Linked List add to head/tail by robgmerrill - https://repl.it/FodU/19

FodU-19.js

// linked list constructor function
function LinkedList() {
  // initial creation has no nodes
  this.head = null;
  this.tail = null;
}

// node constructor function
// properties value, next previous
function Node(value, next, prev) {
  this.value = value;
  this.next = next;
  this.prev = prev;
}

//create an instance of our Node
var node1 = new Node(100, 'node2', null);

// adding new value to prototype
LinkedList.prototype.addToHead = function(value) {
  // create new node to add to head of linked list
  // value is a parameter that is passed to the function; this.head refers to the previous head of the node because that is now next, null will be the previous value because this is the head
  var newNode = new Node(value, this.head, null);
  // already nodes present
  if (this.head) {
    // give old head a new head
    this.head.prev = newNode;
  } else {
    // if no head node then this is both the head and the tail node
    this.tail = newNode;
  }
  // set the head
  this.head = newNode;
};

var ll = new LinkedList();

// creation of first node 
ll.addToHead(100);
// console.log(ll);
// creation of new head
ll.addToHead(200);
// console.log(ll);
ll.addToHead(300);
// console.log(ll);

// add to tail method

LinkedList.prototype.addToTail = function(value) {
  var newNode = new Node(value, null, this.tail);
  // already nodes present
  if(this.tail) {
    //give old tail a new tail
    this.tail.next = newNode;
  } else {
    this.head = newNode;
  }
  // set the tail
  this.tail = newNode;
}

ll.addToTail(500);
// console.log(ll);
var myLL = new LinkedList();

myLL.addToTail(10);
myLL.addToTail(20);
myLL.addToTail(30);
// console.log(myLL.tail.prev.prev);

myLL.addToHead(100);
console.log(myLL);