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

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.

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 socketuseEffect(() => {    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.