code: function (data){
      
          //create a new item object, place data in
          var node = { 
                  data: data, 
                  next: null,
                  prev: null
              };
      
          //special case: no items in the list yet
          if (this._length == 0) {
              this._head = node;
              this._tail = node;
          } else {
          
              //attach to the tail node
              this._tail.next = node;
              node.prev = this._tail;
              this._tail = node;
          }        
          
          //don't forget to update the count
          this._length++;
      
      }
docs: Appends some data to the end of the list. This method traverses
  the existing list and places the value at the end in a new item.
  @param {variant} data The data to add to the list.
  @return {Void}
  @method add