TypeScript Functions (ফাংশন)

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

// add("10", 5);    // ❌ string দেওয়া যাবে না
// add(1, 2, 3);    // ❌ অতিরিক্ত argument
console.log(add(2, 3));   // 5

function greet(name: string): string {
  return "হ্যালো, " + name;
}

// Optional — '?' থাকলে দেওয়াও যায়, না-দেওয়াও যায়
function introduce(name: string, title?: string): string {
  return title ? title + " " + name : name;
}

// Default — না দিলে ডিফল্ট মান বসে
function multiply(a: number, b: number = 2): number {
  return a * b;
}

console.log(multiply(5));    // 10
console.log(multiply(5, 4)); // 20

function sum(...numbers: number[]): number {
  return numbers.reduce((total, n) => total + n, 0);
}

console.log(sum(1, 2, 3));         // 6
console.log(sum(1, 2, 3, 4, 5));   // 15

type Calculator = (a: number, b: number) => number;

const addFn: Calculator = (a, b) => a + b;
const mulFn: Calculator = (a, b) => a * b;

function operate(op: Calculator, x: number, y: number) {
  return op(x, y);
}

console.log(operate(addFn, 2, 3));   // 5

function fetchUserName(callback: (name: string) => void) {
  // সিমুলেশন: পরে ডেটা আসবে
  const name = "রহিম";
  callback(name);
}

fetchUserName((name) => {
  console.log("ব্যবহারকারী:", name);
});

// Inline type
const double = (n: number): number => n * 2;

// Object টাইপ ফেরত দেয়
const makeUser: (name: string, age: number) => { name: string; age: number } =
  (name, age) => ({ name, age });

function format(value: string): string;
function format(value: number): string;
function format(value: string | number): string {
  return typeof value === "number"
    ? value.toFixed(2)
    : value.toUpperCase();
}

console.log(format(3.14159));   // "3.14"
console.log(format("hello"));   // "HELLO"

interface Counter {
  count: number;
  add(this: Counter, amount: number): void;
}

const counter: Counter = {
  count: 0,
  add(this: Counter, amount: number) {
    this.count += amount;
  },
};

counter.add(5);
console.log(counter.count);   // 5

function throwError(message: string): never {
  throw new Error(message);
}

function assertNever(value: never): never {
  throw new Error("অপ্রত্যাশিত মান: " + value);
}