Excellent, I'll read up on that link. Thank you for your reply.

I'm curious about the difference between adding the method to the constructor versus using the prototype method as shown in this answer. What's the advantage? Many users have voted this solution a best practice so I imagine the difference is significant. Thanks.

Ok thank you, I appreciate your quick reply. I'll continue to write tests and try to figure out why I'm getting a false match.

I'm unclear on what "unfindables" are. I've written about 20 or so tests that pass and when I submit my solution everything passes except the second of the two "It should return Unknown for unfindables". The submission test result stated "Expected: Unknown, instead got: Mastercard". I wrote a new batch of tests to try and nail down variations of the Mastercard rules that shouldn't pass but cannot replicate this failure. Am I missing something in the definition of "unfindables"?