fix(express): make toFetchHandler always resolve to a Response (#9)

Since 0.2.3, toFetchHandler is built on connectToWeb, whose WebHandler can
resolve to undefined (the "no middleware handled the request" pass-through
signal). That undefined is not assignable to Server['fetch'], breaking the
documented `fetch: toFetchHandler(app)` pattern.

Wrap connectToWeb with a 404 fallback so toFetchHandler always resolves to a
Response, mirroring @universal-middleware's own nextOr404 convention for
terminal handlers, while keeping the cancel-event propagation that motivated
the move away from srvx's toFetchHandler.


Claude-Session: https://claude.ai/code/session_01GcNoNA6CzTQiSAS589U6Yu

Co-authored-by: Claude <noreply@anthropic.com>

Author: brillout

.changeset/express-tofetchhandler-response.md

@@ -0,0 +1,9 @@
+---
+"@vikejs/express": patch
+---
+
+fix: make `toFetchHandler` always resolve to a `Response`
+
+Since `0.2.3`, `toFetchHandler` is built on `connectToWeb`, whose `WebHandler` can resolve to `undefined` (the "no middleware handled the request" signal). That `undefined` is not assignable to `Server['fetch']`, breaking the documented `fetch: toFetchHandler(app)` pattern.
+
+`toFetchHandler` now wraps `connectToWeb` with a `404` fallback so it always resolves to a `Response` (mirroring `@universal-middleware`'s own `nextOr404` convention), while keeping the cancel-event propagation that motivated the move away from srvx's `toFetchHandler`.

packages/express/src/index.test.ts

@@ -3,11 +3,15 @@ import { describe, expect, it, vi } from "vitest";
 
 vi.mock("vike/universal-middleware", () => ({ default: () => {} }));
 
-import { connectToWeb, toFetchHandler } from "./index.js";
+import { toFetchHandler } from "./index.js";
 
 describe("toFetchHandler", () => {
-  it("is connectToWeb", () => {
-    expect(toFetchHandler).toBe(connectToWeb);
+  it("falls back to 404 when the app doesn't handle the request", async () => {
+    const app = express();
+
+    const response = await toFetchHandler(app)(new Request("http://localhost/missing"));
+    expect(response).toBeInstanceOf(Response);
+    expect(response.status).toBe(404);
   });
 
   it("handles GET request", async () => {

packages/express/src/index.ts

@@ -2,7 +2,23 @@ import { type App, apply, connectToWeb } from "@universal-middleware/express";
 import vikeMiddleware from "vike/universal-middleware";
 
 export * from "@universal-middleware/express";
-export const toFetchHandler = connectToWeb;
+
+/**
+ * Convert an Express app into a web `fetch` handler.
+ *
+ * `connectToWeb()` resolves to `undefined` when the request is passed through
+ * without being handled (Connect's `next()`, or a middleware returning `false`).
+ * A terminal `fetch` handler must always return a `Response`, so we fall back to
+ * a `404` — mirroring the `nextOr404` convention of `@universal-middleware`'s own
+ * handlers. This keeps the return type assignable to `Server['fetch']`.
+ *
+ * (`@vikejs/express` used to re-export srvx's `toFetchHandler`, but that one
+ * deadlocks and doesn't work on Deno/Bun — see srvx#132.)
+ */
+export function toFetchHandler(app: Parameters<typeof connectToWeb>[0]): (request: Request) => Promise<Response> {
+  const handler = connectToWeb(app);
+  return async (request) => (await handler(request)) ?? new Response(null, { status: 404 });
+}
 
 type EnhancedMiddlewareExpress = Parameters<typeof apply>[1][number];