const flatten = array => {
  const inner = (result, arr) => {
    if (!Array.isArray(arr)) {
      return [...result, arr]
    }
    return arr.reduce((acc, item) => [ ...acc, ...inner([], item)], result)
  }
  return inner([], array)
}

console.log(flatten([1, 2, [3, 4], [[5, 6, [7], [8, [9]]]]]))
// [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]