// RxJS v6+
import { iif, of, interval } from 'rxjs';
import { mergeMap } from 'rxjs/operators';
​
const r$ = of('R');
const x$ = of('X');
​
interval(1000)
  .pipe(mergeMap(v => iif(() => v % 4 === 0, r$, x$)))
  .subscribe(console.log);
​
// output: R, X, X, X, R, X, X, X, etc...

// RxJS v6+
import { fromEvent, iif, of } from 'rxjs';
import { mergeMap, map, throttleTime, filter } from 'rxjs/operators';
​
const r$ = of(`I'm saying R!!`);
const x$ = of(`X's always win!!`);
​
fromEvent(document, 'mousemove')
  .pipe(
    throttleTime(50),
    filter((move: MouseEvent) => move.clientY < 210),
    map((move: MouseEvent) => move.clientY),
    mergeMap(yCoord => iif(() => yCoord < 110, r$, x$))
  )
  .subscribe(console.log);

// RxJS v6+
import { fromEvent, iif, of, interval, pipe } from 'rxjs';
import { mergeMap } from 'rxjs/operators';
​
interval(1000)
  .pipe(
    mergeMap(v =>
      iif(
        () => !!(v % 2),
        of(v)
        // if not supplied defaults to EMPTY
      )
    )
    // output: 1,3,5...
  )
  .subscribe(console.log);