const treeView = new TreeViewPlugin(viewer, {
  // ...
  sortNodes: true,

  // sort numerically if node titles are numeric
  defaultSortFunction: (node1, node2) => { 
    const isNumber1 = !isNaN(Number(node1.title));
    const isNumber2 = !isNaN(Number(node2.title));

    if (isNumber1 && isNumber2) {
      return Number(node1.title) - Number(node2.title);
    } else {
      return node1.title.localeCompare(node2.title);
    }
  }
});