I've just realized how trivial my question was.
Thanks for clarification!

If I type in my function something like:
return arr.join(',') it returns ''1,1,3,3,7,2,2,2'' instead of '20, 37, 21'

If I try:
return arr.split(',') it returns '['20', '37', '21']'

Is this some kind of local IDE issue? I've checked the documentation for join(',') and split(',') and it didn't suggest that the output would look like this.
I've used these methods several times, and they worked like they are suppoused to, now they don't. I will appreciate any advice.