Hello World

Set up the development environment and print to the console.

• The template folder structure
• A minimal extension that prints to the browser console
• Building and Installing an Extension

The template folder structure

Writing a JupyterLab extension usually starts from a configurable template. It can be downloaded with the cookiecutter tool and the following command:

cookiecutter https://github.com/jupyterlab/extension-cookiecutter-ts

cookiecutter asks for some basic information that could for example be setup like this:

author_name []: tuto
extension_name [myextension]: hello-world
project_short_description [A JupyterLab extension.]: minimal lab example
repository [https://github.com/my_name/myextension]:

The cookiecutter creates the directory hello-world [or your extension name] that looks like this:

hello-world/
│   .eslintignore
│   .eslintrc.js
│   .gitignore
│   .prettierignore
│   .prettierrc
│   LICENSE
│   package.json
│   README.md
│   tsconfig.json
│
├───.github
│   └───workflows
│           build.yml
│
├───src
│       index.ts
│
└───style
        index.css

Those files can be separated in 3 groups:

• Information about the extension:
  • README.md contains some instructions
  • LICENSE contains your extension code license; BSD-3 Clause by default (but you can change it).
• Extension code (those files are mandatory):
  • package.json contains information about the extension such as dependencies
  • tsconfig.json contains information for the typescript compilation
  • src/index.ts this contains the actual code of your extension
  • style/index.css contains style elements that you can use
• Validation:
  • .prettierrc and .prettierignore specify the code formatter prettier configuration
  • .eslintrc.js and .eslintignore specify the code linter eslint configuration
  • .github/workflows/build.yml sets the continuous integration tests of the code using GitHub Actions

The following sections will walk you through the extension code files.

A minimal extension that prints to the browser console

Start with the file src/index.ts. This typescript file contains the main logic of the extension. It begins with the following import section:

// src/index.ts#L1-L4

import {
  JupyterFrontEnd,
  JupyterFrontEndPlugin
} from '@jupyterlab/application';

JupyterFrontEnd is the main Jupyterlab application class. It allows you to access and modify some of its main components. JupyterFrontEndPlugin is the class of the extension that you are building. Both classes are imported from a package called @jupyterlab/application. The dependency of your extension on this package is declared in the file package.json:

// package.json#L36-L38

"dependencies": {
  "@jupyterlab/application": "^2.0.0"
},

With this basic import setup, you can move on to construct a new instance of the JupyterFrontEndPlugin class:

// src/index.ts#L9-L12

const extension: JupyterFrontEndPlugin<void> = {
  id: 'hello-world',
  autoStart: true,
  activate: (app: JupyterFrontEnd) => {

// src/index.ts#L13-L13

console.log('the JupyterLab main application:', app);

// src/index.ts#L17-L17

export default extension;

A JupyterFrontEndPlugin contains a few attributes:

• id: the unique id of the extension
• autoStart: a flag to start the extension automatically or not
• activate: a function (() => {} notation) that takes one argument app of type JupyterFrontEnd and will be called by the main application to activate the extension.

app is simply the main JupyterLab application. The activate function acts as an entry point into the extension. In this example, it calls the console.log function to output something into the browser developer tools console.

Your new JupyterFrontEndPlugin instance has to be finally exported to be visible to JupyterLab, which is done with the line export default extension.

Now that the extension code is ready, you need to install it within JupyterLab.

Building and Installing an Extension

These are the instructions on how your extension can be installed for development:

The jlpm command is JupyterLab's pinned version of yarn that is installed with JupyterLab. You may use yarn or npm in lieu of jlpm below.

# Clone the repo to your local environment
# Move to hello-world directory
# Install dependencies
jlpm
# Build Typescript source
jlpm build
# Link your development version of the extension with JupyterLab
jupyter labextension link .

The first command installs the dependencies that are specified in package.json. Among the dependencies are also all of the JupyterLab components that you want to use in your project.

The second step runs the build script. In this step, the TypeScript code gets converted to javascript using the compiler tsc and stored in a lib directory. Finally, the module is linked to JupyterLab.

After all of these steps are done, running jupyter labextension list should show something like:

   local extensions:
        @jupyterlab-examples/hello-world: [...]/basics/hello-world

Now let's check inside of JupyterLab if it works. Run [can take a while]:

jupyter lab --watch

Your extension writes something to the browser console. In most web browsers you can open the console pressing the F12 key. You should see something like:

JupyterLab extension hello-world is activated

Your extension works but it is not doing much. Let's modify the source code a bit. Simply replace the activate function with the following lines:

// src/index.ts#L12-L14

activate: (app: JupyterFrontEnd) => {
  console.log('the JupyterLab main application:', app);
}

To update the module, simply go to the extension directory and run jlpm build again. Since you used the --watch option when starting JupyterLab, you just have to refresh the JupyterLab website in the browser and should see in the browser console:

the JupyterLab main application:
Object { _started: true, _pluginMap: {…}, _serviceMap: Map(...), _delegate: {…}, commands: {…}, contextMenu: {…}, shell: {…}, registerPluginErrors: [], _dirtyCount: 0, _info: {…}, … }

This is the main application JupyterLab object and you will see how to interact with it in the other examples.

Checkout how the core packages of JupyterLab are defined on this page. Each package is structured similarly to the extension that you are writing. This modular structure makes JupyterLab very adaptable.

An overview of the classes and their attributes and methods can be found in the JupyterLab documentation. The @jupyterlab/application module documentation is here and here is the JupyterFrontEnd class documentation.

Where to Go Next

JupyterLab is built on top of three major concepts. It is advised to look through the corresponding examples in the following order:

• command: Function to be executed from UI elements. See the commands example
• widget: UI based brick. See the widgets example
• signal: Observer pattern between JupyterLab elements. See the signals example