When to Use `type` vs `interface` in TypeScript

✅ Use interface when:

• You’re defining data models (User, Product, Order)
• You want to extend later (e.g. interface Admin extends User)
• You’re working with classes
• You need declaration merging (e.g. in libraries or configs)

interface Product {
  id: string;
  name: string;
}

✅ Use type when:

• You need a union or intersection
• You’re working with primitives, tuples or functions
• You want to compose types (e.g. type A = B & C)
• You’re defining utility types

type ID = string | number;
type ProductWithStock = Product & { stock: number };

🧭 Simple rule of thumb:

• For data models → use interface
• For type combinations → use type

✍️ Part of my personal cheatsheet at buildlogmmd – for fast recall in work situations.