Array to linked list in javascript

arrayToLinkedList.js

function ListNode(x) {
    this.value = x;
    this.next = null;
}
  
function SinglyList(){
    this.head = null;
}
  
SinglyList.prototype.add = function(data) {
  currentNode = this.head;
  for (var i in data){
    var newNode = new ListNode(data[i]);
    if(currentNode == null){
      this.head = newNode;
      this._lenght++;
      currentNode = this.head;
    }
    else {
      currentNode.next = newNode;
      currentNode = currentNode.next;
    }
  }
  return this.head;
};

var arrayToLinkedlist = function(arr){
  if (arr == null) {
    return [];
  }

  if (!Array.isArray(arr)) {
    throw new TypeError('array-to-linkedlist expects an array.');
  }

  var singly = new SinglyList();
  return singly.add(arr);
}