Time and randomness

The test that fails at midnight

Real time in a test means: a test that waits, a test that is different at the end of the month, a test that fails when the machine is slow, and a test nobody can reason about. The cure is always the same — the code asks something for the time, rather than taking it.

and passes for the rest of the year

function isExpired(quote) {
  return Date.now() - quote.issuedAt > 30 * 24 * 60 * 60 * 1000;
}

it("expires after thirty days", () => {
  const quote = { issuedAt: Date.now() - 31 * 24 * 60 * 60 * 1000 };

  expect(isExpired(quote)).toBe(true);      // fine — but the test depends on the real clock
});

Inject the clock

Two lines of production code, and the test becomes an exact statement about a boundary. This is the same move as the dependency injection in the last lesson, applied to the most-used global in the language.

a default argument, and the whole problem goes away

export function isExpired(quote, now = Date.now()) {
  return now - quote.issuedAt > THIRTY_DAYS;
}

it("expires after thirty days", () => {
  expect(isExpired({ issuedAt: 0 }, THIRTY_DAYS + 1)).toBe(true);
  expect(isExpired({ issuedAt: 0 }, THIRTY_DAYS - 1)).toBe(false);
});

Fake timers, for code that schedules

Every real runner has this — vi.useFakeTimers(), jest.useFakeTimers() — and it replaces setTimeout, setInterval and Date.now globally for the duration of the test. The effect is the same: a debounce test that takes a microsecond and proves the boundary, instead of one that sleeps 300 ms and proves roughly nothing.

move the clock by hand instead of waiting

const clock = fakeTimers(0);

const save = spy();
const debounced = debounce(save, 300, clock);

debounced("a");
debounced("b");

clock.tick(299);
expect(save.callCount).toBe(0);      // not yet

clock.tick(1);
expect(save.callCount).toBe(1);      // exactly once
expect(save.calledWith("b")).toBe(true);

Restore them afterwards

Fake timers are global state. A suite that installs them and forgets to restore leaves every later test in a world where setTimeout never fires — which shows up as a completely unrelated test hanging. afterEach(() => vi.useRealTimers()).

Randomness

The same applies to ids and tokens: crypto.randomUUID() in the middle of a function is untestable, and a function that takes an id generator is testable and no harder to call.

seeded, or injected — never left to chance

// the code takes its source of randomness
export function pickFitter(fitters, random = Math.random) {
  return fitters[Math.floor(random() * fitters.length)];
}

// the test decides what "random" means
expect(pickFitter(["a", "b", "c"], () => 0)).toBe("a");
expect(pickFitter(["a", "b", "c"], () => 0.99)).toBe("c");

// or a seeded generator, when you need a whole sequence
const random = seededRandom(42);

Time zones and formatting

A test that asserts a formatted date fails on a machine in another zone unless the zone is fixed. Set TZ=UTC in the test environment, store everything in UTC as the web course said, and assert on the ISO string rather than the localised one — or pass an explicit locale and zone to Intl.DateTimeFormat and assert that.

The test that fails at midnight

Inject the clock

Fake timers, for code that schedules

Randomness

Time zones and formatting

Practice