import 'rxjs/add/observable/interval';
import 'rxjs/add/observable/of';
import 'rxjs/add/operator/take';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mergeMap';

mergeOperator () {
    const numbers$ = Observable.interval(1000);
    const letters$ = Observable.of('a', 'b', 'c', 'd', 'e');
    letters$.mergeMap(x => numbers$.take(5).map(i => i + x))
      .subscribe(x => console.log(x));
  }