24 — Server Components & Concurrent React

Modern React patterns: React Server Components (RSC) and Concurrent features. These work primarily in frameworks like Next.js App Router.

Server Components vs Client Components

Feature	Server Component	Client Component
Runs on	Server only	Server (SSR) + Client
Bundle size	Zero JS sent to client	Included in bundle
Can use hooks	❌ No	✅ Yes
Can use browser APIs	❌ No	✅ Yes
Can access DB/filesystem	✅ Yes	❌ No
Can use async/await	✅ Yes	❌ No (use useEffect)
Default in Next.js App Router	✅ Yes	Only with "use client"

Concurrent Features

Feature	What It Does
useTransition	Mark state updates as non-urgent
useDeferredValue	Defer a value update to keep UI responsive
Suspense	Show fallback while content loads
startTransition	Non-hook version of useTransition
use()	Read promises and context in render (React 19)

File

• server-concurrent.tsx — Server Components, client boundary, Suspense patterns, streaming, concurrent features, React 19 patterns