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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.
String(num) is actually more versatile and consistent than num.toString(). For example, toString() will throw an error if num is null or undefined, while String(null) safely returns "null". Even though it’s not needed here, using String() is more robust and still very readable.
[...] is called the spread syntax. When we write [...str], it spreads the string into an array of its individual characters. This is necessary because reverse() only works on arrays.
Without the spread (just [str]), you'd get an array with the entire string as a single element — not what we want in this case.