fastify-socket.io: TS2339 'io' does not exist on type 'FastifyInstance' with Fastify v5.6.2

[UBU11]

When using Fastify v5.6.2 with TypeScript and fastify-socket.io, the io property is not recognized by TypeScript, even though it exists at runtime.

Error:

error TS2339: Property 'io' does not exist on type 'FastifyInstance<Server<typeof IncomingMessage, typeof ServerResponse>, IncomingMessage, ServerResponse<IncomingMessage>, FastifyBaseLogger, FastifyTypeProviderDefault> & PromiseLike<...> & { ...; }'

error reproduce:

import fastify from "fastify";
import fastifyIO from "fastify-socket.io";

const server = fastify();
server.register(fastifyIO);

server.get("/", (req, reply) => {
  server.io.emit("hello");
});

server.ready().then(() => {
  // we need to wait for the server to be ready, else `server.io` is undefined
  server.io.on("connection", (socket) => {
    // ...
  });
});

server.listen({ port: 3000 });

official socket.io docs: link

[shubhamkumar13]

The way I got around is by forking the repo and using the master branch as an override.

The master branch works but the current release is a bit old so the current tutorial don't work.

[AngelZatch]

If you sill need help with this, I had to make a fastify.d.ts type file to add the decorator:

import { Socket } from "socket.io"

declare module "fastify" {
  interface FastifyInstance {
    io: Socket
  }
}

I'd rather not have done this, but it works (and it feels like cheating lol).

I also had to cast the fastifySocketIo when registering it in my server.ts file:

import Fastify, {
  FastifyBaseLogger,
  FastifyPluginCallback,
  FastifyRequest,
  FastifyTypeProvider,
  RawServerDefault,
} from "fastify"
import fastifySocketIo from "fastify-socket.io"

export const initializeServer = async () => {
// Other imports

  await server.register(
    fastifySocketIo as unknown as FastifyPluginCallback<
      object,
      RawServerDefault,
      FastifyTypeProvider,
      FastifyBaseLogger
    >,
    {}
  )

// server.ready()
}

I'm using fastify^5.0.0 and fastify-socket.io^5.1.0 and I haven't gotten any issues yet.

[shubhamkumar13]

Thanks @AngelZatch I had to switch to the nodejs httpserver instance for now.

But if I ever come back to this I will try it.

Thanks!