Can get type codec's output types by programs?

[beiifeng]

for same case, if we want to declare a Date property, but this property is string in http request, so we can do

const $User = t.Object({
  createdAt: t
    .Codec(
      t.Optional(
        t.String({
          format: "date-time",
          description: "The creation date of the user",
          example: "2024-06-05T12:00:00Z",
          "@type": "http://www.w3.org/2001/XMLSchema#dateTime",
        }),
      ),
    )
    .Decode((value) => new Date(value))
    .Encode((value) => value.toISOString()),
});

For createdAt, the input type is string, the output type is Date.

I can use

t.IsCodec($User.properties.createdAt);
t.IsOptional($User.properties.createdAt);
t.IsString($User.properties.createdAt);

to determine it is Codec, Optional and String, but can determine the output type. I add a "@type" property to mark it.

const IsDateTime = (schema: t.TSchema) => {
  return schema["@type"] === "http://www.w3.org/2001/XMLSchema#dateTime";
};

Do we have a better plan to do this?

[potatohoney-p]

For runtime schema inspection, keeping an explicit marker is a reasonable approach here. The codec stores the encoded schema plus decode/encode functions; it does not store a separate schema describing the callback's return value. Consequently, IsString() still describes the encoded string, not the resulting Date.

If you need the TypeScript type instead, TypeBox provides StaticDecode<typeof YourCodec> (Date in this case) and StaticEncode<typeof YourCodec> (string). Those are compile-time types, not values that a runtime schema walker can inspect. See the codec documentation.

For a runtime walker, your marker can stay next to the codec definition. Treat that metadata as your application's contract rather than evidence inferred from the callback. Calling the decoder with a sample value would only tell you the result for that sample, and might execute arbitrary application code.

Tested with TypeBox 1.4.2 and TypeScript 5.9.3 (strict: true, skipLibCheck: true), the marker check needs an existence guard:

const IsDateTime = (schema: t.TSchema) =>
  '@type' in schema &&
  schema['@type'] === 'http://www.w3.org/2001/XMLSchema#dateTime';

I checked that the marker survives codec construction, IsString and IsCodec both return true, and Value.Decode/Value.Encode round-trip the timestamp through a Date. Your original Codec(Optional(String(...))) placement also preserves optionality in the version tested.

[beiifeng]

Thank you for your reply.
Since this is a reasonable approach, I will continue to use it this way.
Thanks again.