thank you @cwps, the problem is that this is happening in test.js file which is not accesible for me. I do not call map function in my code, that is why I do not know, how to continue right now :)

Any hints guys? I really do not know what I am missing here - this is the only error I get. Everything else is ok. - TypeError: arr.map is not a function at addOneArr (test.js:78:18)

Thank you for this post, can you explain it more for us? THE BEST solution above is just O(n) complex am I right?