Fix: Calling io.emit("event") with events that have no arguments results in TypeScript errors

lib/typed-events.ts

@@ -52,9 +52,11 @@ export type EventNamesWithoutAck<
 > = IfAny<
   Last<Parameters<Map[K]>> | Map[K],
   K,
-  K extends (
-    Last<Parameters<Map[K]>> extends (...args: any[]) => any ? never : K
-  )
+  K extends (Parameters<Map[K]> extends never[] ? K : never)
+    ? K
+    : K extends (
+        Last<Parameters<Map[K]>> extends (...args: any[]) => any ? never : K
+      )
     ? K
     : never
 >;

test/connection-state-recovery.ts

@@ -222,11 +222,11 @@ describe("connection state recovery", () => {
 
     class DummyAdapter extends Adapter {
       override persistSession(session) {
-        expect.fail();
+        expect().fail();
       }
 
       override restoreSession(pid, offset) {
-        expect.fail();
+        expect().fail();
         return Promise.reject("should not happen");
       }
     }

test/messaging-many.ts

@@ -526,7 +526,7 @@ describe("messaging many", () => {
     ]).then(async () => {
       try {
         await io.timeout(200).emitWithAck("some event");
-        expect.fail();
+        expect().fail();
       } catch (err) {
         expect(err).to.be.an(Error);
         // @ts-ignore

test/namespaces.ts

@@ -6,6 +6,7 @@ import {
   createClient,
   successFn,
   createPartialDone,
+  assert,
 } from "./support/util";
 
 describe("namespaces", () => {
@@ -417,6 +418,7 @@ describe("namespaces", () => {
     socket1.on("a", successFn(done, io, socket1, socket2));
 
     socket2.on("connect", () => {
+      assert(socket2.id);
       io.except(socket2.id).emit("a");
     });
   });
@@ -435,6 +437,7 @@ describe("namespaces", () => {
     socket1.on("a", successFn(done, io, socket1, socket2));
 
     socket2.on("connect", () => {
+      assert(socket2.id);
       nsp.except(socket2.id).emit("a");
     });
   });

test/socket-timeout.ts

@@ -62,7 +62,7 @@ describe("timeout", () => {
     io.on("connection", async (socket) => {
       try {
         await socket.timeout(50).emitWithAck("unknown");
-        expect.fail();
+        expect().fail();
       } catch (err) {
         expect(err).to.be.an(Error);
         success(done, io, client);

test/socket.io.test-d.ts

@@ -187,6 +187,7 @@ describe("server", () => {
     Ev extends keyof Map = keyof Map
   > = (ev: Ev, ...args: Parameters<Map[Ev]>) => ReturnType<Map[Ev]>;
   interface ClientToServerEvents {
+    noArgs: () => void;
     helloFromClient: (message: string) => void;
     ackFromClient: (
       a: string,
@@ -196,6 +197,7 @@ describe("server", () => {
   }
 
   interface ServerToClientEvents {
+    noArgs: () => void;
     helloFromServer: (message: string, x: number) => void;
     ackFromServer: (
       a: boolean,
@@ -212,12 +214,14 @@ describe("server", () => {
   // While these could be generated using the types from typed-events,
   // it's likely better to just write them out, so that both the types and this are tested properly
   interface ServerToClientEventsNoAck {
+    noArgs: () => void;
     helloFromServer: (message: string, x: number) => void;
     ackFromServer: never;
     ackFromServerSingleArg: never;
     onlyCallback: never;
   }
   interface ServerToClientEventsWithError {
+    noArgs: () => void;
     helloFromServer: (message: string, x: number) => void;
     ackFromServer: (
       a: boolean,
@@ -233,6 +237,7 @@ describe("server", () => {
   }
 
   interface ServerToClientEventsWithMultiple {
+    noArgs: () => void;
     helloFromServer: (message: string, x: number) => void;
     ackFromServer: (a: boolean, b: string, ack: (c: boolean[]) => void) => void;
     ackFromServerSingleArg: (
@@ -243,6 +248,7 @@ describe("server", () => {
     onlyCallback: (a: () => void) => void;
   }
   interface ServerToClientEventsWithMultipleAndError {
+    noArgs: () => void;
     helloFromServer: (message: string, x: number) => void;
     ackFromServer: (
       a: boolean,
@@ -279,7 +285,7 @@ describe("server", () => {
         sio.send(1, "2", [3]);
         // @ts-expect-error - ServerToClientEvents doesn't have a message event
         nio.send(1, "2", [3]);
-        // This should likely be an error, but I don't know how to make it one
+        // This correctly becomes an error in TS 5.3.2, so when updating typescript, this should expect-error
         sio.send();
         nio.send();
       });
@@ -383,6 +389,9 @@ describe("server", () => {
       });
       it("has the correct types for `emit`", () => {
         const sio = new Server<ClientToServerEvents, ServerToClientEvents>();
+        expectType<ToEmit<ServerToClientEventsWithMultipleAndError, "noArgs">>(
+          sio.timeout(0).emit<"noArgs">
+        );
         expectType<
           ToEmit<ServerToClientEventsWithMultipleAndError, "helloFromServer">
         >(sio.timeout(0).emit<"helloFromServer">);
@@ -406,6 +415,8 @@ describe("server", () => {
         expectType<never>(
           undefined as Parameters<typeof sansTimeout["emitWithAck"]>[0]
         );
+        // @ts-expect-error - "noArgs" doesn't have a callback and is thus excluded
+        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
@@ -428,7 +439,12 @@ describe("server", () => {
       it("Infers correct types", () => {
         const sio = new Server<ClientToServerEvents, ServerToClientEvents>();
         const nio = sio.of("/test");
-
+        expectType<ToEmit<ServerToClientEventsNoAck, "noArgs">>(
+          sio.emit<"noArgs">
+        );
+        expectType<ToEmit<ServerToClientEventsNoAck, "noArgs">>(
+          nio.emit<"noArgs">
+        );
         expectType<ToEmit<ServerToClientEventsNoAck, "helloFromServer">>(
           // These errors will dissapear once the TS version is updated from 4.7.4
           // the TSD instance is using a newer version of TS than the workspace version
@@ -439,6 +455,7 @@ describe("server", () => {
           nio.emit<"helloFromServer">
         );
         sio.on("connection", (s) => {
+          expectType<ToEmit<ServerToClientEvents, "noArgs">>(s.emit<"noArgs">);
           expectType<ToEmit<ServerToClientEvents, "helloFromServer">>(
             s.emit<"helloFromServer">
           );
@@ -474,6 +491,8 @@ describe("server", () => {
       it("Infers correct types", () => {
         const sio = new Server<ClientToServerEvents, ServerToClientEvents>();
         sio.on("connection", (s) => {
+          // @ts-expect-error - "noArgs" doesn't have a callback and is thus excluded
+          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
@@ -506,6 +525,10 @@ describe("server", () => {
         srv.listen(() => {
           sio.on("connection", (s) => {
             expectType<Socket<ClientToServerEvents, ServerToClientEvents>>(s);
+            s.on("noArgs", (...args) => {
+              expectType<[]>(args);
+              done();
+            });
             s.on("helloFromClient", (message) => {
               expectType<string>(message);
               done();
@@ -526,6 +549,10 @@ describe("server", () => {
         const sio = new Server<ClientToServerEvents, ServerToClientEvents>(srv);
         srv.listen(() => {
           sio.on("connection", (s) => {
+            // @ts-expect-error - shouldn't accept emit events
+            s.on("noArgs", (message, number) => {
+              done();
+            });
             // @ts-expect-error - shouldn't accept emit events
             s.on("helloFromServer", (message, number) => {
               done();
@@ -538,14 +565,17 @@ describe("server", () => {
 
   describe("listen and emit event maps for the serverSideEmit method", () => {
     interface ClientToServerEvents {
+      noArgs: () => void;
       helloFromClient: (message: string) => void;
     }
 
     interface ServerToClientEvents {
+      noArgs: () => void;
       helloFromServer: (message: string, x: number) => void;
     }
 
     interface InterServerEvents {
+      noArgs: () => void;
       helloFromServerToServer: (message: string, x: number) => void;
       ackFromServerToServer: (foo: string, cb: (bar: number) => void) => void;
     }
@@ -563,20 +593,29 @@ describe("server", () => {
           Server<ClientToServerEvents, ServerToClientEvents, InterServerEvents>
         >(sio);
         srv.listen(async () => {
+          sio.serverSideEmit("noArgs");
           sio.serverSideEmit("helloFromServerToServer", "hello", 10);
           sio
             .of("/test")
             .serverSideEmit("helloFromServerToServer", "hello", 10);
 
+          sio.on("noArgs", (...args) => {
+            expectType<[]>(args);
+          });
           sio.on("helloFromServerToServer", (message, x) => {
             expectType<string>(message);
             expectType<number>(x);
           });
+          sio.of("/test").on("noArgs", (...args) => {
+            expectType<[]>(args);
+          });
           sio.of("/test").on("helloFromServerToServer", (message, x) => {
             expectType<string>(message);
             expectType<number>(x);
           });
 
+          //@ts-expect-error - "helloFromServerToServer" does not have a callback
+          sio.serverSideEmitWithAck("noArgs");
           //@ts-expect-error - "helloFromServerToServer" does not have a callback
           sio.serverSideEmitWithAck("helloFromServerToServer", "hello");