Attempt to create custom Subject

index.js

const { rxObserver } = require('api/v0.3');
const { Subject, of, timer } = require('rxjs');
const { finalize, takeUntil } = require('rxjs/operators');

class CountSubject extends Subject {
  constructor(){
    super();
    this.subscribersCount = 0;
  }

  subscribe(...args) {
    this.subscribersCount++;
    console.log('count', this.subscribersCount);

    const subscription = super.subscribe(...args);
    const unsubscribe = subscription.unsubscribe;

    subscription.unsubscribe = (...args)=>{
      this.subscribersCount--;
      console.log('count', this.subscribersCount);
      
      return unsubscribe.call(subscription, ...args);
    };
  
    return subscription;
  }
}



const a$ = new CountSubject();

const subscription1 = a$.pipe(
  takeUntil(timer(500))
).subscribe(rxObserver());

// emit an event
setTimeout(()=>{
  a$.next(1);
}, 250);