It cannot. It still has to check whether there is a local variable in any other scope that is above the one currently being executed, all the way to the top whe

It cannot. It still has to check whether there is a local variable in any other scope that is above the one currently being executed, all the way to the top where it finds global. Same with window in a browser context. You could still have a "window" variable placed between your execution context and the global context.