Polling jQuery ajax for each. Check if element does not exist, if so prepend to div - performance

Question

I have a div #notification-data which on $(document).ready gets populated with multiple <li></li> from $.post.
The $.post then gets called setTimeout(poll_n,9000); so data is up-to-date.
So im not updating all the data every time, I would like to do check if the <li></li> already exists in #notification-data, if it does not exist then I would like to prepend() it to #notification-data.
The data comes in the form of:
<li id="new_notification_1" class="seen_0 li_notification">blah</li>
<li id="new_notification_2" class="seen_0 li_notification">bleh</li>
As an extra question, is this the correct way of long polling?
Here is my code:
function poll_n(){
$.post('<?php echo $siteUrl ?>notifications.php?x=' + (new Date()).getTime() +'', function(data) {
$(data).find(".li_notification").each(function () {
var li_id = $(this).attr('id');
if ($(li_id).closest('#notification-data').length) {
//do nothing
} else {
$('#notification-data').append(??not_sure_what_goes_here??); // process results here
}
});
setTimeout(poll_n,9000);
});
}
EDIT - After answer I have now got this but it does not work (I get nothing in the console).
success: function(data){
$(data).find(".li_notification").each(function() {
var id = $(this).attr('id'),
notification = $('#notification-data');
console.log(id);
console.log('hello');
if (notification.find('#' + id).length === 0) {
// notification doesn't exists yet then lets prepend it
notification.prepend('#' + id);
}
});
},

Answer 1

You can try this:
function poll_n() {
$.ajax({
type: 'POST',
url: 'your url',
success: function(data){
var notification = $('#notification-data');
$.each($(data), function(){
var id = this.id;
if (notification.find('#' + id).length === 0) {
// notification doesn't exists yet then lets prepend it
notification.prepend(this);
}
});
},
complete: function(jqXHR, status) {
if (status === 'success') {
setTimeout(poll_n, 9000);
}
}
});
}
You must call poll_n() again after the request has been completed.