Linked List with Search Method by robgmerrill - https://repl.it/FodU/36

FodU-36.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;
};



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;
}


LinkedList.prototype.removeHead = function() {
  if (!this.head) return null;
  
    var val = this.head.value;
    this.head = this.head.next;

  if (this.head) this.head.prev = null;
  else this.tail = null;
  return val;
};


LinkedList.prototype.removeTail = function() {
  if(!this.tail) return null;
  var val = this.tail.value;
  this.tail = this.tail.prev;
  if (this.tail) this.tail.next = null;
  else this.head = null;
  return val;
}

// var ll = new LinkedList();
// ll.addToHead('1');
// ll.addToHead('2');
// ll.addToHead('3');
// console.log(ll.removeTail());

LinkedList.prototype.search = function(searchValue) {
  var currentNode = this.head;
  while (currentNode) {
    if (currentNode.value === searchValue) {
      return currentNode.value;
    } else {
    currentNode = currentNode.next;
    }
  }
  return null;
};

var myLL = new LinkedList();

myLL.addToHead(123);
myLL.addToHead(70);
myLL.addToHead('hello');
myLL.addToTail(19);
myLL.addToTail('world');
myLL.addToTail(20);

myLL.search(70);