index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Hello World with Socket.io</title>
  </head>
  <body>
    <script src="/socket.io/socket.io.js"></script>
    <script>
      var socket = io("http://localhost:3000");

      var send = function(){
          var text = document.getElementById('m').value;
          socket.emit('chat', text)
      }
       var receive = function(msg){
            var li = document.createElement('li');
            li.innerText = msg;
            document.getElementById('messages').appendChild(li);
       }
       
       socket.on('chat' , receive);
        
    </script>
    <div>
        <ul id="messages"> </ul>
        <div class="zone_saisie">
            <input type="text" id="m" />
            <button onclick="send()"> send </button>
        </div>
    </div> 
  </body>
</html>