Service worker: cache static assets safely

import { CACHE_VERSION, PRECACHE_URLS, APP_SHELL } from './assets.js';

const STATIC_CACHE = `static-${CACHE_VERSION}`;
const EXPECTED_CACHES = [STATIC_CACHE];

self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(STATIC_CACHE).then((cache) => cache.addAll(PRECACHE_URLS))
  );
});

self.addEventListener('activate', (event) => {
  event.waitUntil(
    caches.keys().then((keys) => {
      const stale = keys.filter((key) => !EXPECTED_CACHES.includes(key));
      return Promise.all(stale.map((key) => caches.delete(key)));
    }).then(() => self.clients.claim())
  );
});

self.addEventListener('fetch', (event) => {
  const { request } = event;
  const url = new URL(request.url);

  if (request.method !== 'GET' || url.origin !== self.location.origin) {
    return;
  }

  if (request.mode === 'navigate') {
    event.respondWith(handleNavigation(request));
    return;
  }

  event.respondWith(cacheFirst(request));
});

async function handleNavigation(request) {
  try {
    const fresh = await fetch(request);
    const cache = await caches.open(STATIC_CACHE);
    cache.put(APP_SHELL, fresh.clone());
    return fresh;
  } catch (err) {
    const cache = await caches.open(STATIC_CACHE);
    return (await cache.match(APP_SHELL)) || cache.match('/offline.html');
  }
}

async function cacheFirst(request) {
  const cache = await caches.open(STATIC_CACHE);
  const hit = await cache.match(request);
  if (hit) return hit;

  const response = await fetch(request);
  if (response.ok && response.type === 'basic') {
    cache.put(request, response.clone());
  }
  return response;
}

export function registerServiceWorker() {
  if (!('serviceWorker' in navigator)) return;

  window.addEventListener('load', async () => {
    try {
      const reg = await navigator.serviceWorker.register('/sw.js', {
        scope: '/'
      });

      reg.addEventListener('updatefound', () => {
        const incoming = reg.installing;
        if (!incoming) return;

        incoming.addEventListener('statechange', () => {
          const hasController = Boolean(navigator.serviceWorker.controller);
          if (incoming.state === 'installed' && hasController) {
            promptForUpdate(reg);
          }
        });
      });
    } catch (err) {
      console.error('SW registration failed:', err);
    }
  });

  navigator.serviceWorker.addEventListener('controllerchange', () => {
    window.location.reload();
  });
}

function promptForUpdate(reg) {
  const accepted = window.confirm('A new version is available. Reload now?');
  if (accepted && reg.waiting) {
    reg.waiting.postMessage({ type: 'SKIP_WAITING' });
  }
}