[dart:js_interop] Cannot specify signatures of functions passed as arguments to js_interop extension types

[felix-ht]

With the old js interop one could define function arguments with strong typing:

@JS()
class FooOld{
  external void bar(void Function(String e) callback);
}

With the new js_interop this doens't seem to be possible

@JS()
extension type Foo._(JSObject _) implements JSObject {
  // compile error:
  //  Type 'void Function(String)' is not a valid type in the signature of
  // 'dart:js_interop' external APIs or APIs converted via 'toJS'.
  external void bar(void Function(String e) callback);
  // fine but not type safe 
  external void baz(JSFunction callback);
}

How can i still specify function signatures for function arguments? Or do i just have to live without?

[felix-ht]

One options seems to be to just warp the external function in a normal one. With this the unsound part is contained (but still exposed to the user via the original function name).

@JS()
extension type Foo._(JSObject _) implements JSObject {
  external void baz(JSFunction callback);

  void bazDart(void Function(String e) callback) {
    baz(callback.toJS);
  }
}

Or with a container class - but this does add overhead and somewhats defeats the (zero overhead) purpose of type extensions:

@JS('Foo')
extension type JSFoo._(JSObject _) implements JSObject {
  external void baz(JSFunction callback);
}

class Foo {
  final JSFoo jsFoo;

  Foo({required this.jsFoo});

  void baz(void Function(String e) callback) => jsFoo.baz(callback.toJS);
}

[srujzs]

Duplicate of #54557, so gonna close in favor of that (which should capture what you want here).

The best answer with the current library is indeed a forwarding function like you have, but it isn't ideal.

[srujzs]

Duplicate of #54557