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


    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.

    You may have a question: why choose this polyfill if there already is a well-known and popular @ungap/custom-elements-builtin? Unfortunately, it has some problems that make it hard to use. For example, with the @ungap/custom-elements-builtin you cannot use class constructors or create new elements with the new operator.

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



    $ npm install @corpuscule/custom-builtin-elements


    $ yarn add @corpuscule/custom-builtin-elements


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


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


    class ClickCounter extends HTMLButtonElement {
      constructor() {
        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'});


    npm i @corpuscule/custom-builtin-elements

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