There are no locals in JS (honest question)? If that the case you have to assume that everything can be mutated everywhere unless for those functions that explicitly and fully document their side effects (and yield would certainly be a side effect).
variables declared with the var keyword are functionally scoped in JS so they are local to the function they are declared in and are lexically scoped to the closure of that function. So your local variables you can be fairly confident are not mutated anywhere else.
That being said you can totally have globals and whatnot that can be mutated anywhere (e.g. parameters passed to a function or literal globals installed in the global scope) or things declared at a lower scope that can still be modified by multiple functions.
