JavaScript is a platform-independent, object-oriented scripting language designed for the web, originally created by Netscape Communications (though the name is a trademark of Sun Microsystems). It was designed to add interactivity to web sites and has since grown into a fundamental tool for web content developers. JavaScript programs—called scripts—are usually embedded in HTML. Features like dynamic typing and event handling, and its interface with a web page’s Document Object Model (DOM), all make JavaScript a very useful extension to HTML.
JavaScript is not a compiled language; rather, it is interpreted during the parsing of an HTML page by a web client—it is not interpreted on the server side. Despite their similar names, JavaScript has no functional equivalence to the Java language; however, technologies like LiveConnect create interoperability between the two.
Scripts can be placed anywhere within an HTML file, but most commonly are placed in the <head> section, where the page’s title and stylesheet definitions usually reside:
<head> |
<title>My Page</title> |
<!-- a script in another file --> |
<script language="JavaScript" TYPE="text/javascript" |
src="myscript.js"></script> |
<!-- a script inline --> |
<script language="JavaScript" TYPE="text/javascript"><!-- |
function myFunction() { |
... |
} |
// --> |
</script> |
</head> |
The script’s content is enclosed in an HTML comment by convention. The <script> tag is notably the only tag in HTML whose contents are not supposed to be treated as content to display. Putting comments around these inline scripts ensures that older browsers (and non-browser tools) that do not understand the <script> tag do not mistake that content for ordinary text.
Warning: In XHTML, data within the <script> tag is treated as parsed character data (PCDATA) instead of raw character data (CDATA). This means that special characters are parsed like ordinary HTML, and thus must be properly entity encoded. For example, instead of using &, you must use &.
Because entity encoding makes scripts difficult to read, as a general rule, you should generally use external script files when working in XHTML. (As an added bonus, using separate script files reduces network bandwidth and improves page load performance because the scripts are cached independently.)
Apple’s WebKit framework, and the Safari web browser based on it, both support the latest versions of JavaScript. Since the support is built into the framework, you can use all the features of JavaScript within anything that uses WebKit, including Safari, Dashboard, and any WebKit-based OS X application.
The Document Object Model (DOM) is a standardized software interface that allows code written in JavaScript and other languages to interact with the contents of an HTML document. The Document Object Model consists of a series of classes that represent HTML elements, events, and so on, each of which contains methods that operate on those elements or events.
With the Document Object Model, you can manipulate the contents of an HTML document in any number of ways, including adding, removing, and changing content, reading and altering the contents of a form, changing CSS styles (to hide or show content, for example), and so on.
By taking advantage of the Document Object Model, you can create much more dynamic websites that adapt as the user takes actions, such as showing certain form fields depending on selections in other fields, organizing your content based on what pages the viewer has recently visited, adding dynamic navigation features such as pull-down menus, and so on.
Last updated: 2010-01-20