TypeScript Interfaces vs Types

The Short Answer

Both interface and type describe the shape of data. For object shapes they are almost interchangeable. Use interface for objects/classes, and type for unions, primitives, and tuples.

Defining an Object

interface User {
  id: number;
  name: string;
}

type UserT = {
  id: number;
  name: string;
};

What Only "type" Can Do

// Union types
type Status = "active" | "inactive" | "banned";

// Primitive alias
type ID = string | number;

// Tuple
type Point = [number, number];

What Interfaces Do Better

// Declaration merging — two interfaces with the same name combine
interface Window { customProp: string; }
interface Window { anotherProp: number; }
// Both props now exist on Window

// Extending
interface Admin extends User {
  role: string;
}

Comparison Table

Feature	interface	type
Object shapes	✅	✅
Unions	❌	✅
Tuples	❌	✅
Declaration merging	✅	❌
extends / implements	✅	via &

FAQs

Which should I default to?

A common convention: interface for public API object shapes, type for everything else. Consistency matters more than the choice.

Can they reference each other?

Yes — an interface can extend a type and vice versa. More in our TypeScript section.