Interfaces vs. Type Aliases: When to Use Which

1. Two Ways to Define Shapes

TypeScript provides two primary keywords for defining object shapes: interface and type. While they overlap significantly, they possess distinct capabilities.

2. Defining Interfaces & Inheritance (extends)

An interface defines an object contract that can be extended using the extends keyword:

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

// Extending an interface
interface AdminUser extends User {
  permissions: string[];
}

const admin: AdminUser = {
  id: 1,
  name: "Sarah",
  permissions: ["read", "write", "delete"],
};

3. Declaration Merging (Interfaces Only)

Multiple interface declarations with the same name in the same scope automatically merge their property definitions:

interface Window {
  customTitle: string;
}

interface Window {
  customVersion: number;
}

// Window interface now contains BOTH properties!

4. Type Aliases & Intersection Types (&)

Type aliases use Intersection Types (&) to combine object shapes:

type AuditLog = { timestamp: Date };
type UserLog = User & AuditLog; // Intersection combining shapes

5. Summary Comparison Matrix

6. Common Mistakes

• Attempting Declaration Merging with Type Aliases: Declaring type User = {...} twice throws a duplicate identifier compiler error. Use interfaces when library authors need expandable shapes!

7. Next Steps

Default to interface for OOP/library object shapes, and type for unions and utility types. Next, let's explore Union Types and Type Narrowing!