Unfortunately I also can't figure that out...

The autogenerated bindings from JS <-> ObjC/Swift are pretty opaque and if it doesn't handle your use case then there isn't much you can do without manually binding everything you need.

I might recommend using a custom class with getters/setters that don't use the subscript operator. I think on the JS side you can run some code to override [] and point it to the correct functions but I've never tried it out so ymmv.