SOURCE

function Node(number) {
  this.element = number;
  this.prev = null;
  this.next = null;
}

function DoubleLinkedList() {
  this.head = null;
  this.tail = null;
  this.length = 0;
}


DoubleLinkedList.prototype = {
  constructor: DoubleLinkedList,
  insert: function(pos, number) {
    if (pos >= 0 && pos <= this.length) {
      var node = new Node(number);
      var currentNode = this.head;
      var previousNode;
      var index = 0;
      if (pos === 0) {
        if (!this.head) {
          this.head = node;
          this.tail = node;
        } else {
          node.next = this.head;
          currentNode.prev = node;
          this.head = node;
        }
      } else if (pos === this.length) {
        currentNode = this.tail;
        currentNode.next = node;
        node.prev = currentNode;
        this.tail = node;
      } else {
        while (index++ < pos) {
          previousNode = currentNode;
          currentNode = currentNode.next;
        }
        node.next = currentNode;
        currentNode.prev = node;
        node.prev = previousNode;
        previousNode.next = node;
      }
      this.length++;
      return true;
    } else {
      return false;
    }
  }
}