Fun with tuple types in Typescript 3.0

tuples.ts

type Length<T extends any[]> = T["length"];
type Head<T extends any[]> = T[0];
type Tail<T extends any[]> = ((...xs: T) => any) extends (x: any, ...t: infer T) => any ? T : never;

type Last<T extends any[]> = {
    0: never;
    1: T[0];
    ">1": Last<Tail<T>>;
}[Length<T> extends 0 ? 0 : Length<T> extends 1 ? 1 : ">1"];

type T1 = [number, string, boolean, string, { a: number }];
type LengthT1 = Length<T1>; // 5
type LastT1 = Last<T1>; // { a: number }
type HeadT1 = Head<T1>; // number
type TailT1 = Tail<T1>; // [string, boolean, string, { a: number }]

// Validations

const lengthT1: LengthT1 = 5;
const lastT1: LastT1 = { a: 1 };
const headT1: HeadT1 = 42;
const tailT1: TailT1 = ["hello", true, "world", { a: 2 }];