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JavaScript and Cookies

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What are Cookies ?

Web Browser and Server use HTTP protocol to communicate and HTTP is a stateless protocol. But for a commercial website it is required to maintain session information among different pages. For example one user registration ends after completing many pages. But how to maintain user's session information across all the web pages.

In many situations, using cookies is the most efficient method of remembering and tracking preferences, purchases, commissions, and other information required for better visitor experience or site statistics.

How It Works ?

Your server sends some data to the visitor's browser in the form of a cookie. The browser may accept the cookie. If it does, it is stored as a plain text record on the visitor's hard drive. Now, when the visitor arrives at another page on your site, the browser sends the same cookie to the server for retrieval. Once retrieved, your server knows/remembers what was stored earlier.

Cookies are a plain text data record of 5 variable-length fields:

Cookies were originally designed for CGI programming and cookies' data is automatically transmitted between the web browser and web server, so CGI scripts on the server can read and write cookie values that are stored on the client.

JavaScript can also manipulate cookies using the cookie property of the Document object. JavaScript can read, create, modify, and delete the cookie or cookies that apply to the current web page.

Storing Cookies:

The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this:

Syntax:

document.cookie = "key1=value1;key2=value2;expires=date";

Here expires attribute is option. If you provide this attribute with a valid date or time then cookie will expire at the given date or time and after that cookies' value will not be accessible.

Note: 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 will also have to use the corresponding unescape() function when you read the cookie value.

Example:

Following is the example to set a customer name in input cookie.

<html>
<head>
<script type="text/javascript">
<!--
function WriteCookie()
{
   if( document.myform.customer.value == "" ){
      alert("Enter some value!");
      return;
   }

   cookievalue= escape(document.myform.customer.value) + ";";
   document.cookie="name=" + cookievalue;
   alert("Setting Cookies : " + "name=" + cookievalue );
}
//-->
</script>
</head>
<body>
<form name="myform" action="">
Enter name: <input type="text" name="customer"/>
<input type="button" value="Set Cookie" onclick="WriteCookie();"/>
</form>
</body>
</html>

This will produce following result. Now enter something in the text box and press the button "Set Cookie" to set the cookies.

Enter name:

Now your machine has a cookie called name. You can set multiple cookies using multiple key=value pairs separated by comma.

You will learn how to read this cookie in next section.

Reading Cookies:

Reading a cookie is just as simple as writing one, because the value of the document.cookie object is the cookie. So you can use this string whenever you want to access the cookie.

The document.cookie string will keep a list of name=value pairs separated by semicolons, where name is the name of a cookie and value is its string value.

You can use strings' split() function to break the string into key and values as follows:

Example:

Following is the example to get the cookies set in previous section.

<html>
<head>
<script type="text/javascript">
<!--
function ReadCookie()
{
   var allcookies = document.cookie;
   alert("All Cookies : " + allcookies );

   // Get all the cookies pairs in an array
   cookiearray  = allcookies.split(';');

   // Now take key value pair out of this array
   for(var i=0; i<cookiearray.length; i++){
      name = cookiearray[i].split('=')[0];
      value = cookiearray[i].split('=')[1];
      alert("Key is : " + name + " and Value is : " + value);
   }
}
//-->
</script>
</head>
<body>
<form name="myform" action="">
<input type="button" value="Get Cookie" onclick="ReadCookie()"/>
</form>
</body>
</html>

Note: Here length is a method of Array class which returns the length of an array. We will discuss Arrays in a separate chapter. By that time please try to digest it.

This will produce following result. Now press the button "Get Cookie" to see the cookies which you have set in previous section.

Note: There may be some other cookies already set on your machine. So above code will show you all the cookies set at your machine.

Setting the Cookies Expiration Date:

You can extend the life of a cookie beyond the current browser session by setting an expiration date and saving the expiration date within the cookie. This can be done by setting the expires attribute to a date and time.

Example:

The following example illustrates how to set cookie expiration date after 1 Month :

<html>
<head>
<script type="text/javascript">
<!--
function WriteCookie()
{
   var now = new Date();
   now.setMonth( now.getMonth() + 1 ); 
   cookievalue = escape(document.myform.customer.value) + ";"
   document.cookie="name=" + cookievalue;
   document.cookie = "expires=" + now.toUTCString() + ";"
   alert("Setting Cookies : " + "name=" + cookievalue );
}
//-->
</script>
</head>
<body>
<form name="formname" action="">
Enter name: <input type="text" name="customer"/>
<input type="button" value="Set Cookie" onclick="WriteCookie()"/>
</form>
</body>
</html>

Deleting a Cookie:

Sometimes you will want to delete a cookie so that subsequent attempts to read the cookie return nothing. To do this, you just need to set the expiration date to a time in the past.

Example:

The following example illustrates how to delete cookie by setting expiration date one Month in past :

<html>
<head>
<script type="text/javascript">
<!--
function WriteCookie()
{
   var now = new Date();
   now.setMonth( now.getMonth() - 1 ); 
   cookievalue = escape(document.myform.customer.value) + ";"
   document.cookie="name=" + cookievalue;
   document.cookie = "expires=" + now.toUTCString() + ";"
   alert("Setting Cookies : " + "name=" + cookievalue );
}
//-->
</script>
</head>
<body>
<form name="formname" action="">
Enter name: <input type="text" name="customer"/>
<input type="button" value="Set Cookie" onclick="WriteCookie()"/>
</form>
</body>
</html>

Note: Instead of setting date, you can see new time using setTime() function.


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