import { from } from 'rxjs';
import { map, switchMap } from 'rxjs/operators';

const featureObservable = from([‘A’, ‘B’]);
featureObservable.pipe(
    map(feature => getRequirement(feature)),
    switchMap(req => getPlanning(req))
).subscribe( work => console.log(work));



// Console Output
$ Work A #1 is Done!
$ Work A #2 is Done!
$ Work B #1 is Done!
$ Work B #2 is Done!
$ Work B #3 is Done!