How does this work?

Where is the category Best of Best Practices? Right here.

Trying the same (to my eyes) code, I keep getting a 'cannot find symbol' error on variable imp? (Never mind, found the typo, it was variable 'inp' ...)

Should be same result, but wouldn't it be slightly faster to shortcut the extra operation?

A nice demonstration of the dangers of coding for the test.

I agree it's not really 8 kyu, but rather than the problem itself, which isn't that difficult, the instructions which start with:

"Define String.prototype.toAlternatingCase (or a similar function/method such as to_alternating_case/toAlternatingCase/ToAlternatingCase in your selected language; see the initial solution for details)"

make this sound much more difficult than it actually is.

I'd suggest replacing that text with something simple like:

"Define a function..."

Then it's 8 kyu.

You should loop that.

I use the "Fork" button, then "Run" and it shows the time. There can be quite a bit of variation between separate runs of the same code though.

Same. Well, might have been a holey 45 minutes.

Do replies count?

Of several tested, this was fastest.