What is service worker caching strategy?

A service worker is a JavaScript file that runs in the background, intercepting network requests and enabling programmatic caching. Key caching strategies: Cache First: check cache first, fall back to network. Best for: static assets, rarely-changing resources. Network First: try network, fall back to cache if offline. Best for: HTML, API data that must be fresh but also work offline. Stale While Revalidate: serve from cache immediately (fast), then update cache in background. Best for: non-critical content that can be slightly stale. Cache Only: only serve from cache, never go to network. Best for: pre-cached resources that are definitely available. Network Only: bypass cache entirely. Best for: real-time data, analytics. Workbox (by Google): library that implements these strategies declaratively: registerRoute(({ request }) => request.destination === "image", new CacheFirst({ cacheName: "images" })). Service workers also enable offline functionality, push notifications, and background sync. Next.js, Gatsby, and Astro have Workbox integration via plugins.