Perluasan dan Penggabungan Antarmuka

interface Animal { name: string; }
interface Dog extends Animal {
  breed: string;
}
const rex: Dog = { name: 'Rex', breed: 'Labrador' };

interface Flyable { fly(): void; }
interface Swimmable { swim(): void; }
interface Duck extends Flyable, Swimmable {
  quack(): void;
}

interface Base { value: string | number; }
interface Derived extends Base { value: string; } // OK — string is narrower
// interface Bad extends Base { value: boolean; } // Error

interface Window { title: string; }
interface Window { location: Location; }
// Merged: Window has both title and location

// express.d.ts
declare namespace Express {
  interface Request {
    user?: User;
  }
}

type Named = { name: string };
interface User extends Named { email: string; }

interface A { a: number; }
interface B extends A { b: number; }
interface C extends B { c: number; }
const obj: C = { a: 1, b: 2, c: 3 };

interface Serializable { serialize(): string; }
class Model implements Serializable {
  serialize() { return JSON.stringify(this); }
}