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

16.3 Reading Cookies

When you use the cookie
property in

a
JavaScript expression, the value it returns is a string that contains
all the cookies that apply to the current document.[2] The
string is a list of
name=value
pairs separated by semicolons, where name
is the name of a cookie and value is its
string value. This value does not include any of the attributes that
may have been set for the cookie. To determine the value of a
particular named cookie, you can use the String.indexOf(
) and
String.substring( ) methods, or you can use
String.split( ) to break the string into
individual cookies.

[2] In Internet Explorer 3, the cookie
property works only for Document
objects that were retrieved using the HTTP protocol. Documents
retrieved from the local filesystem or via other protocols, such as
FTP, cannot have cookies associated with them.

Once you have extracted the value of a cookie from the
cookie property, you must interpret that value
based on whatever format or encoding was used by the cookie's
creator. For example, the cookie might store multiple pieces of
information in colon-separated fields. In this case, you would have
to use appropriate string methods to extract the various fields of
information. Don't forget to use the unescape(
) function on the cookie value if it was encoded using the
escape( ) function.

The following code shows how you might read the
cookie property, extract a single cookie from it,
and use the value of that cookie:

// Read the cookie property. This returns all cookies for this document.
var allcookies = document.cookie;
// Look for the start of the cookie named "version"
var pos = allcookies.indexOf("version=");
// If we find a cookie by that name, extract and use its value
if (pos != -1) {
var start = pos + 8;                       // Start of cookie value
var end = allcookies.indexOf(";", start);  // End of cookie value
if (end == -1) end = allcookies.length;
var value = allcookies.substring(start, end);  // Extract the value
value = unescape(value);                       // Decode it
// Now that we have the cookie value, we can use it.
// In this case, the cookie was previously set to the modification
// date of the document, so we can use it to see if the document has
// changed since the user last visited.
if (value != document.lastModified)
alert("This document has changed since you were last here");
}

Note that the
string
returned when you read the value of the cookie
property does not contain any information about the various cookie
attributes. The cookie property allows you to set
those attributes, but it does not allow you to read them.