Detecting if a browser is using Private Browsing mode

Update June 2019

Google is removing the ability to detect Private Browsing Mode permanently in Chrome 76 onwards. So, if you're wanting to detect private browsing it's now impossible (unless you find a way to do it that Google hasn't found). The ability to detect private browsing mode has been acknowledged as a bug and was never intended.

To anyone else coming across this question, please note as of 2014, there is no reliable or accurate way to detect if someone is browsing in an incognito/private/safe browsing mode through Javascript or CSS. Previous solutions that once worked like the CSS history hack have since been rendered unusable by all browser vendors.

There should never be a situation where needing to detect private browsing mode on a normal day-to-day website is ever needed. People are choosing to browse anonymously and or not anonymously for their own reasons.

Browsers like Chrome and Firefox do not disable functionality like localStorage any more. They simply namespace it in a temporary location to prevent websites that use it from erroring out. Once you're finished browsing, the namespace is erased and nothing is saved. If you are testing for localStorage support regardless of mode, it will always return true for browsers that support it.

Other means of detecting private mode in Chrome specifically have been completely patched and will no longer work.

If it is required internally by a company, you should develop a browser plugin. Chrome and Firefox, in particular, expose internal API's which allow plugins to check if the user is in private browsing/incognito mode and action accordingly. It cannot be done outside of a plugin.

Here's an easier way to do detect privacy mode. This works in Safari only. I created it because a web app I am developing uses localStorage. LocalStorage is not available in Safari when in privacy mode, thus my app will not work. On page load, run the script below. It shows an alert box if we cannot use localStorage.

try {  // try to use localStorage  localStorage.test = 2;        } catch (e) {  // there was an error so...  alert('You are in Privacy Mode\nPlease deactivate Privacy Mode and then reload the page.');}

Current state

Google Chrome has developed further and leaves no more space for detection when using incognito mode. Same might apply for other browsers.

Old solutions (might partially work)

It is possible to detect enabled private browsing modes for the majority of used browsers. This includes Safari, Firefox, IE10, Edge and Google Chrome.

Firefox

When the private browsing mode of Firefox is enabled, the IndexedDB throws an InvalidStateError because it is not available in private browsing mode.

To very if that:

var db = indexedDB.open("test");db.onerror = function(){/*Firefox PB enabled*/};db.onsuccess =function(){/*Not enabled*/};

Safari

For Safari, the key is the local storage service. It is disabled in privacy mode. So try to access it and use a try-catch clause.The following method works on both, OSX and iOS devices. Credits for this method are going to this question and answer

var storage = window.sessionStorage;try {    storage.setItem("someKeyHere", "test");    storage.removeItem("someKeyHere");} catch (e) {    if (e.code === DOMException.QUOTA_EXCEEDED_ERR && storage.length === 0) {        //Private here    }}

IE10/Edge

Internet Explore is even going to disable the IndexedDB when in privacy mode. So check for existence. But that's not sufficient enough, because older browsers maybe don't even have an IDB. So do another check, e.g. for events that only IE10 and subsequent browser have/trigger. A related question on CodeReview can be found here

if(!window.indexedDB && (window.PointerEvent || window.MSPointerEvent)){ //Privacy Mode}

Chrome

Update: This doesn't work since Chrome 76 (thanks to @jLynx)

Chromes Incognito mode can be verified by the file system. A great explanation can be found here on SO

var fs = window.RequestFileSystem || window.webkitRequestFileSystem;if (!fs) {    console.log("FS check failed..");    return;}fs(window.TEMPORARY, 100, function (fs) {}, function (err) {//Incognito mode});