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Why teach global impurity?
A for loop is great, but why teach to hold global variables outside of the code block or a function? I understand simplicity, but if you teach it correctly from the start, and just tell them why later, it will be easier to understand later.
Otherwise put, we learn practices that we then have to unlearn later because they're not best practices. Why not start with best practices up front and make it an easy rule. Then when you explain why, there is no massive unlearning and frustration.
Steven Parker228,991 Points
This might just be a version issue.
Many of the courses were developed before ES2015 was widely adopted. To my knowledge, those released since all adhere to the principles you describe.
As the existing courses get revised, I would expect everything in the library will eventually get updated and conform also.