/typescript-patterns

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

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