JupyterLab Tutorial

Virtual DOM

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Virtual DOMΒΆ

JupyterLab is based on PhosphorJS, which provides a flexible Widget class that handles the following:

  • Resize events that propagate down the Widget heirarchy.
  • Lifecycle events (onBeforeDetach, onAfterAttach, etc.).
  • Both CSS-based and absolutely postioned layouts.

The idea of virtual DOM rendering, which became popular in the ReactJS community, is a very elegant and efficient way of rendering and updating DOM content in response to model/state changes. The base Widget class in PhosphorJS does not use virtual DOM rendering, however Phosphor provides a lightweight virtual DOM API that can be used to render leaf content inside a Widget. However, the virtual DOM API in PhosphorJS is implemented in a manner that is unopinionated about how it is used.

In JupyterLab, we provide a more opinionated API for using virtual DOM rendering following best practices. This API can be found in common/vdom.ts and offers two classes:

  • VDomModel
  • VDomRenderer

To use these classes, we recommend the following approach.

First, import the virtual DOM API from PhosphorJS and JupyterLab:

import {
  h, VNode
} from 'phosphor/lib/ui/vdom';

import {
  VDomModel, VDomRenderer
} from '../common/vdom'; // From another JupyterLab plugin

Second, create a subclass of VDomModel that contains all of the state needed to render the content:

class TestModel extends VDomModel {
  get myvalue(): string {
    return this._myvalue;

  set myvalue(newValue: string) {
    this._myvalue = newValue;
    this.stateChanged.emit(void 0);

  private _myvalue = '';

For each attribute that is part of your model, you will implement a get and set method as we have done here for the myvalue attribute. All of the set methods in your model should end by calling this.stateChanged.emit(void 0).

Third, create a subclass of VDomRenderer that has a render() method that uses Phosphor’s virtual DOM API and returns a PhosphorJS VNode or an array of them:

class TestWidget extends VDomRenderer<TestModel> {
  protected render(): VNode | VNode[] {
    return h.span(this.model.myvalue);

Now anytime the model is changed, the content will be rendered on the page efficiently using the virtual DOM approach.