javascript - How to create new instance of an extended class of custom elements

Question

I'm trying the example from google developer site and I'm getting Error: "TypeError: Illegal constructor. What's wrong and How to fix it?

class FancyButton extends HTMLButtonElement {
  constructor() {
    super(); // always call super() first in the ctor.
    this.addEventListener('click', e => this.drawRipple(e.offsetX,e.offsetY));
  }

  // Material design ripple animation.
  drawRipple(x, y) {
    let div = document.createElement('div');
    div.classList.add('ripple');
    this.appendChild(div);
    //    div.style.top = `${y - div.clientHeight/2}px`;
    //    div.style.left = `${x - div.clientWidth/2}px`;
    div.style.backgroundColor = 'currentColor';
    div.classList.add('run');
    div.addEventListener('transitionend', e => div.remove());
  }
}

customElements.define('fancy-button', FancyButton, {extends: 'button'});
let button = new FancyButton();
button.textContent = 'Fancy button!';
button.disabled = true;

Answer 1

Blink, the web engine that currently implements Custom Element v1 (in Chrome v53+ for example) only supports autonomous custom elements: see open Blink bug.

If you want to define customized built-in elements (i.e. <button> extension), you'll need to use a polyfill like the one from Web Reflection.

Alternatly, you can still use the Custom Element v0 syntax (document.registerElement).

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Update #3

Since october 2018, they work natively with Chrome 67+ and Firefox 63+ :-)