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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.
but then you'd have to add extra logic for the right side, his way is easier
I'd be inclined to agree if he uses === instead of ==
No, splice is mutative. Never mutate when writing functional code, which is clearly what this problem is asking for.