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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 starts as the char code of the first entry, it will increment if it matches in the .map to be the one more (the i++). If it doesnt match at any point then there is a missing letter, and i will stop increasing which will leave i as the char code of the missing letter.
It's useful to go along with pen and paper to see what each operation is doing. Remember that i++ mutates i, and notice that the map is just acting as a forEach here