JSFiddle - React, Tailwind, and code Playground

JavaScript

class MyComponent extends HTMLElement {
  connectedCallback() {
    console.log('connectedCallback: this.hasAttribute("foo")', this.hasAttribute('foo'));
  	console.log('connectedCallback: this.hasAttribute("bar")', this.hasAttribute('bar'));
  }
}

customElements.define('my-component', MyComponent);

let el = document.createElement('my-component');
el.setAttribute('foo', '...');
document.body.append(el);
el.setAttribute('bar', '...');