Why Typescript?

function add(a: number, b: number) {
  return a + b;
}

add(10, 20);     // ✅ OK
add(10, "20");   // ❌ Error: Argument of type 'string' is not assignable to parameter of type 'number'

interface User {
  id: string;
  name: string;
  email?: string; // optional
}

const user: User = {
  id: "u1",
  name: "Aminul",
};

type ApiResponse<T> = {
  success: boolean;
  data: T;
  message?: string;
};

type Patient = {
  id: string;
  name: string;
  phone: string;
};

const res: ApiResponse<Patient> = {
  success: true,
  data: { id: "p1", name: "Rahim", phone: "017xxxxxxx" },
};

type Status = "PENDING" | "PAID" | "UNPAID";

function getBadge(status: Status) {
  if (status === "PAID") return "✅ Paid";
  if (status === "PENDING") return "⏳ Pending";
  return "❌ Unpaid";
}