Everything after the line containing '<-' happens in a callback. Since it's a callback, I assume it's up to the function whether to call it, when to call it, a

Everything after the line containing '<-' happens in a callback. Since it's a callback, I assume it's up to the function whether to call it, when to call it, and how many times to call it, so this can implement control statements. I would guess that it also allows it to be async (when the callback isn't called until after an I/O operation).