This seems to be caused by f6ef267

@lts20050703 - from #4813 (comment)

I think most of the issues come from the fact that calling .timeout() is (at least currently) mandatory on the server side, since there is no default timeout value (the client has an ackTimeout option though).

So, for example:

// INVALID (will time out directly anyway)
io.emit('withAck', 'hello', (...args) => { });
// VALID
io.timeout(1).emit('withAck', 'hello', (...args) => { });

For emitting with ack from the server, it is correctly an error now, because it wouldn't do what you'd expect from the beginning and would timeout immediately as darrachequesne pointed out. Plus the type definition was misleading for it in 4.7.2 anyway. e would actually be a number[] rather than a number - if it was valid at all.

io.emit("withAck", "4", (e) => {}); // <- Error

I will however look into the no-callback one, since that should be valid.

@lts20050703, PR is up with the fix for the noArgs issue.

Until the fix is merged in and a new version is cut, you can use patch-package (or something similar) to update the dist/typed-events.d.ts file.

- export declare type EventNamesWithoutAck<Map extends EventsMap, K extends EventNames<Map> = EventNames<Map>> = IfAny<Last<Parameters<Map[K]>> | Map[K], K, K extends (Last<Parameters<Map[K]>> extends (...args: any[]) => any ? never : K) ? K : never>;
+ export declare type EventNamesWithoutAck<Map extends EventsMap, K extends EventNames<Map> = EventNames<Map>> = IfAny<Last<Parameters<Map[K]>> | Map[K], 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>;

I believe it would be line 32, but I don't have the current version installed anywhere to double check that line number (also, the line might not be exactly the same, but you are replacing the declaration of EventNamesWithoutAck).

This should be fixed by cb6d2e0, included in version 4.7.4.

Could you please check?