From 5b41258247f1dddf0f883b46c5e988084712cf93 Mon Sep 17 00:00:00 2001
From: Varun Chawla <varun_6april@hotmail.com>
Date: Sun, 15 Feb 2026 02:10:22 -0800
Subject: [PATCH] fix(types): allow emitWithAck for events with void callbacks

EventNamesWithAck previously excluded events whose callback had no
non-error arguments (e.g. `(cb: () => void) => void` or
`(cb: (err: Error) => void) => void`). This made it impossible to use
emitWithAck as a simple acknowledgement mechanism without data.

The fix removes the FirstNonErrorArg void check while keeping the guard
against events with no parameters at all, so events like `() => void`
(no callback) are still correctly excluded.

Closes #5257
---
 packages/socket.io/lib/typed-events.ts      | 12 +++++-------
 packages/socket.io/test/socket.io.test-d.ts | 19 +++++++++----------
 2 files changed, 14 insertions(+), 17 deletions(-)

diff --git a/packages/socket.io/lib/typed-events.ts b/packages/socket.io/lib/typed-events.ts
index 0a04e67577..bbdf5a633c 100644
--- a/packages/socket.io/lib/typed-events.ts
+++ b/packages/socket.io/lib/typed-events.ts
@@ -22,8 +22,6 @@ export type EventNames<Map extends EventsMap> = keyof Map & (string | symbol);
 
 /**
  * Returns a union type containing all the keys of an event map that have an acknowledgement callback.
- *
- * That also have *some* data coming in.
  */
 export type EventNamesWithAck<
   Map extends EventsMap,
@@ -32,11 +30,11 @@ export type EventNamesWithAck<
   Last<Parameters<Map[K]>> | Map[K],
   K,
   K extends (
-    Last<Parameters<Map[K]>> extends (...args: any[]) => any
-      ? FirstNonErrorArg<Last<Parameters<Map[K]>>> extends void
-        ? never
-        : K
-      : never
+    Parameters<Map[K]> extends never[]
+      ? never
+      : Last<Parameters<Map[K]>> extends (...args: any[]) => any
+        ? K
+        : never
   )
     ? K
     : never
diff --git a/packages/socket.io/test/socket.io.test-d.ts b/packages/socket.io/test/socket.io.test-d.ts
index 5c6ef3b187..625f42a3c4 100644
--- a/packages/socket.io/test/socket.io.test-d.ts
+++ b/packages/socket.io/test/socket.io.test-d.ts
@@ -265,15 +265,11 @@ describe("server", () => {
   interface ServerToClientEventsWithMultipleWithAck {
     ackFromServer: (a: boolean, b: string) => Promise<boolean[]>;
     ackFromServerSingleArg: (a: boolean, b: string) => Promise<string[]>;
-    // This should technically be `undefined[]`, but this doesn't work currently *only* with emitWithAck
-    // you can use an empty callback with emit, but not emitWithAck
     onlyCallback: () => Promise<undefined>;
   }
   interface ServerToClientEventsWithAck {
     ackFromServer: (a: boolean, b: string) => Promise<boolean>;
     ackFromServerSingleArg: (a: boolean, b: string) => Promise<string>;
-    // This doesn't work currently *only* with emitWithAck
-    // you can use an empty callback with emit, but not emitWithAck
     onlyCallback: () => Promise<undefined>;
   }
   describe("Emitting Types", () => {
@@ -420,8 +416,9 @@ describe("server", () => {
         sio.timeout(0).emitWithAck("noArgs");
         // @ts-expect-error - "helloFromServer" doesn't have a callback and is thus excluded
         sio.timeout(0).emitWithAck("helloFromServer");
-        // @ts-expect-error - "onlyCallback" doesn't have a callback and is thus excluded
-        sio.timeout(0).emitWithAck("onlyCallback");
+        expectType<
+          ToEmitWithAck<ServerToClientEventsWithMultipleWithAck, "onlyCallback">
+        >(sio.timeout(0).emitWithAck<"onlyCallback">);
         expectType<
           ToEmitWithAck<
             ServerToClientEventsWithMultipleWithAck,
@@ -496,10 +493,12 @@ describe("server", () => {
           s.emitWithAck("noArgs");
           // @ts-expect-error - "helloFromServer" doesn't have a callback and is thus excluded
           s.emitWithAck("helloFromServer");
-          // @ts-expect-error - "onlyCallback" doesn't have a callback and is thus excluded
-          s.emitWithAck("onlyCallback");
-          // @ts-expect-error - "onlyCallback" doesn't have a callback and is thus excluded
-          s.timeout(0).emitWithAck("onlyCallback");
+          expectType<
+            ToEmitWithAck<ServerToClientEventsWithAck, "onlyCallback">
+          >(s.emitWithAck<"onlyCallback">);
+          expectType<
+            ToEmitWithAck<ServerToClientEventsWithAck, "onlyCallback">
+          >(s.timeout(0).emitWithAck<"onlyCallback">);
           expectType<
             ToEmitWithAck<ServerToClientEventsWithAck, "ackFromServerSingleArg">
           >(s.emitWithAck<"ackFromServerSingleArg">);