Question 1

What caching layers exist in Next.js App Router?

Accepted Answer

Request memoization, Data Cache, Full Route Cache, and Router Cache (client). Each layer has different lifetimes and invalidation. Interviews often probe whether you can name and distinguish them. Misconfiguring one causes 'why is this stale?' bugs.

Question 2

How does fetch caching work by default in Next.js?

Accepted Answer

fetch responses can be cached in the Data Cache unless opted into no-store/dynamic behavior (defaults evolved by version — know your version). Use cache: 'no-store' for dynamic, next.revalidate for ISR-like data, and tags for invalidation. Always verify against current Next docs for default changes.

Question 3

What is React cache() used for?

Accepted Answer

Deduplicate pure data calls within a single server request/render tree. It is not a cross-request cache. Use it to avoid repeat DB hits when multiple components need the same user/session.

Question 4

How do you prevent caching of personalized pages?

Accepted Answer

Use dynamic APIs (cookies/headers), no-store, and private cache headers; never put user HTML in shared full-route cache. Audit CDN behavior. Segment personalized islands. Prefer caching shared public fragments only.

Question 5

Client Router Cache vs Full Route Cache — difference?

Accepted Answer

Router Cache is client-side soft-navigation cache; Full Route Cache is server/CDN stored RSC payload/HTML for static routes. Stale client navigations often come from Router Cache. Understand refresh/revalidate behaviors when debugging 'UI didn't update after mutation'.

Question 6

How do cache tags design for a commerce site?

Accepted Answer

Tag by entity and relationships: product, category, collection; invalidate the minimal set on publish. Avoid one mega-tag that busts everything. Document tag ownership. Combine with path revalidation for critical URLs.

Question 7

Lead: production shows intermittent stale dashboards. How do you diagnose?

Accepted Answer

Map which cache layer, check personalization leakage, verify revalidation hooks, add cache telemetry, and fix defaults with tests. Reproduce with cache keys and cookies. Confirm CDN vs app cache. Establish runbooks for purge. Treat cache bugs as P0 when auth data leaks.

Caching Next.js Interview Questions

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