README.md

@corpuscule/custom-builtin-elements

This package is a polyfill for the Customized Built-in Elements specification.

Rationale

Unfortunately, Safari is not going to support this part of the Custom Elements spec, as well as IE 11 and Edge do not have this feature on board. This polyfill aims to fill this gap and provide support for this feature for all browsers that miss it.

Differently from @ungap/custom-elements-builtin, this polyfills allow the usage of the constructor, which is apparently a caveat of that alternative library.

This polyfill provides a solution that issue and makes it possible to use the customized built-in elements in all browsers that lack support for them: Safari, Edge, IE 11.

Installation

npm:

$ npm install @corpuscule/custom-builtin-elements

Yarn:

$ yarn add @corpuscule/custom-builtin-elements

https://unpkg.com

import 'https://unpkg.com/@corpuscule/custom-builtin-elements';

Support

To use this polyfill with IE 11 you need the following tools:

• Symbol polyfill (with support for Symbol.hasInstance).
• Promise polyfill.
• @babel/plugin-transform-instanceof applied to your code that uses instanceof against any built-in constructor (like HTMLButtonElement etc.).

Also, for all browsers that do not support native web components, you need an implementation of the customElements registry existing. You have the following choices:

• Use @webcomponents/webcomponentsjs, that will also add support for autonomous custom elements.
• Use the minimal implementation of customElements provided by this polyfill that would allow you to use the customized built-in elements only.

Note that both minimal implementation or @webcomponents/webcomponentsjs polyfill should be executed before the main polyfill because it will override methods of the customElements registry.

Examples of adding minimal implementation:

• via npm and ESM:

import '@corpuscule/custom-builtin-elements/lib/customElementsBase';
import '@corpuscule/custom-builtin-elements';

• via https://unpkg.com and script:

<script src="https://unpkg.com/@corpuscule/custom-builtin-elements/lib/customElementsBase.js"></script>
<script src="https://unpkg.com/@corpuscule/custom-builtin-elements"></script>

Example

class ClickCounter extends HTMLButtonElement {
  constructor() {
    super();
    this._count = 0;
    this.increase = this.increase.bind(this);
  }

  get count() {
    return this._count;
  }

  connectedCallback() {
    this.addEventListener('click', this.increase);
  }

  disconnectedCallback() {
    this.removeEventListener('click', this.increase);
  }

  increase() {
    this._count += 1;
  }
}

customElements.define('x-click-counter', ClickCounter, {extends: 'button'});