Event visualizer

A visualization tool to better understand how events propagate in the shadow DOM.

Overview

The way DOM events propagate in the shadow DOM is not intuitive for developers onboarding with web components. Event configuration, DOM structure and closed vs. opened shadow trees are many factors influencing event propagation.

This project is an attempt to bring clarity to this subject by offering a visual playground explaining how events propagates step-by-step in the shadow DOM.

Try it out: Demo / Playground

Installation

This package can be consumed as an NPM package.

$ npm install --save @pmdartus/event-visualizer

Alternatively for drop-in consumption this package can directly be loaded from Skypack CDN.

<script type="module" src="https://cdn.skypack.dev/@pmdartus/event-visualizer"></script>

<event-visualizer>

Properties / Attributes

Property	Attribute	Type	Default	Description
label	label	string	"Event propagation"	The label name.
eventBubbles	event-bubbles	boolean	false	Indicates wether the dispatched event should bubbles or not.
eventComposed	event-composed	boolean	false	Indicates wether the dispatched event should be composed or not.

Slots

Name	Description
default	Accepts a single <template> element representing the DOM tree to visualize. Refer to the DOM tree definition section for more details.

DOM tree definition

The visualized DOM tree is configured by passing a <template> element in the default slot. The content of the template tag is interpreted by the <event-visualizer> custom element to render the visual previous of the DOM tree and emulate event dispatching:

• Shadow trees can be defined directly in HTML via the declarative shadow DOM syntax with the difference difference that the shadow-root attribute is renamed to data-shadow-root.
• The original event target is defined by adding a target attribute.
• A label can be added to any element using the id attribute.
• Restrictions:
  • The template content should have root element.
  • The DOM should have a one element with the target attribute.

<event-visualizer label="Simple tree">
  <template>
    <div id="a">
      <div id="b" target></div>
    </div>
  </template>
</event-visualizer>

<event-visualizer label="Simple shadow tree">
  <template>
    <div id="a">
      <template data-shadowroot="open">
        <div id="b" target></div>
      </template>
      <div id="c">
    </div>
  </template>
</event-visualizer>