typescript-patterns

# TypeScript Patterns ## Core Rules - Strict mode always (`"strict": true`) - No `any` — use `unknown` for dynamic values - Explicit return types on all functions - `const` over `let`, never `var` ## Type Definitions ### Interfaces vs Types ```typescript // ✅ Interface for objects/classes (extensible) interface User { id: string email: string name: string } // ✅ Type for unions, primitives, computed type Status = 'active' | 'inactive' | 'pending' type UserOrAdmin = User | Admin type ReadonlyUser = Readonly<User> ``` ### Generics ```typescript // ✅ Reusable generic types type ApiResponse<T> = { data: T error: string | null status: number } type PaginatedResponse<T> = { items: T[] total: number page: number limit: number } // ✅ Generic functions const findById = <T extends { id: string }>(items: T[], id: string): T | undefined => items.find(item => item.id === id) ``` ## Utility Types ```typescript // Pick specific fields type UserPreview = Pick<User, 'id' | 'name'> // Omit sensitive fields type PublicUser = Omit<User, 'password' | 'salt'> // Make all optional (for partial updates) type UpdateUserDto = Partial<User> // Make all required type RequiredUser = Required<User> // Make all readonly type FrozenUser = Readonly<User> // Extract from union type ActiveStatus = Extract<Status, 'active' | 'pending'> // Record type type UserMap = Record<string, User> ``` ## Discriminated Unions ```typescript // ✅ Type-safe error handling type Result<T...