new Promise(function (resolve) {
    var db,
    on = function(){ resolve(true) },
    off = function(){ resolve(false) },
    tryls = function tryls() {
        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();
        }
    }

    // Blink (chrome & opera)
    window.webkitRequestFileSystem ? webkitRequestFileSystem(0, 0, off, on)
    // FF
    : "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('is private mode: ' + isPrivateMode)
});