src/murja-newui/test_ws.html

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<html>
  <body>
    <textarea id="msgs"></textarea>
    <br />
    <button id="send">Send stuff </button>

    <script>
      window.addEventListener('DOMContentLoaded', _ => {
	  const socket = new WebSocket("http://localhost:12345/newui");
	  const msgs = document.querySelector('#msgs');
	  const sendbtn = document.querySelector('#send');

	  // Connection opened
	  socket.addEventListener("open", (event) => {
	      msgs.value += event;
	      msgs.value += "\n";
	  });

	  // Listen for messages
	  socket.addEventListener("message", (event) => {
	      msgs.value += event.data + "\n";
	  });

	  sendbtn.addEventListener('click', _ => {
	      socket.send('Aslan on hassu kissa');
	  });
      });
    </script>
  </body>
</html>