unfold.js

function unfold(p, f, g, seed, reduce, tailgen) {
  function recursive(iter) {
    if (p(iter)) return tailgen(iter);
    return reduce(f(iter), recursive(g(iter)));
  }

  return recursive(seed);
}

function sum(a) {
  return unfold(
    ([iter, value]) => value.done,
    ([iter, value]) => value.value,
    ([iter, value]) => [iter, iter.next()],
    (() => { const iter = a[Symbol.iterator](); return [iter, iter.next()]; })(),
    (head, tail) => head + tail,
    ([iter, value]) => 0,
  );
}

console.log(sum([1,2,3]));