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:
type Result<T> = { ok: true; value: T } | { ok: false; error: Error };Type Guards — narrow at runtime:
function isUser(obj: unknown): obj is User {
return typeof obj === 'object' && obj !== null && 'id' in obj;
}Generic Constraints:
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— useunknownand narrow- Type assertions (
as) — use type guards !operator — handle null properly- Missing return types on complex functions