Comment by mikebelanger

You can invoke custom methods and pass in any kind of data into them. As in

  class SomeElement extends HTMLElement {
    constructor() {
      super();
    }

    someMethod(x) {
      this.innerHTML = `<h1>${x}</h1>`;
    }
  }

  // ignore registry stuff

  const customElement = document.getElementById('custom-element-id');

  customElement.someMethod(42);

But you won't learn that from most mainstream custom element tutorials though, for whatever reason.