Why do sites name static files with a content hash, like app.3f9a2c.js?

Answer

A file with a given name never changes, so browsers and CDNs can cache it for a year (immutable). A new release produces new file names, so users get it immediately, with no purge and no stale mixes of old and new files. Only the small HTML page that links them needs a short TTL.

Review this in your daily deck → All cards in Caching