Whenever an Ajax request is about to be sent, jQuery checks whether there are any other outstanding Ajax requests. If none are in progress, jQuery triggers the ajaxStart event. Any and all handlers that have been registered with the .ajaxStart() method are executed at this time.

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

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

We can attach our event handler to any element:

$('.log').ajaxStart(function() {
  $(this).text('Triggered ajaxStart handler.');
});

Now, we can make an Ajax request using any jQuery method:

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

When the user clicks the button and the Ajax request is sent, the log message is displayed.

Note: Because .ajaxStart() is implemented as a method of jQuery object instances, we can use the this keyword as we do here to refer to the selected elements within the callback function.