Detect if the browser is running in Private mode

is-private-mode.js

// uncomment if you are transpiling with Babel + Webpack
// const { window, document } = global;

/**
 * Lightweight script to detect whether the browser is running in Private mode.
 * 
 * You should use a polyfill for Promise.
 * @see https://ourcodeworld.com/articles/read/316/top-5-best-javascript-promises-polyfills
 *
 * @returns {Promise}
 */
function isPrivateMode() {
  return new Promise((resolve) => {
    const yes = () => resolve(true); // is in private mode
    const not = () => resolve(false); // not in private mode
    const testLocalStorage = () => {
      try {
        if (localStorage.length) not();
        else {
          localStorage.x = 1;
          localStorage.removeItem('x');
          not();
        }
      } catch (e) {
        // 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();
      }
    };
    // Chrome & Opera
    if (window.webkitRequestFileSystem) {
      return void window.webkitRequestFileSystem(0, 0, not, yes);
    }
    // Firefox
    if ('MozAppearance' in document.documentElement.style) {
      if (indexedDB === null) return yes();
      const db = indexedDB.open('test');
      db.onerror = yes;
      db.onsuccess = not;
      return void 0;
    }
    // Safari
    const isSafari = navigator.userAgent.match(/Version\/([0-9\._]+).*Safari/);
    if (isSafari) {
      const version = parseInt(isSafari[1], 10);
      if (version < 11) return testLocalStorage();
      try {
        window.openDatabase(null, null, null, null);
        return not();
      } catch (_) {
        return yes();
      };
    }
    // IE10+ & Edge InPrivate
    if (!window.indexedDB && (window.PointerEvent || window.MSPointerEvent)) {
      return yes();
    }
    // default navigation mode
    return not();
  });
}

test.js

isPrivateMode().then((isPrivate) => {
  console.log('browsing in private mode? ', isPrivate);
});