tldr; Reassigning parameters deoptimizes in many engines, specifically v8 - it's a horrible idea to do it. Variables are free, and creating new ones rather than reusing old ones makes code much clearer.

It would make no difference making a new variable just to assign to for storing the new array, considering the old parameter is never used again. It will only make the code less readable.

Considering that the block of code is more maintainable as-is, such a practice rule does not apply here.

best practice is not to mutate the initial argument of the function. str in this case.

.map can work better here (more declarative)