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JavaScript: The Definitive Guide, 4th Edition
By
David Flanagan
Publisher
: O'Reilly
Pub Date
: November 2001
ISBN
: 0-596-00048-0
Pages
: 936
Slots
: 1
This fourth edition of the definitive reference to
JavaScript, a scripting language that can be embedded
directly in web pages, covers the latest version of the
language, JavaScript 1.5, as supported by Netscape 6 and
Internet Explorer 6. The book also provides complete
coverage of the W3C DOM standard (Level 1 and Level 2),
while retaining material on the legacy Level 0 DOM for
backward compatibility.
1.5 Client-Side JavaScript: Executable Content in Web Pages
When a web browser is
augmented with a JavaScript interpreter, it allows executable content
to be distributed over the Internet in the form of JavaScript
scripts. Example 1-1 shows a simple JavaScript
program, or script, embedded in a web page.
Example 1-1. A simple JavaScript program
<html>
<body>
<head><title>Factorials</title></head>
<script language="JavaScript">
document.write("<h2>Table of Factorials</h2>");
for(i = 1, fact = 1; i < 10; i++, fact *= i) {
document.write(i + "! = " + fact);
document.write("<br>");
}
</script>
</body>
</html>
When loaded into a JavaScript-enabled browser, this script produces
the output shown in Figure 1-1.
Figure 1-1. A web page generated with JavaScript
As you can see in this example, the
<script> and
</script> tags are used to embed JavaScript
code within an HTML file.
We'll learn more about the <script>
tag in Chapter 12.
The main feature of JavaScript
demonstrated by this example is the use of the
document.write( ) method.[2] This method is used to dynamically output
HTML text that is parsed and displayed by the web browser;
we'll encounter it many more times in this book.
[2] "Method" is the
object-oriented term
for function or procedure; you'll see it used throughout this
book. Besides allowing control over the content of web pages, JavaScript
allows control over the browser and over the content of the HTML
forms that appear in the browser. We'll learn about these
capabilities of JavaScript in more detail later in this chapter and
in much more detail later in this book.
JavaScript
can control not only the content of HTML documents, but also the
behavior of those documents. That is, a JavaScript program might
respond in some way when you enter a value in an input field or click
on an image in a document. JavaScript does this by defining
event handlers for the document -- pieces of
JavaScript code that are executed when a particular event occurs,
such as when the user clicks on a button. Example 1-2 shows the definition of a simple
HTML form that includes an event
handler that is executed in response to a button click.
Example 1-2. An HTML form with a JavaScript event handler defined
<form>
<input type="button"
value="Click here"
onclick="alert('You clicked the button');">
</form>
Figure 1-2 illustrates the result of clicking the
button.
Figure 1-2. The JavaScript response to an event
The onclick
attribute shown in Example 1-2 was originally a
Netscape extension added to HTML specifically for client-side
JavaScript. Now, however, this and other event handler attributes
have been standardized in HTML Version 4.0. All JavaScript event
handlers are defined with HTML attributes like this one. The value of
the onclick attribute is a string of JavaScript
code to be executed when the user clicks the button. In this case,
the onclick event handler calls the
alert( ) function. As you can see in Figure 1-2, alert( ) pops up a
dialog box to display the specified message.
Example 1-1 and Example 1-2 highlight only the simplest features of
client-side JavaScript. The real power of JavaScript on the client
side is that scripts have access to a hierarchy of
objects that are based on the content
of the web page. For example, client-side JavaScript programs can
access and manipulate each of the images that appear in a document
and can communicate and interact with Java applets and other objects
embedded within an HTML document. Once you have mastered the core
JavaScript language, the key to using JavaScript effectively in web
pages is learning to use the features of the DOM exposed by the
browser.
