Make it deterministic

All labs

This suite passes, then fails, then passes. The engine runs it under a hundred seeds, varying the seeded randomness and the order in which same-instant timers fire, and reports how often it disagreed with itself. Every entry in the flake taxonomy has a deterministic fix, and the fix is almost always to make the varying thing an input rather than something the test reaches for.

goalGet the failure rate to zero across a hundred seeds — without deleting the assertions.

The module

export function delay(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

export function shuffleInto(target, items) {
  for (const item of items) {
    setTimeout(() => target.push(item), 0);
  }
  return target;
}

export function pickWinner(entrants, random) {
  const draw = typeof random === "function" ? random : Math.random;
  return entrants[Math.floor(draw() * entrants.length)];
}

Your tests

// Both of these pass sometimes. Run it and see how often.

test("collects the items", async () => {
  const collected = shuffleInto([], ["a", "b", "c"]);
  await delay(1);
  expect(collected).toEqual(["a", "b", "c"]);
});

test("picks a winner", () => {
  expect(pickWinner(["ana", "bo", "cy"])).toBe("ana");
});

Failure rate across a hundred seeds

Run the suite to measure it.