Whenever an Ajax request is about to be sent, jQuery triggers the ajaxSend event. Any and all handlers that have been registered with the .ajaxSend() method are executed at this time.

To observe this method in action, set up a basic Ajax load request:

<div class="trigger">Trigger</div>
<div class="result"></div>
<div class="log"></div>

Attach the event handler to the document:

$( document ).ajaxSend(function() {
  $( ".log" ).text( "Triggered ajaxSend handler." );
});

Now, make an Ajax request using any jQuery method:

$( ".trigger" ).click(function() {
  $( ".result" ).load( "ajax/test.html" );
});

When the user clicks the element with class trigger and the Ajax request is about to begin, the log message is displayed.

All ajaxSend handlers are invoked, regardless of what Ajax request is to be sent. If you must differentiate between the requests, use the parameters passed to the handler. Each time an ajaxSend handler is executed, it is passed the event object, the jqXHR object (in version 1.4, XMLHttpRequestobject), and the settings object that was used in the creation of the Ajax request. For example, you can restrict the callback to only handling events dealing with a particular URL:

$( document ).ajaxSend(function( event, jqxhr, settings ) {
  if ( settings.url == "ajax/test.html" ) {
    $( ".log" ).text( "Triggered ajaxSend handler." );
  }
});

• As of jQuery 1.9, all the handlers for the jQuery global Ajax events, including those added with the .ajaxSend() method, must be attached to document.
• If $.ajax() or $.ajaxSetup() is called with the global option set to false, the .ajaxSend() method will not fire.