Thanks for your input. After reading I went to look into it and learned a lot and better practices. Thank you!

Example: the teacher gave the task to find a favorite quote from the Harry Potter book and show it to the whole class.

Solutions without mutation: copy the quote from the book with chalk onto the school board, scan the book page, print the quote from the electronic version of the book and stick it on the board.

Solution with mutation: take scissors, cut out the quote from the book and stick it on the board.

can you please explain why for the benefit of people who are most likely new and don't realise the implications of this (or what mutating even means)

Mutating an argument is the worst thing you can do.

it's not.

list don't change

Please advise as we all know, we should avoid changes in original array. So how this is the best practice?

Ah, that's what I should have done! Good work.

thanks for creating this kata