How do browser caching strategies work?

Direct answer

What a strong answer covers

How it works

HTTP caching has two independent decisions: how long a response may be reused without asking the server, set by max-age or s-maxage, and how the cache checks whether a stale copy is still valid, done with ETag or Last-Modified and a conditional request. Versioned static assets can be treated as immutable because a change produces a new URL; documents whose URL never changes need short freshness plus validation. Service workers add a programmable layer on top with their own strategies.

Example

Serve hashed JS with long max-age and immutable; serve the HTML shell with revalidation so it can point to the next asset version. An API response that changes per user needs Cache-Control: private with no-store or a short max-age, otherwise a shared CDN cache can serve one user’s data to another.

Common mistakes

• Caching HTML for a long time so users keep loading assets that no longer exist.
• Confusing no-cache, which allows storing but requires revalidation, with no-store.
• Forgetting Vary when the same URL returns different bodies by Accept-Encoding or cookie.

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