<custom-menu id="menu">
  <span slot="title">Candy menu</span>
</custom-menu>

<script>
customElements.define('custom-menu', class extends HTMLElement {
  connectedCallback() {
    this.attachShadow({mode: 'open'});
    this.shadowRoot.innerHTML = `<div class="menu">
      <slot name="title"></slot>
      <ul><slot name="item"></slot></ul>
    </div>`;

    // shadowRoot can't have event handlers, so using the first child
    this.shadowRoot.firstElementChild.addEventListener('slotchange',
      e => alert("slotchange: " + e.target.name)
    );
  }
});

setTimeout(() => {
  menu.insertAdjacentHTML('beforeEnd', '<li slot="item">Lollipop</li>')
}, 1000);

setTimeout(() => {
  menu.querySelector('[slot="title"]').innerHTML = "New menu";
}, 2000);
</script>