localStorage Pollyfill for Safari / Chrome Private / Incognito Mode & Others

localStoragePollyfill.js

try {
    localStorage.setItem('test', true);
} catch (e) {
    if (e.code == 22) { //localStorage exists but size limit -> Probably Safari Private Mode.
        localStorage.__proto__ = Object.create(Storage.prototype);
        localStorage.__proto__._data = {};
        localStorage.__proto__.setItem = function (id, val) {
            return this._data[id] = String(val)
        };
        localStorage.__proto__.getItem = function (id) {
            return this._data.hasOwnProperty(id) ? this._data[id] : undefined
        };
        localStorage.__proto__.removeItem = function (id) {
            return delete this._data[id]
        };
        localStorage.__proto__.clear = function () {
            return this._data = {}
        };
    }
} finally {
   localStorage.removeItem('test');
}

You should also add

try{<...>}
catch(e){<...>}
finally {
   localStorage.removeItem('test');
}

to clean the test item

Added, thanks! :)