jquery ajax function not working

put the event handler function inside $(document).ready(function(){…}). it shall work now

also add preventDefault() to restrict page refreshing

$(document).ready(function() {

            $("#postcontent").submit(function(e) {
                e.preventDefault();
                $.ajax({
                    type : "POST",
                    url : "add_new_post.php",
                    data : $("#postcontent").serialize(),
                    beforeSend : function() {
                          $(".post_submitting").show().html("<center><img src='images/loading.gif'/></center>");
                    },
                    success : function(response) {
                        alert(response);
                        $("#return_update_msg").html(response);
                        $(".post_submitting").fadeOut(1000);
                    }
                });
                e.preventDefault();
            });

        });