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

Workaround: what we ended up doing

We figured it was to do with firewall issues, and so we came up with a workaround that did the trick. If anyone has this same issue, here's what we did:

1. We still write the data to a text file on the local hard disk as we previously did, using an HTA.

2. When the user clicks "send data back to server", the HTA reads in the data and writes out an HTML page that includes that data as an XML data island (actually using a SCRIPT LANGUAGE=XML script block).

3. The HTA launches a link to the HTML page in the browser.

4. The HTML page now contains the javascript that posts the data to the server (using Microsoft.XMLHTTP).

Hope this helps anyone with a similar requirement. In this case it was a Flash game used on a laptop at tradeshows. We never had access to the laptop and could only email it to the client as this tradeshow was happening in another country.

Answer 2

Many of the answers here are wrong. It seems people figure out what was causing status==0 in their particular case and then generalize that as the answer.

Practically speaking, status==0 for a failed XmlHttpRequest should be considered an undefined error.

The actual W3C spec defines the conditions for which zero is returned here: https://fetch.spec.whatwg.org/#concept-network-error

As you can see from the spec (fetch or XmlHttpRequest) this code could be the result of an error that happened even before the server is contacted.

Some of the common situations that produce this status code are reflected in the other answers but it could be any or none of these problems:

1. Illegal cross origin request (see CORS)
2. Firewall block or filtering
3. The request itself was cancelled in code
4. An installed browser extension is mucking things up

What would be helpful would be for browsers to provide detailed error reporting for more of these status==0 scenarios. Indeed, sometimes status==0 will accompany a helpful console message, but in others there is no other information.

Answer 3

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.

Answer 4

It should be noted that an ajax file upload exceeding the client_max_body_size directive for nginx will return this error code.

Answer 5

In case anyone else comes across this problem, this was giving me issues due to the AJAX request and a normal form request being sent. I solved it with the following line:

<form onsubmit="submitfunc(); return false;">

The key there is the return false, which causes the form not to send. You could also just return false from inside of submitfunc(), but I find explicitly writing it to be clearer.

Answer 6

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!