Re: [w3c/webcomponents] Custom Element - untrackable upgrade (#671)

### Update

The only way to intercept/interact before `attributeChangedCallback` or `connectedCallback` are triggered is by using `DOMNodeInsterted` event on the `document`.

However, these events have been deprecated, but `MutationObserver` would not be notified before the component upgrade, always after.

```js
check('sync template'); // fails
customElements.whenDefined('test-case').then(() => {
  check('async template');  // fails

  document.addEventListener('DOMNodeInsterted', e => {
    if (e.target === target) console.log('got it before checks');
  });

  // triggers attribute and conected
  document.body.appendChild(target);

  check('sync live'); // OK
  customElements.whenDefined('test-case').then(() => {
    check('async live');  // OK
    console.log(target.outerHTML);
  });
});
```

Accordingly, there's still no way to intercept a Custom Element upgrade.

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/671#issuecomment-332837080