React Component Design

React Component Design

Purpose

React components MUST be pure, composable, and reusable abstractions. This skill enforces TypeScript-first design, separation of concerns, and deterministic rendering to ensure maintainable, testable UI code that works reliably across different contexts.

When to use

• Building a new UI element from scratch
• Refactoring a large, monolithic React component
• Extracting shared UI patterns into a component library
• Adding new features to an existing component hierarchy
• Preparing components for cross-team reuse

When NOT to use

• Styling implementation details (use DOM Security Hardening skill instead)
• State management architecture decisions (use State Management Patterns skill)
• Performance optimization via memo/useMemo (handle at call site, not in component)

Inputs required

• Existing React codebase with TypeScript
• Clear understanding of component's single responsibility
• Props interface definition (before implementation)

Workflow

1. Define the API: Write the TypeScript interface for props BEFORE the component body. This clarifies the contract.
2. Verify Props: Ensure props contain ONLY what the component needs—no unnecessary inherited types.
3. Isolate Logic: Extract all complex state and side-effects into custom hooks (NEVER in component body).
4. Render JSX: Build static JSX structure based solely on props and hook return values.
5. Apply Styles: Use CSS classes via className prop. NEVER use inline style={{...}}.
6. Export Type: Export both the component AND its Props interface for consumers.
7. Verify Size: Confirm file does NOT exceed 150 lines. If it does, extract sub-components.

Rules

• MUST be a pure function: identical props = identical output
• MUST export a strictly typed TypeScript interface named [ComponentName]Props
• MUST NOT exceed 150 lines per file (extract sub-components if larger)
• MUST NOT use inline styles (style={{...}})
• MUST NOT drill props more than 2 levels deep
• MUST NOT fetch data directly (use hooks or parent component)
• MUST support className prop for customization

Anti-patterns

• Prop Drilling: Passing props down more than 2 levels deep (use Context, composition, or compound components)
• Inline Styles: Using style={{...}} objects instead of CSS classes
• God Components: Handling data fetching, business logic, and UI rendering in one file
• State in Props: Copying prop values into local state with useState(props.val)
• Implicit Dependencies: Accessing globals or services without prop parameters
• Class Components: Using class components instead of functional components with hooks

Failure conditions

• Component file exceeds 200 lines
• Props interface is not exported
• Component performs side effects without useEffect
• More than 2 levels of prop drilling detected

Validation checklist

• Props interface is explicitly exported and named [ComponentName]Props
• Component is a pure function with no render-time side effects
• No inline style={{...}} objects anywhere
• No prop drilling beyond 2 levels
• File is ≤ 150 lines
• All complex logic is extracted to custom hooks
• Component supports className prop for customization
• TypeScript types are strict (no any)
• No direct API/data fetching in component body

Output format

• File count: 1 primary .tsx file (+ additional files if refactored)
• Exports: One default export (component) + named export for Props interface
• Structure: Props interface → Component function → Export both
• Styling: CSS classes only, no inline styles
• Size: ≤ 150 lines per component file

Security considerations

• Components do NOT execute arbitrary user input; text content is always escaped
• Props are never used in DOM insertion without sanitization
• Event handlers are never created from strings or user input
• No use of dangerouslySetInnerHTML (violates DOM Security Hardening)

Agent execution notes

• Agent MAY: Create new components, refactor large components, add TypeScript interfaces
• Agent MUST NEVER: Add inline styles, drill props beyond 2 levels, add data fetching to component
• Agent MUST ASK: Before exceeding 150 lines, before changing prop contracts
• Agent MUST VALIDATE: All TypeScript is strict, no prop drilling, component is pure functional

Example

❌ Anti-pattern (Prop drilling, inline styles, state mirroring):

// Bad: mirrors props, inline styles, multiple responsibilities
export const Modal = ({ title, onClose, userId, userName, isOpen }) => {
  const [name, setName] = useState(userName); // ANTI-PATTERN: state from props
  
  return (
    <div style={{ position: 'fixed', top: 0 }}
      <h1>{title}</h1>
      <input value={name} => setName(e.target.value)} />
      <UserProfile userId={userId} name={name} /> {/* 2 levels */}
    </div>
  );
};

export const UserProfile = ({ userId, name }) => {
  return <UserBio userId={userId} name={name} />; {/* 3 levels: VIOLATION */}
};

✅ Correct pattern (Pure, CSS classes, no drilling):

interface ModalProps {
  title: string;
  onClose: () => void;
  isOpen: boolean;
  children: React.ReactNode;
}

export const Modal = ({ title, onClose, isOpen, children }: ModalProps) => {
  if (!isOpen) return null;
  
  return (
    <div className="modal-overlay"
      <div className="modal-content">
        <h1>{title}</h1>
        {children}
      </div>
    </div>
  );
};

interface UserProfileProps {
  userId: string;
}

export const UserProfile = ({ userId }: UserProfileProps) => {
  const user = useUser(userId); // Use hook, not props
  return (
    <div>
      <h2>{user?.name}</h2>
      <p>{user?.bio}</p>
    </div>
  );
};