Careeroza

Defining Interfaces

The interface Keyword

An interface is another way to name an object type. It can describe properties, optional and readonly properties, and methods.

Example

interface Product {
  readonly id: number;
  name: string;
  price: number;
  discountPercent?: number;
  finalPrice(): number;
}

const phone: Product = {
  id: 11,
  name: "ShopEase Phone X",
  price: 19999,
  discountPercent: 10,
  finalPrice() {
    return this.price - (this.price * (this.discountPercent ?? 0)) / 100;
  },
};

console.log(phone.name, "->", phone.finalPrice());

Output

ShopEase Phone X -> 17999.1

Missing Method

Example

An object must implement everything the interface requires (TS2741).

interface Greeter {
  name: string;
  greet(): string;
}

const g: Greeter = { name: "Bot" };

Compiler output (tsc)

Line 6: error TS2741: Property 'greet' is missing in type '{ name: string; }' but required in type 'Greeter'.