Detect if a browser is in Private Browsing mode

is-private-mode.js

/**
 * Detect if the browser is running in Private Browsing mode
 */
function isPrivateMode() {
  return new Promise((resolve) => {
    const on = () => resolve(true); // is in private mode
    const off = () => resolve(false); // not private mode
    const testLocalStorage = () => {
      try {
        if (localStorage.length) off();
        else {
          localStorage.x = 1;
          localStorage.removeItem('x');
          off();
        }
      } 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 ? on() : off();
      }
    };
    // Chrome & Opera
    if (window.webkitRequestFileSystem) {
      return void window.webkitRequestFileSystem(0, 0, on, off);
    }
    // Firefox
    if ('MozAppearance' in document.documentElement.style) {
      const db = indexedDB.open('test');
      db.onerror = on;
      db.onsuccess = off;
      return void 0;
    }
    // Safari
    if (/constructor/i.test(window.HTMLElement)) {
      return testLocalStorage();
    }
    // IE10+ & Edge
    if (!window.indexedDB && (window.PointerEvent || window.MSPointerEvent)) {
      return on();
    }
    // others
    return off();
  });
}

test.js

// for IE: Promises aren't natively supported in IE10 or 11

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