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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.
numbers[i + 1] dont become undefined, look at loop condition arr.length - 1.
I can not say its very clean , numbers[i + 1] becomes undefined at one point.
Is kindof dirty solition but works anyway.