Comment by goldenkek

Because tests should not be more verbose than code. Refactoring the sumArray method to use map instead of a for loop shouldnt break tests. it is terrible terrible practice to take internal implementations and then stub their callees to make sure that the code that was written got executed. I saw this a lot at Amazon. Its a fundamental type of moronic testing and is absoltely useless. All it serves is to make any kind of refactoring or changes twice as burdensome.