Service Workers & Offline Caching

Also Known As

TL;DR

Explanation

Service Workers run in a separate thread, intercept fetch events, and implement caching strategies: Cache First (serve from cache, fall back to network — ideal for static assets), Network First (try network, fall back to cache — for fresh content), Stale While Revalidate (serve cache immediately, update in background — balance of speed and freshness), Cache Only, and Network Only. Workbox (Google) simplifies service worker patterns. Lifecycle: install (precache static assets), activate (clean old caches), fetch (intercept requests). Service workers require HTTPS (or localhost).

Common Misconception

Why It Matters

Common Mistakes

• Cache First for HTML pages — users see stale content after deploys.
• No cache versioning — old caches never cleaned, storage grows indefinitely.
• Caching third-party resources without CORS headers — cached responses may be opaque.
• Not handling service worker update lifecycle — new service worker waits for old tabs to close.

Code Examples

// Cache everything forever — stale content after deploy:
self.addEventListener('fetch', e => {
    e.respondWith(
        caches.match(e.request).then(r => r || fetch(e.request))
    );
    // HTML pages cached forever — users see old version after deploy!
});

// Workbox — strategy per resource type:
import { registerRoute } from 'workbox-routing';
import { CacheFirst, NetworkFirst, StaleWhileRevalidate } from 'workbox-strategies';

// Static assets: cache forever (content-hashed filenames):
registerRoute(({url}) => url.pathname.match(/\.(js|css|woff2)$/),
    new CacheFirst({ cacheName: 'assets-v1' }));

// HTML pages: network first, cache fallback:
registerRoute(({request}) => request.mode === 'navigate',
    new NetworkFirst({ cacheName: 'pages-v1', networkTimeoutSeconds: 3 }));

// API: stale while revalidate:
registerRoute(({url}) => url.pathname.startsWith('/api/'),
    new StaleWhileRevalidate({ cacheName: 'api-v1' }));

References

• ↗ https://developer.chrome.com/docs/workbox/

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