Getting an arbitrary property from a JavaScript object in Dart

Question

Edit: Here is a minimal project that illustrates my issue. You can see the error described by serving it to the browser: pub get and then either pub serve (dartium) or pub build --mode=debug (other browsers).

How can I access an arbitrary JavaScript property from Dart through a JsObjectImpl? I am using the ace.js library with an interop to Dart that I've adapted from a typescript interface, and the method I am calling returns a plain javascript object with key-value pairs.

Dart gives me a JsObjectImpl, which cannot be casted to a Map or a JsObject, both of which have [] accessors. It confusingly seems to inherit from the deprecated JSObject (note the 's' is capitalized in the latter) which does not have the [] accessor, so I can't get the data out.

Some error messages:

1. When attempting a cast from JsObjectImpl to JsObject:
   ORIGINAL EXCEPTION: type 'JSObjectImpl' is not a subtype of type 'JsObject' of 'obj' where JSObjectImpl is from dart:js JsObject is from dart:js. I get a similar message when using Map as well.

2. Looking at the object in the debugger, I can frustratingly see the property in JS view but not in the Dart object:
   
   The 4: Object is the data I want.

Ceporah

can you create a small example which demonstrates this issue? It'll be easier to help if there's something to debug – Chayachayote

preferred using dartpad.dartlang.org – Holliholliday

pub.dartlang.org/packages/js is now a better option than dart:js for dart-js-interop – Holliholliday

@GünterZöchbauer unfortunately I still get the issue even when importing only 'package:js/js.dart' – Ceporah

Just importing another package is not enough. Read the readme of the package pub.dartlang.org/packages/js about how to use it. – Holliholliday

I'll help you out - let me just clone your package. – Carleencarlen

Would you mind taking a look at my javascript and dart facade files? github.com/ahirschberg/dart-js-interop-toy/blob/master/web/… and github.com/ahirschberg/dart-js-interop-toy/blob/master/web/… ? Reading through the readme, nothing jumps out to me as obviously incorrect, although its definitely possible I'm missing something. – Ceporah

Answer 1

Ok, this was a fun one, happy holidays :)

It looks like Map is not a supported auto-conversion for package:js. So a couple of things:

1. Filed https://github.com/dart-lang/sdk/issues/28194
2. Sent your a PR introducing a workaround

For interested parties, we can use the browser-native Object.keys:

@JS()
library example;

import 'package:js/js.dart';

/// A workaround to converting an object from JS to a Dart Map.
Map jsToMap(jsObject) {
  return new Map.fromIterable(
    _getKeysOfObject(jsObject),
    value: (key) => getProperty(jsObject, key),
  );
}

// Both of these interfaces exist to call `Object.keys` from Dart.
//
// But you don't use them directly. Just see `jsToMap`.
@JS('Object.keys')
external List<String> _getKeysOfObject(jsObject);

And call it once we have an arbitrary JavaScript object:

var properties = jsToMap(toy.getData());
print(properties);

Carleencarlen

Answer 2

I had to modify @matanlurey solution so it works on dart 2.12 and is recursive.

import 'dart:js';

/// A workaround to deep-converting an object from JS to a Dart Object.
Object jsToDart(jsObject) {
  if (jsObject is JsArray || jsObject is Iterable) {
    return jsObject.map(jsToDart).toList();
  }
  if (jsObject is JsObject) {
    return Map.fromIterable(
      getObjectKeys(jsObject),
      value: (key) => jsToDart(jsObject[key]),
    );
  }
  return jsObject;
}

List<String> getObjectKeys(JsObject object) => context['Object']
    .callMethod('getOwnPropertyNames', [object])
    .toList()
    .cast<String>();

Vomiturition