/**

 * Lightweight script to detect whether the browser is running in Private mode.

 * @returns {Promise<boolean>}

 *

 * This snippet uses Promises. If you want to run it in old browsers, polyfill it:

 * @see https://cdn.jsdelivr.net/npm/es6-promise@4/dist/es6-promise.auto.min.js

 *

 * More Promise Polyfills:

 * @see https://ourcodeworld.com/articles/read/316/top-5-best-javascript-promises-polyfills

 *

 * Source:

 * @see https://gist.github.com/jherax/a81c8c132d09cc354a0e2cb911841ff1

 */

function isPrivateMode() {

  return new Promise(function detect(resolve) {

    var yes = function() { resolve(true); }; // is in private mode

    var not = function() { resolve(false); }; // not in private mode

​

    function detectChromeOpera() {

      // https://developers.google.com/web/updates/2017/08/estimating-available-storage-space

      var isChromeOpera = /(?=.*(opera|chrome)).*/i.test(navigator.userAgent) && navigator.storage && navigator.storage.estimate;

      if (isChromeOpera) {

        navigator.storage.estimate().then(function(data) {

          return data.quota < 120000000 ? yes() : not();

        });

      }

      return !!isChromeOpera;

    }

​

    function detectFirefox() {

      var isMozillaFirefox = 'MozAppearance' in document.documentElement.style;

      if (isMozillaFirefox) {

        if (indexedDB == null) yes();

        else {

          var db = indexedDB.open('inPrivate');

          db.onsuccess = not;

          db.onerror = yes;

        }

      }

      return isMozillaFirefox;

    }

​

    function detectSafari() {

      var isSafari = navigator.userAgent.match(/Version\/([0-9\._]+).*Safari/);

      if (isSafari) {

        var testLocalStorage = function() {

          try {

            if (localStorage.length) not();

            else {

              localStorage.setItem('inPrivate', '0');

              localStorage.removeItem('inPrivate');

              not();

            }

          } catch (_) {

            // Safari only enables cookie in private mode

            // if cookie is disabled, then all client side storage is disabled

            // if all client side storage is disabled, then there is no point

            // in using private mode

            navigator.cookieEnabled ? yes() : not();

          }

          return true;

        };

​

        var version = parseInt(isSafari[1], 10);

        if (version < 11) return testLocalStorage();

        try {

          window.openDatabase(null, null, null, null);

          not();

        } catch (_) {

          yes();

        }

      }

      return !!isSafari;

    }

​

    function detectEdgeIE10() {

      var isEdgeIE10 = !window.indexedDB && (window.PointerEvent || window.MSPointerEvent);

      if (isEdgeIE10) yes();

      return !!isEdgeIE10;

    }

​

    // when a browser is detected, it runs tests for that browser

    // and skips pointless testing for other browsers.

    if (detectChromeOpera()) return;

    if (detectFirefox()) return;

    if (detectSafari()) return;

    if (detectEdgeIE10()) return;

    // default navigation mode

    return not();

  });

}

​

// -------------------------------

​

setTimeout(function() {

  isPrivateMode().then(function(isPrivate) {

    console.log('Browsing in private mode? ', isPrivate);

    alert('Browsing in private mode? ' + isPrivate);

  });

}, 100);

​