JavaScript data structure bidirectional linked list

// Create a doubly linked list constructor function DoublyLinkedList() {
 // Create a node constructor function Node(element) {
  this.element = element
  this.next = null
  this.prev = null // Newly added}

 // Define the property this.length = 0
 this.head = null
 this.tail = null // Newly added // Define related operation methods // Append data at the tail DoublyLinkedList.prototype.append = function (element) {
  // 1. Create a node based on the element var newNode = new Node(element)

  // 2. Determine whether the list is an empty list if (this.head == null) {
   this.head = newNode
   this.tail = newNode
  } else {
   this.tail.next = newNode
   newNode.prev = this.tail
   this.tail = newNode
  }

  // 3.length+1
  this.length++
 }

 //Insert data at any position DoublyLinkedList.prototype.insert = function (position, element) {
  // 1. Determine the problem of out-of-bounds if (position < 0 || position > this.length) return false

  // 2. Create a new node var newNode = new Node(element)

  // 3. Determine the insertion position if (position === 0) { // Insert data at the first position // Determine whether the linked list is empty if (this.head == null) {
    this.head = newNode
    this.tail = newNode
   } else {
    this.head.prev = newNode
    newNode.next = this.head
    this.head = newNode
   }
  } else if (position === this.length) { // Insert to the end // Think: In this case, do we need to check if the linked list is empty? The answer is no, why?
   this.tail.next = newNode
   newNode.prev = this.tail
   this.tail = newNode
  } else { // Insert data in the middle // Define attribute var index = 0
   var current = this.head
   var previous = null

   // Find the correct position while (index++ < position) {
    previous = current
    current = current.next
   }

   //Exchange the pointing order of nodes newNode.next = current
   newNode.prev = previous
   current.prev = newNode
   previous.next = newNode
  }

  // 4.length+1
  this.length++

  return true
 }

 // Delete the corresponding element according to the position DoublyLinkedList.prototype.removeAt = function (position) {
  // 1. Determine the problem of out-of-bounds if (position < 0 || position >= this.length) return null

  // 2. Determine the location to be removed var current = this.head
  if (position === 0) {
   if (this.length == 1) {
    this.head = null
    this.tail = null
   } else {
    this.head = this.head.next
    this.head.prev = null
   }
  } else if (position === this.length -1) {
   current = this.tail
   this.tail = this.tail.prev
   this.tail.next = null
  } else {
   var index = 0
   var previous = null

   while (index++ < position) {
    previous = current
    current = current.next
   }

   previous.next = current.next
   current.next.prev = previous
  }

  // 3.length-1
  this.length--

  return current.element
 }

 // Get the position of the element in the linked list DoublyLinkedList.prototype.indexOf = function (element) {
  // 1. Define variables to save information var current = this.head
  var index = 0

  // 2. Find the correct information while (current) {
   if (current.element === element) {
    return index
   }
   index++
   current = current.next
  }

  // 3. If it is not found at this position, return -1
  return -1
 }

 // Delete according to the element DoublyLinkedList.prototype.remove = function (element) {
  var index = this.indexOf(element)
  return this.removeAt(index)
 }

 // Check if it is empty DoublyLinkedList.prototype.isEmpty = function () {
  return this.length === 0
 }

 // Get the length of the linked list DoublyLinkedList.prototype.size = function () {
  return this.length
 }

 // Get the first element DoublyLinkedList.prototype.getHead = function () {
  return this.head.element
 }

 // Get the last element DoublyLinkedList.prototype.getTail = function () {
  return this.tail.element
 }

 // Implementation of traversal method // Forward traversal method DoublyLinkedList.prototype.forwardString = function () {
  var current = this.head
  var forwardStr = ""

  while (current) {
   forwardStr += "," + current.element
   current = current.next
  }

  return forwardStr.slice(1)
 }

 // Reverse traversal method DoublyLinkedList.prototype.reverseString = function () {
  var current = this.tail
  var reverseStr = ""

  while (current) {
   reverseStr += "," + current.element
   current = current.prev
  }

  return reverseStr.slice(1)
 }

 // Implement the toString method DoublyLinkedList.prototype.toString = function () {
  return this.forwardString()
 }
}