Type inference for async handler about fastify-typebox

[sinclairzx81]

@Jnig Hi, that would be pretty cool, unfortunately I'm not sure I can add this to fastify-typebox as it would conflict with typical reply.status(...).send(...) usage (where users would need to return the same data they sent with the reply).

However, if you would like to experiment, here is the types to make it happen. This creates a union of all status code response types, so you would need to return one of them on the request handler.

// New: Extracts a union of all possible status codes. If no response is specified, this will
// return unknown, if no status codes are specified, then it also returns unknown. 
export type FastifyTypeBoxHandlerMethodReturnType<T extends FastifyTypeBoxSchema> =
    T['response'] extends undefined ? unknown :
    keyof T['response'] extends never ? unknown : 
    ({ 
        [K in keyof T['response']] : Static<T['response'][K]>
    })[keyof T['response']]


export type FastifyTypeBoxHandlerMethod<
    Server extends RawServerBase, 
    Schema extends FastifyTypeBoxSchema,
    Url    extends string
> = (
    request: FastifyTypeBoxRequest<Server, Schema, Url>, 
    reply:   FastifyTypeBoxReply<Schema>
) => FastifyTypeBoxHandlerMethodReturnType<Schema> |  Promise<FastifyTypeBoxHandlerMethodReturnType<Schema>>

Which enables the following type checking behavior.

fastify.get('/', {
    schema: {
        response: {
            200: Type.String(),
            201: Type.Tuple([Type.String(), Type.Number()]),
            202: Type.Object({ x: Type.Number(), y: Type.Number() })
        }
    },
}, async () => {
    // return 42                                // fail: not in union
    // return ['hello', 42]                     // fail: needs const assertion
    
    // return 'hello'                           // ok
    // return ['hello', 42] as [string, number] // ok
     return { x: 1, y: 2 }                      // ok
})

Perhaps one possible solution to there being multiple ways to return a response may be to have .send(T) return T, so implementations require users to return on .send(..), as follows.

{
   schema: {
      response: {
        200: Type.String()
      }
   }
}, (req, reply) => {
   return reply.status(200).send('foo') // send returns 'string'
})

However, I feel this might change fastify's interface a bit too much, but it is worth exploring.

If you're interested, I'm currently working trying to bring some of this functionality into Fastify itself. There is an active PR in process which you can find here which implements a new Fastify feature called Type Providers. As such I may transition this project into a prototyping library to help explore how static typing could enhance the overall Fastify experience for TypeScript / TypeBox users.

Hey, thanks for having a look at the types and experimenting around, all suggestions are worth discussing and thinking about!

Cheers
S

[Jnig]

Hi,

thank you for your fast and kind reply. I understand that this would conflict with the typical use case. It's probably just a matter of getting used to reply.status(...).send(...) .

Based on your code snippet I have experimented to build a wrapper function that uses only the status code 200. This seems to work well, even if it looks a bit hacky.

So it is possible to create a shortcut if needed and I think the issue can be closed.

The Type Providers PR also looks very promising. Looking forward to the PR being merged!

const fastify = Fastify();

export type FastifyTypeBoxHandlerMethodReturnType<T extends FastifyTypeBoxSchema> =
  T['response'] extends undefined ? unknown : Static<T['response']>;

export type FastifyTypeBoxHandlerMethodSimple<
  Server extends RawServerBase,
  Schema extends FastifyTypeBoxSchema,
  Url extends string,
> = (
  request: FastifyTypeBoxRequest<Server, Schema, Url>,
  reply: FastifyTypeBoxReply<Schema>,
) =>
  | FastifyTypeBoxHandlerMethodReturnType<Schema>
  | Promise<FastifyTypeBoxHandlerMethodReturnType<Schema>>;

function simple200<Url extends string, Schema extends FastifyTypeBoxSchema>(
  url: Url,
  options: FastifyTypeBoxRouteOptions<Server, Schema, any>,
  handler: FastifyTypeBoxHandlerMethodSimple<Server, Schema, Url>,
) {
  if (options?.schema?.response) {
    options.schema.response = {200: options.schema.response as TSchema};
  }

  fastify.get(url, options, async (request, reply) => {
    reply.status(200).send(await handler(request, reply));
  });
}

simple200(
  '/users/:userId',
  {
    schema: {
      querystring: Type.Object({
        x: Type.Number(),
        y: Type.Number(),
      }),
      response: Type.Object({
        result: Type.Number(),
      }),
    },
  },
  async (request, reply) => {
    const { x, y } = request.query;

    return { resu2lt: x + y };
  },
);

simple200(
  '/users/:userId',
  {
    schema: {
      querystring: Type.Object({
        x: Type.Number(),
        y: Type.Number(),
      }),
      response: Type.Object({
        result: Type.Number(),
      }),
    },
  },
  async (request, reply) => {
    const { x, y } = request.query;

    return { result: x + y };
  },
)