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JavaScript in the Browser

In the preceding chapters, you learned about JavaScript’s core, ECMAScript, and how the basics of

the language work. Beginning with this chapter, the focus switches to using JavaScript inside its

natural habitat: the Web browser.

Web browsers have come a long way since JavaScript was first introduced in Netscape Navigator

2.0. Browsers today are capable of handling a variety of file formats, not just conventional HTML.

Ironically enough, JavaScript is used in most of these file formats as a way to dynamically change

content on the client. This chapter explores how JavaScript fits into HTML and other languages, as

well as introduces you to some basic concepts of the Browser Object Model (BOM).

JavaScript in HTML

Of course, it was HTML that first made use of embedded JavaScript, so the natural first discussion

point is how JavaScript is used in HTML. The evolution of HTML to include JavaScript began with

the introduction of tags to be used in conjunction with JavaScript, as well as the addition of new

attributes for several common parts of HTML.

The <script/> tag

JavaScript is included in HTML pages by using the

tag. Typically located within the

<head/>

tag of a page, the

tag was originally defined to have one or two attributes:

language

that indicates the scripting language being used and, optionally,

src

that indicates

an external JavaScript file to include in the page. The

language

attribute is traditionally set

JavaScript

, but it can also be used to indicate the exact version of JavaScript, such as

JavaScript1.3

(if the

language

attribute is omitted, the browser defaults to the most current

version of JavaScript available).

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