dart2js should not allow js-interop field

[johnniwinther]

Current dart2js accepts js-interop fields but doesn't recognize these as js-interop fields. Js-interop fields should not be allowed and should result in a compile-time error.

[vsmenon]

@johnniwinther - can you clarify with an example?

[johnniwinther]

Currently fields are not recognized as js-interop members, but are also not disallowed.

So for instance in this code

@JS('Class')
class Class {
   Foo field;
}

we do not consider Class.field to be a js-interop member (but accidentally as a native member because Class is considered native).

Js-interop should be external (which fields cannot be) and the example should have be written as

@JS('Class')
class Class {
   external Foo get field;
   external void set field(Foo value);
}

The use of fields also doesn't fit well with CFE since a read-only field would be written as

@JS('Class')
class Class {
  final Foo field;
}

and CFE (which doesn't recognize js-interop) would complain about the missing initialization of Class.field.

[vsmenon]

thanks!

[jmesserly]

Just discovered this bug in a dart2js TODO comment. It's great to know that dart2js doesn't support it, because I'd like to remove/repurpose this in the JS interop spec. 👍

What I'd like: fields defined in this way would mean Dart fields, not JS-interop properties. For example:

@JS('Class')
class Class {
  // JS interop getter/setter pair (draft spec)
  //
  // This is just syntactic sugar for:
  //
  // external Foo get field1;
  // external void set field1(Foo value);
  //
  @JS()
  Foo field1;

  // Dart extension field (draft spec)
  Foo field2;
}

The syntactic sugar is helpful when type names are long, or when the field was renamed:

@JS('Class')
class Class {
   @JS('theJSFieldName')
   external SomeLongTypeName get field;
   @JS('theJSFieldName')
   external void set field(SomeLongTypeName value);
}
// vs
@JS()
class Class {
  @JS('theJSFieldName')
  SomeLongTypeName field;
}

[greglittlefield-wf]

Related: #27561