LocalStorage/SessionStorage polyfill

polyfill.js

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;
  }
};

if (!isStorageAvailable(window.localStorage) || isStorageAvailable(window.sessionStorage)) (function () {

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 (!isStorageAvailable(window.localStorage)) window.localStorage = new Storage('local');
if (!isStorageAvailable(window.sessionStorage)) window.sessionStorage = new Storage('session');

})();

On line 14 you're missing a !.

It should be

if (!isStorageAvailable(window.localStorage) || !isStorageAvailable(window.sessionStorage)) (function () {