Part 3.3Interfaces and object shapes

Interfaces name the shape expected from an object. They are compile-time descriptions, not JavaScript classes or values created at runtime.

Interfaces

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

const user: User = { id: 1, name: "Ada" };

TypeScript uses structural typing: an object is suitable when it has the required shape, even if it was created elsewhere.

Optional properties

Add ? when a property may be absent. With strict checking, test it before assuming it is present.

interface User { name: string; email?: string; }