Changed attributes in objects are not Export()ed

[calmh]

I have the following use case, around templating: take an object which comes from JSON (i.e., map[string]interface{} etc.), pass it to some Javascript code, receive the maybe changed object, and JSON it back again.

The following test case illustrates what I do, and what I see as the failure when I do that:

func TestChangeAttribute(t *testing.T) {
	in := map[string]interface{}{
		"attr": []interface{}{"string"},
	}
	expected := map[string]interface{}{
		"attr": []interface{}{"changed"},
	}

	vm := otto.New()
	vm.Set("In", in)
	result, err := vm.Run(`(function fn() {
		var tmp = In;
		tmp.attr = ["changed"];
		return tmp;
	})()`)
	if err != nil {
		t.Fatal(err)
	}

	actual, err := result.Export()
	if err != nil {
		t.Fatal(err)
	}

	if !reflect.DeepEqual(actual, expected) {
		t.Errorf("Returned object does not match expected:\n\tactual: %#v\n\texpected: %#v", actual, expected)
	}

	expBs, _ := json.Marshal(expected)
	actBs, err := json.Marshal(actual)
	if err != nil {
		t.Fatal(err)
	}
	if !bytes.Equal(actBs, expBs) {
		t.Errorf("JSON serialization differs:\n\tactual: %s\n\texpected: %s", actBs, expBs)
	}
}

I would expect the test to pass, but the first check fails:

template_test.go:130: Returned object does not match expected:
  actual: map[string]interface {}{"attr":otto.Value{kind:5, value:(*otto._object)(0xc4201a0960)}}
  expected: map[string]interface {}{"attr":[]interface {}{"changed"}}

Here the changed attr is an otto.Value where I would have expected a []interface{} (or possibly slice of string or something). The next case, the JSON serialization:

template_test.go:139: JSON serialization differs:
  actual: {"attr":{}}
  expected: {"attr":["changed"]}

The slice here is gone and replaced by an empty object. Note that the serialization itself didn't fail, which is also an error in itself...