The fact that you can redefine a function gives me a terribly unsettled feeling, probably because I remember the assembly language days when we were told not to

The fact that you can redefine a function gives me a terribly unsettled feeling, probably because I remember the assembly language days when we were told not to write self-modifying code, as it's not re-entrant. (If JS were ever in a concurrent environment, would re-defining functions lead to problems?)