http form to get data

Here is a snippet of code from the script section of an HTML document. A button press event triggers the JSONpost function.
As written it sends an 'A'. The Espruino code sees the 'A' and sends some data back in S.

I think there is a way to insert the message length as well, but that work is currently archived.

function JSONpost(){
 //code to post the Postobj to the server and get a reply
 var yhttp = new XMLHttpRequest();
 yhttp.addEventListener("error",JSONpost1);
 yhttp.open("POST","CMD", true);
 yhttp.send("A"); // text to send

 yhttp.onreadystatechange = function() { 
  if (yhttp.readyState == 4 && yhttp.status == 200) { 
   var S=JSON.parse(yhttp.responseText);
// the reply is in S
  }//endif 
 };//end onreadystate function 
}//end JSONpost 

Additional info can be had here:
Add Event Listener