How to know which element in body triggered AJAX request in jQuery

Another generic way that requires least setup is to use activeElement property of the event target:

$(document).ajaxStart(function (e) {
    try {
        var $el = $(e.target.activeElement);
        var isButton = $el.hasClass('btn');

        // just a precautionary check
        if (!isButton) {
            return;
        }

        $el.button('loading'); // or whatever else you want to do

    } catch (ex) {
        console.log(ex);
    }
});

$(document).ajaxStop(function (e) {
    try {
        var $el = $(e.target.activeElement);
        var isButton = $el.hasClass('btn');

        if (!isButton) {
            return;
        }

        $el.button('reset');

    } catch (ex) {
        console.log(ex);
    }
});

The reason this works is because when a button is clicked, it receives focus. And e.target (which is document at this point tells us about the current element in focus via activeElement property.

Note that the try...catch is completely optional. It is usually a good idea to wrap any global handlers in try...catch just to ensure that any exception here doesn't cause any side-effects. The console.log will let you know about the exception, if any, so that you can fix it later.

If you want to use the ajax request handlers, an id will need to be assigned (manually or automatically) to the element that triggered the event. You could then send this id as an additional key-value pair in the request (element_id:THEID) and grab it with various substring methods in the ajax request handlers.

Example:

<script type="text/javascript">

    $(document).ready(function(){

        $("input").click(function(){

            var newid = (create a custom id with date and time);

            $(this).attr("id", newid);
            $.post("handler.php", {element_id : $(this).attr("id")});
        });

   }).ajaxSend(function(e, xhr, settings){

        var start_position = settings.data.indexOf("element_id");
        var equalsign_position = settings.data.indexOf("=", start_position);
        var ampersand_position = settings.data.indexOf("&", start_position);

        var element_id;
        if(ampersand_position == -1){
            element_id = settings.data.substr(equalsign_position+1);
        } else {
            element_id = settings.data.substr(equalsign_position+1, ampersand_position-equalsign_position-1);
        }

        $("#"+element_id).after("<div id='div"+element_id+"'><img src='loading_start.png' /></div>");

    }).ajaxComplete(function(e, xhr, settings) {

        var start_position = settings.data.indexOf("element_id");
        var equalsign_position = settings.data.indexOf("=", start_position);
        var ampersand_position = settings.data.indexOf("&", start_position);

        var element_id;
        if(ampersand_position == -1){
            element_id = settings.data.substr(equalsign_position+1);
        } else {
            element_id = settings.data.substr(equalsign_position+1, ampersand_position-equalsign_position-1);
        }

        $("#div"+element_id).remove();


    });

    </script>

    <input type="button" value="Save" />

The alternative is handling the appearing and disappearing of the loading image in each event handler (but I seem to understand you have too many of them?), like this:

<script type="text/javascript">

    $(document).ready(function(){

        $("input").click(function(){


            var element_id = (generate custom id);

            $(this).after("<div id='div"+element_id+"'><img src='loading_start.png' /></div>");

            $.post("handler.php", {var:"hello"}, function(){
                $("#div"+element_id).remove();
            });

        });
    })
</script>