Detect if the browser is running in Private mode - Promise based (last update: Feb 2020)

is-private-mode.js

/**
 * Lightweight script to detect whether the browser is running in Private mode.
 * @returns {Promise<boolean>}
 *
 * Live demo:
 * @see https://output.jsbin.com/tazuwif
 *
 * 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
 *
 * Disclaimer:
 * No longer maintained, last updated: Feb 2020
 */
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();
  });
}

is-private-mode.ts

/**
 * Lightweight script to detect whether the browser is running in Private mode.
 *
 * Live demo:
 * @see https://output.jsbin.com/tazuwif
 *
 * 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
 *
 * Disclaimer:
 * No longer maintained, last updated: Feb 2020
 */
export function isPrivateMode(): Promise<boolean> {
  return new Promise<boolean>(resolve => {
    const yes = () => resolve(true); // is in private mode
    const not = () => resolve(false); // not in private mode

    function detectChromeOpera(): boolean {
      // https://developers.google.com/web/updates/2017/08/estimating-available-storage-space
      const isChromeOpera = /(?=.*(opera|chrome)).*/i.test(navigator.userAgent) && navigator.storage?.estimate;
      if (isChromeOpera) {
        navigator.storage.estimate().then(({ quota }) => {
          quota < 120000000 ? yes() : not();
        });
      }
      return !!isChromeOpera;
    }

    function detectFirefox(): boolean {
      const isMozillaFirefox = 'MozAppearance' in document.documentElement.style;
      if (isMozillaFirefox) {
        if (indexedDB == null) yes();
        else {
          const db = indexedDB.open('inPrivate');
          db.onsuccess = not;
          db.onerror = yes;
        }
      }
      return isMozillaFirefox;
    }

    function detectSafari(): boolean {
      const isSafari = navigator.userAgent.match(/Version\/([0-9\._]+).*Safari/);
      if (isSafari) {
        const testLocalStorage = () => {
          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;
        };

        const version = parseInt(isSafari[1], 10);
        if (version < 11) return testLocalStorage();
        try {
          (window as any).openDatabase(null, null, null, null);
          not();
        } catch (_) {
          yes();
        }
      }
      return !!isSafari;
    }

    function detectEdgeIE10(): boolean {
      const 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();
  });
}

test.js

/**
 * Live demo:
 * @see https://output.jsbin.com/tazuwif
 *
 * Disclaimer:
 * No longer maintained, last updated: Feb 2020
 */
isPrivateMode().then(function (isPrivate) {
  console.log('Browsing in private mode? ', isPrivate);
});

If anyone is looking for a solution in 2022, I have a repo available: https://github.com/Joe12387/detectIncognito