fix(typings): properly type emits with timeout (#1570)

When emitting with a timeout (added in version 4.4.0), the "err"
argument was not properly typed and would require to split the client
and server typings. It will now be automatically inferred as an Error
object.

Workaround for previous versions:

```ts
type WithTimeoutAck<isEmitter extends boolean, args extends any[]> = isEmitter extends true ? [Error, ...args] : args;

interface ClientToServerEvents<isEmitter extends boolean = false> {
    withAck: (data: { argName: boolean }, callback: (...args: WithTimeoutAck<isEmitter, [string]>) => void) => void;
}

interface ServerToClientEvents<isEmitter extends boolean = false> {

}

const io = new Server<ClientToServerEvents, ServerToClientEvents<true>>(3000);

io.on("connection", (socket) => {
    socket.on("withAck", (val, cb) => {
        cb("123");
    });
});

const socket: Socket<ServerToClientEvents, ClientToServerEvents<true>> = ioc("http://localhost:3000");

socket.timeout(100).emit("withAck", { argName: true }, (err, val) => {
  // ...
});
```

Related: #1555

Author: engina

Diff for: lib/socket.ts

@@ -12,6 +12,40 @@ import debugModule from "debug"; // debug()
 
 const debug = debugModule("socket.io-client:socket"); // debug()
 
+type Last<T extends any[]> = T extends [...infer I, infer L] ? L : any;
+type AllButLast<T extends any[]> = T extends [...infer I, infer L] ? I : any[];
+
+type PrependTimeoutError<T> = T extends (...args: infer Params) => infer Result
+  ? (err: Error, ...args: Params) => Result
+  : T;
+
+/**
+ * Utility type to decorate the acknowledgement callbacks with a timeout error.
+ *
+ * This is needed because the timeout() flag breaks the symmetry between the sender and the receiver:
+ *
+ * @example
+ * interface Events {
+ *   "my-event": (val: string) => void;
+ * }
+ *
+ * socket.on("my-event", (cb) => {
+ *   cb("123"); // one single argument here
+ * });
+ *
+ * socket.timeout(1000).emit("my-event", (err, val) => {
+ *   // two arguments there (the "err" argument is not properly typed)
+ * });
+ *
+ */
+export type DecorateAcknowledgements<E> = {
+  [K in keyof E]: E[K] extends (...args: infer Params) => infer Result
+    ? (
+        ...args: [...AllButLast<Params>, PrependTimeoutError<Last<Params>>]
+      ) => Result
+    : E[K];
+};
+
 export interface SocketOptions {
   /**
    * the authentication payload sent when connecting to the Namespace
@@ -677,7 +711,9 @@ export class Socket<
    *
    * @returns self
    */
-  public timeout(timeout: number): this {
+  public timeout(
+    timeout: number
+  ): Socket<ListenEvents, DecorateAcknowledgements<EmitEvents>> {
     this.flags.timeout = timeout;
     return this;
   }

Diff for: test/typed-events.test-d.ts

@@ -59,6 +59,17 @@ describe("typed events", () => {
 
         socket.emit("random", 1, "2", [3]);
         socket.emit("no parameters");
+
+        socket.emit("ackFromClient", "1", 2, (c, d) => {
+          expectType<any>(c);
+          expectType<any>(d);
+        });
+
+        socket.timeout(1000).emit("ackFromClient", "1", 2, (err, c, d) => {
+          expectType<any>(err);
+          expectType<any>(c);
+          expectType<any>(d);
+        });
       });
     });
   });
@@ -111,6 +122,12 @@ describe("typed events", () => {
   describe("listen and emit event maps", () => {
     interface ClientToServerEvents {
       helloFromClient: (message: string) => void;
+      ackFromClient: (
+        a: string,
+        b: number,
+        ack: (c: string, d: boolean) => void
+      ) => void;
+      ackFromClientNoArg: (ack: () => void) => void;
     }
 
     interface ServerToClientEvents {
@@ -142,6 +159,23 @@ describe("typed events", () => {
         const socket: Socket<ServerToClientEvents, ClientToServerEvents> = io();
 
         socket.emit("helloFromClient", "hi");
+
+        socket.emit("ackFromClient", "1", 2, (c, d) => {
+          expectType<string>(c);
+          expectType<boolean>(d);
+        });
+
+        socket.timeout(1000).emit("ackFromClient", "1", 2, (err, c, d) => {
+          expectType<Error>(err);
+          expectType<string>(c);
+          expectType<boolean>(d);
+        });
+
+        socket.emit("ackFromClientNoArg", () => {});
+
+        socket.timeout(1000).emit("ackFromClientNoArg", (err) => {
+          expectType<Error>(err);
+        });
       });
 
       it("does not accept arguments of wrong types", () => {