I had exactly the same issue, I tested this on multiple environments, using a similar sorting function and it is not at all the code's fault. It's probably a problem with the environment (Node.js 6 I believe it is) having a bit of an undefined behavior. MDN highlights that this issue might come up in certain environments: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort

Too bad, that was a pretty elegant solution in my opinion.

It's your code's problem. Please review how to use a sorting comparison function properly and raise future code problems as a question.