Sams Teach Yourself JavaScript in 24 Hours

Learning Web Scripting Basics

In the world of science fiction movies (and many other movies that have no excuse), computers are often seen obeying commands in English. Although this might indeed happen in the near future, computers currently find it easier to understand languages such as BASIC, C, and Java.

If you know how to use HTML (Hypertext Markup Language) to create a web document, you've already worked with one computer language. You use HTML tags to describe how you want your document formatted, and the browser obeys your commands and shows the formatted document to the user.

Because HTML is a simple text markup language, it can't respond to the user, make decisions, or automate repetitive tasks. Interactive tasks such as these require a more sophisticated language: a programming language, or a scripting language.

Although many programming languages are complex, scripting languages are generally simple. They have a simple syntax, can perform tasks with a minimum of commands, and are easy to learn. Web scripting languages enable you to combine scripting with HTML to create interactive web pages.

Scripts and Programs

A movie or a play follows a scripta list of actions (or lines) for the actors to perform. A web script provides the same type of instructions for the web browser. A script in JavaScript can range from a single line to a full-scale application. (In either case, JavaScript scripts usually run within a browser.)

Some programming languages must be compiled, or translated, into machine code before they can be executed. JavaScript, on the other hand, is an interpreted language: The browser executes each line of script as it comes to it.

There is one main advantage to interpreted languages: Writing or changing a script is very simple. Changing a JavaScript script is as easy as changing a typical HTML document, and the change is enacted as soon as you reload the document in the browser.

Introducing JavaScript

JavaScript was developed by Netscape Communications Corporation, the maker of the Netscape web browser. JavaScript was the first web scripting language to be supported by browsers, and it is still by far the most popular.

JavaScript is almost as easy to learn as HTML, and it can be included directly in HTML documents. Here are a few of the things you can do with JavaScript:

• Display messages to the user as part of a web page, in the browser's status line, or in alert boxes

• Validate the contents of a form and make calculations (for example, an order form can automatically display a running total as you enter item quantities)

• Animate images or create images that change when you move the mouse over them

• Create ad banners that interact with the user, rather than simply displaying a graphic

• Detect the browser in use or its features and perform advanced functions only on browsers that support them

• Detect installed plug-ins and notify the user if a plug-in is required

• Modify all or part of a web page without requiring the user to reload it

• Display or interact with data retrieved from a remote server

You can do all this and more with JavaScript, including creating entire applications. We'll explore the uses of JavaScript throughout this book.