Why does `AssertJ`’s `isCloseTo` return `true` when comparing `Double.NaN` to a numeric value?

General Discussions
1

AssertJ unexpectedly returns true when checking if a double is close to NaN.

```java
assertThat(Double.NaN).isCloseTo(0.00811, Percentage.withPercentage(0.1));


Is this a bug? The version used is 3.6.2.
2

I ran into this as well and realized it’s not exactly a bug but a quirk in older versions of AssertJ (like 3.6.2). In that version, isCloseTo doesn’t handle NaN properly—it treats it as “not comparable” and can return true unexpectedly. Upgrading to a newer AssertJ (3.24+) fixed this in my projects because newer versions explicitly check for NaN and Infinity.

3

I also noticed that waitFor sometimes fails in headless CI runs because the component renders asynchronously. Switching to findBy queries made the tests more reliable:

for (const status of statuses) {
  const element = await screen.findByText(status, {}, { timeout: 3000 });
  expect(element).toBeInTheDocument();
}

findBy internally waits for the element to appear, which avoids flaky waitFor usage. It’s cleaner and often fixes CI-only failures.

4

From my experience, any comparison using Percentage or Offset with NaN can give unexpected results, because NaN is unordered (NaN != NaN). I usually guard my tests like this:

double value = calculateSomething();
if (Double.isNaN(value)) {
    assertThat(value).isNaN();
} else {
    assertThat(value).isCloseTo(0.00811, Percentage.withPercentage(0.1));
}

This pattern prevents false positives when NaN sneaks in and keeps numeric comparisons safe.