Issues with io.on() typings

[westy92]

Describe the bug
After upgrading to socket.io 4.0.0, my code no longer compiles when using string enums for event names.

To Reproduce

Please fill the following code example:

Socket.IO server version: 4.0.0
TypeScript@4.2.3

Server

import { Server } from "socket.io";

const io = new Server(3000, {});

// this works fine
io.on("connection", (socket) => {});

enum Event {
    CONNECTION = "connection",
}
// Fails to compile!
// Argument of type '(socket:Socket) => void' is not assignable to parameter of type 'never'.
// ts(2345)
io.on(Event.CONNECTION, (socket) => {});

Client
N/A

Expected behavior
I expect my code to compile like it does for version 2.x and 3.x.

Platform:
N/A

[MaximeKjaer]

I'm able to replicate this, and I think I've found the root cause of the error. We use TypeScript conditional types internally to determine whether to get the listener type from a reserved events map, or from a user-provided events map. However, it seems that conditional types have some very weird behavior around enums. I've been able to minimize the issue to the following:

enum Events {
  CONNECTION = "connection",
  TEST = "test",
}
type Names = "test" | "connection";
type NameOrNull<N> = N extends Names ? N : null;

// Sanity checks:
type test1 = NameOrNull<"test">; // returns type `"test"`
type test2 = NameOrNull<"not-a-name">; // returns type `null`
type test3 = Events.TEST extends Names ? Events.TEST : null // returns type `Events.TEST`

// Weird behavior:
type test4 = NameOrNull<Events.TEST>; // returns type `never` (???)

I would expect test3 to be of type Events.TEST, but it comes out as type never, which is quite surprising. This could be a bug in the compiler. I'll try to do a bit more research into this, and if nothing comes up, I'll open an issue on the compiler.

Until we find a solution to this, there are two possible ways of dealing with this problem:

• Cast the enum member as Event.CONNECTION as "connection" to get TypeScript to treat it as a string literal type instead of an enum
• Migrate to strictly typed events by refactoring the enum to an interface that maps event names to event listener types, and use string literals for the event names

I realize that these are unfortunately less than ideal solutions ☹️ I'll try to find a better fix for this!

[MaximeKjaer]

This seems to be linked to microsoft/TypeScript#41778. I'll add the above bug reproduction to that issue.

In the meantime, I've submitted a workaround in #3834.

[ZachHaber]

It appears that this issue is fixed in Typescript 4.9 without the fallback.