Type Narrowing (টাইপ ন্যারোয়িং)

function printValue(value: string | number): void {
  if (typeof value === "string") {
    console.log("টেক্সট:", value.toUpperCase());
  } else {
    console.log("সংখ্যা:", value.toFixed(2));
  }
}

printValue("hello");
printValue(3.1416);

function getLength(text?: string): number {
  if (text) {
    return text.length;    // এখানে text: string
  }
  return 0;
}

function pick(x: string | number, y: string | boolean) {
  if (x === y) {
    // এখানে x ও y দুইটিই string
    console.log(x.toUpperCase());
  }
}

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

function move(animal: Fish | Bird): void {
  if ("swim" in animal) {
    animal.swim();    // narrowing: Fish
  } else {
    animal.fly();     // narrowing: Bird
  }
}

function addOrLog(value: string[] | Date): void {
  if (value instanceof Date) {
    console.log(value.toISOString());
  } else {
    console.log(value.join(", "));
  }
}

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; size: number }
  | { kind: "rect"; width: number; height: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.size ** 2;
    case "rect":
      return shape.width * shape.height;
  }
}

console.log(area({ kind: "circle", radius: 3 }));   // ≈ 28.27