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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.
"el" is a parameter of the function that is used in filter, it will save the elements of the array in each iteration of filter, so he uses "el" in the function to have each character in the array. I think he named the parameter "el" because of "element". The reason it maybe look's something weird it's because he use a "function expresion"( function(){} ) instead of a "arrow function"( () => {} )