Observable5.ts

class SafeSubscriber<T> {
  closed = false;
 
  constructor(private destination: Partial<Observer<T>>) {}
 
  next(value: T) {
    if (!this.closed) {
      this.destination.next?.(value); // Note the ?. check here.
    }
  }
 
  complete() {
    if (!this.closed) {
      this.closed = true;
      this.destination.complete?.(); // And here.
    }
  }
}