Help with code

I am currently working on some code in JSFiddle.net but nothing is displaying and no errors are coming up when I debug. Also, I set parameters to correct No-Library (pure JS) which is what was recommended by user who made code. Any help would be greatly appreciated.

HTML

Javascript

function LinkedList() { this.head = null; this.tail = null; this.length = 0; }

function Node() { this.next = null; this.prev = null; this.content = null; }

LinkedList.prototype.add = function(_content) { var node = new Node(); node.content = _content;

if (this.head == null) { this.head = node; this.length = 1; return node; }

if (this.tail == null) { this.tail = node; this.tail.prev = this.head; this.head.next = this.tail; this.length = 2; return node; }

this.tail.next = node; node.prev = this.tail; this.tail = node; this.length++; return node; }

LinkedList.prototype.print = function() { if (this.head == null) return "Empty List"; var s = ""; var node = this.head; while (node != null) { s += node.content + " "; node = node.next; } return s; }

var aList = new LinkedList();

function addNode() { var c = document.getElementbyId("v").value; aList.add(c); document.getElementById("output").innerHTML = aList.print(); }