verify(input: string): string[] {
        const isWeekend = [SUNDAY, SATURDAY]
            .filter(x => x === this._timeProvider.getDay())
            .length > 0;
        
        if (isWeekend) {
            throw new Error("It's the weekend!");
        }
        return [];
    }
}

class FakeTimeProvider implements TimeProviderInterface {
    fakeDay: number;
    getDay(): number {
        return this.fakeDay;
    }
}

describe('password verifier with interfaces', () => {
    test('on weekends, throws exceptions', () => {
        const stubTimeProvider = new FakeTimeProvider();
        stubTimeProvider.fakeDay = SUNDAY;
        const verifier = new PasswordVerifier([], stubTimeProvider);
        expect(() => verifier.verify('anything'))
            .toThrow("It's the weekend!");
    });
});