CustomElementRegistry.get()

constructor = customElements.get(name);

customElements.define('my-paragraph',
  class extends HTMLElement {
    constructor() {
      let templateContent = document.getElementById('my-paragraph').content;
      super() // returns element this scope
        .attachShadow({mode: 'open'}) // sets AND returns this.shadowRoot
        .append(templateContent.cloneNode(true));
  }
})

// Return a reference to the my-paragraph constructor
let ctor = customElements.get('my-paragraph');