Extending EventTarget Classes for Polymorphic Controller Subscriptions in Svelte Components

export class FormStateController extends EventTarget {
  #state = Object.seal({ valid: false, value: '' });
 
  constructor(initialValue) {
    super();
    this.#state.value = initialValue;
  }
 
  updateValue(val) {
    this.#state.value = val;
    this.#state.valid = val.trim().length > 0;
    this.dispatchEvent(new CustomEvent('statechange', { detail: { ...this.#state } }));
  }
 
  get snapshot() {
    return Object.freeze({ ...this.#state });
  }
}