--- name: language-patterns description: Provides language-specific patterns for TypeScript, Python, and React including idioms, best practices, and common patterns. Use when implementing features in these languages. user-invocable: false disable-model-invocation: false --- # Language Patterns This skill provides language-specific patterns and best practices. Apply patterns that match the project's language and framework. --- ## TypeScript Patterns ### Type Safety **Use strict types over `any`:** ```typescript // Bad function process(data: any): any { return data.value; } // Good interface DataItem { value: string; count: number; } function process(data: DataItem): string { return data.value; } ``` **Use discriminated unions for variants:** ```typescript type Result = | { success: true; data: T } | { success: false; error: Error }; function handleResult(result: Result) { if (result.success) { // TypeScript knows result.data exists console.log(result.data); } else { // TypeScript knows result.error exists console.error(result.error); } } ``` **Use `unknown` over `any` for external data:** ```typescript async function fetchData(): Promise { const response = await fetch('/api/data'); return response.json(); } // Then validate/parse const data = await fetchData(); if (isValidData(data)) { // Now safely typed } ``` ### Null Handling **Use optional chaining and nullish coalescing:** ```typescript // Optional chaining const userName = user?.profile?.name; // Nullish coalescing (only for null/undefined) const displayName = userName ?? 'Anonymous'; // Combine them const city = user?.address?.city ?? 'Unknown'; ``` **Use type guards:** ```typescript function isUser(obj: unknown): obj is User { return ( typeof obj === 'object' && obj !== null && 'id' in obj && 'email' in obj ); } ``` ### Async Patterns **Prefer async/await over raw promises:** ```typescript // Good async function fetchUser(id: string): Promise { const response = await fetch(`/api/users/${id}`); if (!response.ok) { throw new Error(`Failed to fetch user: ${response.status}`); } return response.json(); } ``` **Handle errors properly:** ```typescript async function safeOperation(): Promise> { try { const data = await riskyOperation(); return { success: true, data }; } catch (error) { return { success: false, error: error as Error }; } } ``` **Parallel operations:** ```typescript // Run in parallel const [users, posts] = await Promise.all([ fetchUsers(), fetchPosts() ]); // With error handling const results = await Promise.allSettled([ fetchUsers(), fetchPosts() ]); ``` --- ## Python Patterns ### Type Hints **Use type hints for clarity:** ```python from typing import Optional, List, Dict def process_users( users: List[dict], filter_active: bool = True ) -> List[str]: """Process users and return their names.""" result: List[str] = [] for user in users: if filter_active and not user.get("active"): continue result.append(user["name"]) return result ``` **Use dataclasses for data containers:** ```python from dataclasses import dataclass from typing import Optional @dataclass class User: id: int email: str name: str active: bool = True profile: Optional[dict] = None ``` **Use Pydantic for validation:** ```python from pydantic import BaseModel, EmailStr class UserCreate(BaseModel): email: EmailStr name: str age: int class Config: extra = "forbid" # Reject unknown fields ``` ### Pythonic Patterns **Use comprehensions:** ```python # List comprehension names = [user.name for user in users if user.active] # Dict comprehension user_map = {user.id: user for user in users} # Generator for large data active_users = (user for user in users if user.active) ``` **Context managers for resources:** ```python # File handling with open("file.txt", "r") as f: content = f.read() # Database connections with get_db_connection() as conn: conn.execute(query) # Custom context manager from contextlib import contextmanager @contextmanager def timer(name: str): start = time.time() yield print(f"{name}: {time.time() - start:.2f}s") ``` **Use `pathlib` for paths:** ```python from pathlib import Path config_path = Path(__file__).parent / "config" / "settings.yaml" if config_path.exists(): content = config_path.read_text() ``` ### Error Handling ```python class ValidationError(Exception): """Raised when input validation fails.""" pass class NotFoundError(Exception): """Raised when a resource is not found.""" pass def get_user(user_id: int) -> User: user = db.query(User).get(user_id) if user is None: raise NotFoundError(f"User {user_id} not found") return user ``` --- ## React Patterns ### Component Patterns **Functional components with hooks:** ```tsx interface UserCardProps { user: User; onEdit: (user: User) => void; } function UserCard({ user, onEdit }: UserCardProps) { const handleClick = useCallback(() => { onEdit(user); }, [user, onEdit]); return (

{user.name}

); } ``` **Custom hooks for logic reuse:** ```tsx function useUser(userId: string) { const [user, setUser] = useState(null); const [loading, setLoading] = useState(true); const [error, setError] = useState(null); useEffect(() => { setLoading(true); fetchUser(userId) .then(setUser) .catch(setError) .finally(() => setLoading(false)); }, [userId]); return { user, loading, error }; } ``` ### State Management **Use appropriate state level:** ```tsx // Local state - component only const [isOpen, setIsOpen] = useState(false); // Lifted state - shared between siblings // Put in common parent // Context - deeply nested sharing const ThemeContext = createContext("light"); // External store - complex app state // Use Redux, Zustand, or similar ``` **Derive state when possible:** ```tsx // Bad: redundant state const [items, setItems] = useState([]); const [totalCount, setTotalCount] = useState(0); // Good: derive from source of truth const [items, setItems] = useState([]); const totalCount = items.length; ``` ### Performance **Memoization:** ```tsx // Memoize expensive computations const sortedItems = useMemo( () => items.sort((a, b) => a.name.localeCompare(b.name)), [items] ); // Memoize callbacks passed to children const handleClick = useCallback(() => { doSomething(id); }, [id]); // Memoize components const MemoizedChild = memo(ChildComponent); ``` **Lazy loading:** ```tsx const HeavyComponent = lazy(() => import('./HeavyComponent')); function App() { return ( }> ); } ``` ### Error Boundaries ```tsx class ErrorBoundary extends Component { state = { hasError: false, error: null }; static getDerivedStateFromError(error: Error) { return { hasError: true, error }; } componentDidCatch(error: Error, info: ErrorInfo) { logError(error, info); } render() { if (this.state.hasError) { return ; } return this.props.children; } } ``` --- ## General Best Practices ### Naming Conventions | Language | Variables | Functions | Classes | Constants | |----------|-----------|-----------|---------|-----------| | TypeScript | camelCase | camelCase | PascalCase | UPPER_SNAKE | | Python | snake_case | snake_case | PascalCase | UPPER_SNAKE | | React | camelCase | camelCase/use* | PascalCase | UPPER_SNAKE | ### File Organization **TypeScript/React:** ``` src/ components/ Button/ Button.tsx Button.test.tsx index.ts hooks/ utils/ types/ ``` **Python:** ``` src/ package/ __init__.py models.py services.py utils.py tests/ test_models.py test_services.py ``` ### Import Organization **TypeScript:** ```typescript // 1. External packages import React from 'react'; import { useState } from 'react'; // 2. Internal modules (absolute) import { Button } from '@/components'; import { useAuth } from '@/hooks'; // 3. Relative imports import { helper } from './utils'; import type { Props } from './types'; ``` **Python:** ```python # 1. Standard library import os from pathlib import Path # 2. Third-party packages import requests from pydantic import BaseModel # 3. Local imports from .models import User from .utils import helper ```

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