Thank you, it was in the first test.

They work fine, your problem is somewhere else.

toUpperCase()/toLowerCase() is not working for me, any help please?

I thought it was a kata issue, thank you for the tip

You have to put it like string.Join(''), as in some other methods you have to write System.X instead of X. Annoying and common on codewars.

the problem was in my code, I fixed it and everything went smooth, thank you for the help

For JS I think there's 500 random tests. With basic approach I'm passing in ~1.5s. You can always add console.log(n) at top of your function.

Im able to pass only 80 test maximum, i think the rest of the 20 tests has big numbers which will take more than 12 seconds to show in output

Either you got an infinite loop, or your code is really slow. Whatever the case, that's not a kata issue.

I optimized my code to use only 1 loop, but i still get the same error

One last error i get is

STDERR
Execution Timed Out (12000 ms)

Why did my code time out?
Our servers are configured to only allow a certain amount of time for your code to execute. In rare cases the server may be taking on too much work and simply wasn't able to run your code efficiently enough. Most of the time though this issue is caused by inefficient algorithms. If you see this error multiple times you should try to optimize your code further.

Sorry I didn't pay attention to this, thank you for the explanation.

if(num <= 0) return "";

String is not an Array in JavaScript, they're entirely different datatypes.