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react-expert

Skill18 starsChanged 5 months agoMITView on GitHub

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.

We split this skill into the rules it gives an agent

11 rules. 8 are common, 1 is distinctive, and 2 are very specific to this project.

  • 8

    Common

    Most of the 15 edited versions we read keep these.

  • 1

    Distinctive

    This author's own choices. Worth a look.

  • 2

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Common:

    Implement: - Write TypeScript components with proper types

    line 34
  2. Very specific:

    Validate: - Run tsc --noEmit; if it fails, review reported errors, fix all type issues, and re-run until clean before proceeding

    line 35
  3. Distinctive:

    Test: - Write tests with React Testing Library.

    line 37
  4. Common:

    Use TypeScript with strict mode

    line 124
  5. Very specific:

    Use key props correctly (stable, unique identifiers)

    line 126
  6. Common:

    Use semantic HTML and ARIA for accessibility

    line 128
  7. Common:

    Use Suspense boundaries for async operations

    line 130
  8. Common:

    Use array index as key for dynamic lists

    line 134
  9. Common:

    Create functions inside JSX (causes re-renders)

    line 135
  10. Common:

    When implementing React features, provide:

    line 142
  11. Common:

    Test file if non-trivial logic

    line 144

“Common” is measured: kept by at least 6 in 10 of the 15 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
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

  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:

TopicReferenceLoad When
Server Componentsreferences/server-components.mdRSC patterns, Next.js App Router
React 19references/react-19-features.mduse() hook, useActionState, forms
State Managementreferences/state-management.mdContext, Zustand, Redux, TanStack
Hooksreferences/hooks-patterns.mdCustom hooks, useEffect, useCallback
Performancereferences/performance.mdmemo, lazy, virtualization
Testingreferences/testing-react.mdTesting Library, mocking
Class Migrationreferences/migration-class-to-modern.mdConverting 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)

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