From a8444ff6e1eca6a212609a48e5b326bcc50ad855 Mon Sep 17 00:00:00 2001
From: Rob Hogan <2590098+robhogan@users.noreply.github.com>
Date: Fri, 23 Oct 2026 21:02:58 +0100
Subject: [PATCH] Tolerate configs without the newer server middleware options

https://github.com/react/metro/pull/2031 added `server.unstable_onServerCreated`, `unstable_priorityMiddleware` and `unstable_middleware`, which `createConnectMiddleware` assumes are set. A config from an older `metro-config`, or built by hand, crashes `runServer`:

```
TypeError: config.server.unstable_onServerCreated is not a function
```

This treats them as optional.

Changelog: Internal

Test plan:
- New integration test, failing without this change.
---
 packages/metro/src/index.flow.js                   |  7 ++++---
 .../__tests__/server-extraMiddleware-test.js       | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/packages/metro/src/index.flow.js b/packages/metro/src/index.flow.js
index addfcbfc3c..466340f632 100644
--- a/packages/metro/src/index.flow.js
+++ b/packages/metro/src/index.flow.js
@@ -242,7 +242,8 @@ export const createConnectMiddleware = async function (
   options?: RunMetroOptions,
 ): Promise<MetroMiddleWare> {
   const metroServer = await runMetro(config, options);
-  config.server.unstable_onServerCreated(metroServer);
+  // Absent from configs built by older metro-config versions.
+  config.server.unstable_onServerCreated?.(metroServer);
 
   let enhancedMiddleware: Middleware = metroServer.processRequest;
 
@@ -276,9 +277,9 @@ export const createConnectMiddleware = async function (
     },
     metroServer,
     middleware: composeMiddleware(
-      config.server.unstable_priorityMiddleware,
+      config.server.unstable_priorityMiddleware ?? [],
       enhancedMiddleware,
-      config.server.unstable_middleware,
+      config.server.unstable_middleware ?? [],
     ),
     async end(): Promise<void> {
       await metroServer.end();
diff --git a/packages/metro/src/integration_tests/__tests__/server-extraMiddleware-test.js b/packages/metro/src/integration_tests/__tests__/server-extraMiddleware-test.js
index dacd450a3c..1bd7df7f2a 100644
--- a/packages/metro/src/integration_tests/__tests__/server-extraMiddleware-test.js
+++ b/packages/metro/src/integration_tests/__tests__/server-extraMiddleware-test.js
@@ -96,6 +96,20 @@ describe('runServer custom middleware', () => {
     ]);
   });
 
+  test('serves with a config that lacks the newer server options', async () => {
+    const config = await loadConfig();
+    // As from an older metro-config.
+    const {
+      unstable_middleware: _middleware,
+      unstable_onServerCreated: _onServerCreated,
+      unstable_priorityMiddleware: _priorityMiddleware,
+      ...server
+    } = config.server;
+    await startServer({...config, server});
+
+    expect((await request(BUNDLE_PATH)).status).toBe(200);
+  });
+
   test('mounts deprecated unstable_extraMiddleware before unstable_priorityMiddleware', async () => {
     const calls = [];
     await startServer(await loadConfig(), {