Narrowing TString

[Ghirigoro]

I have a variety of string-based schemas (e.g. custom urls, ids of various kinds, etc.). I use the schemas for more than type validation, so I often access their properties (e.g. maxLength, pattern, etc.).

The issue I'm running in to is that TString seems to be entirely "open" as a type. TS will allow you to reference any property on a TString and it will assume it's an any. For example: mySchema.foozle = "this is going to fail at runtime since I don't have foozle defined";

I've tried to narrow the types by defining them like this:

const MySchema = Type.String({
  maxLength: 99;
}) as TString & {maxLength:number};

But that doesn't work (it casts the optional away from maxLength but doesn't narrow the type further). Is there a way to have a "narrow" string schema that doesn't allow properties that aren't defined in string options (ideally it wouldn't allow properties that are not declared in the options that created the schema - i.e. no optional properties)