What is the best practice for using templates in a constructor versus connectedCallback?

Should I apply template in the constructor or connectedCallback of a custom element? In my experience, when I apply it in connectedCallback, sometimes attributeChangedCallback is called before and I can't query for elements.

export class TestElement extends HTMLElement {
  constructor() {
    super();
    //apply template here ?
  }

  connectedCallback() {
    //apply template here ?
  }
}

I'm curious to understand where and why it's better to apply the template.

Here is an example snippet of applying the template:

let t = document.createElement('template');
t.innerHTML = require('template.html');
this.appendChild(t.content.cloneNode(true));

Answer №1

If you opt not to utilize Shadow DOM, refrain from inserting the template in the constructor() callback.

Instead, ensure that you append it only within the connectedCallback().

In any case, attributeChangedCallback() may be invoked either before or after the aforementioned callbacks based on how your custom element is utilized. It is advisable to conduct a test prior to attempting to query inner elements.

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