debatable as to whether this is good practice or not, but history proved you wrong by adding full-fledged classes to JavaScript, with private and static properties

The value in list[0].language can be anything, but as he specified the [0] index and the loop runs through every object, it will only return true if all the other objects has the same language of the [0] index

You should refer to sample tests and throw the corresponding message instead of an empty string

There can be valid reasons to mutate inputs ( eg. performance. did you ever stop and wonder why eg. default JS sorting behaviour is modification in place? ). So if a kata specifically specifies ( and tests! ) this behaviour, you can have your doubts, but it makes a valid kata.

I fully agree side-effect-free programming has its advantages ( as well as its disadvantages, but you're not even mentioning those ). But single-mindedly forbidding valid language features in kata, or even kata about valid language features, is just closed-minded. Everything's a trade-off, including side-effect-free programming.

Yeah, that confused me for a while. I just directly followed the examples

^didn't knew about that. Thanks man.

very clever

It creates a local copy so that sort() doesn't mutate the input array.

just print the given array and average and your answer in your solution, you will get to know all the test case

This issue is completely valid - a static method, equivalent to (for example) Arrays.sort in Java ,would simply be Array.remove_ in JavaScript. Attaching to the prototype means the method is available on each instance and each instance only. It would be the equivalent of sorting arrays in Java in the following manner

int[] arr = {};
int[] sortedArray = arr.sort(someOtherArray);

This is a suggestion.

"The problem is, you ether learned this lesson already, then this is trivial, or you do not know where the problem is. "

A lot of bug fixing is like that, obvious when you have seen it before, mindnumbingly frustating otherwise.

Thanks