Tools for building JupyterLab extensions

jupyter, extension, plugin
npm install jupyterlab-extension-builder@0.6.2


JupyterLab Extension Builder

Tools for building JupyterLab extensions

A JupyterLab extension provides additional, optional functionality to JupyterLab's built-in capabilities. An extension is a module that provides one or more plugins to the JupyterLab application. To streamline third-party development of extensions, this library provides a build script for generating third party extension JavaScript bundles.

Simple extensions can be created by using the buildExtension function with the default options. More advanced extensions may require additional configuration such as custom loaders or WebPack plugins.

A video tutorial walkthrough for building JupyterLab extensions can be found on YouTube.

Package Install


npm install --save @jupyterlab/extension-builder

Source Build


git clone https://github.com/jupyterlab/extension-builder.git
cd extension-builder
npm install
npm run build

Rebuild Source

npm run clean
npm run build


npm install -g devtool

Insert a debugger; statement in the source where you want the execution to stop in the debugger. Then execute an extension build with

devtool <my_build_script.js>

or if running WebPack directly,

devtool <path_to_webpack_in_node_modules/bin>


Three major usage steps include:

The full API docs can be found here.

Extension entry point

A simple extension entry point that exposes a single application plugin could look like:

module.exports = [{
    id: 'my-cool-extension',
    activate: function(app) {

The extension entry point must be a CommonJS module where the default export is an array of plugin objects. If writing in ES6 format use the default export syntax export default myPlugin for a single plugin, and the following pattern for multiple exports (remove the type declaration if not using TypeScript):

import { JupyterLabPlugin } from 'jupyterlab/lib/application';
// Plugins defined here
const plugins: JupyterlabPlugin<any>[] = [ ... ];
export default plugins;


Build the above example using the following script:

var buildExtension = require('@jupyterlab/extension-builder').buildExtension;

    name: 'my-cool-extension',
    entry: './index.js',
    outputDir: './build'

The name is a string that will be used for the output filename. The entry is the module that exports a plugin definition or array of plugin definitions. The outputDir is the directory in which the generated plugin bundle, manifest, and related files will be stored.

Several optional arguments are also available; see the options at the bottom of the builder.ts file.

In this case the builder script will create the following files in the build directory:


jupyter labextension

Other extensions may produce additional files in the build directory depending on the complexity of extension. The two files above, my-cool-extension.js and my-cool-extension.js.manifest, are used by the JupyterLab server to determine the entry point file(s) and entry point module(s) for the extension. The extension must also be registered, using the command jupyter labextension, in order to be added to the JupyterLab application. See the documentation for labextension

Technical overview

The extension bundles are created using WebPack, and the modules produced by WebPack are modified to use JupyterLab's custom module registration and loading mechanism.

JupyterLab's custom module registration and loading mechanism uses a define function that registers modules by name, where the name contains the package name, version number, and the full path to the module. For example, 'phosphor@0.6.1/lib/ui/widget.js'. Within a define function, a required module is referenced by package name, semver range, and the full path to the module. For example, require('phosphor@^0.6.0/lib/ui/tabpanel.js'). The semver range is determined by the following criteria (see the getModuleSemverPath function in plugin.ts:

  1. If the dependency is in the same package, the exact version of the dependency is used.
  2. If the dependency is a local package (i.e., module given by file://...), the semver is the patch-level range (~) starting from the installed version.
  3. If the dependency is in the dependency list of the module's package.json, then the semver range requested there is used.
  4. Otherwise the installed version of the dependency is used exactly. Note that not listing an external dependency in the package metadata is a bad practice that leads to almost no deduping.

By using a semver range, JupyterLab can perform client-side deduplication of modules, where the registered module that maximally satisfies a semver range is the one returned by the require function call. This also enables us to perform server-side deduplication of modules prior to serving the bundles, and the client-side lookup will still load the correct modules.

Reasons to deduplicate code include:

  • being able to use instanceof() on an object to determine if it is the same class (a technique used by phosphor's drag-drop mechanism)
  • sharing of module-private state between different consumers, such as a list of client-side running kernels in @jupyterlab/services.

All client-side require() calls are synchronous, which means that the bundles containing the define() modules must be loaded prior to using any of the bundles' functions. The loader in JupyterLab provides an ensureBundle() function to load a particular bundle or bundles prior to calling require() on a module.

Custom WebPack Configuration and JupyterLabPlugin

A completely custom WebPack configuration may be needed if there is a case where the buildExtension function is not sufficient to build the extension. If a custom WebPack configuration is needed, the JupyterLabPlugin must be used as part of the WebPack config to ensure proper handling of module definition and requires.

Publishing your extension

Before you publish your extension to npm, add the following keywords attribute to your extension's package.json:

    "keywords": ["jupyterlab", "jupyterlab extension"],

Adding these keywords will allow other users to discover your extension with npm search.