import { interval } from 'rxjs/observable/interval';
import { takeUntil, filter, scan, map, withLatestFrom } from 'rxjs/operators';
const source = interval(1000);
const isEven = val => val % 2 === 0;
//only allow values that are even
const evenSource = source.pipe(filter(isEven));
//keep a running total of the number of even numbers out
const evenNumberCount = evenSource.pipe(scan((acc, _) => acc + 1, 0));
//do not emit until 5 even numbers have been emitted
const fiveEvenNumbers = evenNumberCount.pipe(filter(val => val > 5));
const example = evenSource.pipe(
//also give me the current even number count for display
withLatestFrom(evenNumberCount),
map(([val, count]) => `Even number (${count}) : ${val}`),
//when five even numbers have been emitted, complete source observable
takeUntil(fiveEvenNumbers)
const subscribe = example.subscribe(val => console.log(val));