const System = imports.system;
System.exit(0);

produces

** (gjs:19889): CRITICAL **: gjs_parse_args: assertion `arg_location != NULL' failed
JS_EvaluateScript() failed but no exception message?

while 

const System = imports.system;
System.exit();

produces

    JS ERROR: !!!   Exception was: Error: Error invoking exit: Expected 1 arguments, got 0
    JS ERROR: !!!     message = '"Error invoking exit: Expected 1 arguments, got 0"'
    JS ERROR: !!!     fileName = '"main.js"'
    JS ERROR: !!!     lineNumber = '17'
    JS ERROR: !!!     stack = '"@main.js:17

I think either the number of args should be 1 in the JS_DefineFunction() call on l.158 of system.c (https://git.gnome.org/browse/gjs/tree/modules/system.c#n158) or gjs_exit() shouldn't call gjs_parse_args() on l.116 (https://git.gnome.org/browse/gjs/tree/modules/system.c#n116).

I don't know what your desired API for system.exit() is, so I didn't prepare a patch, but either way it's a one or two-line fix.

Personally, the API with the exit code argument makes more sense to me.