JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    Number to send: <input id="source" type="text" />
    <button id="sendit" onclick="ChangeIt();">Send It</button>
</div>
<div>
    Sent number: <input id="test1" type="text" />
</div>

JavaScript

function ChangeIt() {
    alert('here');
    var xmlhttp=false;
    var response;
    xmlhttp = new XMLHttpRequest();
    var theSource = document.getElementById("source"),
      val;
    if(theSource) {
      val = theSource.value;
        alert(val);
    }
     xmlhttp.open("GET", "/ajax_json_echo/?x=" + val,true);
     xmlhttp.onreadystatechange=function() {
      if (xmlhttp.readyState==4) {
          response = new Function("return " + xmlhttp.responseText)();
          //alert(xmlhttp.responseText);
          //alert(response.get_response.x);
         
            var theText = document.getElementById("test1");
            if(theText) {
              theText.value = response.get_response.x;
            } else {
                alert('not found');
            }
        }
     }
     xmlhttp.send(null);
}