Doubly linked lists

There are different types of linked list. In this section, we are going to cover the doubly linked list. The difference between a doubly linked list and a normal linked list is that in a linked list we make the link from one node to the next one only, while in a doubly linked list, we have a double link: one for the next element and one for the previous element, as shown in the following diagram:

Let's get started with the changes that are needed to implement the DoublyLinkedList class:

class DoublyNode extends Node { // {1}
  constructor(element, next, prev) {
    super(element, next); // {2}
    this.prev = prev; // {3} NEW
  }
}

class DoublyLinkedList extends LinkedList { // {4}
  constructor(equalsFn = defaultEquals) {
    super(equalsFn); // {5}
    this.tail = undefined; // {6} NEW
  }
}  

As the DoublyLinkedList class is a special type of LinkedList, we will extend the LinkedList class ({4}). This means the DoublyLinkedList class will inherit (will have access to...