Ошибка undefined error код ошибки 0

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.

I believe the error code indicates that the response was empty, (as not even headers were returned). This means the connection was accepted and then closed gracefully (TCP FIN).
There are a number of things which could cause this, but based off of your description, some form of firewall seems the most likely culprit.

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);

answered Aug 5, 2013 at 11:05

I found a new and undocumented reason for status == 0. Here is what I had:

XMLHttpRequest.status === 0
XMLHttpRequest.readyState === 0
XMLHttpRequest.responseText === ''
XMLHttpRequest.state() === 'rejected'

It was not cross-origin, network, or due to cancelled requests (by code or by user navigation). Nothing in the developer console or network log.

I could find very little documentation on state() (Mozilla does not list it, W3C does) and none of it mentioned «rejected».

Turns out it was my ad blocker (uBlock Origin on Firefox).

answered Oct 18, 2017 at 15:27

In my case, it was because the AJAX call was being blocked by the browser because of the same-origin policy. It was the least expected thing, because all my HTMLs and scripts where being served from 127.0.0.1. How could they be considered as having different origins?

Anyway, the root cause was an innocent-looking <base> tag:

<base href='<%=request.getScheme()%>://<%=request.getServerName() + ":" + request.getServerPort() + request.getContextPath()%>/'/>

I removed the <base> tag, which I did not need by the way, and now it works fine!

answered Jan 27, 2013 at 17:48

In addition to Lee’s answer, you may find more information about the real cause by switching to synchronous requests, as you’ll get also an exception :

function request(url) {
    var request = new XMLHttpRequest();
    try {
        request.open('GET', url, false);
        request.send(null);
    } catch (e) {
        console.log(url + ': ' + e);
    }
}

For example :

NetworkError: A network error occurred.

answered Mar 30, 2018 at 11:07

answered Aug 10, 2015 at 16:40

answered Aug 15, 2019 at 21:37

In my case, the error occurred in a page requested with HTTP protocol, with a Javascript inside it trying to make an HTTPS request. And vice-versa.

After page loading, press F12 (or Ctrl + U) and take a look at the HTML code of your page. If you see something like that in your code:

<!-- javascript request inside the page -->
<script>
var ajaxurl = "https://example.com/wp-admin/admin-ajax.php";
(...)
</script>

And your page was requested this way:

http://example.com/example-page/2019/09/13/my-post/#elf_l1_Lw

You certainly will face this error.

To fix it, set the protocol of the Javascript request equal to the protocol of page request.

This situation involving different protocols, for page and js requests, was mentioned before in the answer of Brad Parks but, I guess the diagnostic technique presented here is easier, for the majority of users.

answered Sep 16, 2019 at 17:47