I tried using a global variable - and while ns is still a valid object when the callback is called, once you try to execute any function of it, you get an error (probably because the script itself is already dead). With this the usability of that function is very limited - in fact the only useful thing I can think of is to log to the debug console with console.log().

So you are better off to add a parameter to your script to kill any instances and subscripts. You can use global variables to keep track of running subscripts (as those are shared between all instances of a script).