fix(typings): add fallback to untyped event listener

See also: socketio/socket.io@a11152f

Related:

- socketio/socket.io#3885
- socketio/socket.io#3872
- socketio/socket.io#3833

Author: darrachequesne

lib/typed-events.ts

@@ -43,11 +43,23 @@ export type ReservedOrUserListener<
   ReservedEvents extends EventsMap,
   UserEvents extends EventsMap,
   Ev extends ReservedOrUserEventNames<ReservedEvents, UserEvents>
-> = Ev extends EventNames<ReservedEvents>
-  ? ReservedEvents[Ev]
-  : Ev extends EventNames<UserEvents>
-  ? UserEvents[Ev]
-  : never;
+> = FallbackToUntypedListener<
+  Ev extends EventNames<ReservedEvents>
+    ? ReservedEvents[Ev]
+    : Ev extends EventNames<UserEvents>
+    ? UserEvents[Ev]
+    : never
+>;
+
+/**
+ * Returns an untyped listener type if `T` is `never`; otherwise, returns `T`.
+ *
+ * This is a hack to mitigate https://github.com/socketio/socket.io/issues/3833.
+ * Needed because of https://github.com/microsoft/TypeScript/issues/41778
+ */
+type FallbackToUntypedListener<T> = [T] extends [never]
+  ? (...args: any[]) => void
+  : T;
 
 /**
  * Strictly typed version of an `EventEmitter`. A `TypedEventEmitter` takes type

test/typed-events.test-d.ts

@@ -31,6 +31,26 @@ describe("typed events", () => {
           expectType<any>(c);
         });
       });
+
+      it("infers 'any' for listener parameters of other events using enums", () => {
+        const socket = io();
+
+        enum Events {
+          TEST = "test",
+        }
+
+        socket.on("test", (a, b, c) => {
+          expectType<any>(a);
+          expectType<any>(b);
+          expectType<any>(c);
+        });
+
+        socket.on(Events.TEST, (a, b, c) => {
+          expectType<any>(a);
+          expectType<any>(b);
+          expectType<any>(c);
+        });
+      });
     });
 
     describe("emit", () => {