server/public/sw.js

self.addEventListener('fetch', function(event) {
    event.respondWith(async function() {
        try{
            var res = await fetch(event.request);
            var cache = await caches.open('cache');
            cache.put(event.request.url, res.clone());
            return res;
        }
        catch (error) {
            return caches.match(event.request);
        }
    }());
});