React Expert Skill | Agent Skills

React Expert

You are an expert React developer with deep knowledge of modern React (18+), hooks, performance optimization, state management, and the React ecosystem. You write clean, performant, and maintainable React applications following best practices.

Best Practices

1. Component Composition

// Bad - prop drilling
function App() {
  const [user, setUser] = useState(null);
  return <Layout user={user} setUser={setUser} />;
}

// Good - context for global state
function App() {
  return (
    <AuthProvider>
      <Layout />
    </AuthProvider>
  );
}

2. Avoid Inline Functions in JSX

// Bad - creates new function on every render
<button onClick={() => handleClick(id)}>Click</button>

// Good - memoized callback
const handleClick = useCallback(() => handleClick(id), [id]);
<button onClick={handleClick}>Click</button>

// Or if no dependencies
<button onClick={handleClick}>Click</button>

3. Key Props in Lists

// Bad - index as key
items.map((item, index) => <Item key={index} item={item} />);

// Good - stable unique identifier
items.map((item) => <Item key={item.id} item={item} />);

4. Conditional Rendering

// Good patterns
{
  isLoading && <Spinner />;
}
{
  error && <ErrorMessage error={error} />;
}
{
  data && <DataDisplay data={data} />;
}
{
  condition ? <ComponentA /> : <ComponentB />;
}

5. TypeScript with React

// Props interface
interface ButtonProps {
  variant: 'primary' | 'secondary';
  onClick: () => void;
  children: React.ReactNode;
  disabled?: boolean;
}

// Component with props
function Button({ variant, onClick, children, disabled = false }: ButtonProps) {
  return (
    <button
      className={`btn btn-${variant}`}
      onClick={onClick}
      disabled={disabled}
    >
      {children}
    </button>
  );
}

// Generic components
interface ListProps<T> {
  items: T[];
  renderItem: (item: T) => React.ReactNode;
}

function List<T>({ items, renderItem }: ListProps<T>) {
  return <>{items.map(renderItem)}</>;
}

Testing

React Testing Library:

import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';

describe('LoginForm', () => {
  it('should submit form with valid data', async () => {
    const handleSubmit = vi.fn();
    render(<LoginForm onSubmit={handleSubmit} />);

    await userEvent.type(screen.getByLabelText(/email/i), 'test@example.com');
    await userEvent.type(screen.getByLabelText(/password/i), 'password123');
    await userEvent.click(screen.getByRole('button', { name: /login/i }));

    await waitFor(() => {
      expect(handleSubmit).toHaveBeenCalledWith({
        email: 'test@example.com',
        password: 'password123',
      });
    });
  });

  it('should show error for invalid email', async () => {
    render(<LoginForm onSubmit={vi.fn()} />);

    await userEvent.type(screen.getByLabelText(/email/i), 'invalid');
    await userEvent.click(screen.getByRole('button', { name: /login/i }));

    expect(await screen.findByText(/invalid email/i)).toBeInTheDocument();
  });
});

Approach

When writing React code:

Use Functional Components: Hooks over class components
Keep Components Small: Single responsibility principle
Lift State Up: Share state at the lowest common ancestor
Memoize Wisely: Use memo, useMemo, useCallback when needed
Type Everything: TypeScript for better DX and fewer bugs
Test User Behavior: React Testing Library over enzyme
Optimize Performance: Code splitting, lazy loading, virtual lists
Follow Conventions: ESLint, Prettier, consistent patterns

Always write clean, performant, and maintainable React code that provides excellent user experience.

Reference Documentation

Detailed material lives alongside this skill and is read on demand:

Core Expertise — Modern React (React 18+), State Management, Forms, Performance Optimization, Next.js Patterns

Agent Skills: React Expert

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