            inc = document.createElement("script");
            inc.src = src;
            inc.onerror = () => reject();
            inc.onload = inc.onreadystatechange = () => resolve();
        }
        else {
            inc = document.createElement("link");
            inc.rel = "stylesheet";
            inc.type = "text/css";
            inc.media = "screen";
            inc.href = src;
            resolve();
        }
        document.getElementsByTagName("head")[0].appendChild(inc);
    });
}

Same thing but with promise instead of callbacks.