Is there a way to make a custom implementation of Nashorn JSObject work with Object.keys()? - java-8

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])");
}
}