Update Handler typing with generic request and response types

[jgornick]

In order to get better typing on the arguments for the Handler type, does it make sense to use generics for the request and response arguments?

For example, what if there was:

import { Request as ExpressRequest, Response as ExpressResponse } from 'express'
import { Params, ParamsDictionary } from 'express-serve-static-core'
import { Request } from 'openapi-backend'
import { Context } from 'openapi-backend/backend'

export type Handler<
  TRequest extends Request = any,
  TResponse = any
> = (context: Context, req: TRequest, res: TResponse, ...args: any[]) => any | Promise<any>;

export type OpenApiExpressRequest<
  P extends Params = ParamsDictionary,
  ResBody = any,
  ReqBody = any
  > = ExpressRequest<P, ResBody, ReqBody> & Request

export type ExpressHandler<
  P extends Params = ParamsDictionary,
  ResBody = any,
  ReqBody = any
> = Handler<OpenApiExpressRequest<P, ResBody, ReqBody>, ExpressResponse<ResBody>>

This way, when writing handlers for an express-backed service, they could be typed as:

import { ExpressHandler } from 'openapi-backend/backend'

export const getById: ExpressHandler = (c, req, res) => {
  res.status(200).json({ message: `getById: ${req.path}` })
}

Thoughts?

[anttiviljami]

I like this idea! PRs welcome :)

[henhal]

Personally I think adding generic typing is a good idea, but I don't think this module should be tightly coupled with e.g. Express or other frameworks; such conversion layers could be provided separately. I see generic typing as more useful for actually creating typed operations (so that getById could be typed to have a certain request body type etc). I doubt that the Request type defined in openapi-backend would be type compatible with all frameworks anyway - is it actually even type compatible with Express' Request type?

BTW, @anttiviljami , I've been working on a layer on top of openapi-backend that adds much of this (and some other nice features). I wasn't previously able to open source it, but I'm looking into doing so now. However, in retrospect, some of these features could probably have been contributed to this module instead of being implemented in an additional layer, but that wasn't possible at the time. I'd rather be a collaborator than a parasite you know. :) Anyway, I'll see what I end up having time to do but I'm curious about your view on such layers built on top of your module.

[henhal]

@anttiviljami Just wanted to mention that my openapi-ts-backend package with full TypeScript type support is now released at https://www.npmjs.com/package/openapi-ts-backend. It heavily relies on openapi-backend of course, so a lot of credit goes to you. :)
It also comes with an executable that creates TypeScript types for the operation handlers corresponding to an OpenAPI definition, to enable really writing a type safe OpenAPI backend service in TypeScript.