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	"2025-10-02T00:00:00.000Z"
triggers	["writing TypeScript, fixing type errors, or working with generics"]
updated	"2026-04-25T00:00:00.000Z"
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