JupyterLab can be extended in two ways via:

  • application plugins (top level): Application plugins extend the functionality of JupyterLab itself, and this tutorial focuses on them.
  • document widget extensions (lower level): Document widget extensions extend the functionality of document widgets added to the application, and we cover them in the “Documents” tutorial.

A JupyterLab application is comprised of:

  • A core Application object
  • Plugins

A full example of an application is contained here. Looking at the index.js file, you can see the extensions used in the tutorial example.


A plugin adds a core functionality to the application:

  • A plugin can require other plugins for operation.
  • A plugin is activated when it is needed by other plugins, or when explicitly activated.
  • Plugins require and provide Token objects, which are used to provide a typed value to the plugin’s activate() method.
  • The module providing plugin(s) must meet the JupyterLab.IPluginModule interface, by exporting a plugin object or array of plugin objects as the default export.

The default plugins in the JupyterLab application include:

  • Terminal - Adds the ability to create command prompt terminals.
  • Shortcuts - Sets the default set of shortcuts for the application.
  • Images - Adds a widget factory for displaying image files.
  • Help - Adds a side bar widget for displaying external documentation.
  • File Browser - Creates the file browser and the document manager and the file browser to the side bar.
  • Editor - Add a widget factory for displaying editable source files.
  • Console - Adds the ability to launch Jupyter Console instances for interactive kernel console sessions.
  • Services - An application-specific interface to @jupyterlab/services.
  • RenderMime - The registry for adding kernel display_data renderers.
  • Document Registry - Used to add functionality around widgets backed by files.

Application Object

The JupyterLab Application object is given to each plugin in its activate() function. The Application object has a:

  • commands - used to add and execute commands in the application.
  • keymap - used to add keyboard shortcuts to the application.
  • shell - a JupyterLab shell instance.

JupyterLab Shell

The JupyterLab shell is used to add and interact with content in the application. The application consists of:

  • A top area for things like top level menus and toolbars
  • Left and right side bar areas for collapsable content
  • A main area for user activity.
  • A bottom area for things like status bars


The Phosphor library is used as the underlying architecture of JupyterLab and provides many of the low level primitives and widget structure used in the application. Phosphor provides a rich set of widgets for developing desktop-like applications in the browser, as well as patterns and objects for writing clean, well-abstracted code. The widgets in the application are primarily Phosphor widgets, and Phosphor concepts, like message passing and signals, are used throughout. Phosphor messages are a many-to-one interaction that allows information like resize events to flow through the widget hierarchy in the application. Phosphor signals are a one-to-many interaction that allow listeners to react to changes in an observed object.