Conditional Types (শর্তাধীন টাইপ)

// T extends U ? X : Y
type Wrapper<T> = T extends string ? "string-type" : "other-type";

type A = Wrapper<string>;     // "string-type"
type B = Wrapper<number>;     // "other-type"

// Custom utility — নিজের IsArray
type IsArray<T> = T extends unknown[] ? true : false;

type C = IsArray<string[]>;   // true
type D = IsArray<string>;     // false

type ToArray<T> = T extends unknown ? T[] : never;

// union distributive হয় — প্রতিটি member আলাদা
type Result = ToArray<string | number>;
// → string[] | number[]  (হয় string[], না হয় number[])

type Unwrap<T> = T extends Promise<infer U> ? U : T;

type A = Unwrap<Promise<string>>;   // string
type B = Unwrap<number>;            // number

// ফাংশনের return type বের করা
type Return<T> = T extends (...args: any[]) => infer R ? R : never;

const g = () => "হ্যালো";
type RT = Return<typeof g>;    // string

type GetData<T> = T extends { data: infer D } ? D : never;

interface ProductResponse {
  data: Product[];
  total: number;
}

interface Product {
  id: number;
  title: string;
}

type ProductData = GetData<ProductResponse>;   // Product[]