The switch statement in the fizzBuzz function switches on a tuple containing two expressions—number % 3 (or “the remainder after dividing number by three”) and number % 5 (“the remainder after dividing number by 5”). Each case of the switch statement compares a pattern against the output of this two-value tuple, and checks for a match.

For the number 15, the result of (number % 3, number % 5) is (0, 0)—that is, the number divides exactly by both 3 and 5. This matches the first switch case and returns "FizzBuzz!"