JavaScript and Forms icon

Using Cookies

Richard L. Bowman, PhD ( )
Harrisonburg, VA, USA

I. Introduction

Since values for variables only last as long as the particular web page is displayed, it would be useful to have a method to store small amounts of information on the users computer that can be accessed by a web page as necessary. Cookies are the answer! Browsers do not have to retain more than 300 cookies at any one time nor more than 20 cookies per any one server nor more than 4 KB of information per cookie. So cookies are not the solution for storing large amounts of data. In our interest in JavaScript, cookies are useful to store information about a user's preferences for how our web pages are displayed or as a temporary storage for values while a web page is re-rendered by the browser.

II. Things to Remember

  1. A cookie will have a name and a stored value (a string of information) plus it may have four optional attributes. The first attribute is expires which gives the maximum length of time the cookie is to survive on the user's computer if not deleted by some other means. The second attribute is path. A cookie is useful only for the web page in which it is created or any web page in the same directory. A path can be specified which will overwrite this limitation, e.g., using a "/" allows any web page on the same server to use that cookie. The domain attribute can be used to allow any server at a particular domain to use that cookie. For example, a domain of "" would let all servers at Bridgewater College to use a particular cookie. The secure attribute when set tells the server to exchange cookie information only through a secure encryption scheme. For client-side JavaScript, this attribute does not need to be set.
  2. In order not to run up against the limit of 20 cookies per server on a user's machine, the preferred protocol is not to use more than one cookie per web page. Instead, several items of information can be appended into one string using a colon, semi-colon or comma to separate the individual items of info.
  3. The value for a particular cookie may not contain commas, semi-colons or spaces. However, these can be stored easily in by encrypting the value for the cookie using the escape() function and then using the unescape() function to decipher the value of a stored cookie before it is used.

III. An Example and an Assignment

All of the simulations on my "Interactive Science Activities on the Web" site at: use cookies to pass information from a previous page to the next page. Go to that site and examine the "Planetary Orbit Simulation." It passes five numbers to the next page which does the actual plotting of the planetary orbit.

When you are ready for a challenge and some fun, take this simulation and modify it for your purposes. Begin by stripping out a lot of the non-critical code, e.g., the Site Meter code at the bottom. Hope you enjoy it and do not get too many headaches!

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Created and maintained by: Richard L. Bowman ( ); last updated: 31-Mar-11.