fix(event-handler): moved the response mutation logic to the composeMiddleware function

packages/event-handler/src/rest/Router.ts

@@ -308,19 +308,11 @@ class Router {
       ]);
 
       requestContext.params = route?.params ?? {};
-      const middlewareResult = await middleware({
+      await middleware({
         reqCtx: requestContext,
         next: () => Promise.resolve(),
       });
 
-      // middleware result takes precedence to allow short-circuiting
-      if (middlewareResult !== undefined) {
-        requestContext.res = handlerResultToWebResponse(middlewareResult, {
-          statusCode: getStatusCode(middlewareResult),
-          resHeaders: requestContext.res.headers,
-        });
-      }
-
       return requestContext;
     } catch (error) {
       this.logger.debug(`There was an error processing the request: ${error}`);

packages/event-handler/src/rest/utils.ts

@@ -33,6 +33,7 @@ import {
   SAFE_CHARS,
   UNSAFE_CHARS,
 } from './constants.js';
+import { handlerResultToWebResponse } from './converters.js';
 
 export function getPathString(path: Path): string {
   return isString(path) ? path : path.source.replaceAll(/\\\//g, '/');
@@ -230,6 +231,11 @@ export const isExtendedAPIGatewayProxyResult = (
  * follows the onion model where middleware executes in order before `next()` and in
  * reverse order after `next()`.
  *
+ * When a middleware returns a value(short-circuits), that result becomes the response
+ * and the `res` object in the `RequestContext` is mutated with that result converted
+ * to a Web Response preserving any existing headers while applying the status code
+ * from the middleware result.
+ *
  * @param middleware - Array of middleware functions to compose
  * @returns A single middleware function that executes all provided middleware in sequence
  *
@@ -294,8 +300,13 @@ export const composeMiddleware = (middleware: Middleware[]): Middleware => {
         );
       }
 
+      // middleware result takes precedence to allow short-circuiting
       if (middlewareResult !== undefined) {
         result = middlewareResult;
+        reqCtx.res = handlerResultToWebResponse(middlewareResult, {
+          statusCode: getStatusCode(middlewareResult),
+          resHeaders: reqCtx.res.headers,
+        });
       }
     };
 

packages/event-handler/tests/unit/rest/Router/middleware.test.ts

@@ -777,5 +777,33 @@ describe('Class: Router - Middleware', () => {
       expect(result.statusCode).toBe(403);
       expect(result.body).toBe('Route middleware response');
     });
+
+    it('allows post processing middleware to access the response returned early by a pre-processing middleware', async () => {
+      // Prepare
+      const app = new Router();
+      let message = '';
+      app.get(
+        '/test',
+        [
+          async ({ reqCtx, next }) => {
+            await next();
+            const clonedRes = reqCtx.res.clone();
+            message = (await clonedRes.json()).message;
+          },
+          () => {
+            return Promise.resolve({ message: 'Middleware applied' });
+          },
+        ],
+        () => {
+          return { message: 'Handler applied' };
+        }
+      );
+
+      // Act
+      await app.resolve(createTestEvent('/test', 'GET'), context);
+
+      // Assess
+      expect(message).toEqual('Middleware applied');
+    });
   });
 });