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A localStorage polyfill, that enables persistent storage of data in every browser.


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A localStorage polyfill that makes the window object localStorage available in both modern and old browsers. This is done using a lot of different techniques, that enables persistent synchronous storage in one way or another.

How to use

To use this polyfill you simply have to include the localStorage.js file to your site. Furthermore you have to specify the URL of the localStorage.swf as the swfURL parameter to the file source. One example could be:

<script type="text/javascript"


The localStorage polyfill is available at the following addresses:


  • //
  • //
  • //


  • //
  • //
  • //
  • //
  • //


To ensure that the localStorage polyfill is fully loaded a method has been added to the polyfill. The method isLoaded on the localStorage object is supposed to be run before any use of the localStorage object. If the object is an instance of the FlashStorage, the method is used to ensure that the flash file is loaded and ready. In any other case the method will call the callback function immediately. If the isLoaded method is not available in the localStorage object, it is safe to assumme that it is the native object.

var func = function () {

if ( window.localStorage.isLoaded ) {
} else {;

Supported Browsers

The polyfill support a wide range of browsers. The use of the techniques mentioned in the Techniques section, implies that all browsers with cookies or flash enabled will successfully implement the localStorage object. The other techniques enable browser support as listed:

  • localStorage
    • Chrome 4+
    • Firefox 3.5+
    • Safari 4+
    • Opera 10+
    • IE 8+
  • globalStorage
    • Firefox 2 - 13
  • userData behaviour
    • IE 5 - 10 (IE 9 and IE 10 should work, but my own - and others - tests have proven otherwise)


As per 20/08-2014 the polyfill has been tested successfully in:

  • IE11 (Win7)
  • IE10 (Win7)
  • IE9 (Win7)
  • IE8 (WinXP)
  • IE7 (WinVista)
  • IE6 (WinXP)
  • Firefox 3 (WinXP)
  • Firefox 6 (WinXP)
  • Firefox 31 (Ubuntu 14.04)
  • Chromium 36 (Ubuntu 14.04)
  • Opera 9.0 (WinXP)
  • Opera 12.16 (Ubuntu 14.04)

There has been put a lot of efford in creating unit tests and providing code coverage. To provide these thing the libraries QUnit and Blanket has been used.


The techniques in the polyfill are used such that the best and most reliable solution is tested for support first. If that technique is not available in the browser, the next technique is tested and so forth.

Below is a list of the techniques used in the polyfill. The list is ordered by the best and most reliable solution first.

  • localStorage
  • globalStorage
  • userData behavior
  • Flash
  • Cookies

As per version 2.0, it was decided to remove the google gears solution, as it was not possible to do any testing on that solution.

Storage object

The original localStorage object implements the Storage interface, which looks as follows:

interface Storage {
	readonly attribute unsigned long length;
	[IndexGetter] DOMString key(in unsigned long index);
	[NameGetter] DOMString getItem(in DOMString key);
	[NameSetter] void setItem(in DOMString key, in DOMString data);
	[NameDeleter] void removeItem(in DOMString key);
	void clear();

The polyfill object imitate the interface, which means that any of the methods above is also available in the polyfill object, no matter what technique is used to create the polyfill.

Furthermore as of version 2.0 the localStorage polyfill is created using Object Oriented Programming, meaning that the localStorage object will be an instance of an imitated Storage object.

The localStorage object is furthermore instance of one of four classes if the object is not the native localStorage:

  • GlobalStorage
  • UserDataStorage
  • FlashStorage
  • CookieStorage

Hence it is possible to use the following features:

localStorage instanceof Storage
localStorage instanceof GlobalStorage
localStorage instanceof UserDataStorage
localStorage instanceof FlashStorage
localStorage instanceof CookieStorage

Each of these storages can be created by their own as well.


The polyfill does not handle any exceptions. Instead it just pass on the exception to the callee-function which then can choose to catch it. The cookie, userdata, and flash solutions throws a new exception named CookieQuotaExceeded, UserDataQuotaExceeded and FlashQuotaExceeded, which are thrown when the data stored is greater than what is allowed for the solution.

The best practice of handling the polyfill element is to have a try-catch around any setItem call, as this method potentially will throw exceptions when the Quota of the localStorage is full. Most browsers use the term "quota" in one way or another in their exception name or message and IE simply use "Error". To conclude that the localStorage is full you would have to write something like:

var regexp_quota = /quota/i;

try {                                                           
} catch (exc) {                                                 
	if ( regexp_quota.test( || 
	     regexp_quota.test(exc.message)|| === "Error" ) {
		alert("The localStorage is full");


As per version 2.0, the dependency to swfobject has been removed. Instead it was decided to implement the flash object creation as a part of the flash solution.


A localStorage polyfill, that enables persistent storage of data in every browser.