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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.
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Yeah, you don't technically have to, but if you write production code, it is a good idea to leave the structure there for a couple of reasons. This example is trivial, but you may want to have a default parameter in place, in which case you need the parenthesis (ie: (n=10) => ...), also the parenthesis clearly marks where there function param is, and for anything but one-liners like this, highlighting the name of that parameter argument is helpful, and worth the time to write the parenthesis.
I prefer parenthesis for clarity.
You don't have to use parenthesis when you have only one argument - n - in this case.