CustomElementRegistry

// Create a class for the element
class WordCount extends HTMLParagraphElement {
  constructor() {
    // Always call super first in constructor
    super();

    // count words in element's parent element
    const wcParent = this.parentNode;

    function countWords(node){
      const text = node.innerText || node.textContent;
      return text.trim().split(/\s+/g).filter(a => a.trim().length > 0).length;
    }

    const count = `Words: ${countWords(wcParent)}`;

    // Create a shadow root
    const shadow = this.attachShadow({mode: 'open'});

    // Create text node and add word count to it
    const text = document.createElement('span');
    text.textContent = count;

    // Append it to the shadow root
    shadow.appendChild(text);

    // Update count when element content changes
    setInterval(function() {
      const count = `Words: ${countWords(wcParent)}`;
      text.textContent = count;
    }, 200);
  }
}

// Define the new element
customElements.define('word-count', WordCount, { extends: 'p' });