for (.. in) might iterate of object prototype keys

Same, i mean for this kata its ok, but i dont think its a good practice

Agree

empty object isn't neccessary here

Fixed.

I believe that modifying element in filter is wrong and should be considered as an antipattern.

Maybe few additional examples and tests would be good

In the description it's clearly stated that if NaN is one of the arguments it should return false, but it's different in random tests where, for such a case NaN is asked for to be returned. Many coders pointed this out but nothing's changed

Test for NaN is not valid (in the instructions we return false if we encounter "wrong" valu while test for NaN value expects NaN in result)

Additionally it only happens once per x tests as it is happening during random tests.

Iiiiiiiii po co plusik w wyrażeniu regularnym?;)