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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.
My guess is age.
Number.isIntegeris ECMAScript 2015, and this kata is from early 2015, when that may not have been supported yet.It's not the canonical polyfill either though, so the test may not be perfectly accurate (then again, it very well may be).