Advanced Generics – Variadic Tuple Types

Variadic Tuples (TS 4.0+)

function concat<T extends any[], U extends any[]>(...args: [...T, ...U]): [...T, ...U] {
  return args as any;
}

const result = concat([1, 2], ['a', 'b', 'c']); // inferred as [number, number, string, string, string]

Generic Constraints using extends and infer

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;
type Fn = (x: number) => string;
type R = ReturnType<Fn>; // string

Mapping over tuple elements

type MapTuple<T extends any[], F> = { [K in keyof T]: F };
type Numbers = MapTuple<[1, 2, 3], string>; // [string, string, string]