---

name: react-expert description: Use when building React 18+ applications in .jsx or .tsx files, Next.js App Router projects, or create-react-app setups. Creates components, implements custom hooks, debugs rendering issues, migrates class components to functional, and implements state management. Invoke for Server Components, Suspense boundaries, useActionState forms, performance optimization, or React 19 features. license: MIT metadata: author: https://github.com/Jeffallan version: "1.1.0" domain: frontend triggers: React, JSX, hooks, useState, useEffect, useContext, Server Components, React 19, Suspense, TanStack Query, Redux, Zustand, component, frontend role: specialist scope: implementation output-format: code related-skills: fullstack-guardian, playwright-expert, test-master

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

1. Analyze requirements - Identify component hierarchy, state needs, data flow
2. Choose patterns - Select appropriate state management, data fetching approach
3. Implement - Write TypeScript components with proper types
4. Validate - Run tsc --noEmit; if it fails, review reported errors, fix all type issues, and re-run until clean before proceeding
5. Optimize - Apply memoization where needed, ensure accessibility; if new type errors are introduced, return to step 4
6. 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:

1. Component file with TypeScript types
2. Test file if non-trivial logic
3. 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)