Server's socket.io listener fires twice for no apparent reason

Question

I'm building an app with a chat feature using express.js, react.js and socket.io. Here is a minimal version with the bug reproduced on Github.

The problem is the server's socket.on() always fires twice, for no reason that is apparent to me.

As far as I can tell the client only ever sends a single .emit, but the server's .on always fires twice (and I'm fairly certain that event handler isn't bound twice), regardless of how many clients are connected, or indeed how I've refactored the code.

The sending client's form (input + button) appends the message to its local messages list and then dispatches MESSAGE_NEW to the reducer.

Here's first the Socket.js file.

import React from 'react'
import socketio from 'socket.io-client'

export const socket = socketio.connect('ws://localhost:3001')
export const SocketContext = React.createContext()

And here is the submit handler code from Chat.js (this seems to work correctly, dispatching just once):

const handleSubmit = e => {
  e.preventDefault()

  if (message === '') return

  setMessages(messages => [...messages, message])
  dispatch({ type: 'MESSAGE_NEW', payload: message })
  setMessage('')
}

Here's /Reducer.js, which .emit's the signal (also seems to work fine, firing just once).

import { useContext } from 'react'
import { SocketContext } from './Socket'

export default function Reducer(state, action) {
  const socket = useContext(SocketContext)

  switch (action.type) {
    case 'MESSAGE_NEW':
      // this only fires once, as it should
      console.log('emitting socket')
      socket.emit('message:new', action.payload)
      break

    default:
      return state
  }
}

But here is the relevant bit from /server/index.js, this is where things go wrong. No matter what I've tried, the .on always fires twice.

io.on('connection', socket => {
  console.log('New client connected:', socket.id)

  socket.on('message:new', message => {
    // this always fires twice, i do not understand why
    console.log('message:new: ', message, socket.id)
    socket.broadcast.emit('message:new', message)
  })
})

Update: I found out that moving the socket.emit from Reducer.js to handleSubmit in Chat.js makes it work just fine. I don't understand why since the Reducer doesn't fire twice.

Answer 1

As per React documentation's on strict mode:

Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions:

• Class component constructor, render, and shouldComponentUpdate methods
• Class component static getDerivedStateFromProps method
• Function component bodies
• State updater functions (the first argument to setState)
• Functions passed to useState, useMemo, or useReducer

Notice the last line: functions passed to useReducer will be invoked twice; that is why your message is being sent twice. You can confirm this by updating your reducer like so:

  case "MESSAGE_NEW":
      alert("I'm running");

You'll notice this alert appearing twice, thanks to React.StrictMode.

If you remove the <React.StrictMode> wrapper in src/App.js the test message will no longer repeat; however, it's not fixing the underlying issue here.

The underlying issue is that you're making your reducer (which should be pure) perform an impure side-effect (in this case, sending a message via a websocket). Additionally, the reducer function also has a useContext call – this is also a no-op from React's perspective and is not good practice.

As you've come across in your question, Simply move the socket.emit into your handleSubmit and you should be good to go. Alternatively, you can consider a more sophisticated setup if you wish to go the reducer route; some popular configurations include Redux + Redux Thunk, or Redux + Redux Saga. Your mileage may vary.