Web Storage API

The Web Storage API provides mechanisms by which browsers can store key/value pairs, in a much more intuitive fashion than using cookies.

Web Storage concepts and usage

The two mechanisms within Web Storage are as follows:

  • sessionStorage maintains a separate storage area for each given origin that's available for the duration of the page session (as long as the browser is open, including page reloads and restores).
    • Stores data only for a session, meaning that the data is stored until the browser (or tab) is closed.
    • Data is never transferred to the server.
    • Storage limit is larger than a cookie (at most 5MB).
  • localStorage does the same thing, but persists even when the browser is closed and reopened.
    • Stores data with no expiration date, and gets cleared only through JavaScript, or clearing the Browser cache / Locally Stored Data.
    • Storage limit is the maximum amongst the two.

These mechanisms are available via the Window.sessionStorage and Window.localStorage properties (to be more precise, the Window object implements the WindowLocalStorage and WindowSessionStorage objects, which the localStorage and sessionStorage properties hang off) — invoking one of these will create an instance of the Storage object, through which data items can be set, retrieved and removed. A different Storage object is used for the sessionStorage and localStorage for each origin — they function and are controlled separately.

Note: In Firefox, when the browser crashes/restarts, to avoid memory issues caused by excessive usage of web storage, the amount of data saved per origin is limited to 10MB. See storage quotas and eviction criteria for more information.

Note: Access to Web Storage from third-party IFrames is denied if the user has disabled third-party cookies.

Determining storage access by a third party

Each origin has its own storage — this is true for both web storage and shared storage). However, access of third-party (i.e., embedded) code to shared storage depends on its browsing context. The context in which a third-party code from another origin runs determines the storage access of the third-party code.

A box diagram showing a top-level browsing context called publisher.com, with third-party content embedded in it

Third-party code can be added to another site by injecting it with a <script> element or by setting the source of an <iframe> to a site that contains third-party code. The method used for integrating third-party code determines the browsing context of the code.

  • If your third-party code is added to another site with a <script> element, your code will be executed in the browsing context of the embedder. Therefore, when you call Storage.setItem() or SharedStorage.set(), the key/value pair will be written to the embedder's storage. From the browser's perspective, there is no difference between first-party code and third-party code when a <script> tag is used.
  • When your third-party code is added to another site within an <iframe>, the code inside the <iframe> will be executed with the origin of the <iframe>'s browsing context. If the code inside the <iframe> calls Storage.setItem(), data will be written into the local or session storage of the <iframe>'s origin. If the <iframe> code calls SharedStorage.set(), the data will be written into the shared storage of the <iframe>'s origin.

Web Storage interfaces

Storage

Allows you to set, retrieve and remove data for a specific domain and storage type (session or local).

Window

The Web Storage API extends the Window object with two new properties — Window.sessionStorage and Window.localStorage — which provide access to the current domain's session and local Storage objects respectively, and a storage event handler that fires when a storage area changes (e.g., a new item is stored).

StorageEvent

The storage event is fired on a document's Window object when a storage area changes.

Examples

To illustrate some typical web storage usage, we have created a simple example, imaginatively called Web Storage Demo. The landing page provides controls that can be used to customize the color, font and decorative image. When you choose different options, the page is instantly updated; in addition your choices are stored in localStorage, so that when you leave the page then load it again later on your choices are remembered.

In addition, we have provided an event output page — if you load this page in another tab, then make changes to your choices in the landing page, you'll see the updated storage information outputted as the StorageEvent is fired.

Specifications

Specification
HTML Standard
# dom-localstorage-dev
HTML Standard
# dom-sessionstorage-dev

Browser compatibility

api.Window.localStorage

BCD tables only load in the browser

api.Window.sessionStorage

BCD tables only load in the browser

Private Browsing / Incognito modes

Private windows, incognito mode, and similarly named privacy browsing options, don't store data like history and cookies. In private mode, localStorage is treated like sessionStorage. The storage APIs are still available and fully functional, but all data stored in the private window is deleted when the browser or browser tab is closed.

See also