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Cloned Custom HTML Element does not become custom element until it's attached to the main DOM?

Consider the following scenario where I have a list of custom elements which is dynamically created. The CustomElement has a custom property displayName that should do something when set:

class CustomElement extends HTMLElement {
  set displayName(v) {
    this.querySelector(".name").textContent = v;
  }
}
customElements.define("custom-element", CustomElement)

const template = document.querySelector("template");
const list = document.querySelector(".list");
document.querySelector("button").addEventListener("click", () => {
  const customElTemplate = template.content.firstElementChild;
  console.log("Template constructor: ", customElTemplate.constructor.name);

  const el = template.content.firstElementChild.cloneNode(true);
  console.log("Cloned el constructor:", el.constructor.name);
  
  el.displayName = "Should has content";
  
  const frag = new DocumentFragment();
  frag.appendChild(el);
  console.log("Cloned el in DocumentFragment constructor:", el.constructor.name);
  
  list.appendChild(frag);
  console.log("Cloned el in Document constructor:", el.constructor.name);
});

custom-element {
  display: block;
  margin-bottom: .5rem;
  border: 1px solid black;
}

<template>
  <custom-element><span class="name"></span></custom-element>
</template>

<button>Add Item</button>

<div class="list"></div>

When clicking the button, the console output are:

Template constructor:  HTMLElement
Cloned el constructor: HTMLElement
Cloned el in DocumentFragment constructor: HTMLElement
Cloned el in Document constructor: CustomElement

As you can see, the cloned element doesn't become CustomElement until it's attached to the root document and displayName is simply a plain property that does nothing.

el.displayName = "Should has content"; won't work even after attaching to a DocumentFragment. It only works after attaching to the document (i.e. after list.appendChild(frag); in the above example)

Why is this happening? Is there anyway I can set up the element before attaching it to the main document?

UPDATE: I see there is customElements.upgrade, but even adding it, nothing is changed, unlike the example provided in the article:

customElements.upgrade(el);

7 months ago · Juan Pablo Isaza

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Answer 1 · 2022-06-25T02:08:41.349Z

I found out about customElements.upgrade. Turned out I have to call it after attaching to a shadow DOM:

frag.appendChild(el);
customElements.upgrade(el);

// Now el is CustomElement
console.log(el.constructor.name);

The upgrade() method of the CustomElementRegistry interface upgrades all shadow-containing custom elements in a Node subtree, even before they are connected to the main document.

For the example in my question, I need to:

Attach it to a DocumentFragment,
Call customElements.upgrade
Now I can use custom behavior like setting displayName.

Working code:

class CustomElement extends HTMLElement {
  set displayName(v) {
    this.querySelector(".name").textContent = v;
  }
}
customElements.define("custom-element", CustomElement)

const template = document.querySelector("template");
const list = document.querySelector(".list");
document.querySelector("button").addEventListener("click", () => {
  const customElTemplate = template.content.firstElementChild;
  console.log("Template constructor: ", customElTemplate.constructor.name);

  const el = template.content.firstElementChild.cloneNode(true);
  console.log("Cloned el constructor:", el.constructor.name);
  
  const frag = new DocumentFragment();
  frag.appendChild(el);
  customElements.upgrade(el);
  console.log("Cloned el in DocumentFragment constructor:", el.constructor.name);
  
  el.displayName = "Should has content";
  
  list.appendChild(frag);
  console.log("Cloned el in Document constructor:", el.constructor.name);
});

custom-element {
  display: block;
  margin-bottom: .5rem;
  border: 1px solid black;
}

<template>
  <custom-element><span class="name"></span></custom-element>
</template>

<button>Add Item</button>

<div class="list"></div>