What does it mean when an HTTP request returns status code 0

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

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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  lt base gt  tag    lt base href   lt   request getScheme    gt     lt   request getServerName           request getServerPort     request getContextPath    gt     gt    I removed the  lt base gt  tag  which I did not need by the way  and now it works fine

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It should be noted that an ajax file upload exceeding the client max body size directive for nginx will return this error code

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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

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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

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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 amp 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

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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    lt form onsubmit  submitfunc    return false   gt    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

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As detailed by this answer on this page  a status code of 0 means the request failed for some reason  and a javascript library interpreted the fail as a status code of 0   To test this you can do either of the following   1  Use this chrome extension  Requestly to redirect your url from the https version of your url to the http version  as this will cause a mixed content security error  and ultimately generate a status code of 0  The advantage of this approach is that you don t have to change your app at all  and you can simply  rewrite  your url using this extension   2  Change the code of your app to optionally make your endpoint redirect to the http version of your url instead of the https version  or vice versa   If you do this  the request will fail with status code 0

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If you are testing on local PC  it won t work  To test Ajax example you need to place the HTML files on a web server

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In my case the status became 0 when i would forget to put the WWW in front of my domain  Because all my ajax requests were hardcoded http  WWW mydomain com and the webpage loaded would just be http   mydomain com it became a security issue because its a different domain  I ended up doing a redirect in my  htaccess file to always put www in front

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For what it is worth  depending on the browser  jQuery-based AJAX calls will call your success callback with a HTTP status code of 0   We ve found a status code of  0  usually means the user navigated to a different page before the AJAX call completed   Not the same technology stack as you are using  but hopefully useful to somebody

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wininet dll returns both standard and non-standard status codes that are listed below   401 - Unauthorized file 403 - Forbidden file 404 - File Not Found 500 - some inclusion or functions may missed 200 - Completed  12002 - Server timeout 12029 12030  12031 - dropped connections  either web server or DB server  12152 - Connection closed by server  13030 - StatusText properties are unavailable  and a query attempt throws an exception   For the status code  zero  are you trying to do a request on a local webpage running on a webserver or without a webserver   XMLHttpRequest status   0 and XMLHttpRequest statusText   unknown can help you if you are not running your script on a webserver

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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

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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    We still write the data to a text file on the local hard disk as we previously did  using an HTA  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   The HTA launches a link to the HTML page in the browser  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

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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    Illegal cross origin request   see CORS  Firewall block or filtering The request itself was cancelled in code 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

User · Answer

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    lt  -- javascript request inside the page -- gt   lt script gt  var ajaxurl    https   example com wp-admin admin-ajax php          lt  script gt    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

[ajax] What does it mean when an HTTP request returns status code 0?

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