const flattenArr = ([head, ...tail], resultingArray) =>
  (head === undefined)  // When there is no more input array
    ? resultingArray     // send back the result
    : (Array.isArray(head)) // Else is the head an array
      ? flattenArr([...head, ...tail])
      : flattenArr(tail, [...resultingArray, head])

const flatten = (n) => flattenArr(n, []);