typescript-patterns

# TypeScript Patterns ## Type vs Interface | Use Case | Prefer | |----------|--------| | Object shape | interface | | Union types | type | | Extending shapes | interface | | Intersection | type | | Tuple/primitives | type | ## Key Patterns **Discriminated Unions** — model state with tagged types: ```typescript type Result<T> = { ok: true; value: T } | { ok: false; error: Error }; ``` **Type Guards** — narrow at runtime: ```typescript function isUser(obj: unknown): obj is User { return typeof obj === 'object' && obj !== null && 'id' in obj; } ``` **Generic Constraints:** ```typescript function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] { return obj[key]; } ``` ## Utility Types | Type | Purpose | |------|---------| | Partial\<T\> | All optional | | Required\<T\> | All required | | Pick\<T,K\> | Select properties | | Omit\<T,K\> | Remove properties | | Record\<K,V\> | Key-value map | | ReturnType\<T\> | Function return type | ## Avoid - `any` — use `unknown` and narrow - Type assertions (`as`) — use type guards - `!` operator — handle null properly - Missing return types on complex functions