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 * No longer maintained, last updated: Feb 2020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);
});

@jherax The snippet does not work for Safari 13.0.3. window.openDatabase(null, null, null, null) returns {} in both cases.

@jherax Updated snippet for chrome & opera. For this to work, you will need to add async before (resolve) => {

    // Chrome & Opera
    // https://developers.google.com/web/updates/2017/08/estimating-available-storage-space
    if (/(?=.*(opera|chrome)).*/i.test(navigator.userAgent) && 'storage' in navigator && 'estimate' in navigator.storage) {
        const {quota} = await navigator.storage.estimate();
        return (quota < 120000000) ? yes() : not();
    }

💡 Thanks @kchecoMP.
I have implemented the suggested solution, keeping it with promises instead of async-await.
It is working as expected! 🎉

I also downgraded the ES syntax to be compatible with IE10+.
If you want to test new ES2015+ syntax, you can transpile it with Babel:

https://babeljs.io/repl

Additionally, I added a TypeScript source file, which can be transpiled with TSC:

https://www.typescriptlang.org/play/

@jherax The snippet does not work for Safari 13.0.3. window.openDatabase(null, null, null, null) returns {} in both cases.

Same for me on Safari 13.3

It doesn't work with iOS 13

Update for Chrome and Safari?

Error in line https://gist.github.com/jherax/a81c8c132d09cc354a0e2cb911841ff1#file-is-private-mode-js-L33 with Tor Browser:

Uncaught (in promise) DOMException: The operation is insecure.

Always return false on Chrome 89.x

While this is a very cool bit of code...

I worry about the ethics of detecting a user trying to see an unfiltered view of the world and maintaining their privacy. Perhaps it's best it doesn't work now?

tested on chrome 92, doesn't work anymore.

Doesn't work on Safari anymore either.

Chrome 100, 2022, not working

For everyone, please read the following:

https://gist.github.com/jherax/a81c8c132d09cc354a0e2cb911841ff1?permalink_comment_id=2986681#gistcomment-2986681

This script was created to detect the access to WebStorage API, which most browsers changed in last years. The script was last updated on Feb 2020. The recommendation for everyone is to follow a "detect API" approach, working with fallbacks.

E.g.

async function isAvailableSomeApi (params) {
  // detect access to the API according to your use case.
  const isChrome = ...;
  const isFirefox = ...;
  const isSafari = ...;
  const isIE10 = ...;

  if (isChrome) {
    return await testApiForChrome(params); // Promise<boolean>
  }
  if (isFirefox) {
    return await testApiForFirefox(params); // Promise<boolean>
  }
  if (isSafari) {
    return await testApiForSafari(params); // Promise<boolean>
  }
  if (isIE10) {
    return await testApiForIE10(params); // Promise<boolean>
  }

  // default is treated as "api not available"
  return Promise.resolve(false);
}

async function getSomeApiWithFallback() {
  // use the fallback of the API according to your use case
  const isAvailable = await isAvailableSomeApi(params);
  if (!isAvailable) {
    return buildSomeApiFallback();
  }
  return theRealWorkingAPI;
}

function buildSomeApiFallback() {
  // implement the fallback of your missing API
  // or just throw custom managed errors
}

function mainEntryPoint() {
  const apiProxy = getSomeApiWithFallback();
  // use safely your API
  // ...
}

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