Socket.io双向通信教程，如何理解使用命名空间？

<script src="/socket.io/socket.io.js"></script>
<script>
const socket = io();
//...
</script>

const socket = io("/sunan");

const socket = io("ws://localhost:3000/sunan");

const socket = io("http://localhost:3000/sunan");

//...
const socketio = require("socket.io");
const io = socketio(expressServer);
io.of("/sunan").on("connection", (socket) => {
  //...
});

业务(namespace)：<select id="namespace" onchange="switch__io(this.value)">
  <option value="/">默认</option>
  <option value="/milk">牛奶(/milk)</option>
  <option value="/beer">啤酒(/beer)</option>
</select>
分组(room)：<select id="room" onchange="join(this.value)">
  <option value="0">--</option>
  <option value="1">绩效监控组</option>
  <option value="2">碌碌无为组</option>
</select>
<button onclick="go(1)">业务代码(+1)</button>
<button onclick="go(3)">业务代码(+3)</button>
<script src="/socket.io/socket.io.js"></script>
<script>
  let namespaces = document.getElementById('namespace');
  let all_io = [];
  for (let i = 0; i < namespaces.options.length; i++) {
    const _io = io(namespaces.options.item(i).value);
    _io.on('message', message => {
      console.log(message);
    });
    all_io.push(_io);
  }
  let current_io = all_io[0];
  function switch__io(namespace) {
    current_io = all_io.find(io => io.nsp === namespace);
    document.getElementById('room').value = '0'; // Reset room dropdown to default
  }
  function go(num) {
    current_io.emit('message', num);
  }
  function join(room) {
    // all_io[0].emit('join', room);
    current_io.emit('join', room);
  }
</script>

let express = require("express");
let app = express();
app.use(express.static('public'));
let expressServer = app.listen(80, () => console.log('http://localhost:%s', expressServer.address().port));
const socketio = require("socket.io");
const io = socketio(expressServer);
io.on("connection", (socket) => {
    socket.on("join", (room) => {
        room = parseInt(room) || 0;
        socket.leave(1);
        socket.leave(2);
        if (room > 0) {
            socket.join(room);
            io.to(socket.id).emit("message", "欢迎" + socket.id + "加入房间" + room);
            console.log("加入房间" + room);
        }
    });
    socket.on("message", (msg) => {
        console.log("收到消息：" + msg);
        io.to(1).emit("message", "监控消息："+socket.id + "卖了" + msg + "单");
    });
});
io.of("/milk").on("connection", (socket) => {
    socket.on("join", (room) => {
        room = parseInt(room) || 0;
        socket.leave(1);
        socket.leave(2);
        if (room > 0) {
            socket.join(room);
        }
    });
    socket.on("message", (num) => {
        io.emit("message", "全体广播：牛奶组" + socket.id + "出了" + num + "单");
        io.of("/beer").emit("message", "啤酒组发来全局贺电");
        io.of("/milk").to(1).emit("message", "监控消息：" + socket.id + "卖了牛奶" + num + "单");
    });
});
io.of("/beer").on("connection", (socket) => {
    socket.on("join", (room) => {
        room = parseInt(room) || 0;
        socket.leave(1);
        socket.leave(2);
        if (room > 0) {
            socket.join(room);
        }
    });
    socket.on("message", (num) => {
        io.emit("message", "全体广播：啤酒组" + socket.id + "出了" + num + "单");
        io.of("/milk").emit("message", "牛奶组发来全局贺电");
        io.of("/beer").to(1).emit("message", "监控消息：" + socket.id + "卖了啤酒" + num + "单");
    });
});