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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.
tldr; Reassigning parameters deoptimizes in many engines, specifically v8 - it's a horrible idea to do it. Variables are free, and creating new ones rather than reusing old ones makes code much clearer.
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best practice is not to mutate the initial argument of the function. str in this case.
.map can work better here (more declarative)