Custom Elements API

<custom-counter start="10" step="2"></custom-counter>

<script>
  class CustomCounter extends HTMLElement {
    static get observedAttributes() {
      return ['start', 'step'];
    }

    constructor() {
      super();
      this._count = 0;
      this._step = 1;
      const shadow = this.attachShadow({ mode: 'open' });
      shadow.innerHTML = `
        <style>
          :host { display: inline-flex; gap: 0.5rem; align-items: center; }
          button { padding: 0.25rem 0.5rem; cursor: pointer; }
          .value { font-weight: bold; min-width: 2rem; text-align: center; }
        </style>
        <button id="dec">-</button>
        <span class="value">0</span>
        <button id="inc">+</button>
      `;
    }

    connectedCallback() {
      this._count = parseInt(this.getAttribute('start') || '0');
      this._step = parseInt(this.getAttribute('step') || '1');
      this._update();

      this.shadowRoot.getElementById('inc')
        .addEventListener('click', () => this._increment());
      this.shadowRoot.getElementById('dec')
        .addEventListener('click', () => this._decrement());
    }

    attributeChangedCallback(name, oldVal, newVal) {
      if (name === 'step') this._step = parseInt(newVal || '1');
    }

    disconnectedCallback() {
      console.log('Counter removed from DOM');
    }

    _increment() { this._count += this._step; this._update(); }
    _decrement() { this._count -= this._step; this._update(); }
    _update() {
      this.shadowRoot.querySelector('.value').textContent = this._count;
      this.dispatchEvent(new CustomEvent('change', {
        detail: { value: this._count }
      }));
    }
  }

  customElements.define('custom-counter', CustomCounter);

  document.querySelector('custom-counter')
    .addEventListener('change', e => console.log('Count:', e.detail.value));
</script>