How can I debug "/mysite/null" resource request?

Question

I've been trying to work out why our pages load slowly, and one thing I've noticed in Fiddler is that each page load for /mysite/mypage.aspx brings with it a request for GET /mysite/null HTTP/1.1, resulting in a 404 error.

I'm guessing this 404 error on every page load is slowing the site down even further, are there any ways of debugging what might be causing this? We use lots of Ajax/Javascript.

Similar question: http://stackoverflow.com/questions/539177/what-is-the-source-of-path-null-requests, ours is slightly different because it reliably happens on every page load

Answer 1

Get FireFox and download FireBug as you can use this to debug XHR.

Without seeing your code this is the best advice I can give you as tracking down these bugs are usually tedious and require some intimate knowledge of the working domain. FireBug is your best bet for debugging these sorts of issues within a page.

UPDATE:

Debugging the XmlHttpRequest object in IE can be done using Julien Couvreur's bookmarklet.

Answer 2

If you're using IE8, you can simply use the developer toolbar (hit F12) to step through your code.

The most likely explanation is that you have a malformed URL constructed by JavaScript somewhere in your page.