Can you determine if Chrome is in incognito mode via a script?

Question

Is it possible to determine if Google Chrome is in incognito mode via a script?

Edit: I actually meant is it possible via user-script, but the answers assume JavaScript is running on a web page. I've re-asked the question here in regards to user scripts.

Answer 1

The documentation for incognito mode specifically says that websites won't behave differently. I believe this means that the answer is no.

Answer 2

One way is to visit a unique URL and then check to see whether a link to that URL is treated as visited by CSS.

You can see an example of this in "Detecting Incognito".

In main.html add an iframe,

 <iframe id='testFrame' name='testFrame' onload='setUniqueSource(this)' src='' style="width:0; height:0; visibility:hidden;"></iframe>

, and some JavaScript code:

function checkResult() {
  var a = frames[0].document.getElementById('test');
  if (!a) return;

  var color;
  if (a.currentStyle) {
    color = a.currentStyle.color;
  } else {
    color = frames[0].getComputedStyle(a, '').color;
  }

  var visited = (color == 'rgb(51, 102, 160)' || color == '#3366a0');
  alert('mode is ' + (visited ? 'NOT Private' : 'Private'));
}

function setUniqueSource(frame) {
  frame.src = "test.html?" + Math.random();
  frame.onload = '';
}

Then in test.html that are loaded into the iFrame:

<style> 
   a:link { color: #336699; }
   a:visited { color: #3366A0; }
</style> 
<script> 
  setTimeout(function() {
    var a = document.createElement('a');
    a.href = location;
    a.id = 'test';
    document.body.appendChild(a);
    parent.checkResult();
  }, 100);
</script> 

NOTE: trying this from the filesystem can make Chrome cry about "Unsafe Javascript". It will, however, work serving from a webserver.

Answer 3

You can, in JavaScript, see JHurrah's answer. Except for not highlighting links, all incognito mode does is not save browse history and cookies. From google help page:

• Webpages that you open and files downloaded while you are incognito aren't recorded in your browsing and download histories.
• All new cookies are deleted after you close all incognito windows that you've opened.

As you can see the differences between normal browsing and incognito happen after you visit the webpage, hence there is nothing that browser communicates to the server when it's in this mode.

You can see what exactly your browser sends to the server using one of many HTTP request analysers, like this one here. Compare the headers between normal session and incognito and you will see no difference.

Answer 4

If you are developing an Extension then you can use the tabs API to determine if a window/tab incognito.

More information can be found on code.google.com.

If you are just working with a webpage, it is not easy, and it is designed to be that way. However, I have noticed that all attempts to open a database (window.database) fail when in incongnito, this is because when in incognito no trace of data is allowed to be left on the users machine.

I haven't tested it but I suspect all calls to localStorage fail too.