Sometimes you want your web pages to do a little more than just display text and graphics. If you want your pages to offer some sort of interaction with the user then you need to add a piece of program code (or script) that either runs on the computer that the user is using (a client-side script) or runs on the main computer belonging to the ISP (a server-side script).

There are different programming languages for writing these scripts ... this page deals with a language called JavaScript (originally developed by Netscape Communications).

JavaScript is an interpreted language. You type the script in as part of the HTML code for that web page and the browser interprets that script (along with the HTML tags) to produce a finished web page.

The HTML code that you type in is interpreted by the browser to produce a web page. The same HTML code will always produce the same page: so, generally speaking, to change the web page you have to change the underlying HTML.
A piece of JavaScript, however, will allow you to change small parts of the web page without having to change the HTML. This means that the page can respond to the user without having to reload.

The text shown here is very similar to the JavaScript used to run the Fortune Cookie machine shown above.

1. The first line of code <script> indicates that the following lines will be in JavaScript.
2. The line function Fortune() { defines the start of a module of code called Fortune.
3. Then var rn_value=0; defines a special container in the computer's memory and calls it rn_value.
4. Then a random number (between 0 and 1) is generated and stored in the rn_value container: rn_value = Math.random();

The remaining lines all do similar things. The value of the random number is checked - and depending on its size a different fortune cookie is chosen:

5. The line if (rn_value < 0.2) { tests the random number to see if it is less than 0.2
6. If it is less than 0.2 then the next line, document.frm.fort_image.src="fort1.jpg", is run and the flow of control jumps to the </script> line.
   This line selects the cookie image and changes it to one containg an appropriate piece of text.
7. If the value is greater or equal to 0.2 then the program jumps to the next else line and so on.