I find it sad that so many frameworks leave the developer to duplicate server-side and client-side validation. There are obviously some things you can't reasona

I find it sad that so many frameworks leave the developer to duplicate server-side and client-side validation. There are obviously some things you can't reasonably validate on the client, but I'd like to see more automated ways to take backend constraints and check them on the client too. Ideally constraints would also propagate from model definitions, so there could be a single source of truth for "what phone numbers do we accept". Some years back I tried to do this by parsing SQL table definitions, but never got far enough. Django does this, but it lacks pretty much any client-side validation support IIRC. (Or client-side anything, really...)