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.
In https://github.com/Heilemann/sockettest/blob/c1df22349ac1877ed625a01b14f48730b7af122d/client/src/Chat.jsx#L14-L22 you add an event listener and later try to remove it again. But since you write out the handler function
message => {
setMessages([...messages, message])
}
each time, the removal has no effect, since the handler to be removed is not identical to the one that you added. You must assign the handler function to a variable and use that variable in .on and .off.
Even if this does not fix the issue, it is still worth doing.
Your component is rendering twice, calling socket twice, giving two connection messages.
Ensure that it renders once by using your useEffect like this
In your chat.js import the socket.io
import socketio from 'socket.io-client'
let socket
useEffect(() => {
socket = socketio.connect('ws://localhost:3001')
return () => {
socket.off('message:new', listener)
}
},[])
In your submit handler
const handleSubmit = e => {
e.preventDefault()
if (message === '') return
setMessages(messages => [...messages, message])
socket.emit('message:new', message)
//dispatch({ type: 'MESSAGE_NEW', payload: message })
setMessage('')
}
Passing in an empty array as the second argument means the useEffect will only be called once. The bottom line is to ensure that the socket.io client is called once. props and state changes causes the component to re-render which in turns causes the socket to be connected multiple times. You can explore other patterns that cleaner. However, this is what you need to do.
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, andshouldComponentUpdatemethods- Class component static
getDerivedStateFromPropsmethod- Function component bodies
- State updater functions (the first argument to
setState)- Functions passed to
useState,useMemo, oruseReducer
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.
I debugged your code and found the issue with the reducer triggering twice.
The solution is to remove the <React.StrictMode>
tags around your App tag in the index.js.
Further info on StrictMode: https://reactjs.org/docs/strict-mode.html