Marbles

first.js

const { rxObserver } = require('api/v0.3');
const { timer, from, of, concat } = require('rxjs');
const { filter, tap, concatMap, take, map, debounceTime, interval, delay, first } = require('rxjs/operators');

// Try chaning 0 to baa
matching = '0';


$urls = from(['a', 'b', 'c'])
.pipe(
  tap(rxObserver('urls')),
  concatMap(x => of(x + 'a').pipe(delay(1), tap(rxObserver('urls with delay')))),
  tap(rxObserver('with delay')),
  filter(x => x === 'ba'),
  concatMap(x => of(x + 'a')),
  first(),
  tap(rxObserver('filtered urls')),
);

$allUrls = concat(of('0'), $urls, of('1'));

$allUrls.pipe(
  tap(rxObserver('all urls with pre-and-post')),
  concatMap(item => of(item).delay(1)),
  tap(rxObserver('resolved urls')),
  first(x => x == matching),
).subscribe(rxObserver('result'));