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Parallel routes let a single layout render multiple independent pages at once. Combine them with intercepting routes and you get URL-aware modals with zero hacks.

Server Actions collapse form handling into a single server function—no API route, no manual fetch, no useState for loading. Here's the pattern that replaces 80% of your mutation boilerplate.

Four caching layers, three revalidation APIs, and the Next.js 16 rename that turns everything you learned in 15 into the "previous model". Here's the complete mental model for data fetching in the App Router.

If you're still writing pages/api/... files in a new Next.js project, stop. Route Handlers are the App Router's replacement, and they're meaningfully better — typed, streaming-capable, and built on the same Web platform APIs that Cloudflare.

Picture this: a live dashboard that polls for new notifications every 3 seconds. The count increments in state, the UI updates, everything looks right — except the interval callback is silently reading count = 0 on every tick, because it.

Open a modal 10 times without cleanup and you've got 10 stacked timers silently draining memory. Here's what this looks like in a PR and how senior devs catch it.

Cypress runs inside the browser; Playwright runs outside it. That one architectural split explains the debugging experience, the WebKit story, and why parallelism costs money on one and nothing on the other.

Most useMemo and useCallback calls in the wild accomplish nothing. Learn exactly when each hook works, why one unstable prop breaks the whole memoization chain, and when to just delete it.

Here's a scenario I see in almost every codebase I review. A developer writes an async function that reads some state, does a bit of work, then writes back a result. In tests, it's fine. In development, it's fine.