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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.
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I feel like you're splitting hairs. NaN, literally not a number, is of type number because there is no other primal it could be attached to. Would it be attached to a string? Array? Boolean?
From the ECMA 2015 specfications it says that NaN is "number value that is an IEEE 754-2008 'Not-a-Number' value." What about positive and negative infinity? Those are also "number" values that don't directly relate to a number but are under the number primals.
You should either specify that non-numerical number types should be accounted for or admit that your test case was ill-concieved and that it's illogical to try and add NaN, infinity, or -infinity.
According to javascript NaN is a number so... even If I don't like it too the sum of number NaN plus number 2 is NaN. Yes ironically the constant for Not A Number is of type.... number!!!.
typeof NaN === "number" //true