name: typescript-patterns description: TypeScript best practices and patterns. Use when writing TypeScript, fixing type errors, or working with generics. version: 1.0.0 format: 2025-10-02 triggers:

• "writing TypeScript, fixing type errors, or working with generics" updated: 2026-04-25 status: ACTIVE

TypeScript Patterns

Type vs Interface

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

Avoid

• any — use unknown and narrow
• Type assertions (as) — use type guards
• ! operator — handle null properly
• Missing return types on complex functions