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                // built with code from

  class extends HTMLElement {
     * The class constructor object
    constructor() {
      // Always call super first in constructor

      // Track the count
      this.count = 0;

      this.shadow = this.attachShadow({ mode: "closed" });
      let button = document.createElement("button");
      button.innerText = `Clicked ${this.count} times`;

     * Runs each time the element is appended to the DOM
    connectedCallback() {
      const button = this.shadow.firstChild;
      button.addEventListener("click", () => {
        button.innerText = `Clicked ${this.count} times`;

     * Create a list of attributes to observe
    static get observedAttributes() {
      return ["count"];

     * Runs when the value of an attribute is changed on the component
     * @param  {String} name     The attribute name
     * @param  {String} oldValue The old attribute value
     * @param  {String} newValue The new attribute value
    attributeChangedCallback(name, oldValue, newValue) {
      console.log("changed", name, oldValue, newValue, this);

      // Make sure the new value is a number
      // We're not checking the name is "count" for now, but
      // if there's more than one attribute we would need to
      let num = parseFloat(newValue);
      if (isNaN(num)) return;

      // Update the count and render the UI
      this.count = num;
      this.firstElementChild.textContent = `Clicked ${this.count} Times`;

// without shadow DOM, this thing could wipe the contents
// of the element

let counter = document.querySelector("counter-button");
counter.innerHTML = "<p>Changed you</p>";