Web component custom elements

x-info-card.js

    class XInfoCard extends HTMLElement{
      // Monitor the 'message' attribute for changes.
      static get observedAttributes() {return ['message']; }

       // Respond to attribute changes.
       attributeChangedCallback(attr, oldValue, newValue) {
         if (attr == 'message') {
           this.textContent = newValue;
         }
      }
    }