Service Workers and Offline-First Web Applications

Understanding Service Workers

Service workers are JavaScript files that run in a separate thread from your web page, acting as a programmable proxy between your application and the network. They intercept all network requests from your page and can serve cached responses, modify requests, or generate responses entirely offline. Service workers enable features like offline support, background sync, and push notifications. They follow a lifecycle of installation (downloading and caching assets), activation (cleaning up old caches), and fetch handling (intercepting network requests).

Caching Strategies for Offline Support

Different content types require different caching strategies. Cache-first serves content from cache immediately and updates the cache in the background — ideal for static assets like images, fonts, and CSS. Network-first tries the network and falls back to cache — ideal for frequently updated content like API responses. Stale-while-revalidate serves cached content immediately while fetching fresh data in the background — ideal for content that benefits from speed but should be updated when possible.

• Cache-first: Serve instantly from cache, update in background for static assets
• Network-first: Try network for fresh data, fall back to cache when offline
• Stale-while-revalidate: Serve cached version immediately, update cache from network
• Cache-only: Serve only from cache for pre-cached app shell resources

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