Checkpoint: Unit Tests That Matter · Testing JavaScript Apps

Five questions on this section. Answer all of them, then see your score and why each answer is right or wrong.

Splitwise-lite gives free premium to groups that spend $50.00 (5000 cents) or more. Which two inputs best test the rule?

4999 and 5000
1000 and 10000
0 and 5000
5000 and 5001

What should change in this test?

JavaScript

test('rejects a negative amount', async () => {
  expect(saveExpense({ description: 'Taxi', amountCents: -100 }, api))
    .rejects.toThrow('positive');
});

Nothing; it is correct as written.
Replace toThrow with toThrowError.
Wrap the call in try/catch and assert in the catch.
Put await in front of expect(...) so the test waits for the rejection.

While testing computeBalances, a test replaces splitEvenly with vi.fn().mockReturnValue([500, 500]). Later splitEvenly gets a rounding bug. What happens to that test?

It fails, because computeBalances now gets wrong shares.
It stays green, because the real splitEvenly never runs in it.
Vitest warns that the mock no longer matches the implementation.
The test errors because mocks of internal modules are not allowed.

The second row of this table fails. What test name does Vitest report?

JavaScript

test.each([
  { total: 1000, people: 3, first: 334 },
  { total: 1001, people: 4, first: 251 },
])('splits $total between $people', ({ total, people, first }) => {
  expect(splitEvenly(total, people)[0]).toBe(first);
});

splits $total between $people
splits 1001 between 4 (row 2)
splits 1001 between 4
splits [object Object] between undefined

A reminder is scheduled with setTimeout to go out 24 hours after an expense is added. With vi.useFakeTimers() active, which line makes the reminder fire in the test?

await new Promise((r) => setTimeout(r, 86_400_000))
vi.useRealTimers()
vi.setSystemTime(tomorrow)
vi.advanceTimersByTime(24 * 60 * 60 * 1000)

5 questions unanswered