javascript

Chapter 19: Client - Side Storage
Limits and Restrictions
As with other client - side data storage solutions, DOM Storage also has limitations. These limitations are
browser - specific. Generally speaking, the size limit for client - side data is set on a per - domain basis, so
each domain has a fixed amount of space in which to store its data. Analyzing the domain of the page
that is storing the data enforces this restriction, rather than the domain specified as part of
globalStorage .
In Firefox 2, there was no limit to the amount of data that could be saved; Firefox 3 introduced a 5MB
per - domain limit. IE 8 introduced its DOM Storage support with a limit of 10MB per domain.
Summary
The ability to store data on the client is an increasingly important capability for web applications. This
chapter covered the following aspects of client - side storage:
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Traditionally, such storage was limited to using cookies, small pieces of information that could
be set from the client or server and transmitted along with every request.
JavaScript provides access to cookies through document.cookie .
The limitations placed on cookies make them okay for storing small amounts of data, but
inefficient for storing large amounts.
IE provides a behavior called user data that can be applied to an element on the page as follows:
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Once applied, the element can load data from a named data store and make the information
accessible via the getAttribute() , setAttribute() , and removeAttribute() methods.
The data must be explicitly saved to a named data store using the save() method for it to
persist between sessions.
DOM Storage was first defined in the Web Applications 1.0 specification and later was absorbed into the
HTML 5 specification as described here :
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DOM Storage defines two objects to save data: sessionStorage and localStorage . The former
is used strictly to save data within a browser session, because the data is removed once the
browser is closed. The latter is used to persist data across sessions and based on cross -domain
security policies.
There is a third object, globalStorage , which has been implemented by Firefox as it was
defined in earlier versions of HTML 5.
The localStorage object replaced globalStorage in later versions, so the functionality is
very similar.
With all of these options available, it ’ s possible to store a significant amount of data on the client
machine using JavaScript. You should use care not to store sensitive information, because the data cache
isn ’ t encrypted.
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