Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

1.3 Client-Side JavaScript

When a JavaScript
interpreter is embedded in a web
browser, the result is client-side JavaScript. This is by far the
most common variant of JavaScript; when most people refer to
JavaScript, they usually mean client-side JavaScript. This book
documents client-side JavaScript, along with the core JavaScript
language that client-side JavaScript incorporates.

We'll discuss client-side JavaScript and its capabilities in
much more detail later in this chapter. In brief, though,
client-side JavaScript combines
the scripting ability of a JavaScript interpreter with the
document object model (DOM) defined
by a web browser. These two distinct technologies combine in a
synergistic way, so the result is greater than the sum of its parts:
client-side JavaScript enables executable content to be distributed
over the Web and is at the heart of a new generation of


Dynamic
HTML (DHTML) documents.

Just as the ECMA-262 specification defined a standard version of the
core JavaScript language, the
World Wide Web Consortium (W3C) has
published a DOM specification (or recommendation) that standardizes
the features a browser must support in its DOM. We'll learn
much more about this standard in Chapter 17, Chapter 18, and Chapter 19. Although the W3C DOM standard is not yet as
well supported as it could be, it is supported well enough that web
developers can start writing JavaScript code that relies on it.

Table 1-2 shows the core language version
and DOM capabilities supported by various
browser versions from

Netscape and Microsoft. Note that the
versions of Internet Explorer listed in the table refer to the
Windows version of that browser. The capabilities of Macintosh
versions of IE often vary (sometimes significantly) from the
same-numbered versions for Windows. Also, bear in mind that IE allows
the JScript interpreter to be upgraded independently of the browser
itself, so it is possible to encounter an installation of IE that
supports a version of the language
greater than that shown here.

Table 1-2. Client-side JavaScript features by browser

The differences and incompatibilities between Netscape's and
Microsoft's client-side versions of JavaScript are much greater
than the differences between their respective implementations of the
core language. However, both browsers do agree upon a large subset of
client-side JavaScript features. For lack of better names,

versions of client-side
JavaScript are sometimes referred to by the version of the core
language on which they are based. Thus, in client-side contexts the
term "JavaScript 1.2" refers to the version of
client-side JavaScript supported by Netscape 4 and Internet Explorer
4. When I use core-language version numbers to refer to client-side
versions of JavaScript, I am referring to the compatible subset of
features supported by both Netscape and Internet Explorer. When I
discuss client-side features specific to one browser or the other, I
refer to the browser by name and version number.

Note that Netscape and Internet Explorer are not the only browsers
that support client-side JavaScript. For example,
Opera (http://www.opera.com) supports client-side
JavaScript as well. However, since Netscape and Internet Explorer
have the vast majority of market share, they are the only browsers
discussed explicitly in this book. Client-side JavaScript
implementations in other browsers should conform fairly closely to
the implementations in these two browsers.

Similarly, JavaScript is not the only programming language that can
be embedded within a web browser. For example, Internet Explorer
supports a language known as
VBScript, a variant of Microsoft's
Visual Basic language that provides many of the same features as
JavaScript but can be used only with Microsoft browsers. Also, the
HTML 4.0 specification uses the Tcl programming language as
an example of an embedded scripting language in its discussion of the
HTML <script> tag. While there are no
mainstream browsers that support Tcl for this purpose, there is no
reason that a browser could not easily support this language.

Previous editions of this book have covered Netscape browsers more
thoroughly than Microsoft browsers. The reason for this bias was that
Netscape was the inventor of JavaScript and (for a time, at least)
held the dominant position in the web-browser market. This bias
toward Netscape has declined in each subsequent edition of the book,
and the current edition is heavily focused on
standards, such as
ECMAScript and the W3C DOM, rather than on particular browsers.
Nevertheless, readers may find that some of the original bias toward
Netscape comes through in the material that remains from older

editions.