Type Guard (টাইপ গার্ড)

type Fish = { name: string; swim: () => void };
type Bird = { name: string; fly: () => void };

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

const pets: (Fish | Bird)[] = [
  { name: "কই", swim: () => console.log("সাঁতার") },
  { name: "টিয়া", fly: () => console.log("উড়া") },
];

const fishes = pets.filter(isFish);   // টাইপ: Fish[]

function feed(pet: Fish | Bird): void {
  if (isFish(pet)) {
    pet.swim();    // pet: Fish
  } else {
    pet.fly();     // pet: Bird
  }
}

// Array filtering-এও টাইপ বজায় থাকে guard-এর কারণে
const names = fishes.map((fish) => fish.name);   // string[]

type Video = { type: "video"; duration: number };
type Article = { type: "article"; wordCount: number };

const isVideo = (item: Video | Article): item is Video =>
  item.type === "video";

function preview(item: Video | Article): string {
  return isVideo(item)
    ? item.duration + " সেকেন্ড"
    : item.wordCount + " শব্দ";
}