Service worker: cache static assets safely

                  const CACHE = 'static-v1';

self.addEventListener('install', (event) => {
  event.waitUntil(caches.open(CACHE));
  self.skipWaiting();
});

self.addEventListener('activate', (event) => {
  event.waitUntil(
    caches.keys().then((keys) => Promise.all(keys.filter((k) => k !== CACHE).map((k) => caches.delete(k))))
  );
  self.clients.claim();
});

self.addEventListener('fetch', (event) => {
  const url = new URL(event.request.url);
  if (url.origin !== location.origin) return;

  if (url.pathname.startsWith('/_next/static/') || url.pathname.startsWith('/assets/')) {
    event.respondWith(
      caches.open(CACHE).then(async (cache) => {
        const cached = await cache.match(event.request);
        if (cached) return cached;
        const res = await fetch(event.request);
        cache.put(event.request, res.clone());
        return res;
      })
    );
  }
});