Extending native HTML element in Angular 6 - angular6

Question

I have recently created a native web component which is working well in all browsers. I moved this web component into an Angular 6 application and all works as expected. I then tried to extend a native HTML element which again worked perfectly except when I brought it into my Angular 6 application.
Using the examples from Mozilla I will try and illustrate my issue. Using the following trying to extend a native 'p' element:
// Create a class for the element
class WordCount extends HTMLParagraphElement {
constructor() {
// Always call super first in constructor
super();
// count words in element's parent element
var wcParent = this.parentNode;
function countWords(node){
var text = node.innerText || node.textContent
return text.split(/\s+/g).length;
}
var count = 'Words: ' + countWords(wcParent);
// Create a shadow root
var shadow = this.attachShadow({mode: 'open'});
// Create text node and add word count to it
var text = document.createElement('span');
text.textContent = count;
// Append it to the shadow root
shadow.appendChild(text);
// Update count when element content changes
setInterval(function() {
var count = 'Words: ' + countWords(wcParent);
text.textContent = count;
}, 200)
}
}
// Define the new element
customElements.define('word-count', WordCount, { extends: 'p' });
<p is="word-count">This is some text</p>
By taking that same code and putting it into an Angular 6 application, the component never runs. I put console log statements in the constructor and connectedCallback methods and they never trigger. If I remove the {extends: 'p'} object and change the extends HTMLParagraphElement and make it an extend HTMLElement to be an autonomous custom element everything works beautifully. Am I doing something wrong or does Angular 6 not support the customized built-in element extension?

Answer 1

I assume the reason is the way that Angular creates those customized built-in elements when parsing component templates - it probably does not know how to properly do that. Odds are it considers is a regular attribute which is fine to add after creation of the element (which it isn't).
First creating the element and then adding the is-attribute will unfortunately not upgrade the element.
See below example: div#d has a non-working example of that customized input.
customElements.define('my-input', class extends HTMLInputElement {
connectedCallback() {
this.value = this.parentNode.id
this.parentNode.classList.add('connected')
}
}, {
extends: 'input'
})
document.addEventListener('DOMContentLoaded', () => {
b.innerHTML = `<input type="text" is="my-input">`
let el = document.createElement('input', {
is: 'my-input'
})
el.type = 'text'
c.appendChild(el)
// will not work:
let el2 = document.createElement('input')
el2.setAttribute('is', 'my-input')
el2.type = 'text'
d.appendChild(el2)
})
div {
border: 3px dotted #999;
padding: 10px;
}
div::before {
content: "#"attr(id)" ";
}
.connected {
background-color: lime;
}
<div id="a"><input type="text" is="my-input"></div>
<div id="b"></div>
<div id="c"></div>
<div id="d"></div>
So to get it to work with Angular, hook into the lifecycle of your Angular component (e.g. onInit() callback) and pick a working way to create your element there.