Why is jest.useFakeTimers not working with RxJs Observable delay

Question

I'm wondering why jest.useFakeTimers is working with setTimeout but not with the delay operator of RxJs:

jest.useFakeTimers();
import {Observable} from 'rxjs/Observable';
import 'rxjs';

describe('timers', () => {
    it('should resolve setTimeout synchronously', () => {
        const spy = jest.fn();
        setTimeout(spy, 20);
        expect(spy).not.toHaveBeenCalled();
        jest.runTimersToTime(20);
        expect(spy).toHaveBeenCalledTimes(1);
    });
    it('should resolve setInterval synchronously', () => {
        const spy = jest.fn();
        setInterval(spy, 20);
        expect(spy).not.toHaveBeenCalled();
        jest.runTimersToTime(20);
        expect(spy).toHaveBeenCalledTimes(1);
        jest.runTimersToTime(20);
        expect(spy).toHaveBeenCalledTimes(2);
    });
    it('should work with observables', () => {
        const delay$ = Observable.of(true).delay(20);
        const spy = jest.fn();
        delay$.subscribe(spy);
        expect(spy).not.toHaveBeenCalled();
        jest.runTimersToTime(2000);
        expect(spy).toHaveBeenCalledTimes(1);
    });
});

FYI: using 20 or 2000 as an argument for jest.runTimersToTime makes no difference. Using jest.runAllTimers() makes the test past

Answer 1

The delay operator does not work with Jest's fake timers because the delay operator's implementation uses its scheduler's concept of time - which is unrelated to Jest's concept of fake time.

The source is here:

while (queue.length > 0 && (queue[0].time - scheduler.now()) <= 0) {
  queue.shift().notification.observe(destination);
}

The current (non-fake) time is assigned to notifications when they are created and the current time is what the scheduler's now method returns. When Jest's fake timers are used, an insufficient amount of actual (non-fake) time will have elapsed and the notifications will remain in the queue.

To write RxJS tests using fake or virtual time, you can use the VirtualTimeScheduler. See this answer. Or you can use the TestScheduler and marble tests.