Comparing HTML with Javascript - php

Question

So I was wondering something.
In a IM/chat website I'm making, I have it check the database every 10 seconds or so to see if any new data has come in. And also, when the user posts a new comment, it automatically sends it to the database and adds it to the comment list without reloading. But it loads all the comments each time.
I was wondering if its possible to add an effect to the new comment (such as fading in) without doing that to all the old comments as well.
function update(){
oldhtml = $('#listposts');
$.ajax({
type: "POST",
data: "",
url: "update.php",
success: function(msg){
$("#listposts").html(msg);
$('.comment_date').each(function(){
$(this).text('[' + prettyDate($(this).text())+']');
if(oldhtml == )
});
}
});
}
var intervalID = window.setInterval(update, 10000);
That's my update code. Here's my post code:
$("#postbutton").click(function () {
if(!$('#post').val()==""){
$.ajax({
type: "POST",
data: "data=" + $("#post").val(),
url: "post.php",
success: function(msg){
$("#listposts").html(msg);
$('.comment_date').each(function(){
$(this).text('[' + prettyDate($(this).text())+']');
});
}
});
$("#post").val("");
}
});
I'm also using prettyDate, as you can see. But that has nothing to do with this problem.
So as the title states, I was gonna try to save the current html in a variable (oldhtml) and then load the new stuff. Then I would compare the two and just use the new comment to fade in. Am I way out there? Am I missing the point?
Oh and please don't down vote me just cause I missed an obvious solution. I thought you're supposed to use it if I don't explain well, which I think I did.

Answer 1

You can do this in your success handler:
var $dv = $('<div />').css('display', 'none').html(msg);
$("#listposts").append($dv);
$dv.fadeIn();
Of course you can use a <span> instead of <div> depending on your needs.

Answer 2

Similar to Blaster's...
$('<div />').html(msg).appendTo('#posts').hide().fadeIn();