Sure. But this is easier to understand, albeit means repeating code:

    class B {
        foo: number;
        bar?: number;
        baz: number;
    }

I know these are toy examples, and I'm sure there are reasons why in the real world you'll be modeling things where this is not a good option. But I'd really need to be convinced that the CRUD webapps a lot of us write actually benefit from having this in our source code.