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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.
Done.
array can has not numeric indexes and skipped indexes.
I would make it even more explicit.
"Write a function, which takes as parameters an array and a callback function. The callback function is a mathematical function that will be applied on each element of this array."
->
"Write the
processArrayfunction, which takes an array and a callback function as parameters. The callback function can be, for example, a mathematical function that will be applied on each element of this array. Optionally, also write tests similar to the examples below."