Give this a go:
<form name="postcontent" id="postcontent">
<input name="postsubmit" type="submit" id="postsubmit" value="POST"/>
<textarea id="postdata" name="postdata" placeholder="What's Up ?"></textarea>
</form>
<script>
(function() {
$("#postcontent").on('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);
}
});
});
})();
</script>