A LocalStorage polyfill that also works in private browsing Safari (by doing window.localStorage.__proto__ = ourPolyfillObject) (where window.localStorage !== 'undefined', but has a quota of 0 and throws an error when you try to setItem)

storagePolyfill.js

//variant of https://gist.github.com/Contra/6368485

(function () {

  var isStorageAvailable = function (storage) {
    if (typeof storage == 'undefined') return false;
    try { // hack for safari incognito
      storage.setItem("storage", "");
      storage.getItem("storage");
      storage.removeItem("storage");
      return true;
    }
    catch (err) {
      return false;
    }
  };

  var localStorageAvailable = isStorageAvailable(window.localStorage),
    sessionStorageAvailable = isStorageAvailable(window.sessionStorage);

  if (!localStorageAvailable || !sessionStorageAvailable) {

    var Storage = function (type) {
      function createCookie(name, value, days) {
        var date, expires;

        if (days) {
          date = new Date();
          date.setTime(date.getTime()+(days*24*60*60*1000));
          expires = "; expires="+date.toGMTString();
        } else {
          expires = "";
        }
        document.cookie = name+"="+value+expires+"; path=/";
      }

      function readCookie(name) {
        var nameEQ = name + "=",
            ca = document.cookie.split(';'),
            i, c;

        for (i=0; i < ca.length; i++) {
          c = ca[i];
          while (c.charAt(0)==' ') {
            c = c.substring(1,c.length);
          }

          if (c.indexOf(nameEQ) == 0) {
            return c.substring(nameEQ.length,c.length);
          }
        }
        return null;
      }

      function setData(data) {
        data = JSON.stringify(data);
        if (type == 'session') {
          window.name = data;
        } else {
          createCookie('localStorage', data, 365);
        }
      }

      function clearData() {
        if (type == 'session') {
          window.name = '';
        } else {
          createCookie('localStorage', '', 365);
        }
      }

      function getData() {
        var data = type == 'session' ? window.name : readCookie('localStorage');
        return data ? JSON.parse(data) : {};
      }


      // initialise if there's already data
      var data = getData();

      return {
        length: 0,
        clear: function () {
          data = {};
          this.length = 0;
          clearData();
        },
        getItem: function (key) {
          return data[key] === undefined ? null : data[key];
        },
        key: function (i) {
          // not perfect, but works
          var ctr = 0;
          for (var k in data) {
            if (ctr == i) return k;
            else ctr++;
          }
          return null;
        },
        removeItem: function (key) {
          if (data[key] === undefined) this.length--;
          delete data[key];
          setData(data);
        },
        setItem: function (key, value) {
          if (data[key] === undefined) this.length++;
          data[key] = value+''; // forces the value to a string
          setData(data);
        }
      };
    };

    if (!localStorageAvailable) window.localStorage.__proto__ = new Storage('local');
    if (!sessionStorageAvailable) window.sessionStorage.__proto__ = new Storage('session');

  }

})();