Chapter 28. Introduction to JavaScript
JavaScript is a client-side scripting
language that adds interactivity to web pages and lets designers
control various aspects of the browser itself. With JavaScript, you
can do such things as display additional information about links,
create mouse rollover effects, change the contents of pages based on
certain conditions, randomly display content on a page, load content
in new browser windows and frames, and (with some help from CSS) move
elements around on the page.
A scripting language is somewhere between a markup language, like
HTML, and a full-blown programming language, like Java. With
JavaScript, you can add extra functionality to your web site using
short snippets of scripting code that has a syntax that's
fairly easy to understand.
This chapter includes material and code by Nick Heinle and Bill
Peña, authors of Designing with
JavaScript (O'Reilly). For a more advanced
JavaScript reference, see JavaScript: The Definitive
Guide by David Flanagan (O'Reilly). Also note that
for simple functionality, you may not need to write your own
JavaScript at all; software like Macromedia's Dreamweaver can
do the coding for you, with built-in behaviors you can drop in like
HTML objects. But as with HTML, if you are going to use JavaScript,
you should be familiar with the basics of the language, whether you
are using a WYSIWYG editor or not.
28.1. JavaScript History
JavaScript was first introduced by Netscape in Navigator 2.0, as a
simple scripting language that could be embedded directly in web
pages. Since then, JavaScript has evolved through five versions and
is now codified in a standard. Note that the name of the language,
JavaScript, is only coincidentally related to the Java programming
language. Although JavaScript and Java share some similarity in
syntax, you don't need to know anything about Java to learn
JavaScript.
JavaScript has evolved from Version 1.0 in Navigator 2.0 to Version
1.5 in Netscape 6. Microsoft's name for its version of
JavaScript is Jscript, and, for the most part,
Jscript mirrors
JavaScript's functionality, but also varies from version to
version. At the time of this writing, the most widely supported
version is 1.2, which is the basis of both Netscape's and
Microsoft's 4.0 browser implementations. If you have a script
that is written for an older version, say JavaScript 1.1, it will
work fine with all of the newer versions. However, if you use
features in a later version, say JavaScript 1.5, older browsers will
not understand your scripts. Later, you'll learn how to use
JavaScript to detect what browser is displaying your page, so you can
customize your scripts to be backwards-compatible.
JavaScript (and Jscript) have also been standardized as
ECMAScript (ECMA-262). Technically,
JavaScript 1.5 is an implementation of the third version of the
ECMAScript standard. Currently, both Netscape 6 and IE 5.5 support
this standard version of JavaScript very well, and hopefully they
will continue to agree in their support.
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