My bad, forgot about it

1. Unnecessary comment
2. Implementation can be simplified

Remove unnecessary parentheses

length can be renamed to width to better match another constant

1. Too difficult implementation for such problem
2. Why do you use let if you don't update value of the variable numbers and halfOfLength, you should prefer using const to declare them
3. firstPart and secondPart are declared as global variables, it's a bad practice!

I think that additional test cases are necessary.
Many solutions have a flow in a simple "bla bla bla!" case.
Output -> "labay labay bla!".

Why it is not "Best Pratice"?

How simple it is!
I feel embarrassed comparing this solution with mine ;)

I think that you should add "Don't mutate the input!" to the description.

Very readable