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Make custom element definition trigger in-document upgrades #1204

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May 10, 2016
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Make custom element definition trigger in-document upgrades #1204

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18 changes: 17 additions & 1 deletion source
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Original file line number Diff line number Diff line change
Expand Up @@ -67660,7 +67660,7 @@ console.log(plasticButton2.getAttribute("is")); // will output "plastic-button"<
<code>Window</code> object.</p>

<pre class="idl">interface <dfn>CustomElementsRegistry</dfn> {
void <span data-x="dom-CustomElementsRegistry-define">define</span>(DOMString name, Function constructor, optional ElementDefinitionOptions options);
[<span>CEReactions</span>] void <span data-x="dom-CustomElementsRegistry-define">define</span>(DOMString name, Function constructor, optional ElementDefinitionOptions options);
any <span data-x="dom-CustomElementsRegistry-get">get</span>(DOMString name);
Promise&lt;void&gt; <span data-x="dom-CustomElementsRegistry-whenDefined">whenDefined</span>(DOMString name);
};
Expand Down Expand Up @@ -67815,6 +67815,22 @@ dictionary <dfn>ElementDefinitionOptions</dfn> {

<li><p>Add <var>definition</var> to this <code>CustomElementsRegistry</code>.</p></li>

<li><p>Let <var>document</var> be this <code>CustomElementsRegistry</code>'s <span
data-x="concept-relevant-global">relevant global object</span>'s <span
data-x="concept-document-window"><code>Document</code> object</span>.</p></li>

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This surprised me a little bit since it means that a script that runs during parsing can remove the is="" attribute and it'll have an effect. Should we not store it on an internal slot?

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Hmm, what scenario are you specifically concerned about? Here is my guess:

<!DOCTYPE html>
<script>
customElements.define("x-a", class extends HTMLElement {
    constructor() {
        super();
        document.querySelector("p").removeAttribute("is");
    }
});
</script>

<p is="x-b"></p>
<x-a></x-a>

<script>
customElements.define("x-b", class extends HTMLParagraphElement {}, { extends: "p" })''
</script>

In this scenario, as you say, the x-a constructor runs and removes the is="x-b" attribute from the <p>. Then, when we later run customElements.define("x-b", ...), the <p> is not upgraded.

That seems totally expected to me. I guess it is a little contradictory with the idea behind the current sentence:

After a custom element is created, changing the value of the is attribute does not change the element's behaviour.

But I don't think it's surprising, and it doesn't even contradict a strict reading of that sentence.

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That or simply <p is="x-b"></p><script>document.querySelector("p").removeAttribute("is")</script>. I think I misremembered that we planned on guarding against that.

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Ah, right, that's of course a simpler repro.

I think there was a general sentiment that "is only gets read once" and that changing it doesn't impact things. But I don't think it was ever formalized, and for edge cases like this I don't think we specifically wanted to preserve the originally-parsed is value.

I'll rebase.

<li><p>Let <var>upgrade candidates</var> be all elements in <var>document</var> whose namespace
is the <span>HTML namespace</span> and whose local name is <var>localName</var>, in tree
order. Additionally, if <var>extends</var> is non-null, only include elements that have an
attribute named <code data-x="attr-is">is</code> whose value is <var>name</var>.</p></li>

<li><p>For each element <var>element</var> in <var>upgrade candidates</var>, <span>enqueue a
custom element upgrade reaction</span> given <var>element</var> and
<var>definition</var>.</p></li>

<!-- It is equivalent to just try to upgrade all elements in the document, and let "try to
upgrade" bail out, but this seems a bit more explicit. -->

<li>
<p>If this <code>CustomElementsRegistry</code>'s <span>when-defined promise map</span>
contains an entry with key <var>name</var>:</p>
Expand Down