Exact imitation of the localStorage object, but uses cookies to provide localStorage in browsers that don't support it.

localstorage-shim.js

// https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Storage#Compatibility

// Note: The maximum size of data that can be saved is severely
// restricted by the use of cookies. With this algorithm, use the
// functions localStorage.setItem() and localStorage.removeItem() to
// add, change, or remove a key. The use of methods
// localStorage.yourKey = yourValue; and delete localStorage.yourKey; to
// set or delete a key *is not a secure way with this code*. You can
// also change its name and use it only to manage a document's cookies
// regardless of the localStorage object.
//
// Note: By changing the string
// "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/" to: "; path=/"
// (and changing the object's name), this will become a sessionStorage
// polyfill rather than a localStorage polyfill. However, this
// implementation will share stored values across browser tabs and
// windows (and will only be cleared when all browser windows have been
// closed), while a fully-compliant sessionStorage implementation
// restricts stored values to the current browsing context only.


if (!window.localStorage) {
  Object.defineProperty(window, "localStorage", new (function () {
    var aKeys = [], oStorage = {};
    Object.defineProperty(oStorage, "getItem", {
      value: function (sKey) { return sKey ? this[sKey] : null; },
      writable: false,
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "key", {
      value: function (nKeyId) { return aKeys[nKeyId]; },
      writable: false,
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "setItem", {
      value: function (sKey, sValue) {
        if (!sKey) { return; }
        document.cookie = escape(sKey) + "=" + escape(sValue) + "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/";
      },
      writable: false,
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "length", {
      get: function () { return aKeys.length; },
      configurable: false,
      enumerable: false
    });
    Object.defineProperty(oStorage, "removeItem", {
      value: function (sKey) {
        if (!sKey) { return; }
        document.cookie = escape(sKey) + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/";
      },
      writable: false,
      configurable: false,
      enumerable: false
    });
    this.get = function () {
      var iThisIndx;
      for (var sKey in oStorage) {
        iThisIndx = aKeys.indexOf(sKey);
        if (iThisIndx === -1) { oStorage.setItem(sKey, oStorage[sKey]); }
        else { aKeys.splice(iThisIndx, 1); }
        delete oStorage[sKey];
      }
      for (aKeys; aKeys.length > 0; aKeys.splice(0, 1)) { oStorage.removeItem(aKeys[0]); }
      for (var aCouple, iKey, nIdx = 0, aCouples = document.cookie.split(/\s*;\s*/); nIdx < aCouples.length; nIdx++) {
        aCouple = aCouples[nIdx].split(/\s*=\s*/);
        if (aCouple.length > 1) {
          oStorage[iKey = unescape(aCouple[0])] = unescape(aCouple[1]);
          aKeys.push(iKey);
        }
      }
      return oStorage;
    };
    this.configurable = false;
    this.enumerable = true;
  })());
}