title: On which thread are asynchronous requests like AJAX request occur processed and executed
body: <p>Since Javascript engines in browsers have only 1 thread, when we create an XHR request using jquery, like so</p>
  
  <pre><code>$.ajax({
    url: "http://test.com/test.html",
    context: document.body
  }).done(function() {
    $( this ).addClass( "done" );
  });
  </code></pre>
  
  <p>on which thread does the HTTP request to the remote server occur, and what thread is it that notifies the main thread that the request is completed?</p>