Cookies

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:

ppkcookie1=testcookie

If I want to set another cookie, I again do

document.cookie =
  'ppkcookie2=another test; expires=Fri, 3 Aug 2001 20:47:11 UTC; path=/'

The first cookie is not overwritten, as it would when document.cookie would be a real string. Instead the second one is added to document.cookie, so if we read it out we get

ppkcookie1=testcookie; ppkcookie2=another test

If I reset a cookie

document.cookie =
  'ppkcookie2=yet another test; expires=Fri, 3 Aug 2001 20:47:11 UTC; path=/'

the old cookie is overwritten and document.cookie reads

ppkcookie1=testcookie; ppkcookie2=yet another test

To read out a cookie you have to treat document.cookie as a string and search for certain characters (semicolons, for instance) and for the cookie name. I'll explain how to do it below.

Finally, to remove a cookie, set it with an expiry date before today. The browser sees that the cookie has expired and removes it.

document.cookie =
  'ppkcookie2=yet another test; expires=Fri, 27 Jul 2001 02:47:11 UTC; path=/'

Example

If you're thoroughly confused by all this strange syntax, try the example below. You can set two cookies, ppkcookie1 and ppkcookie2. Fill in the desired value in the text box.

Create cookie 1
Read cookie 1
Erase cookie 1.

Create cookie 2
Read cookie 2
Erase cookie 2.

For comparision, read out document.cookie.

I set the cookies to remain active for seven days. If you return to this page within that time, you'll get an alert that the cookie(s) is/are still active. Try it by setting a cookie, then reloading this page.

The scripts

These are the three scripts you need.

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=/";
}

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;
}

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

Explanation

The functions are not very difficult, the hardest part is creating the correct syntax for setting a cookie.

createCookie

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.

createCookie('ppkcookie','testcookie',7)

If you set the number of days to 0 the cookie is trashed when the user closes the browser. If you set the days to a negative number the cookie is trashed immediately.

The function receives the arguments and starts doing its job.

function createCookie(name,value,days) {

First of all see if there is a days value. If there isn't we don't need to do the time calculation.

	if (days) {

If there is, create a new Date object containing the current date.

		var date = new Date();

Now get the current Time (in milliseconds) and add the required number of days (in milliseconds). Set the Time of the date to this new value, so that it now contains the date in milliseconds that the cookie should expire.

		date.setTime(date.getTime()+(days*24*60*60*1000));

Set the variable expires to this date in the UTC/GMT format required by cookies.

		var expires = "; expires="+date.toGMTString();
	}

If 0 is passed to the function, expires is not set and the cookie expires when the user closes his browser..

	else var expires = "";

Finally write the new cookie into document.cookie in the correct syntax.

	document.cookie = name+"="+value+expires+"; path=/";
}

Cookie created.

readCookie

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.

var x = readCookie('ppkcookie1')
if (x) {
	[do something with x]
}

The function receives the argument and starts.

function readCookie(name) {

We're going to search for the name of the cookie, followed by an =. So create this new string and put it in nameEQ:

	var nameEQ = name + "=";

Then split document.cookie on semicolons. ca becomes an array containing all cookies that are set for this domain and path.

	var ca = document.cookie.split(';');

Then we go through the array (so through all cookies):

	for(var i=0;i < ca.length;i++) {

Set c to the cookie to be checked.

		var c = ca[i];

If the first character is a space, remove it by using the substring() method. Continue doing this until the first character is not a space.

		while (c.charAt(0)==' ') c = c.substring(1,c.length);

Now string c begins with the name of the current cookie. If this is the name of the desired cookie

		if (c.indexOf(nameEQ) == 0)

we've found what we were looking for. We now only need to return the value of the cookie, which is the part of c that comes after nameEQ. By returning this value we also end the function: mission accomplished.

		if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
	}

If, after having gone through all cookies, we haven't found the name we're looking for, the cookie is not present. We return null.

	return null;
}

Cookie read.

eraseCookie

Erasing is extremely simple.

eraseCookie('ppkcookie')

Pass the name of the cookie to be erased

function eraseCookie(name) {

and call createCookie() to set the cookie with an expiry date of one day ago.

	createCookie(name,"",-1);
}

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