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Chapter 7. JavaScript

Looking at the title of this chapter, you probably said to yourself, "JavaScript? What does that have to do with CGI programming or Perl?" It's true that JavaScript is not Perl, and it cannot be used to write CGI scripts.[9] However, in order to develop powerful web applications we need to learn much more than CGI itself. Therefore, our discussion has already covered HTTP and HTML forms and will later cover email and SQL. JavaScript is yet another tool that, although not fundamental to creating CGI scripts, can help us create better web applications.

[9]Some web servers do support server-side JavaScript, but not via CGI.

In this chapter, we will focus on three specific applications of JavaScript: validating user input in forms; generating semiautonomous clients; and bookmarklets. As we will soon see, all three of these examples use JavaScript on the client side but still rely on CGI scripts on the server side.

This chapter is not intended to be an introduction to JavaScript. Since many web developers learn HTML and JavaScript before turning to Perl and CGI, we will assume you've had some exposure to JavaScript already. If you haven't, or if you are interested in learning more, you may wish to refer to JavaScript: The Definitive Guide by David Flanagan (O'Reilly & Associates, Inc.).

7.1. Background

Before we get started, let's discuss the background of JavaScript. As we said, we'll skip the introduction to JavaScript programming, but we should clear up possible confusions about what we mean when we refer to JavaScript and how JavaScript relates to similar technologies.

7.1.1. History

JavaScript was originally developed for Netscape Navigator 2.0. JavaScript has very little to do with Java despite the similarity in names. The languages were developed independently, and JavaScript was originally called LiveScript. However Sun Microsystems (the creator of Java) and Netscape struck a deal, and LiveScript was renamed to JavaScript shortly before its release. Unfortunately, this single marketing decision has confused many who believe that Java and JavaScript are more similar than they are.

Microsoft later created their own JavaScript implementation for Internet Explorer 3.0, which they called JScript . Initially, JScript was mostly compatible with JavaScript, but then Netscape and Microsoft developed their languages in different directions. The dynamic behavior provided in the latest versions of these languages is now very different.

Fortunately, there have been efforts to standardize these languages via ECMAScript and DOM. ECMAScript is an ECMA standard that defines the syntax and structure of the language that JScript and JavaScript will become. ECMAScript itself is not specific to the Web and is not directly useful as a language because it doesn't do anything; it only defines a few very basic objects. That's where the Document Object Model (DOM) comes in. The DOM is a separate standard being developed by the World Wide Web Consortium to define the objects used with HTML and XML documents without respect to a particular programming language.

The end result of these efforts is that JavaScript and JScript should one day adopt both the ECMAScript standard as well as the DOM standard. They will then share a uniform structure and a common model for interacting with documents. At this point they will both become compatible and we can write client-side scripting code that will work across all browsers that support this standard.

Despite the distinction between JavaScript and JScript, most people use the term JavaScript in reference to any implementation of JavaScript or JScript, regardless of browser; we will also use the term JavaScript in this manner.

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