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Understanding JavaScript

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This chapter from HTML, CSS and JavaScript All in One, Sams Teach Yourself: Covering HTML5, CSS3, and jQuery, 2nd Edition introduces the concept of web scripting and the JavaScript language. As the chapter moves ahead, you’ll learn how to include JavaScript commands directly in your HTML documents, and how your scripts will be executed when the page is viewed in a browser. You will work with a simple script, edit it, and test it in your browser, all the while learning the basic tasks involved in creating and using JavaScript scripts.
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The World Wide Web (WWW) began as a text-only medium—the first browsers didn’t even support images within web pages. The Web has come a long way since those early days. Today’s websites include a wealth of visual and interactive features in addition to useful content: graphics, sounds, animation, and video. Web scripting languages, such as JavaScript, are one of the easiest ways to spice up a web page and to interact with users in new ways.

The first part of this chapter introduces the concept of web scripting and the JavaScript language. As the chapter moves ahead, you’ll learn how to include JavaScript commands directly in your HTML documents, and how your scripts will be executed when the page is viewed in a browser. You will work with a simple script, edit it, and test it in your browser, all the while learning the basic tasks involved in creating and using JavaScript scripts.

Learning Web Scripting Basics

You already know how to use one type of computer language: HTML. 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. But 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. JavaScript is a web scripting language that enables you to combine scripting with HTML to create interactive web pages.

Scripts and Programs

A movie or a play follows a script—a 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 nearly 20 years ago by Netscape Communications Corporation, the maker of the long-defunct 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.

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