import { merge, of } from "rxjs";
import { connect, filter, map } from "rxjs/operators";

const chars$ = of("A", "b", "C", "D", "e", "f", "G");

chars$
  .pipe(
    connect(shared$ =>
      merge(
        shared$.pipe(
          filter(x => x.toLowerCase() === x),
          map(x => `lower ${x.toUpperCase()}`)
        ),
        shared$.pipe(
          filter(x => x.toLowerCase() !== x),
          map(x => `upper ${x}`)
        )
      )
    )
  )
  .subscribe(console.log);
// (synchronously) upper A
// (synchronously) lower B
// (synchronously) upper C
// (synchronously) upper D
// (synchronously) lower E
// (synchronously) lower F
// (synchronously) upper G