Extending Interfaces

An interface can extends one or more other interfaces and add its own properties.

interface Entity {
  id: number;
  createdAt: string;
}

interface Person {
  name: string;
  email: string;
}

interface Customer extends Entity, Person {
  loyaltyPoints: number;
}

const c: Customer = {
  id: 1, createdAt: "2026-01-15T10:00:00Z",
  name: "Vikram", email: "[email protected]",
  loyaltyPoints: 320,
};

function welcome(p: Person): string {
  return "Welcome back, " + p.name;
}

console.log(welcome(c)); // a Customer is also a Person
console.log(c.loyaltyPoints);

Welcome back, Vikram
320

TypeScript checks the shape, not the name. Any object that has the needed properties fits, even if it never mentions the interface. This is called structural typing ("duck typing").

interface HasPrice {
  price: number;
}

function withGst(item: HasPrice): number {
  return Math.round(item.price * 1.18);
}

const book = { title: "Clean Code", price: 500 };
const course = { name: "TypeScript", price: 1999, hours: 20 };
console.log(withGst(book), withGst(course));

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