HTML
Living Standard — Last Updated 16 January 2024
Living Standard — Last Updated 16 January 2024
Support in all current engines.
This section is non-normative.
This specification introduces two related mechanisms, similar to HTTP session cookies, for storing name-value pairs on the client side. [COOKIES]
The first is designed for scenarios where the user is carrying out a single transaction, but could be carrying out multiple transactions in different windows at the same time.
Cookies don't really handle this case well. For example, a user could be buying plane tickets in two different windows, using the same site. If the site used cookies to keep track of which ticket the user was buying, then as the user clicked from page to page in both windows, the ticket currently being purchased would "leak" from one window to the other, potentially causing the user to buy two tickets for the same flight without noticing.
To address this, this specification introduces the sessionStorage getter. Sites can add data to the session
storage, and it will be accessible to any page from the same site opened in that window.
For example, a page could have a checkbox that the user ticks to indicate that they want insurance:
< label >
< input type = "checkbox" onchange = "sessionStorage.insurance = checked ? 'true' : ''" >
I want insurance on this trip.
</ label > A later page could then check, from script, whether the user had checked the checkbox or not:
if ( sessionStorage. insurance) { ... } If the user had multiple windows opened on the site, each one would have its own individual copy of the session storage object.
The second storage mechanism is designed for storage that spans multiple windows, and lasts beyond the current session. In particular, web applications might wish to store megabytes of user data, such as entire user-authored documents or a user's mailbox, on the client side for performance reasons.
Again, cookies do not handle this case well, because they are transmitted with every request.
The localStorage getter is used to access a page's local
storage area.
The site at example.com can display a count of how many times the user has loaded its page by putting the following at the bottom of its page:
< p >
You have viewed this page
< span id = "count" > an untold number of</ span >
time(s).
</ p >
< script >
if ( ! localStorage. pageLoadCount)
localStorage. pageLoadCount = 0 ;
localStorage. pageLoadCount = parseInt( localStorage. pageLoadCount) + 1 ;
document. getElementById( 'count' ). textContent = localStorage. pageLoadCount;
</ script > Each site has its own separate storage area.
The localStorage getter provides access
to shared state. This specification does not define the interaction with other agent clusters
in a multiprocess user agent, and authors are encouraged to assume that there is no locking
mechanism. A site could, for instance, try to read the value of a key, increment its value, then
write it back out, using the new value as a unique identifier for the session; if the site does
this twice in two different browser windows at the same time, it might end up using the same
"unique" identifier for both sessions, with potentially disastrous effects.
Support in all current engines.
Storage interface[Exposed =Window ]
interface Storage {
readonly attribute unsigned long length ;
DOMString ? key (unsigned long index );
getter DOMString ? getItem (DOMString key );
setter undefined setItem (DOMString key , DOMString value );
deleter undefined removeItem (DOMString key );
undefined clear ();
};storage.lengthSupport in all current engines.
Returns the number of key/value pairs.
storage.key (n)Support in all current engines.
Returns the name of the nth key, or null if n is greater than or equal to the number of key/value pairs.
value = storage.getItem (key)Support in all current engines.
value = storage[key]Returns the current value associated with the given key, or null if the given key does not exist.
storage.setItem (key, value)Support in all current engines.
storage[key] = valueSets the value of the pair identified by key to value, creating a new key/value pair if none existed for key previously.
Throws a "QuotaExceededError" DOMException exception
if the new value couldn't be set. (Setting could fail if, e.g., the user has disabled storage
for the site, or if the quota has been exceeded.)
Dispatches a storage event on Window objects
holding an equivalent Storage object.
storage.removeItem (key)Support in all current engines.
delete storage[key]Removes the key/value pair with the given key, if a key/value pair with the given key exists.
Dispatches a storage event on Window objects
holding an equivalent Storage object.
storage.clear()Support in all current engines.
Removes all key/value pairs, if there are any.
Dispatches a storage event on Window objects
holding an equivalent Storage object.
A Storage object has an associated:
local" or "session".
To reorder a Storage object
storage, reorder storage's map's
entries in an implementation-defined manner.
Unfortunate as it is, iteration order is not defined and can change upon most mutations.
To broadcast a Storage object
storage, given a key, oldValue, and newValue, run
these steps:
Let thisDocument be storage's relevant global object's associated Document.
Let url be thisDocument's URL.
Let remoteStorages be all Storage objects excluding
storage whose:
and, if type is "session",
whose relevant settings object's associated
Document's node navigable's traversable navigable is thisDocument's node
navigable's traversable navigable.
For each remoteStorage of
remoteStorages: queue a global task on the DOM manipulation task
source given remoteStorage's relevant global object to fire an event named storage at remoteStorage's relevant global
object, using StorageEvent, with key initialized to key, oldValue initialized to oldValue, newValue initialized to newValue, url initialized to url, and storageArea initialized to
remoteStorage.
The Document object associated with the resulting task is not necessarily fully active, but events fired
on such objects are ignored by the event loop until the Document
becomes fully active again.
The length getter
steps are to return this's map's size.
The key(index)
method steps are:
If index is greater than or equal to this's map's size, then return null.
Let keys be the result of running get the keys on this's map.
Return keys[index].
The supported property names on a Storage object storage
are the result of running get the keys on
storage's map.
The getItem(key) method steps are:
The setItem(key,
value) method are:
Let oldValue be null.
Let reorder be true.
If value cannot be stored, then throw a
"QuotaExceededError" DOMException exception.
The removeItem(key) method steps are:
The clear() method
steps are:
sessionStorage getterinterface mixin WindowSessionStorage {
readonly attribute Storage sessionStorage ;
};
Window includes WindowSessionStorage ;window.sessionStorageSupport in all current engines.
Returns the Storage object associated with that window's origin's
session storage area.
Throws a "SecurityError" DOMException if the
Document's origin is an opaque origin or if the request violates a policy decision
(e.g., if the user agent is configured to not allow the page to persist data).
A Document object has an associated session storage holder, which is
null or a Storage object. It is initially null.
The sessionStorage getter steps are:
If this's associated
Document's session storage holder is non-null, then return
this's associated
Document's session storage holder.
Let map be the result of running obtain a session storage bottle
map with this's relevant settings object and "sessionStorage".
If map is failure, then throw a "SecurityError"
DOMException.
Set this's associated
Document's session storage holder to
storage.
Return storage.
After creating a new auxiliary browsing context and document, the session storage is copied over.
localStorage getterinterface mixin WindowLocalStorage {
readonly attribute Storage localStorage ;
};
Window includes WindowLocalStorage ;window.localStorageSupport in all current engines.
Returns the Storage object associated with window's origin's local
storage area.
Throws a "SecurityError" DOMException if the
Document's origin is an opaque origin or if the request violates a policy decision
(e.g., if the user agent is configured to not allow the page to persist data).
A Document object has an associated local storage holder, which is null
or a Storage object. It is initially null.
The localStorage getter steps are:
If this's associated
Document's local storage holder is non-null, then return
this's associated
Document's local storage holder.
Let map be the result of running obtain a local storage bottle map
with this's relevant settings object and "localStorage".
If map is failure, then throw a "SecurityError"
DOMException.
Set this's associated
Document's local storage holder to storage.
Return storage.
StorageEvent interfaceSupport in all current engines.
[Exposed =Window ]
interface StorageEvent : Event {
constructor (DOMString type , optional StorageEventInit eventInitDict = {});
readonly attribute DOMString ? key ;
readonly attribute DOMString ? oldValue ;
readonly attribute DOMString ? newValue ;
readonly attribute USVString url ;
readonly attribute Storage ? storageArea ;
undefined initStorageEvent (DOMString type , optional boolean bubbles = false , optional boolean cancelable = false , optional DOMString ? key = null , optional DOMString ? oldValue = null , optional DOMString ? newValue = null , optional USVString url = "", optional Storage ? storageArea = null );
};
dictionary StorageEventInit : EventInit {
DOMString ? key = null ;
DOMString ? oldValue = null ;
DOMString ? newValue = null ;
USVString url = "";
Storage ? storageArea = null ;
};event.keyReturns the key of the storage item being changed.
event.oldValueReturns the old value of the key of the storage item whose value is being changed.
event.newValueReturns the new value of the key of the storage item whose value is being changed.
event.urlReturns the URL of the document whose storage item changed.
event.storageAreaReturns the Storage object that was affected.
The key,
oldValue,
newValue,
url, and storageArea
attributes must return the values they were initialized to.
The initStorageEvent(type, bubbles,
cancelable, key, oldValue, newValue, url,
storageArea) method must initialize the event in a manner analogous to the
similarly-named initEvent() method. [DOM]
A third-party advertiser (or any entity capable of getting content distributed to multiple sites) could use a unique identifier stored in its local storage area to track a user across multiple sessions, building a profile of the user's interests to allow for highly targeted advertising. In conjunction with a site that is aware of the user's real identity (for example an e-commerce site that requires authenticated credentials), this could allow oppressive groups to target individuals with greater accuracy than in a world with purely anonymous web usage.
There are a number of techniques that can be used to mitigate the risk of user tracking:
User agents may restrict access to the localStorage
objects to scripts originating at the domain of the active
document of the top-level traversable, for instance denying access to the
API for pages from other domains running in iframes.
User agents may, possibly in a manner configured by the user, automatically delete stored data after a period of time.
For example, a user agent could be configured to treat third-party local storage areas as session-only storage, deleting the data once the user had closed all the navigables that could access it.
This can restrict the ability of a site to track a user, as the site would then only be able to track the user across multiple sessions when they authenticate with the site itself (e.g. by making a purchase or logging in to a service).
However, this also reduces the usefulness of the API as a long-term storage mechanism. It can also put the user's data at risk, if the user does not fully understand the implications of data expiration.
If users attempt to protect their privacy by clearing cookies without also clearing data stored in the local storage area, sites can defeat those attempts by using the two features as redundant backup for each other. User agents should present the interfaces for clearing these in a way that helps users to understand this possibility and enables them to delete data in all persistent storage features simultaneously. [COOKIES]
User agents may allow sites to access session storage areas in an unrestricted manner, but require the user to authorize access to local storage areas.
User agents may record the origins of sites that contained content from third-party origins that caused data to be stored.
If this information is then used to present the view of data currently in persistent storage, it would allow the user to make informed decisions about which parts of the persistent storage to prune. Combined with a blocklist ("delete this data and prevent this domain from ever storing data again"), the user can restrict the use of persistent storage to sites that they trust.
User agents may allow users to share their persistent storage domain blocklists.
This would allow communities to act together to protect their privacy.
While these suggestions prevent trivial use of this API for user tracking, they do not block it altogether. Within a single domain, a site can continue to track the user during a session, and can then pass all this information to the third party along with any identifying information (names, credit card numbers, addresses) obtained by the site. If a third party cooperates with multiple sites to obtain such information, a profile can still be created.
However, user tracking is to some extent possible even with no cooperation from the user agent whatsoever, for instance by using session identifiers in URLs, a technique already commonly used for innocuous purposes but easily repurposed for user tracking (even retroactively). This information can then be shared with other sites, using visitors' IP addresses and other user-specific data (e.g. user-agent headers and configuration settings) to combine separate sessions into coherent user profiles.
User agents should treat persistently stored data as potentially sensitive; it's quite possible for emails, calendar appointments, health records, or other confidential documents to be stored in this mechanism.
To this end, user agents should ensure that when deleting data, it is promptly deleted from the underlying storage.
Because of the potential for DNS spoofing attacks, one cannot guarantee that a host claiming to be in a certain domain really is from that domain. To mitigate this, pages can use TLS. Pages using TLS can be sure that only the user, software working on behalf of the user, and other pages using TLS that have certificates identifying them as being from the same domain, can access their storage areas.
Different authors sharing one host name, for example users hosting content on the now defunct
geocities.com, all share one local storage object. There is no feature to
restrict the access by pathname. Authors on shared hosts are therefore urged to avoid using these
features, as it would be trivial for other authors to read the data and overwrite it.
Even if a path-restriction feature was made available, the usual DOM scripting security model would make it trivial to bypass this protection and access the data from any path.
The two primary risks when implementing these persistent storage features are letting hostile sites read information from other domains, and letting hostile sites write information that is then read from other domains.
Letting third-party sites read data that is not supposed to be read from their domain causes information leakage. For example, a user's shopping wishlist on one domain could be used by another domain for targeted advertising; or a user's work-in-progress confidential documents stored by a word-processing site could be examined by the site of a competing company.
Letting third-party sites write data to the persistent storage of other domains can result in information spoofing, which is equally dangerous. For example, a hostile site could add items to a user's wishlist; or a hostile site could set a user's session identifier to a known ID that the hostile site can then use to track the user's actions on the victim site.
Thus, strictly following the origin model described in this specification is important for user security.