How do I debug a JavaScript program using Nashorn?

I need to be able to debug JS code executed by Nashorn ie:

ScriptEngineManager sem = new ScriptEngineManager();
ScriptEngine engine = sem.getEngineByName("nashorn");
engine.eval("load (\"src/com/sap/rdl/runjs/file.js\");");

      

        
        
        
      

    

Now I need to add a breakpoint in the JS file and programmatically jump through the lines (implement a debugger).

I know intelliJ and NetBeans have implemented this, so it should be possible, but I can't find any API in Nashorn that allows stepping and / or displaying strings or variables.

Any advice / links would be most welcome. Thanks in advance.