The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

React Workflow

Tool Grid

Task	Tool	Command
Lint	Biome	biome check .
Format	Biome	biome format --write .
Test	Vitest + Testing Library	vitest
E2E	Playwright	playwright test

---

Component Architecture

Functional Components Only

Components MUST be functional components. Class components MUST NOT be used.

// CORRECT
function UserProfile({ user }: UserProfileProps) {
  return <div>{user.name}</div>;
}

// ALSO CORRECT
const UserProfile = ({ user }: UserProfileProps) => {
  return <div>{user.name}</div>;
};

// INCORRECT - MUST NOT use class components
class UserProfile extends Component { ... }

Component Naming

• Component files MUST use PascalCase: UserProfile.tsx
• Components MUST be named exports for better tree-shaking
• One component per file SHOULD be the default; co-located helpers are OPTIONAL

Props Interface

Props MUST be typed with explicit interfaces:

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

function Button({ variant, disabled = false, children, onClick }: ButtonProps) {
  return (
    <button className={variant} disabled={disabled} onClick={onClick}>
      {children}
    </button>
  );
}

---

React 19 Features

useActionState

Server actions SHOULD use useActionState for form handling:

import { useActionState } from 'react';

function LoginForm() {
  const [state, formAction, isPending] = useActionState(loginAction, null);

  return (
    <form action={formAction}>
      <input name="email" type="email" required />
      <button type="submit" disabled={isPending}>
        {isPending ? 'Logging in...' : 'Log in'}
      </button>
      {state?.error && <p role="alert">{state.error}</p>}
    </form>
  );
}

useFormStatus

Form submission state SHOULD use useFormStatus in child components:

import { useFormStatus } from 'react-dom';

function SubmitButton() {
  const { pending } = useFormStatus();

  return (
    <button type="submit" disabled={pending}>
      {pending ? 'Submitting...' : 'Submit'}
    </button>
  );
}

useOptimistic

Optimistic updates SHOULD use useOptimistic:

import { useOptimistic } from 'react';

function TodoList({ todos, addTodo }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo) => [...state, { ...newTodo, pending: true }]
  );

  async function handleAdd(formData: FormData) {
    const newTodo = { id: crypto.randomUUID(), text: formData.get('text') };
    addOptimisticTodo(newTodo);
    await addTodo(newTodo);
  }

  return (
    <form action={handleAdd}>
      {optimisticTodos.map(todo => (
        <li key={todo.id} style={{ opacity: todo.pending ? 0.5 : 1 }}>
          {todo.text}
        </li>
      ))}
    </form>
  );
}

use() Hook

The use() hook MAY be used to read promises and context:

import { use, Suspense } from 'react';

function UserData({ userPromise }) {
  const user = use(userPromise);
  return <div>{user.name}</div>;
}

// Usage with Suspense
<Suspense fallback={<Skeleton />}>
  <UserData userPromise={fetchUser(id)} />
</Suspense>

---

Server Components (RSC)

Default to Server Components

Components SHOULD be Server Components by default. Client Components MUST use the 'use client' directive.

// Server Component (default) - no directive needed
async function UserList() {
  const users = await db.users.findMany();
  return <ul>{users.map(u => <li key={u.id}>{u.name}</li>)}</ul>;
}

// Client Component - requires directive
'use client';

import { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(c => c + 1)}>{count}</button>;
}

RSC Patterns

• Data fetching SHOULD happen in Server Components
• Interactive elements MUST be Client Components
• Server Components MUST NOT use hooks or browser APIs
• Client Components SHOULD be pushed to the leaves of the component tree

---

React Compiler

Automatic Memoization

React Compiler handles memoization automatically. Manual memoization SHOULD NOT be used unless profiling shows a specific need:

// RECOMMENDED - let React Compiler optimize
function ExpensiveList({ items, filter }) {
  const filtered = items.filter(filter);
  return <ul>{filtered.map(item => <li key={item.id}>{item.name}</li>)}</ul>;
}

// AVOID - unnecessary manual memoization
function ExpensiveList({ items, filter }) {
  const filtered = useMemo(() => items.filter(filter), [items, filter]);
  return <ul>{filtered.map(item => <li key={item.id}>{item.name}</li>)}</ul>;
}

Compiler Requirements

• Components MUST follow the Rules of React (pure rendering, immutable props)
• Side effects MUST be in useEffect or event handlers
• State updates MUST be immutable

---

Testing

Testing Library Patterns

Tests MUST use accessible queries. Priority order:

1. getByRole (RECOMMENDED)
2. getByLabelText (for form fields)
3. getByPlaceholderText (when no label)
4. getByText (for non-interactive elements)
5. getByTestId (last resort)

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

test('submits form with user data', async () => {
  const user = userEvent.setup();
  const onSubmit = vi.fn();

  render(<LoginForm onSubmit={onSubmit} />);

  // CORRECT - uses accessible queries
  await user.type(screen.getByRole('textbox', { name: /email/i }), 'test@example.com');
  await user.type(screen.getByLabelText(/password/i), 'secret123');
  await user.click(screen.getByRole('button', { name: /log in/i }));

  expect(onSubmit).toHaveBeenCalledWith({
    email: 'test@example.com',
    password: 'secret123'
  });
});

User Event

User interactions MUST use @testing-library/user-event over fireEvent:

// CORRECT
const user = userEvent.setup();
await user.click(button);
await user.type(input, 'text');

// AVOID
fireEvent.click(button);
fireEvent.change(input, { target: { value: 'text' } });

Async Queries

Async operations MUST use findBy* or waitFor:

// CORRECT
const successMessage = await screen.findByRole('alert');
expect(successMessage).toHaveTextContent('Saved!');

// CORRECT
await waitFor(() => {
  expect(screen.getByRole('list')).toHaveTextContent('Item 1');
});

---

Accessibility

ARIA Requirements

Interactive elements MUST have accessible names:

// CORRECT
<button aria-label="Close dialog">
  <CloseIcon />
</button>

// CORRECT
<button>
  <CloseIcon aria-hidden="true" />
  <span className="sr-only">Close dialog</span>
</button>

// INCORRECT - no accessible name
<button>
  <CloseIcon />
</button>

Keyboard Navigation

All interactive elements MUST be keyboard accessible:

function Menu({ items }) {
  const [activeIndex, setActiveIndex] = useState(0);

  const handleKeyDown = (e: React.KeyboardEvent) => {
    switch (e.key) {
      case 'ArrowDown':
        e.preventDefault();
        setActiveIndex(i => Math.min(i + 1, items.length - 1));
        break;
      case 'ArrowUp':
        e.preventDefault();
        setActiveIndex(i => Math.max(i - 1, 0));
        break;
      case 'Enter':
      case ' ':
        e.preventDefault();
        items[activeIndex].onSelect();
        break;
    }
  };

  return (
    <ul role="menu" onKeyDown={handleKeyDown}>
      {items.map((item, i) => (
        <li
          key={item.id}
          role="menuitem"
          tabIndex={i === activeIndex ? 0 : -1}
          aria-selected={i === activeIndex}
        >
          {item.label}
        </li>
      ))}
    </ul>
  );
}

Focus Management

Focus MUST be managed appropriately for modals, dialogs, and dynamic content:

function Dialog({ isOpen, onClose, children }) {
  const dialogRef = useRef<HTMLDivElement>(null);

  useEffect(() => {
    if (isOpen) {
      dialogRef.current?.focus();
    }
  }, [isOpen]);

  if (!isOpen) return null;

  return (
    <div
      ref={dialogRef}
      role="dialog"
      aria-modal="true"
      tabIndex={-1}
      onKeyDown={e => e.key === 'Escape' && onClose()}
    >
      {children}
    </div>
  );
}

---

State Management

State Hierarchy

State SHOULD follow this hierarchy (simplest to most complex):

1. Local state (useState) - component-specific state
2. Lifted state - shared between siblings via parent
3. Context - deeply nested or cross-cutting state
4. External store (Zustand, Jotai) - complex global state

Local State First

Simple state MUST use useState:

function Toggle() {
  const [isOn, setIsOn] = useState(false);
  return <button onClick={() => setIsOn(!isOn)}>{isOn ? 'On' : 'Off'}</button>;
}

Context Usage

Context SHOULD be used for theme, auth, or other cross-cutting concerns:

const ThemeContext = createContext<Theme | null>(null);

function useTheme() {
  const theme = useContext(ThemeContext);
  if (!theme) throw new Error('useTheme must be used within ThemeProvider');
  return theme;
}

function ThemeProvider({ children }: { children: React.ReactNode }) {
  const [theme, setTheme] = useState<Theme>('light');
  return (
    <ThemeContext.Provider value={{ theme, setTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

---

Custom Hooks

Extraction Pattern

Reusable logic SHOULD be extracted into custom hooks:

function useLocalStorage<T>(key: string, initialValue: T) {
  const [value, setValue] = useState<T>(() => {
    const stored = localStorage.getItem(key);
    return stored ? JSON.parse(stored) : initialValue;
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(value));
  }, [key, value]);

  return [value, setValue] as const;
}

Hook Naming

Custom hooks MUST start with use:

// CORRECT
function useWindowSize() { ... }
function useFetch<T>(url: string) { ... }

// INCORRECT
function getWindowSize() { ... }
function fetchData<T>(url: string) { ... }

---

Component Composition

Children Pattern

Components SHOULD accept children for flexibility:

function Card({ children, title }: { children: React.ReactNode; title: string }) {
  return (
    <article className="card">
      <h2>{title}</h2>
      <div className="card-content">{children}</div>
    </article>
  );
}

Render Props

Render props MAY be used for sharing stateful logic:

interface MouseTrackerProps {
  render: (position: { x: number; y: number }) => React.ReactNode;
}

function MouseTracker({ render }: MouseTrackerProps) {
  const [position, setPosition] = useState({ x: 0, y: 0 });

  return (
    <div onMouseMove={e => setPosition({ x: e.clientX, y: e.clientY })}>
      {render(position)}
    </div>
  );
}

Compound Components

Related components MAY use the compound component pattern:

const Tabs = ({ children }: { children: React.ReactNode }) => {
  const [activeTab, setActiveTab] = useState(0);
  return (
    <TabsContext.Provider value={{ activeTab, setActiveTab }}>
      {children}
    </TabsContext.Provider>
  );
};

Tabs.List = function TabsList({ children }) { ... };
Tabs.Tab = function Tab({ children, index }) { ... };
Tabs.Panel = function TabPanel({ children, index }) { ... };

---

Error Boundaries

Error Boundary Implementation

Applications MUST have error boundaries for graceful error handling:

'use client';

import { Component, ReactNode } from 'react';

interface Props {
  children: ReactNode;
  fallback: ReactNode;
}

interface State {
  hasError: boolean;
}

class ErrorBoundary extends Component<Props, State> {
  state = { hasError: false };

  static getDerivedStateFromError() {
    return { hasError: true };
  }

  componentDidCatch(error: Error, info: React.ErrorInfo) {
    console.error('Error boundary caught:', error, info);
  }

  render() {
    if (this.state.hasError) {
      return this.props.fallback;
    }
    return this.props.children;
  }
}

Note: Error boundaries MUST be class components (the only exception to the functional-only rule).

---

Suspense and Lazy Loading

Code Splitting

Route-level components SHOULD use lazy loading:

import { lazy, Suspense } from 'react';

const Dashboard = lazy(() => import('./Dashboard'));
const Settings = lazy(() => import('./Settings'));

function App() {
  return (
    <Suspense fallback={<PageSkeleton />}>
      <Routes>
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/settings" element={<Settings />} />
      </Routes>
    </Suspense>
  );
}

Suspense Boundaries

Async operations MUST be wrapped in Suspense boundaries:

function UserProfile({ userId }) {
  return (
    <Suspense fallback={<ProfileSkeleton />}>
      <UserDetails userId={userId} />
      <Suspense fallback={<PostsSkeleton />}>
        <UserPosts userId={userId} />
      </Suspense>
    </Suspense>
  );
}

Loading States

Suspense fallbacks SHOULD be meaningful skeleton states, not spinners:

// RECOMMENDED
<Suspense fallback={<ArticleSkeleton />}>
  <Article />
</Suspense>

// AVOID
<Suspense fallback={<Spinner />}>
  <Article />
</Suspense>