new Promise(function(resolve) {
  var db,
    on = function() { resolve(true) }, // is in private mode
    off = function() { resolve(false) }, // not private mode
    tryls = function() {
      try {
        localStorage.length ? off() : (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
  window.webkitRequestFileSystem ? webkitRequestFileSystem(0, 0, off, on)
    // Firefox
    : 'MozAppearance' in document.documentElement.style ? (db = indexedDB.open('test'), db.onerror = on, db.onsuccess = off)
    // Safari
    : (/constructor/i).test(window.HTMLElement) ? tryls()
    // IE10+ & Edge
    : !window.indexedDB && (window.PointerEvent || window.MSPointerEvent) ? on()
    // Rest
    : off();
}).then(function(isPrivateMode) {
  console.log('Private mode is: ' + isPrivateMode)
});