Copy this code for Polymer Element

gistfile1.txt

import {PolymerElement, html} from '/node_modules/@polymer/polymer/polymer-element.js';

class ImageCarousel extends PolymerElement {
  static get properties() {
    return {
      selected: {
        type: Object,
        observer: '_selectedChanged'
      }
    };
  }
  
  static get template () {
    return html`
      <!-- Styles MUST be inside template -->
      <style>

        :host {
          display: block;
          position: relative;
          overflow: hidden;
        }

        div > ::slotted(:not([selected])) {
          display: none;
        }

        button {
          position: absolute;
          top: calc(50% - 20px);
          padding: 0;
          line-height: 40px;
          border: none;
          background: none;
          color: #DDD;
          font-size: 40px;
          font-weight: bold;
          opacity: 0.7;
        }

        button:hover,
        button:focus {
          opacity: 1;
        }

        #prevBtn {
          left: 12px;
        }

        #nextBtn {
          right: 12px;
        }

        button[disabled] {
          opacity: 0.4;
        }

      </style>

      <div>
        <slot></slot>
      </div>

      <button id="prevBtn" on-click="previous">&#x276E;</button>
      <button id="nextBtn" on-click="next">&#x276F;</button>
    `;
  }
  
  connectedCallback () {
    super.connectedCallback();
    this.shadowRoot.addEventListener('slotchange', this._resetSelected.bind(this));
    this._resetSelected();
  }
  
  _resetSelected () {
    if (!this.selected || this.selected.parentElement !== this) {
      this.selected = this.firstElementChild;
    }
  }
  
  _selectedChanged (selected, oldSelected) {
    if (oldSelected) oldSelected.removeAttribute('selected');
    if (selected) selected.setAttribute('selected', '');
  }
  
  previous () {
    const elem = this.selected && this.selected.previousElementSibling;
    if (elem) {
      this.selected = elem;
    }
  }

  next () {
    const elem = this.selected && this.selected.nextElementSibling;
    if (elem) {
      this.selected = elem;
    }
  }
}

customElements.define('image-carousel', ImageCarousel);