How to create a 'real' JavaScript array in Rhino

Question

Okay, I'm a little stumped. I'm probably missing something blatantly obvious but apparently I just can't see the forest for the trees:

I'm trying to call a JavaScript function that expects its parameter to be an array, i.e. it checks if (arg instanceof Array)... Unfortunately, I (or Rhino) just can't seem to create such an array:

  Context cx = Context.enter();
  Scriptable scope = cx.initStandardObjects();
  String src = "function f(a) { return a instanceof Array; };";

  cx.evaluateString(scope, src, "<src>", 0, null);

  Function f = (Function) scope.get("f", scope);
  Object[] fArgs = new Object[]{ new NativeArray(0) };
  Object result = f.call(cx, scope, scope, fArgs);

  System.out.println(Context.toString(result));

  Context.exit();

And alas, result is false.

What am I missing here?

Edit:
Just a little more information: both [] instanceof Array and new Array() instanceof Array return true as one would expect. If I add elements to the array they show up in the JavaScript code with the right indices (numeric, starting from zero):

  NativeArray a = new NativeArray(new Object[]{ 42, "foo" });

When output using this JavaScript function:

  function f(a) {
      var result = [];
      result.push(typeof a);
      for (var i in a) {
          result.push(i + ' => ' + a[i]);
      }
      return result.join('\\n');
  }

The result is:

  object
  0 => 42
  1 => foo

So it works. Except that I want a 'real' array :)

Answer 1

Almost forgot: Object.prototype.toString.call(a) returns [object Array]

Okay, that's the crucial information. That tells us that the array really is an array, it's just that it's being initialized by an Array constructor in a different scope than the one that the function is testing for, exactly as though you were testing an array from one window against another window's Array constructor in a browser-based app. E.g., there's a scope problem.

Try replacing

Object[] fArgs = new Object[]{ new NativeArray(0) };

with

Object[] fArgs = new Object[]{ cx.newArray(scope, 0) };

...to ensure the correct Array constructor is used. Because you've gone directly to the NativeArray constructor, you've bypassed ensuring that its scope is right, and so the array object's constructor is an Array constructor, but not the same Array constructor as the one on the global object the function sees.