dart2js: toString() breaks for the JS Symbol type

[nex3]

Reproduction:

import 'package:js/js.dart';

@JS("Symbol")
class JSSymbol {}

@JS("Symbol")
external JSSymbol createJSSymbol(String name);

void main() {
  createJSSymbol("foo").toString();
}

Compiling to JS with dart2js (Dart 3.0.7) and running in the browser throws method not found: 'toString$0' (J.getInterceptor$(...).toString$0 is not a function).

[rakudrama]

There is likely a similar issue with the other primitive JavaScript type introduced since ES5: typeof x == "bigint".
/cc @nshahan What does DDC do here?

[nex3]

I can confirm that this happens for BigInt as well.

[rakudrama]

I want to keep open the option of using JavaScript's BigInt for Dart's BigInt.
That would mean having full special-case interceptors like we do for numbers and bools.
What is tricky here is that the primitive JavaScript objects number, string and boolean and now bigint and symbol are not really classes. This means much deeper integration with how dart2js models these things including generating special cases in getInterceptor.

@srujzs - One thing I am not clear on is how @JS() interop and the new static interop is supposed to call the right methods (toString, hashCode etc) for intercepted classes like Uint8Array which have a first-class @Native interpretation, and these primitive objects which are not even JavaScript Objects.

[nshahan]

Currently this crashes in DDC as well when it tries to call the Dart .toString() member on the symbol. Coincidentally the changes I'm about to land to the calling conventions of the Dart Core Object APIs will fix the crash and instead it will call the native JavaScript .toString() member which returns 'Symbol(foo)'.

[srujzs]

One thing I am not clear on is how @JS() interop and the new static interop is supposed to call the right methods (toString, hashCode, etc) for intercepted classes like Uint8Array which have a first-class @Native interpretation, and these primitive objects which are not even JavaScript Objects.

We really haven't specified a behavior here beyond what occurs at runtime (we should). In the case of @staticInterop where it can be typed_data or dart:html objects, we should be getting their corresponding interceptors at runtime and then calling the Object methods from there. For the dart:js_interop types (of which, there are primitives), I'd expect the same. Are you implying they should be different than that behavior?

We do want to expose Symbol and BigInt through dart:js_interop but we should probably wait until we have interceptors for them.

[srujzs]

@nex3 Both DDC and dart2js now intercept Symbols and BigInts as separate interceptors. In DDC, this used to be LegacyJavaScriptObject, but we never had an interceptor in dart2js. This change means two things:

1. toString should now work correctly by calling the external toString/String(#) method.
2. You can't use package:js classes to interop with these types as they're not a subtype of JavaScriptObject but rather Interceptor. You couldn't really do this with dart2js before, but DDC allowed it because of how it intercepted unknown types. We've introduced types in dart:js_interop called JSSymbol and JSBigInt to use instead. If you opt into the extension types experiment, you can interop using those as your representation type. Another alternative is js_util of course.