J is for Javascript

Javascript (not to be confused with the programming language, Java) is a scripting language.

It is also client-side, which means it is executed in your browser, and there are no calls made to the server where your website is stored. So once the webpage is loaded into the viewer's browser, the javascript routines are ready to work.

Javascript gives you the means of introducing some action into a static webpage.

For example, click on the J-shaped safety pin. A light bulb appears. Move the mouse away and the safety pin returns.

Many javascript functions, behaviours, action or routines (call them what you like) are initiated by the mouse, such as on mouse click, on mouse over, on mouse out, on mouse up, and on mouse down.

Some of the most common are to swap an image, pop up a new window, play a sound, and hide a part of the screen.

The rollover button is a very commonly used javascript function.