foo.ts

describe.only('createDataCron', () => {
  it(
    'init',
    marbles(m => {
      const source$ = m.cold('--a|   ');
      const sourceS = '       ^------';
      const ms = m.time('     ------|');
      const expected = '      --a|   ';

      const cron = createDataCron(source$, ms);
      const actual$ = cron.latestData$;

      m.expect(actual$).toBeObservable(expected);
      m.expect(source$).toHaveSubscriptions(sourceS);
    }),
  );
});