HTML

<bitty-7-0 data-connect="AddSubtract">
  <div>
    Value: <span data-receive="add subtract">0</span>
  </div>
  <button data-send="add">Add 1</button>
  <button data-send="subtract">Subtract 1</button>
</bitty-7-0>

JavaScript

window.AddSubtract = class {
  #counter = 0;

  add(_, el) {
    this.#counter += 1;
    el.innerHTML = this.#counter;
  }

  subtract(_, el) {
    this.#counter -= 1;
    el.innerHTML = this.#counter;
  }
};