Example Mixin for Polymer element

my-component.js

/**
 * Copyright 2019 PSU
 * @license Apache-2.0, see License.md for full text.
 */
import { html, PolymerElement } from "@polymer/polymer/polymer-element.js";

const SelectedMixin = function (superClass) {
  return class extends superClass {
    static get properties() {
        return {
            isSelected: {
                type: Boolean,
                value: false,
                notify: true,
                observer: '_selectedChanged'
            }
        };
    }
    _selectedChanged(selected) { 
        // do something
    }

    getSelectedItems() {
        // do something
    }
  }
}

/**
 * `glossary-consumer`
 * `Consumer renderless componet for glossary-service`
 *
 * @microcopy - language worth noting:
 *  -
 *
 * @customElement
 * @polymer
 * @demo demo/index.html
 */
class GlossaryConsumer extends SelectedMixin(PolymerElement) {
  /* REQUIRED FOR TOOLING DO NOT TOUCH */

  /**
   * Store the tag name to make it easier to obtain directly.
   * @notice function name must be here for tooling to operate correctly
   */
  static get tag() {
    return "glossary-consumer";
  }
  /**
   * life cycle, element is afixed to the DOM
   */
  connectedCallback() {
    super.connectedCallback();
  }

  disconnectedCallback() {
    super.disconnectedCallback()
  }
}
window.customElements.define(MyComponent.tag, MyComponent);