An exemple of a service worker that always tries to fetch results, fallbacking to cache only if theres a fetch error.

service-worker.js

const CACHE_VERSION = 'cache_version_1';

self.addEventListener('install', (event) => {
    console.log('service worker install');

    if(!caches in self) {
        return;
    }

    event.waitUntil(
        caches.open(CACHE_VERSION)
            .then(cache => {
                return cache.addAll([
                    '/',
                    '/login/pt/'
                ]);
            })
            .catch(error => console.log(error))
    );
});

self.addEventListener('activate', (event) => {
    console.log('service worker activate');
    event.waitUntil(
        caches.keys()
            .then(cacheKeys => {
                return Promise.all(
                    cacheKeys.map(cacheKey => {
                        if(cacheKey !== CACHE_VERSION) {
                            return caches.delete(cacheKey);
                        }
                    })
                );
            })
    );
});

self.addEventListener('fetch', (event) => {
    console.log('Service worker fetching: ', event.request.method);
    if(event.request.method !== 'POST') {

        event.respondWith(
            fetch(event.request)
            .then(response => {

                caches.open(CACHE_VERSION)
                .then(cache => cache.put(event.request, response.clone()));

                return response.clone();
            })
            .catch(() => {
                return caches.match(event.request)
                .then(response => {
                    return response || new Response('App offline');
                })
            })
        )
    }
});