What does it mean when an HTTP request returns sta

Question

What does it mean when JavaScript network calls such as fetch or XMLHttpRequest, or any other type of HTTP network request, fail with an HTTP status code of 0?

This doesn't seem to be a valid HTTP status code as other codes are three digits in HTTP specification.

I tried unplugging the network completely as a test. It may be unrelated, but that resulted in status code 17003 (IIRC), which cursory searching suggests means "DNS server lookup failed".

The same code works fine from some locations and systems, however within certain environments it fails with status code 0 and there is no responseText provided.

This is a typical HTTP POST to an Internet URL. It does not involve file:// which I understand may return 0 indicating success in Firefox.

Answer 1

An HTTP response code of 0 indicates that the AJAX request was cancelled.

This can happen either from a timeout, XHR abortion or a firewall stomping on the request. A timeout is common, it means the request failed to execute within a specified time. An XHR Abortion is very simple to do... you can actually call .abort() on an XMLHttpRequest object to cancel the AJAX call. (This is good practice for a single page application if you don't want AJAX calls returning and attempting to reference objects that have been destroyed.) As mentioned in the marked answer, a firewall would also be capable of cancelling the request and trigger this 0 response.

XHR Abort: Abort Ajax requests using jQuery

var xhr = $.ajax({
    type: "POST",
    url: "some.php",
    data: "name=John&location=Boston",
    success: function(msg){
       alert( "Data Saved: " + msg );
    }
});

//kill the request
xhr.abort()

It's worth noting that running the .abort() method on an XHR object will also fire the error callback. If you're doing any kind of error handling that parses these objects, you'll quickly notice that an aborted XHR and a timeout XHR are identical, but with jQuery the textStatus that is passed to the error callback will be "abort" when aborted and "timeout" with a timeout occurs. If you're using Zepto (very very similar to jQuery) the errorType will be "error" when aborted and "timeout" when a timeout occurs.

jQuery: error(jqXHR, textStatus, errorThrown);
Zepto:  error(xhr, errorType, error);