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You can bake a basic cake is using the following ingredients
Corrected:
You can bake a basic cake by using the following ingredients
A classical question for an interview job and very important to know it. Unfortunately it's a duplicate of a kata previously published: https://www.codewars.com/kata/find-the-duplicated-number-in-a-consecutive-unsorted-list-tougher-version
And many others with very similar.
Description specifies two different rules for which duplicate to return. The second is wrong.
Returning
-1is not a good choice. It might be a valid return value as well ( you never specify that the array only contains nonnegative numbers ).undefinedor in this case maybeNaN(!) might have been better choices, if you don't want to throw anError.map[] === true( in the reference solution ) is a bit of an antipattern, isn't it? It's aBooleanalready.