What is Cookie?

Cookies are pieces of data created when you visit a website, and contain a unique, anonymous number. They are stored in the cookie directory of your hard drive, and do not expire at the end of your session.

Cookies let us know when you return to our website and what pages or services you use when you're there. Our cookies aren't used to store or collect any personal information. It only lets us know that someone with your unique cookie has returned to our website.

By using cookies we will be able to see how our website is being used. This means we'll be able to identify the most popular areas of our website and make it easier for you to access them. Cookies help us to be more efficient as we can learn what information is important to our customers and what isn't.

How cookies work

A cookie is nothing but a small text file that's stored in your browser.That means this file is stored in the user machine.It contains some data:

1. A name-value pair containing the actual data
2. An expiry date after which it is no longer valid
3. The domain and path of the server it should be sent to

As soon as you request a page from a server to which a cookie should be sent, the cookie is added to the HTTP header. Server side programs can then read out the information and decide that you have the right to view the page you requested or that you want your links to be yellow on a green background.

So every time we visit the site the cookie comes from, information about you is available. This is very nice sometimes, at other times it may somewhat endanger your privacy. Fortunately more and more browsers give you the opportunity to manage your cookies (deleting the one from the big ad site, for example).

Cookies can be read by JavaScript too. They're mostly used for storing user preferences.

name-value

Each cookie has a name-value pair that contains the actual information. The name of the cookie is for your benefit, you will search for this name when reading out the cookie information.

If you want to read out the cookie you search for the name and see what value is attached to it. Read out this value. Of course you yourself have to decide which value(s) the cookie can have and to write the scripts Expiry date

Each cookie has an expiry date after which it is trashed. If you don't specify the expiry date the cookie is trashed when you close the browser. This expiry date should be in UTC (Greenwich) time in the format created by the Date.toGMTString() method Domain and path

Each cookie also has a domain and a path. The domain tells the browser to which domain the cookie should be sent. If you don't specify it, it becomes the domain of the page that sets the cookie, in the case of this page www.quirksmode.org. Please note that the purpose of the domain is to allow cookies to cross sub-domains. My cookie will not be read by search.quirksmode.org because its domain is www.quirksmode.org . When I set the domain to quirksmode.org, the search sub-domain may also read the cookie. I cannot set the cookie domain to a domain I'm not in, I cannot make the domain www.microsoft.com . Only quirksmode.org is allowed, in this case.

The path gives you the chance to specify a directory where the cookie is active. So if you want the cookie to be only sent to pages in the directory cgi-bin, set the path to /cgi-bin. Usually the path is set to /, which means the cookie is valid throughout the entire domain. This script does so, so the cookies you can set on this page will be sent to any page in the www.quirksmode.org domain (though only this page has a script that searches for the cookies and does something with them).

document.cookie

Cookies can be created, read and erased by JavaScript. They are accessible through the property document.cookie. Though you can treat document.cookie as if it's a string, it isn't really, and you have only access to the name-value pairs.

If I want to set a cookie for this domain with a name-value pair 'ppkcookie1=testcookie' that expires in seven days from the moment I write this sentence, I do

document.cookie = 'ppkcookie1=testcookie; expires=Thu, 2 Aug 2001 20:47:11 UTC; path=/'

1. First the name-value pair ('ppkcookie1=testcookie')
2. then a semicolon and a space
3. then the expiry date in the correct format
('expires=Thu, 2 Aug 2001 20:47:11 UTC')
4. again a semicolon and a space
5. then the path (path=/)

This is a very strict syntax, don't change it! (Of course the script manages these dirty bits for you)

Also, even though it looks like I'm writing this whole string to the string document.cookie, as soon as I read it out again I only see the name-value pair:

We are giving some examples about the cookies operation.

createCookie

function createCookie(name,value,days) {
if (days) {
var date = new Date();
date.setTime(date.getTime()+(days*24*60*60*1000));
var expires = "; expires="+date.toGMTString();
}
else var expires = "";
document.cookie = name+"="+value+expires+"; path=/";
}

When calling createCookie() you have to give it three bits of information: the name and value of the cookie and the number of days it is to remain active. In this case the name-value pair should become ppkcookie=testcookie and it should be active for 7 days.

readCookie

function readCookie(name) {
var nameEQ = name + "=";
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) { var c = ca[i]; while (c.charAt(0)==' ') c = c.substring(1,c.length); if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length); } return null; }

To read out a cookie, call this function and pass the name of the cookie. Put the name in a variable. First check if this variable has a value (if the cookie does not exist the variable becomes null, which might upset the rest of your function), then do whatever is necessary.

eraseCookie

Erasing is extremely simple.

function eraseCookie(name) {
createCookie(name,"",-1);
}

The browser, seeing that the expiry date has passed, immediately removes the cookie.