zip.ts

function zip(xs, ys) {
  if (xs.length < ys.length) {
    return xs.map((x, idx) => [x, ys[idx]])
  } else {
    return ys.map((y, idx) => [xs[idx], y])
  }
}

// FP Style

function head<T>(xs: T[]): T {
  return xs[0]
}

function tail<T>(xs: T[]): T[] {
  return xs.slice(1)
}

function zip<T, U>(xs: T[], ys: U[], zipped: [T, U][] = []): [T, U][] {
  if (xs.length === 0 || ys.length === 0) return zipped
  return zip(tail(xs), tail(ys), [...zipped, [head(xs), head(ys)]])
}

// - Currying
function zip<U>(ys: U[]) {
  return function forXs<T>(xs: T[]) {
    return function appliedTailCall(zipped: [T, U][] = []): [T, U][] {
      if (xs.length === 0 || ys.length === 0) return zipped
      return zip(tail(ys))(tail(xs))([...zipped, [head(xs), head(ys)]])
    }
  }
}

// - w/o proper tail call
function zip<T, U>(xs: T[], ys: U[]): [T, U][] {
  if (xs.length === 0 || ys.length === 0) return []
  return [[head(xs), head(ys)], ...zip(tail(xs), tail(ys))]
}