I would suggest adding what you wrote: "Empty sum gives 0, no sum gives -1" or what you wrote in another reply: "[-1,-1] returns 0. [] returns -1". I understand your reasoning I'm just hoping to clear things up for beginners.

i enjoyed it too

Could you suggest a revised sentence?

I don't find it unclear. Empty sum gives 0, no sum gives -1. Empty sum is maths as usual, no sum is "empty" ( value ).

For node v8, flat is not available. Will raise new issue about having to update.

Cosnider revising this sentence:

"The task is to return the maximum sum of the elements located between two negative elements, or -1, Nothing, or a similar empty value, if there is no such sum."

It is unclear as to when to return -1 versus returning 0. For a 7kyu I feel this info should be stated explicitly. Otherwise great qustion I enjoyed it!

Codewars runs an older version of node that doesn't support some modern JS methods. Could be why.

Are you running it in node via VScode or in browser?