react-expert

SKILL.md

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)

Documentation
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