What does a service worker let you do, and what would you be careful about before adding one?

A service worker is a proxy between the page and the network, so it can serve cached responses, work offline, precache a shell and handle background sync or push. The caution is lifecycle: it controls pages only after activation, an old worker keeps serving old assets until it is replaced, and a bad cache rule can pin users to a broken build. Always version your caches and ship a clear update path.

It sits between the page and the network as a separate worker, so it can intercept fetches and decide whether to answer from a cache. That gives me offline support, an instant shell on repeat visits, background sync for queued actions and push notifications. What I am careful about is the lifecycle, because it is where the incidents come from. A new worker installs, then waits until every controlled tab is gone before activating, so users can sit on old code far longer than you expect. And if I cache the HTML with a cache first rule and then ship a broken worker, I have effectively bricked returning users, since they never fetch the fix. So I version cache names and delete old ones on activate, I keep HTML network first and only cache first for hashed assets, and I always keep a kill switch path that unregisters and clears caches. If the requirement is only speed on repeat visits and not genuine offline use, I would rather get that from cache headers and a CDN and skip the whole class of problem.