java - JavaScripting API

Question

When I invoked the callScriptFunction() it shows the scriptReturnValue is null. How can I rewrite the function to avoid the null?

public class SimpleInvokeScript {

    public static void main(String[] args) 
      throws FileNotFoundException, ScriptException, NoSuchMethodException
    {
        new SimpleInvokeScript().run(); 
    }

    public void run() throws FileNotFoundException, ScriptException, NoSuchMethodException {
        ScriptEngineManager scriptEngineMgr = new ScriptEngineManager();
        ScriptEngine jsEngine = scriptEngineMgr.getEngineByName("JavaScript");

        jsEngine.put("myJavaApp", this);

        File scriptFile = new File("src/main/scripts/JavaScript.js");

        // Capture the script engine's stdout in a StringWriter. 
        StringWriter scriptOutput = new StringWriter();
        jsEngine.getContext().setWriter(new PrintWriter(scriptOutput));


        Object scriptReturnValue = jsEngine.eval(new FileReader(scriptFile));

        if ( scriptReturnValue == null) {
            System.out.println("Script returned null");
        } else {
            System.out.println(
                "Script returned type " + scriptReturnValue.getClass().getName() +
                ", with string value '" + scriptReturnValue + "'"
            );

        }

        Invocable invocableEngine = (Invocable) jsEngine; 
        System.out.println("Calling 'invokeFunction' on the engine");
       invocableEngine.invokeFunction("callScriptFunction", "name");

        System.out.println("Script output:\n----------\n" + scriptOutput);
        System.out.println("----------");

    /** Method to be invoked from the script to return a string. */
    public String getSpecialMessage() {
        System.out.println("Java method 'getSpecialMessage' invoked");
        return "A special announcement from SimpleInvokeScript Java app";
    }
}

The JavaScript.js will look like this. Also, I have one more doubt, I gave msg = myJavaApp.getSpecialMessage() without var declaration and it's working. Can we avoid var declaration?

function scriptFunction(name){

     var msg = myJavaApp.getSpecialMessage();
     println( msg );
}