Check if third-party cookies are enabled

I have an application that needs to check whether the client browser has third-party-cookies enabled. Does anyone know how to do this in JavaScript?

Technical Background

The third party sets & reads cookies over HTTP (not in JavaScript).

So we need two requests to an external domain to test if third-party cookies are enabled:

One where the third party sets the cookie(s)
The second, with a differing response depending on whether the browser sent the cookie(s) back to the same third party in a second request.

We cannot use XMLHTTPRequest (Ajax) because of the DOM security model.

Obviously you can't load both scripts in parallel, or the second request may be made before the first request’s response makes it back, and the test cookie(s) will not have been set.

Code Example

Given:

The .html file is on one domain, and
The .js.php files are on a second domain, we have:

The HTML test page

Saved as third-party-cookies.html

<!DOCTYPE html>
<html>
<head id="head">
  <meta charset=utf-8 />
  <title>Test if Third-Party Cookies are Enabled</title>
<style type="text/css">
body {
  color: black;
  background: white none;
}
.error {
  color: #c00;
}
.loading {
  color: #888;
}
.hidden {
  display: none;
}
</style>
<script type="text/javascript">
window._3rd_party_test_step1_loaded = function(){
  // At this point, a third-party domain has now attempted to set a cookie (if all went to plan!)
  var step2Url = 'http://third-party.example.com/step2.js.php',
    resultsEl = document.getElementById('3rd_party_cookie_test_results'),
    step2El = document.createElement('script');

  // Update loading / results message
  resultsEl.innerHTML = 'Stage one complete, loading stage 2&hellip;';
  // And load the second part of the test (reading the cookie)
  step2El.setAttribute('src', step2Url);
  resultsEl.appendChild(step2El);
}
window._3rd_party_test_step2_loaded = function(cookieSuccess){
  var resultsEl = document.getElementById('3rd_party_cookie_test_results'),
    errorEl = document.getElementById('3rd_party_cookie_test_error');
  // Show message
  resultsEl.innerHTML = (cookieSuccess ? 'Third party cookies are <b>functioning</b> in your browser.' : 'Third party cookies appear to be <b>disabled</b>.');

  // Done, so remove loading class
  resultsEl.className = resultsEl.className.replace(/\bloading\b/,' ');
  // And remove error message
  errorEl.className = 'hidden';
}
</script>
</head>
<body id="thebody">

  <h1>Test if Third-Party Cookies are Enabled</h1>

  <p id="3rd_party_cookie_test_results" class='loading'>Testing&hellip;</p>
  <p id="3rd_party_cookie_test_error" class="error hidden">(If this message persists, the test could not be completed; we could not reach the third-party to test, or another error occurred.)</p>

  <script type="text/javascript">
  window.setTimeout(function(){
    var errorEl = document.getElementById('3rd_party_cookie_test_error');
    if(errorEl.className.match(/\berror\b/)) {
      // Show error message
      errorEl.className = errorEl.className.replace(/\bhidden\b/,' ');
    } else {
    }
  }, 7*1000); // 7 sec timeout
  </script>
  <script type="text/javascript" src="http://third-party.example.com/step1.js.php"></script>
</body>
</html>

The first third-party JavaScript file

Saved as step1.js.php

This is written in PHP so we can set cookies as the file loads. (It could, of course, be written in any language, or even done in server config files.)

<?php
  header('Content-Type: application/javascript; charset=UTF-8');
  // Set test cookie
  setcookie('third_party_c_t', 'hey there!', time() + 3600*24*2);
?>
window._3rd_party_test_step1_loaded();

The second third-party JavaScript file

Saved as step2.js.php

This is written in PHP so we can read cookies, server-side, before we respond. We also clear the cookie so the test can be repeated (if you want to mess around with browser settings and re-try).

<?php
  header('Content-Type: application/javascript; charset=UTF-8');
  // Read test cookie, if there
  $cookie_received = (isset($_COOKIE['third_party_c_t']) && $_COOKIE['third_party_c_t'] == 'hey there!');
  // And clear it so the user can test it again 
  setcookie('third_party_c_t', '', time() - 3600*24);
?>
window._3rd_party_test_step2_loaded(<?php echo ($cookie_received ? 'true' : 'false'); ?>);

The last line uses the ternary operator to output a literal Javascript true or false depending on whether the test cookie was present.

Test it here.

Available for your testing pleasure at https://alanhogan.github.io/web-experiments/3rd/third-party-cookies.html.

(As a final note — don’t use someone else’s server to test third-party cookies without their permission. It could break spontaneously or inject malware. And it’s rude.)

Here's a pure JS solution not requiring any server-side code, so it can work from a static CDN: https://github.com/mindmup/3rdpartycookiecheck - the first script sets the cookie in the code, then redirects to a second script that will post a message to the parent window.

You can try out a live version using https://jsfiddle.net/tugawg8y/.
Note, this demo does not seem to work any more. Maybe the window.postMessage call is being blocked.

client-side HTML:

third party cookies are <span id="result"/>
<iframe src="https://mindmup.github.io/3rdpartycookiecheck/start.html"
    style="display:none" />

client-side JS:

 var receiveMessage = function (evt) {
   if (evt.data === 'MM:3PCunsupported') {
     document.getElementById('result').innerHTML = 'not supported';
   } else if (evt.data === 'MM:3PCsupported') {
     document.getElementById('result').innerHTML = 'supported';
   }
 };
 window.addEventListener("message", receiveMessage, false);

Of course, this requires that the client runs JavaScript which is a downside compared to the server-based solutions; on the other hand, it's simpler, and you were asking about a JS solution.

Alan H's solution is great, but you don't have to use PHP or any other server-side programming language.

At least if you use nginx. :)

This is a pure* nginx server-side configuration for Alan's solution:

server {

  listen 80;
  server_name third-party.example.com
  
  # Don't allow user's browser to cache these replies
  expires -1;
  add_header Cache-Control "private";
  etag off;
  
  # The first third-party "JavaScript file" - served by nginx
  location = /step1.js.php {
    add_header Content-Type 'application/javascript; charset=UTF-8';
    
    add_header Set-Cookie "third_party_c_t=hey there!;Max-Age=172800";
    
    return 200 'window._3rd_party_test_step1_loaded();';
  }
  
  # The second third-party "JavaScript file" - served by nginx
  location = /step2.js.php {
    add_header Content-Type 'application/javascript; charset=UTF-8';
    
    set $test 'false';
    if ($cookie_third_party_c_t = 'hey there!') {
      set $test 'true';
      # clear the cookie
      add_header Set-Cookie "third_party_c_t=;expires=Thu, 01 Jan 1970 00:00:00 GMT";
    }
    
    return 200 'window._3rd_party_test_step2_loaded($test);';
  }

}

Side notes:

Yeah, yeah, I know that IfIsEvil,
I kept the names ending with ".php" for complete compatibility with Alan's "The HTML test page" (third-party-cookies.html),
You can also move the common "setting the Content-Type header" line of both locations to the server section (scope) of the config - I kept it like this to make it more like Alan H's solution.

My solution works by loading a <script> from an external domain that sets a cookie, checks if it was successful, and then passes the result (1 or 0) as an argument to a callback function.

The HTML:

<script>
function myCallback(is_enabled) {
    if (is_enabled===1) {//third party cookies are enabled
    }
}
</script>
<script src="https://third-party-domain/third-party-cookies.php?callback=myCallback"></script>

If you prefer to run it asynchronously, you may use the async and defer attributes.

This also works with jQuery:

<script>
$.ajax({
    url: 'https://third-party-domain/third-party-cookies.php',
    dataType: 'jsonp',
}).done(function(is_enabled) {
    if (is_enabled===1) {//third party cookies are enabled
    }
})
</script>

Here is the third-party-cookies.php code. This must be hosted on a different domain. The server must support PHP:

<?php

header('Cache-Control: no-store');
header('Content-Type: text/javascript');

if ($_GET['callback']=='') {
    echo 'alert("Error: A callback function must be specified.")';
}
elseif (!isset($_GET['cookieName'])) {// Cookie not set yet
    $cookieName = strtr((string)$_SERVER['UNIQUE_ID'], '@', '_');
    while (isset($_COOKIE[$cookieName]) || $cookieName=='') {
        $cookieName = dechex(mt_rand());// Get random cookie name
    }
    setcookie($cookieName, '3rd-party', 0, '/');
    header('Location: '.$_SERVER['REQUEST_URI'].'&cookieName='.$cookieName);
}
elseif ($_COOKIE[$_GET['cookieName']]=='3rd-party') {// Third party cookies are enabled.
    setcookie($_GET['cookieName'], '', -1, '/'); // delete cookie
    echo $_GET['callback'].'(1)';
}
else {// Third party cookies are not enabled.
    echo $_GET['callback'].'(0)';
}