my-element.js

class MyElement extends HTMLElement {
  constructor() {
    super();
    // We create a shadowDOM
    this.attachShadow({mode: 'open'});
    // Then we inject a html template into the created ShadowDOM
    this.shadowRoot.innerHTML = `
      Hello, I'm a web component
    `;
  }
}

// Register the web component
window.customElements.define('my-element', MyElement);