Test for gist https://gist.github.com/oops-wrong/a9ba9a0382b8c271a52f08b4c5af155b

mute-first-observable.spec.ts

describe('muteFirst', () => {
  const first = 'first';
  const second = 'second';
  const third = 'third';

  it('should return the second observable', () => {
    const first$ = cold('-a', first);
    const second$ = cold('-b', second);

    expect(muteFirst(first$, second$)).toBeObservable(second$);
  });

  it('should subscribe the both observables', () => {
    const first$ = cold('-a', first);
    const second$ = cold('-b', second);

    muteFirst(first$, second$).subscribe();

    expect(first$.getSubscriptions().length).toBe(1);
    expect(second$.getSubscriptions().length).toBe(1);
  });

  it('should not return the second value until the second changed', () => {
    const first$ = cold('-a', first);
    const second$ = cold('-b-c-d', { b: second, c: second, d: third });
    const expected$ = cold('-b---d', { b: second, d: third });

    muteFirst(first$, second$).subscribe();

    expect(muteFirst(first$, second$)).toBeObservable(expected$);
  });
});