[dart:jsinterop] Allow function types in function declarations.

[ykmnkmi]

Current:

import 'dart:js_interop';

typedef EventDispatcher<T> = bool Function(
  String type,
  T detail, {
  bool cancelable,
});

@anonymous
extension type _EventDispatcherOptions._(JSObject _) implements JSObject {
  external factory _EventDispatcherOptions({bool cancelable});
}

@JS('createEventDispatcher')
external JSFunction _createEventDispatcher();

EventDispatcher<T> createEventDispatcher<T>() {
  var jsFunction = _createEventDispatcher();

  return (
    String type,
    T detail, {
    bool cancelable = false,
  }) {
    var jsResult = jsFunction.callAsFunction(
      null,
      type.toJS,
      // ignore: invalid_runtime_check_with_js_interop_types
      ref(detail) as JSAny?,
      _EventDispatcherOptions(cancelable: cancelable),
    ) as JSBoolean;

    return jsResult.toDart;
  };
}

Expected:

import 'dart:js_interop';

typedef EventDispatcher<T> = bool Function(
  String type,
  T detail, {
  bool cancelable,
});

@anonymous
extension type _EventDispatcherOptions._(JSObject _) implements JSObject {
  external factory _EventDispatcherOptions({bool cancelable});
}

typedef _JSEventDispatcher<T> = bool Function(
  String type,
  ExternalDartReference<T> detail, [
  _EventDispatcherOptions options,
]);

@JS('createEventDispatcher')
external _JSEventDispatcher _createEventDispatcher();

EventDispatcher<T> createEventDispatcher<T>() {
  var jsFunction = _createEventDispatcher();

  return (
    String type,
    T detail, {
    bool cancelable = false,
  }) {
    return jsFunction(
      type,
      // ignore: invalid_runtime_check_with_js_interop_types
      ref(detail),
      _EventDispatcherOptions(cancelable: cancelable),
    );
  };
}

PS: I miss and pressed Enter while editing.

[dart-github-bot]

Summary: Users want to use function types (like typedef) within @JS annotated function declarations for better type safety with dart:js_interop. Current limitations prevent this.

[srujzs]

Duplicate of #54557

I'm going to mark this a duplicate, but the proposal in the linked bug is making this a function type parameter of JSExportedDartFunction/JSFunction instead.