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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.
Please when pasting code, use proper markdown and also mark your post as having spoiler content (I did for you this time).
The keys of an object are strings, so you should convert them before pushing to your array to have an array of numbers.
That's not an issue, you're returning an array/list of strings instead of one of numbers. The problem is in your code.