Array|Link

arrayAndLink.js

function ListNode(val, next) {
  this.val = (val === undefined ? 0 : val)
  this.next = (next === undefined ? null : next)
}

function List2Array (list, arr = []) {
  arr.push(list.val)
  if (list.next instanceof ListNode) {
    List2Array(list.next, arr)
  }
  return arr
}

function Array2List(arr, list = new ListNode()) {
  if (arr.length) {
    list.val = arr.shift()
    list.next = arr.length ? new ListNode() : null
    Array2List(arr, list.next)
  }
  return list
}