Offline first Using service workers - Broken

__offlinefirst.js

self.addEventListener('fetch', function (event) {
    // TODO: respond with an entry from the cache if there is one.
    // If there isn't, fetch from the network.    
    const res = caches.open('wittr-static-v1').then(cache => {
        return cache.match(event.request).then(response => { return response; });
    });
    console.log(res);
    if (res !== undefined) {
        event.respondWith(res);
    } else {
        event.respondWith(
            fetch(event.request).then(response => {
                return response;
            })
        );
    }
});

// spaghetti thoughts