confusion about examples/private-messaging

[yangxiao235]

hi, i'm currently hacking on examples/private-messaging, i found there is a little bit confusing on file index.js:
io.on("connection", async (socket) => {
.......
// join the "userID" room
socket.join(socket.userID);
......
}

socket.on("disconnect", async () => {
const matchingSockets = await io.in(socket.userID).allSockets();
const isDisconnected = matchingSockets.size === 0;
........
}

the socket.userID room is only used by the client the socket corresponding to and server, why should check number of allSockets equal to 0 ? is there other clients join the room?

as my understanding, when user select different usename on browser, the client should go there(App.vue):
methods: {
onUsernameSelection(username) {
this.usernameAlreadySelected = true;
socket.auth = { username };
console.log(begin to connect); // logs added
socket.connect();
},
},

but nothing printed on the console. please help me