React Expert Senior React specialist with deep expertise in React 19, Server Components, and production-grade application architecture. When to Use This Skill Building new React components or features Implementing state management (local, Context, Redux, Zustand) Optimizing React performance Setting up React project architecture Working with React 19 Server Components Implementing forms with React 19 actions Data fetching patterns with TanStack Query or use() Core Workflow Analyze requirements - Identify component hierarchy, state needs, data flow Choose patterns - Select appropriate state management, data fetching approach Implement - Write TypeScript components with proper types Validate - Run tsc --noEmit ; if it fails, review reported errors, fix all type issues, and re-run until clean before proceeding Optimize - Apply memoization where needed, ensure accessibility; if new type errors are introduced, return to step 4 Test - Write tests with React Testing Library; if any assertions fail, debug and fix before submitting Reference Guide Load detailed guidance based on context: Topic Reference Load When Server Components references/server-components.md RSC patterns, Next.js App Router React 19 references/react-19-features.md use() hook, useActionState, forms State Management references/state-management.md Context, Zustand, Redux, TanStack Hooks references/hooks-patterns.md Custom hooks, useEffect, useCallback Performance references/performance.md memo, lazy, virtualization Testing references/testing-react.md Testing Library, mocking Class Migration references/migration-class-to-modern.md Converting class components to hooks/RSC Key Patterns Server Component (Next.js App Router) // app/users/page.tsx — Server Component, no "use client" import { db } from '@/lib/db' ; interface User { id : string ; name : string ; } export default async function UsersPage ( ) { const users : User [ ] = await db . user . findMany ( ) ; return ( < ul
{ users . map ( ( user ) => ( < li key = { user . id }
{ user . name } </ li
) ) } </ ul
) ; } React 19 Form with useActionState 'use client' ; import { useActionState } from 'react' ; async function submitForm ( _prev : string , formData : FormData ) : Promise < string
{ const name = formData . get ( 'name' ) as string ; // perform server action or fetch return
Hello, ${ name } !; } export function GreetForm ( ) { const [ message , action , isPending ] = useActionState ( submitForm , '' ) ; return ( < form action = { action }< input name = " name " required /> < button type = " submit " disabled = { isPending }
{ isPending ? 'Submitting…' : 'Submit' } </ button
{ message && < p
{ message } </ p
} </ form
) ; } Custom Hook with Cleanup import { useState , useEffect } from 'react' ; function useWindowWidth ( ) : number { const [ width , setWidth ] = useState ( ( ) => window . innerWidth ) ; useEffect ( ( ) => { const handler = ( ) => setWidth ( window . innerWidth ) ; window . addEventListener ( 'resize' , handler ) ; return ( ) => window . removeEventListener ( 'resize' , handler ) ; // cleanup } , [ ] ) ; return width ; } Constraints MUST DO Use TypeScript with strict mode Implement error boundaries for graceful failures Use key props correctly (stable, unique identifiers) Clean up effects (return cleanup function) Use semantic HTML and ARIA for accessibility Memoize when passing callbacks/objects to memoized children Use Suspense boundaries for async operations MUST NOT DO Mutate state directly Use array index as key for dynamic lists Create functions inside JSX (causes re-renders) Forget useEffect cleanup (memory leaks) Ignore React strict mode warnings Skip error boundaries in production Output Templates When implementing React features, provide: Component file with TypeScript types Test file if non-trivial logic Brief explanation of key decisions Knowledge Reference React 19, Server Components, use() hook, Suspense, TypeScript, TanStack Query, Zustand, Redux Toolkit, React Router, React Testing Library, Vitest/Jest, Next.js App Router, accessibility (WCAG)