I spent a lot of time being confused by the requirement to differentiate between a explicit 'undefined' and an implicit 'undefined'.

Explicit being something like [1, 2, 3, undefined, 4] and implicit being [1, 2, 3, , 4]. Console logging index 3 in each case will give you 'undefined', and that seems like a tricky thing for newer people to understand. And I specify newer because once you know that, the solution isn't too tricky for someone at 5 or 6 kyu to get.

I'd suggest making that more clear in the description. Might save people time and help people realize what they need to google!

I felt like the description on this one was a little dubious.
I sorta guessed what the explanation was trying to say, and luckily I got it correct.