Wikipedia Table Scraper
This looks pretty nice, I only have a few nit-picks, really.

What you call "rows" are actually "columns". It's a bit misleading.
The rows are actually the var arr that you're making.

The ternary is a bit confusing. 
If clarity is important to you (as you mentioned),
then I think a good old-fashioned if-else would be better.

Lastly, the idx function parameters in the inner map calls are unused, so you could just as well drop them.