Array.reduce, map, filter, etc...
Bunch of 10 year old string functions.
My only assumption is that the judge is using CScript or something to run the code which only supported ES 3.0 and doesn't ever and will never be updated.
I'd really like to reach out to my community of JS developers and challenge them with a few of the LeetCode problems, but they'll run into the same issues I'm running into. I hope we can get this fixed so this becomes useful for JS programmers as well.