Professional JavaScript for Web Developers: JavaScript in the Browser, Pt. 1 | WebReference

Professional JavaScript for Web Developers: JavaScript in the Browser, Pt. 1

Professional JavaScript for Web Developers: JavaScript in the Browser, Pt. 1

Reproduced from "Professional JavaScript for Web Developers" by permission of WROX. ISBN 0764579088, copyright 2005. All rights reserved. See WROX for more information.

Chapter 5: 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

The <script/> tag JavaScript is included in HTML pages by using the <script/> tag. Typically located within the <head/> tag of a page, the <script/> 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 to 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).

Although originally created for JavaScript, the <script/> tag can be used to specify any number of different client-side scripting languages with the language attribute indicating the type of code being used. For example, language can be set to VBScript to use Internet Explorer’s VBScript (Windows only).

JavaScript code can be written free form within a <script/> tag, but only if the src attribute isn’t specified; when src is specified, the code inside a <script/> tag may not work (depending on the browser). Example:

This example shows both inline JavaScript code and a link to an external JavaScript file. When using the src attribute, an external JavaScript file is referenced in the same way as images and style sheets.

By convention, external JavaScript files should have a .js extension, although it is not required by most browsers (this leaves open the possibility of dynamically generating JavaScript code using JSP, PHP, or another server-side scripting language).

External file format

External JavaScript files have a very simple format. Essentially, they are just plain text files containing JavaScript code. No <script/> tags are needed inside of external files, because the <script/> tag referencing the file is present in the HTML page. This makes external JavaScript files look very similar to source code files for other programming languages.

For example, consider the following inline code:

To externalize the sayHi() function into a file named external.js, you copy the function text itself (Figure 5-1).

Then the HTML code can be updated to include the external file:

No rules exist about what you can include in a single JavaScript source file, meaning that you are free to include any number of class definitions, functions, and so on, in a single file.

Created: March 27, 2003
Revised: June 20, 2005

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