Use Express post request to emit Socket.io – Blancer.com Tutorials and projects

I want get username when user post it, but io.sockets.emit is not working. However, When i didn’t put io.sockets.emit on app.post, It is working.

my html

<html>
    <body>
        <form action="chat" id="main" method="post">
            <div>
                <input type="text" id="id" name="username" value="" placeholder=" " required />
                <label for="id">暱稱</label>
            </div>
            <div>
                <input type="text" id="key" name="key" value="" placeholder=" " />
                <label for="key">房號</label>
            </div>
            <button type="submit" value="加入房間" class="btn">加入房間</button>
        </form>
    </body>
</html>

my js

io.on('connection', (socket) => {

    socket.on('joinRoom', username => {
        console.log(username);
        console.log("joinRoom");
    });
    socket.on('chat message', msg => {
        const user = getCurrentUser(socket.id);
        console.log(user);
        io.emit('chat message',  msg);
  });
});
app.post('/chat', urlencodedParser, function (req, res) {
    io.sockets.emit('joinRoom', "aa")
    console.log(req.body)
    res.sendFile('public/chat.html', { root: __dirname })
});

‘joinRoom’ seems not receive anything.