Why is AJAX returning HTTP status code 0?

In my experience, you'll see a status of 0 when:

• doing cross-site scripting (where access is denied)
• requesting a URL that is unreachable (typo, DNS issues, etc)
• the request is otherwise intercepted (check your ad blocker)
• as above, if the request is interrupted (browser navigates away from the page)

This article helped me. I was submitting form via AJAX and forgotten to use return false (after my ajax request) which led to classic form submission but strangely it was not completed.

I found another case where jquery gives you status code 0 -- if for some reason XMLHttpRequest is not defined, you'll get this error.

Obviously this won't normally happen on the web, but a bug in a nightly firefox build caused this to crop up in an add-on I was writing. :)

In my case, setting url: '' in ajax settings would result in a status code 0 in ie8.. It seems ie just doesn't tolerate such a setting.

In my case, I was getting this but only on Safari Mobile. The problem is that I was using the full URL (http://example.com/whatever.php) instead of the relative one (whatever.php). This doesn't make any sense though, it can't be a XSS issue because my site is hosted at http://example.com. I guess Safari looks at the http part and automatically flags it as an insecure request without inspecting the rest of the URL.

In my troubleshooting, I found this AJAX xmlhttpRequest.status == 0 could mean the client call had NOT reached the server yet, but failed due to issue on the client side. If the response was from server, then the status must be either those 1xx/2xx/3xx/4xx/5xx HTTP Response code. Henceforth, the troubleshooting shall focus on the CLIENT issue, and could be internet network connection down or one of those described by @Langdon above.

In my case, I was making a Firefox Add-on and forgot to add the permission for the url/domain I was trying to ajax, hope this saves someone a lot of time.

Observe the browser Console while making the request, if you are seeing "The Same Origin Policy disallows reading the remote resource at http ajax..... reason: cors header ‘access-control-allow-origin’ missing" then you need to add "Access-Control-Allow-Origin" in response header. exa: in java you can set this like response.setHeader("Access-Control-Allow-Origin", "*") where response is HttpServletResponse.