JavaScript has two special values 'null' and 'undefined' respectively. It should be possible to assign these values for global (or local) variables. This can be seen by following jrunscript sessions:

./jrunscript
js> var v = null
js> v
script error: sun.org.mozilla.javascript.internal.EcmaError: ReferenceError: "v" is not defined. (<STDIN>#1) in <STDIN> at line number 1
js>quit()

./jrunscript
js> var u = undefined
js> u
script error: sun.org.mozilla.javascript.internal.EcmaError: ReferenceError: "u" is not defined. (<STDIN>#1) in <STDIN> at line number 1
js>quit()
In fact, these are allowed for local variables as shown by following jrunscript sessions:

./jrunscript
js> function f() { var v = null; print(v); }
js> f()
null
js>quit()

./jrunscript
js> function g() { var u = undefined; print(u); }
js> g()
undefined
js>quit()

These are problems with API implementation rather than jrunscript tool. This can be verified by a simple program as below:

import javax.script.*;

public class t {
        public static void main(String[] args) throws Exception {
                ScriptEngineManager m = new ScriptEngineManager();
                ScriptEngine e = m.getEngineByName("js");
                e.eval("x = null; print(x)");
        }
}


or

import javax.script.*;

public class t {
        public static void main(String[] args) throws Exception {
                ScriptEngineManager m = new ScriptEngineManager();
                ScriptEngine e = m.getEngineByName("js");
                e.eval("x = undefined; print(x);");
        }
}


Exception in thread "main" javax.script.ScriptException: sun.org.mozilla.javascript.internal.EcmaError: ReferenceError: "x" is not defined. (<Unknown source>#1) in <Unknown source> at line number 1

is thrown with these programs.