Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

16.2 Storing Cookies

To associate a transient

cookie value
with the current document, simply set the cookie
property to a string of the form:

name=value

For example:

document.cookie = "version=" + escape(document.lastModified);

The next time you read the cookie property, the
name/value pair you stored is included in the list of cookies for the
document. Cookie values may not include semicolons, commas, or
whitespace. For this reason, you may want to
use the JavaScript escape(
) function to encode the value before storing it in the
cookie. If you do this, you'll have to use the corresponding
unescape( ) function when you read the cookie
value.

A cookie written as
described above lasts for the current web-browsing session but is
lost when the user exits the browser. To create a cookie that can
last across browser sessions, include an expiration date by setting
the expires attribute. You can do this by setting
the cookie property to a string of the form:

name=value; expires=date

When setting an expiration date like this,
date should be a date specification in the
format written by Date.toGMTString(
). For example, to create a cookie that
persists for a year, you can use code like this:

var nextyear = new Date(  );
nextyear.setFullYear(nextyear.getFullYear(  ) + 1);
document.cookie = "version=" + document.lastModified +
"; expires=" + nextyear.toGMTString(  );

Similarly, you can set the
path,
domain, and

secure attributes
of a cookie by appending strings of the following format to the
cookie value before that value is written to the
cookie property:

; path=path
; domain=domain
; secure

To change the value of a cookie, set its value again, using the same
name and the new value. Use whatever values are appropriate for
expires, path, and the other
attributes.
To
delete
a cookie, set it again using the same name, an arbitrary value, and
an expiration date that has already passed. Note that the browser is
not required to delete expired cookies immediately, so a cookie may
remain in the browser's cookie file past its expiration date.

16.2.1 Cookie Limitations

Cookies are intended for infrequent
storage of small amounts of data. They are not intended as a
general-purpose communication or data-transfer mechanism, so you
should use them in moderation. Web browsers are not required to
retain more than 300 cookies total, 20 cookies per web server (for
the entire server, not just for your page or site on the server), or
4 kilobytes of data per cookie (both name and value count toward this
4-kilobyte limit). The most restrictive of these is the 20 cookies
per server limit. In order to avoid reaching that limit, you may want
to avoid using a separate cookie for each state variable you want to
save. Instead, you can encode several related state variables into a
single named cookie. Example 16-1, later in this
chapter, shows one way that this can be
done.