LinkedList implemenation in Javascript

linkedlist.js

    var LinkedList = function() 
    {
        this._head = this._tail = null;
        this._transverse = null;
        this._size = 0;
    }

    function QEntry(prev, obj, next) 
    {
        if (typeof obj === 'undefined' || obj === null) {
            throw new Error('Null or Undefined values are not supported');
        }
        this.item = obj;
        this.prev = prev;
        this.next = next;
    }

    LinkedList.prototype.push = function(item) 
    {
        var tail = this._tail,
            entry = new QEntry(tail, item, null);
        if (!this._head) {
            this._head = entry;
        } else {
            tail.next = entry;
        }
        this._tail = entry;
        this._size++;
        return true;
    }

    LinkedList.prototype.offerLast = LinkedList.prototype.push;

    LinkedList.prototype.insertAfter = function(entry, item)
    {
      
      if(!entry.next)
      {
        this.push(item)
      }
      else
      {
        var newEntry = new QEntry(entry,item,entry.next);
        this._size++;
        (entry.next).prev = newEntry;
        entry.next = newEntry;    
      }
    }

     LinkedList.prototype.insertBefore = function(entry, item)
    {
      if(!entry.prev)
      {
        this.offerFirst(item);
      }
      else
      {
      var newEntry = new QEntry(entry.prev,item,entry);
      this._size++;
      (entry.prev).next = newEntry;
      entry.prev = newEntry;
      }    
    }

     LinkedList.prototype.deleteAt = function(list,entry)
     {
      var next = entry.next,
          prev = entry.prev;
      
      if(!prev)
        list._head = next;
      else
        prev.next = next;

      if(!next)
        list._tail = prev;
       else
        next.prev = prev;

      list._transverse = prev;

      this._size--;
      delete entry;
     }

    LinkedList.prototype.pop = function() 
    {
        var ret = this._tail,
            prev = ret.prev;
        if (!prev) {
            this._head = this._tail = null;
        } else {
            prev.next = null;
            ret.prev = null;
        }
        this._tail = prev;
        this._size--;
        return ret.item;
    }

    LinkedList.prototype.popLast = LinkedList.prototype.pop;
    
    LinkedList.prototype.unshift = function(item) 
    {
        var head = this._head,
            entry = new QEntry(null, item, head);
        if (!this._tail) {
            this._tail = entry;
        } else {
            head.prev = entry;
        }
        this._head = entry;
        this._size++;
        return true;
    }
    
    LinkedList.prototype.offerFirst = LinkedList.prototype.unshift;
    
    LinkedList.prototype.shift = function() 
    {
        if (this._size <= 0) return null;
        var ret = this._head,
            next = ret.next;
        if (!next) {
            this._head = this._tail = null;
        } else {
            next.prev = null;
            ret.next = null;
        }
        this._head = next;
        this._size--;
        return ret.item;
    }
    
    LinkedList.prototype.popFirst = LinkedList.prototype.shift;
    
    LinkedList.prototype.toArray = function() 
    {
        var head = this._head,
            arr = [];
        while (head) {
            arr.push(head.item);
            head = head.next;
        }
        return arr;
    };
    
    LinkedList.prototype.next = function()
    {
        if (this._size <= 0) return null;
        if (!this._transverse) {
            this._transverse = this._head;
        } else if (this._transverse.next) {
            this._transverse = this._transverse.next;
        } else {
            return null;
        }
        return this._transverse;
    }
    
    LinkedList.prototype.prev = function() 
    {
       var transverse = this._transverse;
        if (!transverse) {
            return null
        } else if (transverse.prev) {
           this._transverse = transverse.prev;
           transverse = this._transverse;
        }
        else
        {
          this._transverse = null;
          return null;
        }

        return transverse;
    }

    LinkedList.prototype.item = function()
    {
         if (!this._transverse) {
            this._transverse = this._head;
        }
        return this._transverse.item;
    }

    LinkedList.prototype.current = function()
    {
         if (!this._transverse) {
            this._transverse = this._head;
        }
        return this._transverse;
    }

    LinkedList.prototype.first = function()
    {

      return this._head;
    }
    LinkedList.prototype.last = function()
    {
      return this._tail;
    }

    LinkedList.prototype.length = function()
    {
        return this._size;
    }

    

    var ll = new LinkedList();
    var temp = {
        'val':'Ram',
        'type':'String'
    };
    ll.push('1');
    ll.push('2');
    ll.push('3');
    ll.push(temp);