Issue with showing a progress bar using overlay

Requirement : I have a piece of code similar to the code below. I want to show a progress bar in an overlay after the save button is clicked. Once I get the response from the ajax call, I show an alert displaying that the operation was successful.

Issue : The overlay ( progress bar) is visible only after we have got the response from the ajax call and we display the alert. It开发者_StackOverflow is not visible while the ajax call is getting processed.

Am I doing something wrong here or is this the expected behavior? I am using pure javascript. I cannot use any external library to do this.

<script>
  function save(){
      document.getElementById('overlaydiv').style.display = 'block';
      document.getElementById('progressBarDiv ').style.display = 'block';
      var URL = 'some url';
      ajaxFunction(URL, false);
  }
  function ajaxFunction(URL, isAsynchronousCall){
      var xmlHttp;
      var callTypeFlag = true; // is an Asynchronous Call
      if(isAsynchronousCall != null){
         callTypeFlag = isAsynchronousCall;
      }
      try{
          if (window.XMLHttpRequest) {
            xmlHttp = new XMLHttpRequest();
          } else if (window.ActiveXObject) {
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
          }
      }catch (e){
         alert("Your browser does not support AJAX!");
         return false;
      }
      xmlHttp.open("GET", URL, callTypeFlag);
      xmlHttp.onreadystatechange = function(){responseProcessor(xmlHttp)};
      xmlHttp.send(null);
   }
   function responseProcessor(xmlHttp){
    //If the readystate is 4 then check for the request status.
      if (xmlHttp.readyState == 4) {
        if (xmlHttp.status == 200) {
            alert('Settings saved successfully');
            window.close();
            opener.location.href='new URL';
         }
       }
   }
</script>

<div id="overlaydiv" style="display:none"/>  
<div id="progressBarDiv" style="display:none">
  <img border="0" src="progressBar.gif"/>
</div>
<div>
 <table>
   <tr>
     <td>
       <input type="button" onclick="save()"/>
     </td>
   </tr>
 </table>
</div>

Put a little delay when calling ajaxFunction()

function save(){
      document.getElementById('overlaydiv').style.display = 'block';
      document.getElementById('progressBarDiv ').style.display = 'block';

      setTimeout(ajaxFunction, 50);//5,10,20,30,40 will do
}

It appears the issue was because I was using synchronous ajax call. I changed the call to asynchronous and now there is no issue.