flat+deepFlat.js

const isIterable = a => !!(a && a[Symbol.iterator]);

function *flat(iter) {
  for (const a of iter) {
    if (isIterable(a)) yield *a;
    else yield a;
  }
}

function *deepFlat(iter) {
  for (const a of iter) {
    if (isIterable(a)) yield *deepFlat(a);
    else yield a;
  }
}

log([...flat([1, [2], [3, [4]], [[[5], 6]]])]);
// [1, 2, 3, [4], [[5], 6]];

log([...deepFlat([1, [2], [3, [4]], [[[5], 6]]])]);
// [1, 2, 3, 4, 5, 6];