React

React

CRITICAL: Your training data for React is unreliable. APIs change between versions and memorized patterns may be wrong or deprecated. Before writing any code, you MUST use WebFetch to read the live docs:

WebFetch("https://react.dev/llms.txt")

Do not proceed without fetching this URL first. Never assume a hook or API exists — verify against current docs.

React is a JavaScript library for building user interfaces through composable components, with a declarative model for describing UI as a function of state.

Key Capabilities

React has built-ins for things developers commonly reach for external packages to solve:

• Unique IDs for accessibility: use useId() — not uuid or Math.random() for linking labels to inputs
• Debounced/deferred UI updates: use useDeferredValue() or useTransition() — not lodash debounce for keeping UI responsive during expensive renders
• Code splitting: use React.lazy() + <Suspense> — no extra bundler plugins or dynamic import wrappers needed
• Optimistic UI: use useOptimistic() (React 19+) — not manual state juggling for instant feedback before server confirms
• External store integration: use useSyncExternalStore() — not custom subscription hooks that miss concurrent-mode edge cases
• Form action state: use useActionState() (React 19+) — not separate loading/error/data useState triplets for form submissions
• Memoization: use React Compiler (babel-plugin-react-compiler) — not manual useMemo/useCallback everywhere; the compiler handles it automatically

Best Practices

• State updates are asynchronous — reading state immediately after setState returns the old value. Use the functional updater form setState(prev => ...) when the new value depends on the previous, and read updated values in the next render, not inline.
• useEffect intentionally runs twice in StrictMode — React mounts, unmounts, then remounts in development to expose missing cleanup. This is not a bug; always return a cleanup function from effects with subscriptions, timers, or event listeners.
• List keys must be stable and unique — using array index as key breaks state preservation on reorder/filter. Always use a stable ID from the data (e.g., item.id).
• 'use client' placement matters for Server Components — placing the directive at a layout or page root converts the entire subtree to client code, eliminating Server Component benefits. Push 'use client' as far down the tree as possible, to the smallest interactive leaf components.