Type Aliases & Interfaces

Type Aliases (type)

type Point = { x: number; y: number };
let p: Point = { x: 1, y: 2 };

Aliasing works for any type, including unions, intersections, and primitive literals.

Interfaces

interface Person {
  name: string;
  age?: number; // optional property
}

const alice: Person = { name: "Alice" };

Extending Interfaces

interface Employee extends Person {
  employeeId: string;
}

const bob: Employee = { name: "Bob", employeeId: "E123" };

Declaration merging

If you declare the same interface twice, they merge:

interface Window { myProp: string; }
interface Window { another: number; }
// Window now has both properties