Linked List with javascript

linked-list.js

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

var MyLinkedList = function () {
  this.head = null;
};

/**
 * @param {number} index
 * @return {number}
 */
MyLinkedList.prototype.get = function (index) {
  let head = this.head;
  if (head && index >= 0) {
    let i = 0;
    while (head) {
      if (i === index) {
        return head.val;
      }
      head = head.next;
      i++;
    }
  }
  return -1;
};

/**
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtHead = function (val) {
  const newNode = new ListNode(val, this.head);
  this.head = newNode;
};

/**
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtTail = function (val) {
  let head = this.head;
  while (head && head.next) {
    head = head.next;
  }

  if (!head) {
    this.addAtHead(val);
  } else {
    head.next = new ListNode(val, null);
  }
};

/**
 * @param {number} index
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtIndex = function (index, val) {
  const size = this.size();
  if (index >= 0 && index <= size) {
    let head = this.head;
    let before = null;
    let i = 0;
    while (head && index != i) {
      i++;
      before = head;
      head = head.next;
    }
    const node = new ListNode(val, null);
    if (before === null) {
      node.next = this.head;
      this.head = node;
    } else {
      before.next = node;
      node.next = head;
    }
  }
};

/**
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function (index) {
  const size = this.size();
  if (this.head && index >= 0 && index < size) {
    let head = this.head;
    let before = null;
    let i = 0;
    while (head && index != i) {
      i++;
      before = head;
      head = head.next;
    }
    if (before === null) {
      this.head = this.head.next;
    } else {
      before.next = head.next;
    }
  }
};

MyLinkedList.prototype.getList = function () {
  let head = this.head;
  let numbers = [];
  while (head) {
    numbers.push(head.val);
    head = head.next;
  }
  return numbers;
};

MyLinkedList.prototype.size = function () {
  let head = this.head;
  let count = 0;
  while (head) {
    count++;
    head = head.next;
  }
  return count;
};