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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I guess the best is to do this task in one iteration without high level array methods like .filter() or .concat(). Using build in functions is maybe fast and fancy but doesnt actually check your knowledge about solving problems. Its like using jQuery - problem is solved but you dont know how (unless you get into jQuery code and check) :)