A simple JupyterLab widget that adds a button to the Notebook toolbar

mybutton.ts

import {ToolbarButton} from "@jupyterlab/apputils";
import {DocumentRegistry} from "@jupyterlab/docregistry";
import {INotebookModel, NotebookPanel} from "@jupyterlab/notebook";
import {IDisposable} from "@lumino/disposable";

export class ButtonExtension implements DocumentRegistry.IWidgetExtension<NotebookPanel, INotebookModel> {

  createNew(panel: NotebookPanel, context: DocumentRegistry.IContext<INotebookModel>): IDisposable {
    // Create the toolbar button
    let mybutton = new ToolbarButton({
        label: 'My Button',
        onClick: () => alert('you did it!')
    });

    // Add the toolbar button to the notebook
    panel.toolbar.insertItem(9, 'mybutton', mybutton);

    // The ToolbarButton class implements `IDisposable`, so the
    // button *is* the extension for the purposes of this method.
    return mybutton;
  }
}