type vs interface: extends and intersection

Pick the right construct to model and compose your object types without duplicating code.

8 min · 3 questions

You already know how to declare object types. The real question on a project is choosing between type and interface, then composing existing types without duplicating code. Let's start with the most common scenario: modeling entities that look alike.

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

interface Admin extends User {
  permissions: string[];
}

const admin: Admin = {
  id: 1,
  name: "Ada",
  permissions: ["users:write"],
};

extends creates a clear relationship: an Admin is a User with extra powers. With a type alias, you get the same result through the intersection operator &, which merges several shapes into one. It's the perfect tool for cross-cutting concerns, like timestamps present on all your entities.

type Timestamps = {
  createdAt: Date;
  updatedAt: Date;
};

type Product = {
  id: number;
  label: string;
};

type StoredProduct = Product & Timestamps;

So which one should you choose? Two differences really matter. First, only type can name a union like 'active' | 'archived'. Second, two interface declarations sharing the same name merge automatically: handy for augmenting a library's types, dangerous when it happens by accident.

type Status = "active" | "archived";

interface Config {
  locale: string;
}

interface Config {
  debug: boolean;
}

const config: Config = { locale: "fr", debug: true };

Knowledge check

Make sure you remember the key points of this lesson.

1. You need to name the type 'draft' | 'published'. Which construct is required?
   • interface
   • type
   • Both work
   • enum
2. Two interface Config { ... } declarations coexist in the same scope. What happens?
   • An immediate compile error
   • The second one overwrites the first
   • They merge into a single interface
   • The first one takes precedence
3. Which operator combines two object type aliases into a single type requiring the properties of both?
   • |
   • &
   • extends