To be fair, even as someone who has low-level code including writing VMs, because of the heavy use of closures in JavaScript it's not immediately obvious when a

To be fair, even as someone who has low-level code including writing VMs, because of the heavy use of closures in JavaScript it's not immediately obvious when accessing a variable in JS will go through the heap or the hardware stack. What the hardware provides doesn't quite match the semantics of the language and the VM is doing a lot of work/magic to paper over those differences.