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> Basically the output is the unrolled version of the recursion, which cuts down on function invocation (comparatively expensive in dynamic languages).

You're right about why it's included, but this is a bug, not a feature. If this is a hot function it'll be optimized. The recursive call is a known, non-dynamic invocation so the function location will be inlined leaving just the fairly low function call overhead itself. There's a reason nobody does arbitrary-length loop unrolls.

I would definitely bet that this is a bug to be fixed :) Right now you can see this if you call fib(20) in that example...the compiler times out while trying to unroll that far. Clearly that behavior won't stick around.

But should it still unroll the recursive calls and balloon the code out just because a mutatable function call was made, or should it leave it as an actual recursive function call.

"prepack has no idea what Date.now() does" isn't quite true, because there's a typeof check happening on the return to ensure it's a number, meaning that some assumptions are definitely being made.

Also the lack of variable declaration in `y = Date.now()` made y an implicit global, so it couldn't be optimised out no matter what. Even doing `let y = Date.now()` cuts out 400 lines.