demo.html

<!DOCTYPE html>
<html>
  <head>
    <title>Closure demo</title>
  </head>
  <body>
    <h2>Data handling happens below</h2>
    <button id="btn">Clicky</button>
    <h3 id="insertion"></h3>
    
    <script type="text/javascript">
      function fetchContent() {
        var req = new XMLHttpRequest();
        req.onreadystatechange = function () {
          if (req.readyState === 4) {
            if (req.status === 200) {
              // Do something, the request is ready
              document.getElementById('insertion').innerHTML = req.responseText;
            } else {
              alert("Request failure");
            }
          }
        };
        req.open('GET', '/rest/foobar', true);
        req.send(null);
      }
      
      document.getElementById('btn').onclick = fetchContent;
    </script>
  </body>  
</html>