service-worker.js

import { timestamp, files, shell, routes } from "@sapper/service-worker";

const ASSETS = `cache${timestamp}`;

// `shell` is an array of all the files generated by the bundler,
// `files` is an array of everything in the `static` directory
const to_cache = shell.concat(files);
const cached = new Set(to_cache);

self.addEventListener("install", event => {
    event.waitUntil(
        caches
            .open(ASSETS)
            .then(cache => cache.addAll(to_cache))
            .then(() => {
                self.skipWaiting();
            }),
    );
});

self.addEventListener("activate", event => {
    event.waitUntil(
        caches.keys().then(async keys => {
            // delete old caches
            for (const key of keys) {
                if (key !== ASSETS) await caches.delete(key);
            }

            self.clients.claim();
        }),
    );
});

self.addEventListener("fetch", event => {
    if (event.request.method !== "GET" || event.request.headers.has("range")) return;

    const url = new URL(event.request.url);

    // don't try to handle e.g. data: URIs
    if (!url.protocol.startsWith("http")) return;

    // ignore dev server requests
    if (url.hostname === self.location.hostname && url.port !== self.location.port) return;

    // always serve static files and bundler-generated assets from cache
    if (url.host === self.location.host && cached.has(url.pathname)) {
        event.respondWith(caches.match(event.request));
        return;
    }

    // for pages, you might want to serve a shell `service-worker-index.html` file,
    // which Sapper has generated for you. It's not right for every
    // app, but if it's right for yours then uncomment this section
    /*
	if (url.origin === self.origin && routes.find(route => route.pattern.test(url.pathname))) {
		event.respondWith(caches.match('/service-worker-index.html'));
		return;
	}
	*/

    if (event.request.cache === "only-if-cached") return;

    // for everything else, try the network first, falling back to
    // cache if the user is offline. (If the pages never change, you
    // might prefer a cache-first approach to a network-first one.)
    event.respondWith(
        caches.open(`offline${timestamp}`).then(async cache => {
            try {
                const response = await fetch(event.request);
                cache.put(event.request, response.clone());
                return response;
            } catch (err) {
                const response = await cache.match(event.request);
                if (response) return response;

                throw err;
            }
        }),
    );
});