localStorage object provides a
Storage object for an origin.
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 really noticing.
To address this, this specification introduces the sessionStorage IDL attribute. 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 may 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 IDL attribute 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.
Support: namevalue-storageChrome for Android 56+Chrome 4+UC Browser for Android 11+iOS Safari 3.2+Firefox 3.5+IE 8+Samsung Internet 4+Opera Mini NoneAndroid Browser 2.1+Safari 4+Edge 12+Opera 10.5+
Source: caniuse.com
Storage interfaceinterface Storage {
readonly attribute unsigned long length;
DOMString? key(unsigned long index);
getter DOMString? getItem(DOMString key);
setter void setItem(DOMString key, DOMString value);
deleter void removeItem(DOMString key);
void clear();
};
Each Storage object provides access to a list of key/value pairs, which are
sometimes called items. Keys are strings. Any string (including the empty string) is a valid key.
Values are similarly strings.
Each Storage object is associated with a list of key/value pairs when it is
created, as defined in the sections on the sessionStorage
and localStorage attributes. Multiple separate objects
implementing the Storage interface can all be associated with the same list of
key/value pairs simultaneously.
The length attribute must return the number
of key/value pairs currently present in the list associated with the object.
The key(n) method must
return the name of the nth key in the list. The order of keys is user-agent
defined, but must be consistent within an object so long as the number of keys doesn't change.
(Thus, adding or removing a key may change the order of the keys, but merely
changing the value of an existing key must not.) If n is
greater than or equal to the number of key/value pairs
in the object, then this method must return null.
The supported property names on a Storage object are the keys of each
key/value pair currently present in the list associated with the object, in the order that the
keys were last added to the storage area.
The getItem(key) method
must return the current value associated with the given key. If the given key does not exist in the list associated with the object then this method must
return null.
The setItem(key, value) method must first check if a key/value pair with the given key already exists in the list associated with the object.
If it does not, then a new key/value pair must be added to the list, with the given key and with its value set to value.
If the given key does exist in the list, and its value is not equal to value, then it must have its value updated to value. If its previous value is equal to value, then the method must do nothing.
If it couldn't set the new value, the method must throw a
"QuotaExceededError" DOMException exception. (Setting could
fail if, e.g., the user has disabled storage for the site, or if the quota has been exceeded.)
The removeItem(key)
method must cause the key/value pair with the given key to be removed from the
list associated with the object, if it exists. If no item with that key exists, the method must do
nothing.
The setItem() and removeItem() methods must be atomic with respect to failure.
In the case of failure, the method does nothing. That is, changes to the data storage area must
either be successful, or the data storage area must not be changed at all.
The clear() method must atomically cause the
list associated with the object to be emptied of all key/value pairs, if there are any. If there
are none, then the method must do nothing.
When the setItem(), removeItem(), and clear() methods are invoked, events are fired on the
Window objects of other Documents that can access the newly stored or
removed data, as defined in the sections on the sessionStorage and localStorage attributes.
This specification does not require that the above methods wait until the data has been physically written to disk. Only consistency in what different scripts accessing the same underlying list of key/value pairs see is required.
sessionStorage attribute[NoInterfaceObject]
interface WindowSessionStorage {
readonly attribute Storage sessionStorage;
};
Window implements WindowSessionStorage;
The sessionStorage attribute represents the
set of storage areas specific to the current top-level browsing context.
Each top-level browsing context has a unique set of session storage areas, one for each origin.
User agents should not expire data from a browsing context's session storage areas, but may do so when the user requests that such data be deleted, or when the UA detects that it has limited storage space, or for security reasons. User agents should always avoid deleting data while a script that could access that data is running. When a top-level browsing context is destroyed (and therefore permanently inaccessible to the user) the data stored in its session storage areas can be discarded with it, as the API described in this specification provides no way for that data to ever be subsequently retrieved.
The lifetime of a browsing context can be unrelated to the lifetime of the actual user agent process itself, as the user agent can support resuming sessions after a restart.
When a new Document is created in a browsing context which has a
top-level browsing context, the user agent must check to see if that top-level
browsing context has a session storage area for that document's origin. If it
does, then that is the Document's assigned session storage area. If it does not, a
new storage area for that document's origin must be created, and then that
is the Document's assigned session storage area. A Document's assigned
storage area does not change during the lifetime of a Document.
In the case of an iframe being moved to another
Document, the nested browsing context is destroyed and a new one created.
The sessionStorage attribute must return a
Storage object associated with the Document's assigned session storage
area, if any, or null if there isn't one. Each Document object must have a separate
object for its Window's sessionStorage
attribute.
When a new top-level browsing context is created by cloning an existing browsing context, the new browsing context must start with the same session storage areas as the original, but the two sets must from that point on be considered separate, not affecting each other in any way.
When a new top-level browsing context is created by a script in an existing browsing context, or by the user
following a link in an existing browsing context, or in some other way related to a specific
Document, and the creation is not a new start for session storage, then the session
storage area of the origin of that Document must be copied into the new
browsing context when it is created. From that point on, however, the two session storage areas
must be considered separate, not affecting each other in any way.
When the setItem(), removeItem(), and clear() methods are called on a Storage object x that is associated with a session storage area, if the methods did not throw an
exception or "do nothing" as defined above, then for every Document object whose
Window object's sessionStorage attribute's
Storage object is associated with the same storage area, other than x, send a storage notification.
localStorage attribute[NoInterfaceObject]
interface WindowLocalStorage {
readonly attribute Storage localStorage;
};
Window implements WindowLocalStorage;
The localStorage object provides a
Storage object for an origin.
User agents must have a set of local storage areas, one for each origin.
User agents should expire data from the local storage areas only for security reasons or when requested to do so by the user. User agents should always avoid deleting data while a script that could access that data is running.
When the localStorage attribute is accessed, the user
agent must run the following steps, which are known as the
Storage object
initialization steps:
The user agent may throw a "SecurityError"
DOMException and abort these steps instead of returning a Storage
object if the request violates a policy decisions (e.g. if the user agent is configured to not
allow the page to persist data).
If the Document's origin is an opaque origin, then throw a
"SecurityError" DOMException and abort these
steps.
Check to see if the user agent has allocated a local storage area for the
origin of the Document of the Window object on which the
attribute was accessed. If it has not, create a new storage area for that
origin.
Return the Storage object associated with that origin's local storage area.
Each Document object must have a separate object for its Window's localStorage attribute.
When the setItem(), removeItem(), and clear() methods are called on a Storage object x that is associated with a local storage area, if the methods did not throw an
exception or "do nothing" as defined above, then for every Document object whose
Window object's localStorage attribute's
Storage object is associated with the same storage area, other than x, send a storage notification.
The localStorage attribute provides
access to shared state. This specification does not define the interaction with other browsing
contexts 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.
storage eventThe storage event is fired on a Document's
Window object when a storage area changes, as described in the previous two sections
(for session storage, for local
storage).
When a user agent is to send a storage notification for a Document, the
user agent must queue a task to fire an
event named storage at the Document object's Window
object, using StorageEvent.
Such a Document object 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 task source for these tasks is the DOM manipulation task source.
If the event is being fired due to an invocation of the setItem() or removeItem() methods, the event must have its key attribute initialized to the name of the key in question,
its oldValue attribute initialized to the old
value of the key in question, or null if the key is newly added, and its newValue attribute initialized to the new value of the
key in question, or null if the key was removed.
Otherwise, if the event is being fired due to an invocation of the clear() method, the event must have its key, oldValue,
and newValue attributes initialized to null.
In addition, the event must have its url attribute
initialized to the URL of the document whose
Storage object was affected; and its storageArea attribute initialized to the
Storage object from the Window object of the target
Document that represents the same kind of Storage area as was affected
(i.e. session or local).
StorageEvent interface[Constructor(DOMString type, optional StorageEventInit eventInitDict)]
interface StorageEvent : Event {
readonly attribute DOMString? key;
readonly attribute DOMString? oldValue;
readonly attribute DOMString? newValue;
readonly attribute USVString url;
readonly attribute Storage? storageArea;
};
dictionary StorageEventInit : EventInit {
DOMString? key = null;
DOMString? oldValue = null;
DOMString? newValue = null;
USVString url = "";
Storage? storageArea = null;
};
The key attribute must return the value
it was initialized to. It represents the key being changed.
The oldValue attribute must return
the value it was initialized to. It represents the old value of the key being changed.
The newValue attribute must return
the value it was initialized to. It represents the new value of the key being changed.
The url attribute must return the value
it was initialized to. It represents the URL of the document whose key changed.
The storageArea attribute must
return the value it was initialized to. It represents the Storage object that was
affected.
User agents should limit the total amount of space allowed for storage areas, because hostile authors could otherwise use this feature to exhaust the user's available disk space.
User agents should guard against sites storing data under their origin's other affiliated sites, e.g. storing up to the limit in a1.example.com, a2.example.com, a3.example.com, etc, circumventing the main example.com storage limit.
User agents may prompt the user when quotas are reached, allowing the user to grant a site more space. This enables sites to store many user-created documents on the user's computer, for instance.
User agents should allow users to see how much space each domain is using.
A mostly arbitrary limit of five megabytes per origin is suggested. Implementation feedback is welcome and will be used to update this suggestion in the future.
For predictability, quotas should be based on the uncompressed size of data stored.
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 browsing
context, 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 browsing contexts 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 e-mails, 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.