Preserve tuple length when mapping

tuples.ts

interface ReadonlyArray<T> {
  map<U>(
    callbackfn: (value: T, index: number, array: T[]) => U,
    thisArg?: any
  ): { [K in keyof this]: U };
}

type Coord = readonly [number, number, number];

type BiFunction<T> = (a: T, b: T) => T;

function map(a: Coord, f: (i: number) => number): Coord {
  return a.map(f);
}

function zip(a: Coord, b: Coord, f: BiFunction<number>): Coord {
  return a.map((v, i) => f(v, b[i]));
}

var moved = zip([0, 1, 2], [1, 1, 1], (a, b) => a + b);

console.log(moved);