loadAsyncCss.js

// Load CSS file async
export const loadAsyncCss = (href: string | Array<string>, id, cb: () => void = null) => {
    function createElement(d, s, href) {
        var link = d.createElement(s);
        link.href = href;
        link.rel = 'stylesheet';
        return link;
    }
    (function (d, id, href, r) {
        var fjs = d.getElementsByTagName('link')[0];
        var promises: Array<Promise<any>> = [];
        const ID_CSS = id + '-css';

        if (d.getElementById(id) || d.getElementById(ID_CSS)) {
            if (typeof r == 'function') {
                r();
            }
            return;
        }

        if (typeof href == 'string') {
            href = [href];
        }

        href.forEach((href_str, i) => {
            var link = createElement(d, 'link', href_str);
            if (i == href.length - 1) {
                link.id = ID_CSS;
            }
            fjs.parentNode.insertBefore(link, fjs);
            var promise = new Promise((resolve, reject) => {
                link.onload = function () {
                    resolve();
                }
            });
            promises.push(promise);
        });

        if (typeof r == 'function') {
            Promise.all(promises).then(() => r());
        }
    }(document, id, href, cb));
}