wait for Element Upgrade in connectedCallback: FireFox and Chromium differences

问题

Bitten again by this Chrome Element upgrade issue, after spending a week in FireFox.

Forgot to wrap code in a setTimeout before delivering to Chromium browsers.

FireFox prints: ABCD
Chromium prints: ADCD

Question: Why the difference?

<script>
  customElements.define('my-element', class extends HTMLElement {
    connectedCallback() {
      console.log(this.innerHTML);// "A" in FireFox, "" in other Browsers
      if (this.innerHTML == "A")
        this.innerHTML = this.innerHTML + "B";
      else
        setTimeout(() => this.innerHTML = this.innerHTML + "D");
    }
  })
</script>

<my-element>A</my-element><my-element>C</my-element>

Update #1

Apple/Safari: prints: ADCD

note: Chromiums Blink engine is a fork of Apples (WebKit)WebCore code
So the cause may precede Custom Elements

Update #2

With Supersharps reference we found the related threads:

(2016) connectedCallback timing when the document parser creates custom elements
https://github.com/w3c/webcomponents/issues/551
(2019) Need a callback for when children changed or parser finished parsing children
https://github.com/w3c/webcomponents/issues/809

回答1:

I think the Chrome/Safari behaviour is less intuitive for the beginners, but with some more complex scenarios (for example with child custom elements) then it is much more consistant.

See the different examples below. They act strangely in Firefox...

Another use case that I don't have the courage to code: when a document is parsed, maybe you don't have the end of the document yet. Therefore, when a custom element is created, you cannot be sure you get all its children until you get the closing tag (that could never arrive).

According to Ryosuke Niwa for WebKit:

The problem then is that the element won't get connectedCallback until all children are parsed. For example, if the entire document was a single custom element, that custom element would never receive connectedCallback until the entire document is fetched & parsed even though the element is really in the document. That would be bad.

So it's better no to wait and connect the custom element as soon as it is created, that means with no child.

<script>
    customElements.define( 'c-e', class extends HTMLElement {} ) 
    customElements.define('my-element', class extends HTMLElement {
      connectedCallback() {
        console.log(this.innerHTML, this.childNodes.length)
        let span = document.createElement( 'span' )
        if (this.innerHTML.indexOf( 'A' ) >= 0 )
            span.textContent = 'B'
        else
            span.textContent = 'D'
        setTimeout( () => this.appendChild( span ) )
      }
    })
</script>
<my-element>A</my-element><my-element>C</my-element>
<br>
<my-element><c-e></c-e>A</my-element><my-element>A<c-e></c-e></my-element>
<br>
<my-element><c-e2></c-e2>A</my-element><my-element>A<c-e2></c-e2></my-element>

As far as I understand, there was a consensus on it that led to adjust the spec that (Chrome/Safari) way:

Fixes w3c/webcomponents#551 by ensuring that insertions into the DOM trigger connectedCallback immediately, instead of putting the callback reaction on the the backup element queue and letting it get triggered at the next microtask checkpoint. This means connectedCallback will generally be invoked when the element has zero children, as expected, instead of a random number depending on when the next custom element is seen.

We can conclude that Firefox also follow the spec... yes, but we should not rely on the content in connectedCallback for the reasons discussed above.

来源：https://stackoverflow.com/questions/61971919/wait-for-element-upgrade-in-connectedcallback-firefox-and-chromium-differences

标签

google-chrome

web-component

custom-element