debug subscriptions

on-subscribe.js

const onSubscribe = (fn) => stream => Rx.defer(() => {
  fn();
  return stream;
})

const logOnSubscribe = message => onSubscribe(() => console.log(message));

Rx.interval(10e3).pipe(
  switchMap(() => interval(200).pipe(
    logOnSubscribe('subscribed to 200ms interval')
  ))
).subscribe();