How to use Socket.io with Next.js API Routes

The trick is to plug ‘socket.io’ into the http server only once, so checking every access to the api.
Try something like this:

./pages/api/socketio.js

import { Server } from 'socket.io'

const ioHandler = (req, res) => {
  if (!res.socket.server.io) {
    console.log('*First use, starting socket.io')

    const io = new Server(res.socket.server)

    io.on('connection', socket => {
      socket.broadcast.emit('a user connected')
      socket.on('hello', msg => {
        socket.emit('hello', 'world!')
      })
    })

    res.socket.server.io = io
  } else {
    console.log('socket.io already running')
  }
  res.end()
}

export const config = {
  api: {
    bodyParser: false
  }
}

export default ioHandler

./pages/socketio.jsx

import { useEffect } from 'react'
import io from 'socket.io-client'

export default () => {
  useEffect(() => {
    fetch('/api/socketio').finally(() => {
      const socket = io()

      socket.on('connect', () => {
        console.log('connect')
        socket.emit('hello')
      })

      socket.on('hello', data => {
        console.log('hello', data)
      })

      socket.on('a user connected', () => {
        console.log('a user connected')
      })

      socket.on('disconnect', () => {
        console.log('disconnect')
      })
    })
  }, []) // Added [] as useEffect filter so it will be executed only once, when component is mounted

  return <h1>Socket.io</h1>
}