unfold

unfold.js

const unfold = (f, seed) => {
  const go = (f, seed, acc) => {
    const res = f(seed);
    return res ? go(f, res[1], acc.concat([res[0]])) : acc
  }
  return go(f, seed, []);
}
unfold(x => x < 26 ? [String.fromCharCode(x+65), x + 1] : undefined, 0) // Alphabets in capital letter

const range = (begin, end) => {
   return unfold(x => x <= end ? [x, x+1]: undefined, begin)
}
range(67, 100);