React Expert

# 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)
```tsx
// 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`
```tsx
'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
```tsx
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)

[Documentation](https://jeffallan.github.io/claude-skills/skills/frontend/react-expert/)