interface-index-signature.ts

interface Sizes {
  sizes: string[];
}

interface Pizza extends Sizes {
  name: string;
  toppings?: number;
  getAvailableSizes(): void;
  [key: number]: string; // TypeScript index signatures must either be a ´string´ or a ´number´
}

let pizza: Pizza;

function createPizza(name: string, sizes: string[]): Pizza {
  return {
    name,
    sizes,
    getAvailableSizes() {
      return this.sizes;
    },
  };
}

pizza = createPizza("Pepperoni", ["small", "medium", "large"]);
pizza[1] = "PizzaHut";
pizza.toppings = 2;

console.log(`And the Pizza is => ${JSON.stringify(pizza)}`);