preconnect hooker for external links

preconnecter.ts

// https://gist.github.com/KenanSulayman/2da1085913b94add85315e08959bafa6

;(() => {
    const runner = () => {
        const processed: string[] = [];

        const preconn = (type: string, url: string) => {
            if (~processed.indexOf(url)) {
                return;
            }
    
            processed.push(url);
    
            const hint = document.createElement('link');
    
            hint.rel = type;
            hint.as = 'fetch';
            hint.href = url;
    
            document.head.appendChild(hint);
        };
        
        [].slice.call(document.body.getElementsByTagName('a'))
        .forEach((a: HTMLAnchorElement) => {
            const isExternalLink = !/psychonautwiki\.org/.test(a.href);

            a.addEventListener('mouseenter', e => {
                if (isExternalLink) {
                    const match = /(https?:\/\/.*?)\//.exec(a.href)[1];
    
                    preconn('preconnect', match);

                    return;
                }

                preconn('preload', a.href);
                preconn('prerender', a.href);
            });

            if (!isExternalLink) {
                return;
            }

            a.addEventListener('mousedown', e => {    
                preconn('prerender', a.href);
            });
        })
    };

    document.readyState !== 'loading' ? runner() : document.addEventListener('DOMContentLoaded', runner);
})();