Cookies are the JavaScript mechanism for storing information for a long period of time. They are strings that are stored in files on the hard drives of surfers' computers. Cookies are the only mechanism by which JavaScript programs can read and write information to a surfer's hard drive.

Cookies come in handy when you want information generated during one visit to your site to be available during another visit to your site, or when you want information generated on one page of your site to be available to scripts on another page of your site. Let's take a look at the mechanics of setting cookies.

Cookies are created by setting the document.cookie property equal to a properly formatted string. When you create a cookie, you must specify at least its name and its value. Additionally, you can specify some other characteristics of the cookie. If you want to make a cookie persist beyond the current browser session, you must specify an expiry date for the cookie.

The syntax for setting a no-frills cookie with only a name-value pair looks like this:

document.cookie = "name=value";

Then, to make the cookie persistent, you have to specify a date and time, as formatted by the toGMTString() method of the Date object. So, create a date, apply the toGMTString() method to it, and concatenate the results onto the name-value pair, separating the two with a semicolon.

Therefore, if you had stored the results of your toGMTString() operation in a variable called expiryDate, your cookie-setting statement would look like this:

document.cookie = "name=value;" + expiryDate;