feat(express-wrapper): allow custom response encoders

packages/express-wrapper/src/index.ts

@@ -14,43 +14,53 @@ import {
   getServiceFunction,
   RouteHandler,
 } from './request';
+import { defaultResponseEncoder, ResponseEncoder } from './response';
+
+export type { ResponseEncoder } from './response';
 
 const isHttpVerb = (verb: string): verb is 'get' | 'put' | 'post' | 'delete' =>
   verb === 'get' || verb === 'put' || verb === 'post' || verb === 'delete';
 
-export function createServer<Spec extends ApiSpec>(
-  spec: Spec,
-  configureExpressApplication: (app: express.Application) => {
-    [ApiName in keyof Spec]: {
-      [Method in keyof Spec[ApiName]]: RouteHandler<Spec[ApiName][Method]>;
-    };
-  },
-) {
-  const app: express.Application = express();
-  const routes = configureExpressApplication(app);
-
-  const router = express.Router();
-  for (const apiName of Object.keys(spec)) {
-    const resource = spec[apiName] as Spec[string];
-    for (const method of Object.keys(resource)) {
-      if (!isHttpVerb(method)) {
-        continue;
+export const createServerWithResponseEncoder =
+  (encoder: ResponseEncoder) =>
+  <Spec extends ApiSpec>(
+    spec: ApiSpec,
+    configureExpressApplication: (app: express.Application) => {
+      [ApiName in keyof Spec]: {
+        [Method in keyof Spec[ApiName]]: RouteHandler<Spec[ApiName][Method]>;
+      };
+    },
+  ) => {
+    const app: express.Application = express();
+    const routes = configureExpressApplication(app);
+
+    const router = express.Router();
+    for (const apiName of Object.keys(spec)) {
+      const resource = spec[apiName] as Spec[string];
+      for (const method of Object.keys(resource)) {
+        if (!isHttpVerb(method)) {
+          continue;
+        }
+        const httpRoute: HttpRoute = resource[method]!;
+        const routeHandler = routes[apiName]![method]!;
+        const expressRouteHandler = decodeRequestAndEncodeResponse(
+          apiName,
+          httpRoute,
+          // FIXME: TS is complaining that `routeHandler` is not necessarily guaranteed to be a
+          // `ServiceFunction`, because subtypes of Spec[string][string] can have arbitrary extra keys.
+          getServiceFunction(routeHandler as any),
+          encoder,
+        );
+        const handlers = [...getMiddleware(routeHandler), expressRouteHandler];
+
+        const expressPath = apiTsPathToExpress(httpRoute.path);
+        router[method](expressPath, handlers);
       }
-      const httpRoute: HttpRoute = resource[method]!;
-      const routeHandler = routes[apiName]![method]!;
-      const expressRouteHandler = decodeRequestAndEncodeResponse(
-        apiName,
-        httpRoute as any, // TODO: wat
-        getServiceFunction(routeHandler),
-      );
-      const handlers = [...getMiddleware(routeHandler), expressRouteHandler];
-
-      const expressPath = apiTsPathToExpress(httpRoute.path);
-      router[method](expressPath, handlers);
     }
-  }
 
-  app.use(router);
+    app.use(router);
+
+    return app;
+  };
 
-  return app;
-}
+export const createServer = createServerWithResponseEncoder(defaultResponseEncoder);

packages/express-wrapper/src/request.ts

@@ -4,27 +4,11 @@
  */
 
 import express from 'express';
-import * as t from 'io-ts';
 import * as PathReporter from 'io-ts/lib/PathReporter';
 
-import {
-  HttpRoute,
-  HttpToKeyStatus,
-  KeyToHttpStatus,
-  RequestType,
-  ResponseType,
-} from '@api-ts/io-ts-http';
+import { HttpRoute, RequestType } from '@api-ts/io-ts-http';
 
-type NumericOrKeyedResponseType<R extends HttpRoute> =
-  | ResponseType<R>
-  | {
-      [S in keyof R['response']]: S extends keyof HttpToKeyStatus
-        ? {
-            type: HttpToKeyStatus[S];
-            payload: t.TypeOf<R['response'][S]>;
-          }
-        : never;
-    }[keyof R['response']];
+import type { NumericOrKeyedResponseType, ResponseEncoder } from './response';
 
 export type ServiceFunction<R extends HttpRoute> = (
   input: RequestType<R>,
@@ -53,10 +37,11 @@ const createNamedFunction = <F extends (...args: any) => void>(
   fn: F,
 ): F => Object.defineProperty(fn, 'name', { value: name });
 
-export const decodeRequestAndEncodeResponse = <Route extends HttpRoute>(
+export const decodeRequestAndEncodeResponse = (
   apiName: string,
-  httpRoute: Route,
-  handler: ServiceFunction<Route>,
+  httpRoute: HttpRoute,
+  handler: ServiceFunction<HttpRoute>,
+  responseEncoder: ResponseEncoder,
 ): express.RequestHandler => {
   return createNamedFunction(
     'decodeRequestAndEncodeResponse' + httpRoute.method + apiName,
@@ -72,7 +57,7 @@ export const decodeRequestAndEncodeResponse = <Route extends HttpRoute>(
         return;
       }
 
-      let rawResponse: NumericOrKeyedResponseType<Route> | undefined;
+      let rawResponse: NumericOrKeyedResponseType<HttpRoute> | undefined;
       try {
         rawResponse = await handler(maybeRequest.right);
       } catch (err) {
@@ -81,23 +66,7 @@ export const decodeRequestAndEncodeResponse = <Route extends HttpRoute>(
         return;
       }
 
-      const { type, payload } = rawResponse;
-      const status = typeof type === 'number' ? type : (KeyToHttpStatus as any)[type];
-      if (status === undefined) {
-        console.warn('Unknown status code returned');
-        res.status(500).end();
-        return;
-      }
-      const responseCodec = httpRoute.response[status];
-      if (responseCodec === undefined || !responseCodec.is(payload)) {
-        console.warn(
-          "Unable to encode route's return value, did you return the expected type?",
-        );
-        res.status(500).end();
-        return;
-      }
-
-      res.status(status).json(responseCodec.encode(payload)).end();
+      responseEncoder(httpRoute, rawResponse, res);
     },
   );
 };

packages/express-wrapper/src/response.ts

@@ -0,0 +1,50 @@
+import express from 'express';
+import * as t from 'io-ts';
+
+import {
+  HttpRoute,
+  HttpToKeyStatus,
+  KeyToHttpStatus,
+  ResponseType,
+} from '@api-ts/io-ts-http';
+
+export type NumericOrKeyedResponseType<R extends HttpRoute> =
+  | ResponseType<R>
+  | {
+      [Key in keyof R['response'] & keyof HttpToKeyStatus]: {
+        type: HttpToKeyStatus[Key];
+        payload: t.TypeOf<R['response'][Key]>;
+      };
+    }[keyof R['response'] & keyof HttpToKeyStatus];
+
+// TODO: Use HKT (using fp-ts or a similar workaround method, or who knows maybe they'll add
+// official support) to allow for polymorphic ResponseType<_>.
+export type ResponseEncoder = (
+  route: HttpRoute,
+  serviceFnResponse: NumericOrKeyedResponseType<HttpRoute>,
+  expressRes: express.Response,
+) => void;
+
+export const defaultResponseEncoder: ResponseEncoder = (
+  route,
+  serviceFnResponse,
+  expressRes,
+) => {
+  const { type, payload } = serviceFnResponse;
+  const status = typeof type === 'number' ? type : (KeyToHttpStatus as any)[type];
+  if (status === undefined) {
+    console.warn('Unknown status code returned');
+    expressRes.status(500).end();
+    return;
+  }
+  const responseCodec = route.response[status];
+  if (responseCodec === undefined || !responseCodec.is(payload)) {
+    console.warn(
+      "Unable to encode route's return value, did you return the expected type?",
+    );
+    expressRes.status(500).end();
+    return;
+  }
+
+  expressRes.status(status).json(responseCodec.encode(payload)).end();
+};