is_incognito_mode.js

function isIncognitoMode() {
    return new Promise(function(resolve, reject) {
        var fs = window.RequestFileSystem || window.webkitRequestFileSystem;
        if (fs) {
            fs(
                window.TEMPORARY,
                100,
                function (fs) {
                    resolve(false);
                }, function (err) {
                    resolve(true);
                }
            );
            return;
        }

        if (window.indexedDB) {
            var db = indexedDB.open("incognito_test");
            db.onerror = function() {
                resolve(true);
            };
            return;
        }

        if (!window.indexedDB && (window.PointerEvent || window.MSPointerEvent)) {
            resolve(true);
            return;
        }

        var storage = window.sessionStorage;
        try {
            storage.setItem("incognito_test", "test");
            storage.removeItem("incognito_test");
        } catch (e) {
            if (e.code === DOMException.QUOTA_EXCEEDED_ERR && storage.length === 0) {
                resolve(true);
                return;
            }
        }

        resolve(false);
    });
}

isIncognitoMode()
    .then(function(result){
        console.log(result ? "Incognito" : "Not incognito");
    });