miniflux/internal/ui/static/js/service_worker.js

// Incrementing OFFLINE_VERSION will kick off the install event and force
// previously cached resources to be updated from the network.
const OFFLINE_VERSION = 1;
const CACHE_NAME = "offline";

self.addEventListener("install", (event) => {
    event.waitUntil(
        (async () => {
            const cache = await caches.open(CACHE_NAME);

            // Setting {cache: 'reload'} in the new request will ensure that the
            // response isn't fulfilled from the HTTP cache; i.e., it will be from
            // the network.
            await cache.add(new Request(OFFLINE_URL, { cache: "reload" }));
        })()
    );

    // Force the waiting service worker to become the active service worker.
    self.skipWaiting();
});

self.addEventListener("fetch", (event) => {
    // We proxify requests through fetch() only if we are offline because it's slower.
    if (navigator.onLine === false && event.request.mode === "navigate") {
        event.respondWith(
            (async () => {
                try {
                    // Always try the network first.
                    const networkResponse = await fetch(event.request);
                    return networkResponse;
                } catch (error) {
                    // catch is only triggered if an exception is thrown, which is likely
                    // due to a network error.
                    // If fetch() returns a valid HTTP response with a response code in
                    // the 4xx or 5xx range, the catch() will NOT be called.
                    const cache = await caches.open(CACHE_NAME);
                    const cachedResponse = await cache.match(OFFLINE_URL);
                    return cachedResponse;
                }
            })()
        );
    }
});