React

React Production Rules

Common Mistakes

• {count && <Component />} renders "0" when count is 0 — always use {count > 0 && <Component />} for numeric conditions
• Never mutate state directly (array.push() then setState(array)) — React won't detect the change. Always spread: setState([...array, item])
• Keys generated during render (key={Math.random()}) destroy and recreate components every render — generate stable IDs when data is created
• Uninitialized controlled inputs (useState() without default) flip between controlled/uncontrolled — always initialize with empty string or appropriate default

Hooks Traps

• useEffect callback cannot be async directly — define async function inside the effect and call it
• Missing cleanup in effects causes memory leaks — return cleanup function for subscriptions, intervals, and event listeners
• Dependencies array with objects/arrays triggers on every render (new reference each time) — memoize with useMemo or extract primitive values
• useState setter with same reference won't trigger re-render — for objects/arrays, always create new reference

Performance

• Sequential awaits create waterfalls — use Promise.all([fetchA(), fetchB()]) for independent requests
• Barrel imports (import { X } from '@/components') pull entire module — use direct path imports for tree-shaking
• Expensive computations in render body recalculate every render — wrap in useMemo with proper dependencies
• Use startTransition for non-urgent UI updates (filtering, sorting large lists) — keeps input responsive while heavy renders process in background
• Use useRef instead of useState for values that don't need to trigger re-renders (timers, previous values, DOM measurements)

State Architecture

• Colocate state as close as possible to where it's used — lifting state too high causes unnecessary re-renders in the entire subtree
• Derive values from existing state instead of syncing with useEffect — computed values don't need their own state
• When multiple state updates happen together, use useReducer — prevents impossible state combinations that separate useState calls allow
• URL search params are state too — use them for filter/sort/pagination state so users can share and bookmark

Forms (React 19+)

• Prefer form actions over useEffect for mutations — actions handle pending states, errors, and optimistic updates built-in
• Use useActionState for form submission status — replaces manual loading/error state management
• useOptimistic updates UI immediately before server confirms — revert automatically on error

Common Mistakes

• Fetching data in useEffect without abort controller — race conditions when component remounts or deps change fast
• Returning objects from custom hooks without memoizing — consumers re-render on every call even if data hasn't changed
• Using index as key in lists that reorder, filter, or insert — causes subtle bugs where component state gets attached to wrong items