What are service workers and how do they enable PWA features?

A service worker is a JS file the browser runs in a separate thread, scoped to an origin, that can intercept and respond to network requests. It's the engine behind Progressive Web Apps: offline support, push notifications, install-to-home-screen, background sync.

Lifecycle (interviewers love this).

1. Register. navigator.serviceWorker.register("/sw.js", { scope: "/" }) from the page. Scope determines which paths the SW controls. Default scope is the location of the SW file — keep it at the root for site-wide control.
2. Install. self.addEventListener("install", e => e.waitUntil(precache())). Pre-cache critical assets. Promise resolved → installed.
3. Activate. Old SW (if any) is being replaced. Clean up old caches here. The new SW does not control existing pages until they reload, unless you call self.skipWaiting() (in install) and clients.claim() (in activate).
4. Fetch. Every navigation and asset request fires fetch on the SW. You can respond with cache, network, or a synthesized response.

The Cache API. A key-value store of Request → Response, scoped to the origin and named by you. Independent of HTTP cache. Survives reloads.

Caching strategies (Workbox names them).

• Cache-first. Look in cache; if hit, return it; else fetch and cache. Best for hashed static assets.
• Network-first. Fetch; on failure, fall back to cache. Best for HTML / API where freshness matters.
• Stale-while-revalidate. Return cache immediately, fetch in background, update cache. Best for "fast and eventually fresh" — most app shell content.
• Network-only / Cache-only. Self-explanatory. Useful for analytics (network-only) or offline-only assets (cache-only).

// sw.js — stale-while-revalidate
self.addEventListener("fetch", (event) => {
  if (event.request.method !== "GET") return;
  event.respondWith((async () => {
    const cache = await caches.open("v1");
    const cached = await cache.match(event.request);
    const networkPromise = fetch(event.request).then(resp => {
      if (resp.ok) cache.put(event.request, resp.clone());
      return resp;
    }).catch(() => undefined);
    return cached ?? (await networkPromise) ?? new Response("Offline", { status: 503 });
  })());
});

PWA features it unlocks.

• Offline. Pre-cache the app shell; fall back to a cached offline page for navigations.
• Install to home screen. Web App Manifest (manifest.json) + HTTPS + a registered SW = installable. Browser shows the install prompt.
• Push notifications. SW listens for push events and shows notifications even when the page is closed.
• Background sync. registration.sync.register("upload") retries network work when the user comes back online (Chromium only at the moment).
• Periodic background sync (Chromium): refresh data periodically.

Critical gotchas.

• HTTPS only (except localhost). Service workers can hijack requests, so they require a secure context.
• Updates are sticky. A new SW waits until all clients close the old one. Use skipWaiting + clients.claim carefully — can update mid-session and break.
• Cache versioning is your job. When you change the SW, bump the cache name (v1 → v2) and delete old caches in activate — otherwise old assets persist forever.
• fetch runs for every request. Heavy logic in there can slow page loads. Keep handlers lean.
• No DOM access. SW is a Worker — no window, no document. Communicate with pages via postMessage and MessageChannel.
• Range requests / video behave specially — pass through to network; don't try to cache.

Use Workbox in production. Google's library wraps all the strategies, cache versioning, and precaching. Don't hand-write SWs unless you need exotic behavior.

When NOT to use a SW. Static marketing sites that the HTTP cache already handles. Anything where install bugs (sticky old caches) would be catastrophic for low-engagement users. Always have an "unregister" escape hatch.