title: can xmlhttpRequest response wait even after onload
body: <p>I am loading a page through xmlHttpRequest and I am not getting one variable which come into existance after some miliseconds when page loading is done 
  so the problem is when xmlHttpRequest sends back the response I do not get that variable in it.
  I want it to respond back even after onload.</p>
  
  <pre><code> var xhr = new XMLHttpRequest();
              xhr.open("GET", event.url, true);
              xhr.onload = function() { 
                       callback(xhr.responseText); 
                  };              
              xhr.onerror = function() { callback(); };
  
              xhr.followRedirects = true;
              xhr.send();
  </code></pre>
  
  <p>I tried setTimeOut but of no use because may be at that time call is finished</p>
  
  <pre><code>    xhr.onload = function() { 
              console.log('wait for response');
              setTimeout(function(){
                   callback(xhr.responseText); 
              },2000);
              }; 
  </code></pre>
  
  <p>I tried readyStateChange , but no success</p>
  
  <pre><code>    xhr.onreadystatechange = function () {
          if(xhr.readyState === XMLHttpRequest.DONE &amp;&amp; xhr.status === 200) {
              console.log(xhr.responseText);
              callback(xhr.responseText);
          };
      };   
  </code></pre>
  
  <p>by the way, I am trying to load amazon signIn page
  and the variable which is missing everytime is hidden Input Field metadata1,
  I get all other hidden Input fields in response text , except input field, named "metadat1" </p>
  
  <p>I'll be more than Happy, If anyone can help.
  Thanks in advance</p>