Typing functions and objects

function gst(price: number, rate: number): number {
  return (price * rate) / 100;
}

gst(500, 18);   // 90
gst(500);       // Error: Expected 2 arguments, but got 1.
gst("500", 18); // Error: Argument of type 'string' is not assignable to parameter of type 'number'.

function greet(name: string, title?: string): string {
  return title ? `Namaste, ${title} ${name}` : `Namaste, ${name}`;
}

function finalPrice(amount: number, discount = 0): number {
  return amount - discount; // discount is inferred as number
}

function logOrder(id: string): void {
  console.log("Order placed:", id); // returns nothing useful
}

type Product = {
  readonly id: string;  // cannot be changed after creation
  name: string;
  price: number;
  inStock?: boolean;    // optional: may be missing
};

interface Customer {
  name: string;
  phone: string;
  city?: string;
}

const rice: Product = { id: "p1", name: "Basmati rice 1kg", price: 120 };
rice.price = 110;  // OK
rice.id = "p2";    // Error: Cannot assign to 'id' because it is a read-only property.

const cart: Product[] = [
  { id: "p1", name: "Rice", price: 120 },
  { id: "p2", name: "Toor dal", price: 160, inStock: true },
];

function applyToAll(prices: number[], change: (price: number) => number): number[] {
  return prices.map(change);
}

applyToAll([100, 200], (p) => p * 1.18); // p is inferred as number

const stock: Record<string, number> = { rice: 20, dal: 15 };
stock["sugar"] = 8; // any string key, number values

// The same idea written as an index signature:
type Stock = { [item: string]: number };