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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.
The chief advantage is that all the objects will share the same method instead of a new one being created for each instance.
You can find a more detailed difference here: Declaring javascript object method in constructor function vs. in prototype
No you're not. Unfindables are just those numbers that exist outside of valid card numbers - so your solution is matching numbers as Mastercard even though they're outside the expected range/rules for a Mastercard. I might change that message to make it clearer I think.