Is there a way to make a custom implementation of Nashorn JSObject work with Object.keys()?

Question

I recently asked this question How can I pass a proper method reference in so Nashorn can execute it? and got an answer that helped me get much further along with my project, but I discovered a limitation around providing a custom JSObject implementation that I don't know how to resolve.

Given this simple working JSObject that can handle most of the methods JS would invoke on it such as map:

import javax.script.*;
import jdk.nashorn.api.scripting.*;
import java.util.*;
import java.util.function.*;

public class scratch_6 {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager m = new ScriptEngineManager();
        ScriptEngine e = m.getEngineByName("nashorn");

        // The following JSObject wraps this list
        List<Object> l = new ArrayList<>();
        l.add("hello");
        l.add("world");
        l.add(true);
        l.add(1);

        JSObject jsObj = new AbstractJSObject() {
            @Override
            public Object getMember(String name) {
                if (name.equals("map")) {
                    // return a functional interface object - nashorn will treat it like
                    // script function!
                    final Function<JSObject, Object> jsObjectObjectFunction = callback -> {
                        List<Object> res = new ArrayList<>();
                        for (Object obj : l) {
                            // call callback on each object and add the result to new list
                            res.add(callback.call(null, obj));
                        }

                        // return fresh list as result of map (or this could be another wrapper)
                        return res;
                    };
                    return jsObjectObjectFunction;
                } else {
                    // unknown property
                    return null;
                }
            }
        };

        e.put("obj", jsObj);
        // map each String to it's uppercase and print result of map
        e.eval("print(obj.map(function(x) '\"'+x.toString()+'\"'))");

        //PROBLEM
        //e.eval("print(Object.keys(obj))");
    }
}

If you uncomment the last line where Object.keys(obj) is called, it will fail with the error ... is not an Object.

This appears to be because Object.keys() [ NativeObject.java:376 ] only checks whether the object is an instance of ScriptObject or of ScriptObjectMirror. If it is neither of those things, it throws the notAnObject error. :(

Answer 1

Ideally, user implemented JSObject objects should be exactly equivalent to script objects. But, user implemented JSObjects are almost script objects - but not quite. This is documented here -> https://wiki.openjdk.java.net/display/Nashorn/Nashorn+jsr223+engine+notes

Object.keys is one such case where it breaks. However, if you just want for..in javascript iteration support for your objects, you can implement JSObject.keySet in your class.

Example code:

import javax.script.*;
import jdk.nashorn.api.scripting.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager m = new ScriptEngineManager();
        ScriptEngine e = m.getEngineByName("nashorn");

        // This JSObject wraps the following Properties object
        Properties props = System.getProperties();

        JSObject jsObj = new AbstractJSObject() {
            @Override
            public Set<String> keySet() {
                return props.stringPropertyNames();
            }

            @Override
            public Object getMember(String name) {
                return props.getProperty(name);
            }
        };

        e.put("obj", jsObj);
        e.eval("for (i in obj) print(i, ' = ', obj[i])");
    }
}