Frontend Developer

Frontend Developer

Overview

This skill provides expertise in modern frontend development with React, TypeScript, and current best practices. Apply this skill when building components, fixing frontend bugs, optimizing performance, or ensuring accessibility compliance.

Core Principles

React Development Standards

Component Architecture:

• Use functional components with hooks exclusively
• Implement single responsibility principle - separate UI from business logic
• Prefer composition over conditional rendering
• Extract complex logic into custom hooks prefixed with "use"
• Return a cleanup function from useEffect for subscriptions and timers

Custom Hooks:

• Always prefix custom hooks with "use" (e.g., useOnlineStatus)
• Keep hooks focused on a single concern
• Return stable object references to prevent unnecessary re-renders
• Clean up all side effects (subscriptions, timers, async operations)

Example implementation:

function useOnlineStatus() {
  const [isOnline, setIsOnline] = useState(navigator.onLine);

  useEffect(() => {
    const handleOnline = () => setIsOnline(true);
    const handleOffline = () => setIsOnline(false);

    window.addEventListener('online', handleOnline);
    window.addEventListener('offline', handleOffline);

    return () => {
      window.removeEventListener('online', handleOnline);
      window.removeEventListener('offline', handleOffline);
    };
  }, []);

  return isOnline;
}

State Management

Core Principles:

• Derive state, don't sync it - Calculate values during render instead of managing multiple related state variables
• Implement single sources of truth to prevent synchronization bugs
• Model async state with explicit loading and error states
• Use React Context for shared state across component trees

When to use useReducer vs useState:

Use useReducer when:

• State has multiple sub-values that change together
• Next state depends on previous state values
• Complex state transitions with multiple actions
• State logic becomes difficult to test inline
• Calling multiple setState functions in sequence

Use useState when:

• Simple primitive values (boolean, string, number)
• Independent state variables
• Straightforward state updates without complex logic

useReducer Example:

const initialFormState = {
  data: { name: '', email: '', company: '' },
  loading: false,
  errors: {},
  submitted: false
};

function formReducer(state, action) {
  switch (action.type) {
    case 'FIELD_CHANGE':
      return {
        ...state,
        data: { ...state.data, [action.field]: action.value },
        errors: { ...state.errors, [action.field]: null }
      };
    case 'SUBMIT_START':
      return { ...state, loading: true, errors: {}, submitted: false };
    case 'SUBMIT_SUCCESS':
      return { ...state, loading: false, submitted: true };
    case 'SUBMIT_ERROR':
      return { ...state, loading: false, errors: action.errors };
    case 'RESET':
      return initialFormState;
    default:
      return state;
  }
}

State Derivation Example:

// ✅ Good - Single source with derived values
function GameBoard() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  // Derive these instead of managing as separate state
  const winner = calculateWinner(squares);
  const nextPlayer = calculateNextPlayer(squares);
  const status = calculateStatus(winner, squares, nextPlayer);

  return <div>{status}</div>;
}

// ❌ Bad - Manual synchronization of multiple state variables
function GameBoard() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  const [winner, setWinner] = useState(null);
  const [nextPlayer, setNextPlayer] = useState('X');
  const [status, setStatus] = useState('Next player: X');

  // Manually sync all related state - error-prone!
  useEffect(() => {
    const newWinner = calculateWinner(squares);
    const newNextPlayer = calculateNextPlayer(squares);
    const newStatus = calculateStatus(newWinner, squares, newNextPlayer);

    setWinner(newWinner);
    setNextPlayer(newNextPlayer);
    setStatus(newStatus);
  }, [squares]);
}

TypeScript & Type Safety

Strict Typing:

• Use strict typing, avoid any type
• Define interfaces for object shapes and component props
• Use Zod schemas for runtime validation and type inference
• Always use z.infer<typeof schema> to derive types from Zod schemas
• Wrap feature subtrees in error boundaries that render a fallback UI

Performance Optimization

Optimization Strategy:

• Implement code splitting and lazy loading for routes and components
• Use virtualization (react-window, react-virtuoso) for large lists
• Serve images at their displayed size and lazy-load below-the-fold images
• Minimize bundle size through tree shaking and dead code elimination
• Profile performance with React DevTools and browser tools

Memoization Guidelines:

• Use useMemo ONLY for expensive computations - measure first!
• Use useCallback for callback functions passed to memoized child components
• Use React.memo for expensive components to avoid unnecessary re-renders

Example:

// ✅ Good - Use useMemo ONLY for expensive derived calculations
function ExpensiveComponent() {
  const [items, setItems] = useState([]);
  const [filter, setFilter] = useState('');

  // Only for computationally expensive operations - measure first!
  const filteredAndSortedItems = useMemo(() => {
    return items
      .filter(item => item.name.includes(filter))
      .sort((a, b) => expensiveComparisonFunction(a, b));
  }, [items, filter]);

  return <ItemList items={filteredAndSortedItems} />;
}

React SOLID Principles

Single Responsibility: Separate data fetching logic from UI rendering.

// ✅ Good - Custom hook handles data fetching
function useUserData(fetchUser) {
  const [user, setUser] = useState(null);
  useEffect(() => {
    fetchUser().then(setUser);
  }, [fetchUser]);
  return user;
}

function UserCard({ fetchUser }) {
  const user = useUserData(fetchUser);
  if (!user) return <p>Loading...</p>;
  return (
    <div>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
    </div>
  );
}

Open/Closed Principle: Extend behavior via composition, not modification.

// ✅ Good - Composable Alert component
function Alert({ className, children }) {
  return <div className={className}>{children}</div>;
}

const SuccessAlert = ({ message }) => (
  <Alert className="alert-success">{message}</Alert>
);

const ErrorAlert = ({ message }) => (
  <Alert className="alert-error">{message}</Alert>
);

TanStack Query Best Practices

Optimistic Updates:

const mutation = useMutation({
  mutationFn: createItem,
  onMutate: async newItem => {
    // Cancel outgoing refetches
    await queryClient.cancelQueries({ queryKey: ['items'] });

    // Snapshot previous value
    const previousItems = queryClient.getQueryData(['items']);

    // Optimistically update cache
    queryClient.setQueryData(['items'], old => [...old, newItem]);

    // Return context with snapshot
    return { previousItems };
  },
  onError: (err, newItem, context) => {
    // Rollback on error
    queryClient.setQueryData(['items'], context.previousItems);
  },
  onSettled: () => {
    // Refetch after error or success
    queryClient.invalidateQueries(['items']);
  },
});

Accessibility Standards

Essential Requirements:

• Use semantic HTML elements (button, nav, main, section)
• Add ARIA labels and roles to non-semantic interactive elements
• Support keyboard navigation; manage focus on route changes and modal open/close
• Maintain color contrast ratios meeting WCAG AA standards
• Test with screen readers and accessibility tools

Code Organization

Import and File Structure:

• Follow ordered imports: builtin → external → internal → parent → sibling
• Use absolute imports for cleaner dependency management
• Organize by feature domains with clear boundaries
• Extract conditional logic into well-named variables
• Group related functionality in cohesive modules

Implementation Workflow

When applying this skill to frontend development tasks:

1. Analyze Requirements: Understand the specific frontend task - component creation, bug fixing, performance optimization, or accessibility improvements

2. Review Existing Code: Use Read/Grep to examine current implementation and identify patterns or issues

3. Implement: Apply the patterns from the sections above (functional components, TypeScript strict mode, the listed performance and accessibility rules)

Common Anti-Patterns to Avoid

State Management:

• ❌ Using multiple useState calls for interdependent state
• ❌ Syncing state manually with useEffect
• ❌ Creating derived state instead of computing during render

Performance:

• ❌ Premature optimization with useMemo before measuring
• ❌ Using useCallback without React.memo on child components
• ❌ Deeply nested component trees without lazy loading

TypeScript:

• ❌ Using any type to bypass type checking
• ❌ Missing prop interfaces for components
• ❌ Ignoring type errors with @ts-ignore

Accessibility:

• ❌ Using div instead of button for clickable elements
• ❌ Missing alt text on images
• ❌ Insufficient color contrast
• ❌ No keyboard navigation support