code: function() {
    var tree = this.tree;
    var scrollTop = tree.scrollTop;
    var scrollHeight = tree.scrollHeight;
    var offsetHeight = tree.offsetHeight;
    var atBottom = scrollTop >= (scrollHeight - offsetHeight);
  
    if ((atBottom && this.pageIndex < this.pages.length)) {
      var f = document.createDocumentFragment();
  
      util.each(this.pages[this.pageIndex], function(i, item) {
        appendItem(item, f, this.customOption);
      }, this);
  
      tree.appendChild(f);
  
      this.pageIndex++;
  
      this.emit("selectr.paginate", {
        items: this.items.length,
        total: this.data.length,
        page: this.pageIndex,
        pages: this.pages.length
      });
    }
  }
docs: Load the next page of items
  @return {void}