Related: Network Optimization for SPAs and React Apps covers the broader network optimization picture including HTTP caching and API request optimization. A service worker is a JavaScript file that runs in a background thread, separate from your main application. It intercepts every network request your page makes and can respond from cache, modify the request, or let it pass through to the network unchanged. For a returning visitor, a well-configured service worker means many requests never touch the network at all. The page loads from cache at local disk speed. What this covers: What service workers can and cannot do, the three core caching strategies (cache-first, network-first, stale-while-revalidate), when each strategy is correct for which resource type, and how Workbox makes implementing these strategies practical without writing the interception logic by hand. What a service worker actually does A service worker is registered by your application JavaScript and installed by the browser.…