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JavaScript/TypeScript Domain Skill

TypeScript Essentials

Utility types (use these instead of manual definitions):

Type	Effect
Pick<T, K>	Subset of properties
Omit<T, K>	Exclude properties
Partial<T>	All optional
Required<T>	All required
Record<K, V>	Key-value map
ReturnType<typeof fn>	Extract return type
Awaited<Promise<T>>	Unwrap promise type
Extract<T, U> / Exclude<T, U>	Filter union members

Generics:

interface ApiResponse<T> {
  data: T;
  status: number;
}

function fetchData<T>(url: string): Promise<ApiResponse<T>> {
  return fetch(url).then(res => res.json());
}

Advanced TypeScript: See references/typescript-advanced.md for conditional types, mapped types, branded types, infer.

Configuration

TypeScript (tsconfig.json)

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "lib": ["ES2022", "DOM"],
    "strict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "esModuleInterop": true,
    "skipLibCheck": true
  }
}

ESLint

module.exports = {
  parser: '@typescript-eslint/parser',
  extends: [
    'eslint:recommended',
    'plugin:@typescript-eslint/recommended',
  ],
  rules: {
    '@typescript-eslint/no-explicit-any': 'error',
  },
};

Prettier

module.exports = {
  semi: true,
  singleQuote: true,
  trailingComma: 'all',
  printWidth: 100,
};

Package Scripts

{
  "scripts": {
    "dev": "vite",
    "build": "tsc && vite build",
    "test": "vitest",
    "lint": "eslint . --ext .ts,.tsx --fix",
    "format": "prettier --write \"src/**/*.{ts,tsx}\""
  }
}

Testing

import { describe, it, expect, vi } from 'vitest';

describe('UserService', () => {
  it('fetches user data', async () => {
    const user = await service.fetchUser('123');
    expect(user).toBeDefined();
    expect(user.id).toBe('123');
  });

  it('throws on invalid ID', async () => {
    await expect(service.fetchUser('invalid'))
      .rejects.toThrow('User not found');
  });
});

// Mocking
vi.mock('./api', () => ({ fetchUser: vi.fn() }));
vi.mocked(fetchUser).mockResolvedValue({ id: '1', name: 'Alice' });

React component testing:

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

it('submits form', async () => {
  render(<LoginForm onSubmit={vi.fn()} />);
  await userEvent.type(screen.getByLabelText(/email/i), '[email protected]');
  await userEvent.click(screen.getByRole('button', { name: /submit/i }));
  await waitFor(() => {
    expect(screen.getByText(/success/i)).toBeInTheDocument();
  });
});

See references/testing-examples.md for mocking patterns, integration tests, test factories.

React Patterns

Custom hook with cleanup:

function useUser(userId: string) {
  const [user, setUser] = useState<User | null>(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    let cancelled = false;
    fetchUser(userId).then(data => {
      if (!cancelled) setUser(data);
      setLoading(false);
    });
    return () => { cancelled = true; };
  }, [userId]);

  return { user, loading };
}

Performance:

const sorted = useMemo(() => [...items].sort(), [items]);
const handleClick = useCallback(() => doSomething(), []);
const MemoComponent = memo(({ data }: Props) => <div>{data}</div>);

See references/react-patterns.md for compound components, context, forms, code splitting.

Node.js Patterns

Async handler wrapper (Express):

function asyncHandler(fn: (req: Request, res: Response) => Promise<void>) {
  return (req: Request, res: Response, next: NextFunction) => {
    Promise.resolve(fn(req, res)).catch(next);
  };
}

app.get('/users/:id', asyncHandler(async (req, res) => {
  const user = await userService.findById(req.params.id);
  if (!user) { res.status(404).json({ error: 'User not found' }); return; }
  res.json(user);
}));

See references/node-patterns.md for middleware, streams, database patterns, error handling.

Common Patterns

Result type:

type Result<T, E = string> =
  | { success: true; data: T }
  | { success: false; error: E };

Type guard:

function isUser(obj: unknown): obj is User {
  return typeof obj === 'object' && obj !== null && 'id' in obj;
}

Discriminated union:

type Response =
  | { status: 'success'; data: User }
  | { status: 'error'; error: string }
  | { status: 'loading' };

function handle(response: Response) {
  switch (response.status) {
    case 'success': return response.data;  // type-narrowed
    case 'error': throw new Error(response.error);
  }
}

Anti-patterns Quick Reference

Anti-pattern	Fix
any type	Proper types or unknown with validation
Mutating arrays/objects in place	Spread: [...arr, item], { ...obj, key: val }
Nested .then() chains	async/await
Missing error handling on async	try/catch or .catch()
React state mutation arr.push()	setArr(prev => [...prev, item])
Missing useEffect deps	Include all referenced values
Unstable object refs in render	Hoist constants or useMemo
Circular imports	Extract shared code to shared.ts
console.log in production	Structured logger
Non-exhaustive switch on union	Add default: assertNever(x)

Naming: camelCase (variables, functions), PascalCase (classes, types, components), SCREAMING_SNAKE_CASE (constants).

Reference Documentation

• references/typescript-advanced.md - Generics, conditional types, mapped types, branded types
• references/react-patterns.md - Hooks, component patterns, performance, state management, forms
• references/node-patterns.md - Express, async patterns, streams, database patterns
• references/testing-examples.md - Jest/Vitest config, mocking, component testing, integration tests