Caching of Downloaded Code: Testing Results

Today I did some experimenting with the caching of downloaded code (or On-Demand Javascript, whatever you want to call it).

I’ve set up a small testing suite that currently tests 3 different ways of downloading code: script-tag insertion via DOM, XmlHttpRequest as a GET and XHR as a POST.

These are my results for now:

Method IE6 Firefox 1.07 Firefox 1.5b2 Safari 2.0 Opera 8.5
script_dom cached cached cached cached cached
xhr_post not cached not cached not cached not cached not cached
xhr_get cached not cached cached not cached not cached

(Results are the same for Win and OS X where both browsers are available (FF & Opera))

Safari Code Downloading Cache Test

This gives an interesting picture: Firefox does not seem to cache any scripts, neither the ones loaded via DOM nor those loaded via XHR. Only IE loads an XHR GET request from cache.

I’ve got the script in my public testing area, so you can test it for your own browser. Please do so and correct my values if you receive different results.

The sources of my tests are available, too: index.phps and js.phps. I did my testings using the latest prototype.js library. Maybe I will try it later on with another library (e.g. with

I’d be interested in more ways to download code (especially via document.write since I haven’t been able to include this properly to my tests) and in your results for other browsers. Just leave a comment.

UPDATE: I have now included the Expires header field with the Javascript file. Now FireFox in both version caches the script with script_dom, in version 1.5b2 it also caches XHR with GET requests.

, , ,

7 thoughts on “Caching of Downloaded Code: Testing Results

  1. Alexander, have you tried setting headers to encourage caching (in the XHR call as well as the server output)?

    Also, related to document-write, one thing about script tag insertion is you can insert the script in the head or the body. Who knows if some browsers handle those differently.

  2. Pingback: Ajaxian
  3. To hopefully clarify one issue a bit further, while Internet Explorer does cache
    responses to XMLHttpRequest, it might cache more than you want
    it to. I find that a Last-modified date in the response is not enough to stop IE6 from
    caching, but adding an Expires response header persuades it to re-fetch the page.

Leave a Reply

Only people in my network can comment.

This site uses Akismet to reduce spam. Learn how your comment data is processed.