medium-rxjs-multicast-default.ts

function defaultObservableIfEmpty<T>(
  defaultObservable: Observable<T>
): (source: Observable<T>) => Observable<T> {

  return source => source.publish(shared => shared.merge(
    shared.isEmpty().mergeMap(empty => empty ?
      defaultObservable :
      Observable.empty<T>()
    )
  ));
}