Before we proceed here are some terms you should know. Web pages are loaded from Web servers to Web clients, where they're displayed. JavaScript has several built-in functions, but you can extend the list by writing your own. A script is nothing more than a sequence of program instructions or statements. The program steps through the statements one at a time and performs whatever the script tells it.
Scripts tend to have simple rules and require less learning time.
There are several versions of JavaScript supported by certain browsers and browser versions which leads to confusion and incompatibilities. When Netscape introduced JavaScript 1.0, it was supported in Navigator 2.0. Subsequently, Navigator 3.0 supported new enchancements which comprised JavaScript 1.1. At present, Navigator 4.0 supports JavaScript 1.2. Microsoft's JScript was designed to be compatible with Netscape's JavaScript, however there are differences.