Taking the last element in an array in TypeScript type system.

GetLast.ts

// Or just get the last element!
type GetLast<XS extends unknown[]>
  = [never, ...XS][XS['length']];

// => never
type Empty = TakeLast<[]>;

// => 1
type OneEl = TakeLast<[1]>;

// => 5
type MultEls = TakeLast<[1, 2, 3, 4, 5]>

TakeLast.ts

// Util for asserting types
export type Assert<X, T> = X extends T ? X : never;

// Take the last element of a tuple
// and returning the rest
type TakeLast<XS extends unknown[]> = XS extends [infer X, ...infer YS]
  ? YS extends []
    ? [[], X]
    : TakeLast<YS> extends [infer ZS, infer R]
    ? [[X, ...Assert<ZS, unknown[]>], R]
    : never
  : never;

// => never
type Empty = TakeLast<[]>;

// => [[], 1]
type OneEl = TakeLast<[1]>;

// => [[1, 2, 3, 4], 5]
type MultEls = TakeLast<[1, 2, 3, 4, 5]>