Is it possible to run javascript before images begin loading?

Question

I'd like to change the src attribute of images before they are requested by the browser, the aim being to reduce the size of the images using a PHP script like Timthumb. Using jQuery, I thought $(document).ready would do the trick:

$(document).ready(function () {
  var imgs = $('#container img');
  jQuery.each(imgs, function() {
    $(this).replaceWith('<img src="timthumb/timthumb.php?src=' + $(this).attr('src') + '&w=200" />');
    });
});

But the original, unresized image is still downloaded in the background to the browser's cache. Is it possible to do what I'm trying to do on the client side, or is server-side the only option?

Answer 1

Javascript loading and execution is serialized by the browser (unless you use something like head.js), but the problem is that the DOM has to be available for a script to modify it. The jQuery ready event fires after the DOM is available, so the browser has already started requesting the resources that were referenced in the HTML.

So if you put the Javascript before the image tag it won't be able to find the image tags, and once ready fires the download has already started. I'm not aware of any events that fire before image load (just one for aborts), so the cleanest method is to create the HTML with the modified src attributes in the first place.

Alternatively, put the src in a different attribute on the image (like data_orig_src) and run the script to set src to data_orig_src on each image upon document ready. Use CSS to hide the images before changing the src so the user doesn't see a broken image icon. I think this is probably better than adding the images after the fact because you won't need to track where the images need to be placed in the DOM, and it should perform better as well.

Of course if you can change the server to use data_orig_src instead of src, why not just put the proper src in the tag in the first place...