Why do you think the JIT would be able to optimize this? And how would it go about it? I know only about a few rough things and heuristics. I wouldn't expect or assume that this would be optimized.
It would probably have to recognize that the _usage_ of this function can be translated into a local mutation without allocating additional arrays. But from just looking at the function locally it isn't clear whether that is a safe assumption.
What I meant to say is that this definitely isn't a safe assumption and performance of this loop will be less predictable. That said, I wouldn't be surprised that such complex JITs as V8 or JSC can detect this scenario.
It would probably have to recognize that the _usage_ of this function can be translated into a local mutation without allocating additional arrays. But from just looking at the function locally it isn't clear whether that is a safe assumption.