class SafeObserver {

  constructor(destination) {

    this.destination = destination;

  }

  next(value) {

    const destination = this.destination;

    if (destination.next && !this.isUnsubscribed) {

      destination.next && destination.next(value);

    }

  }

  error(err) {

    const destination = this.destination;

    if (!this.isUnsubscribed) {

      if (destination.error) {

        destination.error(error);

      }

      this.unsubscribe();

    }

  }

  complete() {

    const destination = this.destination;

    if (!this.isUnsubscribed) {

      if (destination.complete) {

        destination.complete();

      }

      this.unsubscribe();

    }

  }

  unsubscribe() {

    this.isUnsubscribed = true;

    if (this._unsubscribe) {

      this._unsubscribe();

    }

  }

}

class Observable {

  constructor(_subscribe) {

    this._subscribe = _subscribe;

  }

  subscribe(observer) {

    const safeObserver = new SafeObserver(observer);

    safeObserver._unsubscribe = this._subscribe(safeObserver);

    return () => safeObserver.unsubscribe();

  }

}

const myObservable = new Observable((observer) => {

  let i = 0;

  const id = setInterval(() => {

    if (i < 10) {

      observer.next(i++);

    } else {

      observer.complete();

    }

  }, 100);

  return () => {

    console.log('unsubbed');

    clearInterval(id);

  };

});

const observer = {

  next(value) { console.log('next -> ' + value); },

  error(err) { },

  complete() { console.log('complete'); }

};

const foo = myObservable.subscribe(observer);

foo.unsubscribe();