Using normal classes with vue composition api

how can I use framework-independent JavaScript class instances with reactive state on vue-composition-api.

Given following class:

// CounterService / counter-service.ts
//
// this is a normal javascript class which is independent of vue
export const counterService = new (class CounterService {
  private count: number = 0;

  // should cause all components that use `getCount` to react.
  addOne(): void {
    this.count++;
  }

  // should be reactive
  getCount(): number {
    return this.count;
  }
})()

Kind regards
Rozbeh Chiryai Sharahi