Are there any downsides to prefetching webpages using javascript?

Question

I'm currently experimenting with prefetching pages to increase perceived performance of our website, using the code below (req. jQuery).

Only 0.5% of our visitors use dial-up, I'm excluding querystrings (good old times), externals links (http) and pdfs (our large files are in this format). On a production site, what other possible negative scenario's apply when prefetching that I haven't considered?

<script type="text/javascript">
$(document).ready(function() {
$("a").each(
function(){
    $(this).bind ("mouseover", function() {
        var href=$(this).attr('href');
        if (
            (href.indexOf('?') == -1)&&
            (href.indexOf('http:') ==-1)&&
            ($(this).hasClass('nopreload') == false)&&
            (href.indexOf('.pdf') == -1)
        ) {
            $.ajax({ url:href, cache:true, dataType:"text" });
        }
    });
    $(this).bind ("mousedown", function(btn) {
        if (btn.which==1) {
            var href=$(this).attr('href');
            if ($(this).hasClass('nopreload') == false) {
                window.location.href = href;
                return false;
            }
        }
    });
});
});
</script>

For certain links, when hovered over it will preload the page and on mousedown will navigate (rather then after the button is released).

Answer 1

A right click will trigger a mouse down event too - so you might want to check the events data.

I guess that the speed gain for html source of 20-30kb is rather low. Your function does not preload any image, css or js files but only the pure html code.

Answer 2

On badly coded sites (which there are MANY), clicking a link might have an effect. For example, on many sites a delete button is in fact a link which when clicked, deletes a record. You have to be absolutely sure your site has zero standard vanilla links that when sent a GET request to, have harmful side effects.

You also have to be sure that it isn't possible for users to include similar links. I could also imaging links to external polling services, which e.g. easily allow one to make a poll in some forum by adding clickable links, which update the poll and redirect to REFERER.

Less harmfully, sites may do smart tricks to keep track of activity, thus keeping track of each prefetched page. This may impact your site's statistics or logging, and potentially give you a distorted view of your users' activity.

That said, I like the idea! :-)