You need to either run IDE on JDK8 or have JDK8 added as Platform via Tools|Java Platforms. The create a new Java project. Once it is created, make sure it uses JDK8 platform:

1. right click on project in Projects window


2. select Properties->Libraries and check JDK Platform setting

Next you can:

• create a JavaScript file inside this Java project, if you e.g. right click on the file, you can Run it or debug it. In this case, it would really run/debug single JS file using Nashorn



• or you can start JavaScript code from Java code, e.g.:

  
  
  

      try {
  
      ScriptEngineManager factory = new ScriptEngineManager();
  
      ScriptEngine engine = factory.getEngineByName(nashorn);
  
      engine.eval(function hi(){nvar a = 'PROSPER'.toLowerCase(); nmiddle(); nprint('Live long and' + a)}n function middle(){n var b = 1; for(var i=0, max = 5; i<max;i++){nb++;n}n print('b is '+b);}hi(););
  
  } catch (ScriptException ex) {
  
      Logger.getLogger(SampleRunner.class.getName()).log(Level.SEVERE, null, ex);
  
  }
  
  




• or you can load JS file in Java code using:

  
  
  

      try {
  
      ScriptEngineManager factory = new ScriptEngineManager();
  
      ScriptEngine engine = factory.getEngineByName(nashorn);
  
      engine.eval(load( + src/packageName/file.js + ););
  
  } catch (Exception ex) {
  
      Logger.getLogger(SampleRunner.class.getName()).log(Level.SEVERE, null, ex);
  
  }
  
  

Using this, if you run/debug Java project as such, you can add breakpoints/watches to JavaScript code as well as to Java code as usual and when the JS code is executed, debugger will pause on proper lines, no matter if they belong to JS or Java.

So if you have the file.js JavaScript file, you can open it in IDE, add line breakpoint and when the file is loaded using the 2nd snippet above, debugger will stop on the line breakpoint.

UPDATE: Sample project available here. Check it out, it contains all known (to me) ways to debug and run JS in Nashorn.