Why my server isn't listening the data emitted from client?

I'm learning the socket.io library in Node.js and I can't make the server receive data, only send. During tests I noticed that the io.on listener function isn't even being called, and I don't know where's my mistake. What exactly works and what doesn't is marked with commentaries in the code bellow.

My server-side code:

"use strict";

const http = require("http");
const express = require("express");
const { Server } = require("socket.io");

const { log } = console;

const app = express();
const server = http.createServer(app);
const io = new Server(server);

app.use(express.urlencoded({extended: false}));
app.use(express.json());
app.use(express.static(__dirname + "/public"));

io.on("connect", socket => {
    const { id } = socket;
    io.emit("receiveMsg", id); // This works
})

io.on("sendMsg", msg => { // This doesn't work
    log("sendMsg: " + msg);
});

server.listen(8080);

My client-side code:

const socket = io();
const { log } = console;

socket.on("receiveMsg", msg => { // This works
    log("receiveMsg: " + msg);
});

const sendMsg = () => {
    const msg = document.querySelector("#msg").value;
    socket.emit("sendMsg", msg); // This doesn't work
}

My client-side HTML:

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Test</title>
    </head>
    <body>
        <h1>Hello, world!</h1>
        <input type="text" id="msg" placeholder="msg">
        <button onclick="sendMsg()">send</button>
        <script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/4.1.3/socket.io.js"></script>
        <script>
            // Client-side code here
        </script>
    </body>
</html>

Solution

based on the doc https://socket.io/docs/v4/client-socket-instance/

the connection event is connection on server side, and connect on client side

// server-side
io.on("connection", (socket) => {
  console.log(socket.id); // x8WIv7-mJelg7on_ALbx
});

// client-side
socket.on("connect", () => {
  console.log(socket.id); // x8WIv7-mJelg7on_ALbx
});

for Emit events, if you see here https://socket.io/docs/v4/emitting-events/ it seems requires us to put the subscription event inside connection block

// server-side
io.on("connection", (socket) => {
  socket.on("hello", (arg) => {
    console.log(arg); // world
  });
});

// client-side
socket.emit("hello", "world");

so maybe you can try to change your server code

io.on("connection", (socket) => {
  socket.on("sendMsg", msg => { // This doesn't work
    log("sendMsg: " + msg);
  }
});

it uses socket.on, not io.on on server side