socktest.html

  <script src="https://code.jquery.com/jquery-3.3.1.min.js" crossorigin="anonymous"></script>
  <script src="https://cdn.jsdelivr.net/npm/socket.io-client@2/dist/socket.io.js" crossorigin="anonymous"></script>
  <script>
  const socket = io.connect('http://' + document.domain + ':' + location.port + '/');
  function dlog(etype, msg) {
    if(typeof msg != typeof "")
      msg = JSON.stringify(msg);
    $('#received').append(etype + ":" + msg + "<br />");
    }
  {% autoescape false %}
  const eventList = {{ eventList|default(["connect", "disconnect", "debug", "update"], true) }};
  {% endautoescape %}
  eventList.forEach(eventName =>
    socket.on(eventName, function(msg) {
      dlog(eventName, msg);
      })
    );
  /*
  socket.on('connection', (socket) => {
    socket.use((packet, next) => {
      alert(packet);
      next();
    });
  });
  */
  </script>

<input type="text" id="eventType"></input>
<input type="text" id="msg"></input>
<button type="button" id="send" onclick="socket.emit($('#eventType').val(), $('#msg').val());">Send</button>
<button type="button" id="sendJSON" onclick="socket.emit($('#eventType').val(), JSON.parse($('#msg').val()));">Send as JSON</button>
<br />
<div id="received"></div>