Javascript Intro
Volume Number: 12
Issue Number: 7
Column Tag: Javatech™
Getting the Jump on JavaScript 
Start learning to add life to your Web pages
By Kevin M. Savetz
Note: Source code files accompanying article are located on MacTech CD-ROM orsource code disks.
Static Web pages are a dying breed. Although not obsolete yet, it may be one day soon
that a Web page that doesn’t do anything just won’t get a second glance. Once upon a
time, all Web pages were static. CGI programs started to change that by allowing
powerful searches of Web pages, sites that modified themselves based on the time of
day or the phase of the moon, and even interactive games. Java, a powerful
programming language, is bringing a whole new level of interactivity to the Web.
Somewhere in the middle is JavaScript, a new language that’s simple enough for
beginning programmers to learn, but powerful enough to interact with Web pages and
Java applets in ways previously not possible with any language.
JavaScript is a fledgling language. As I write this, JavaScript isn’t even
complete. The technical specifications of the language have been agreed upon, but no
Web browsers completely support JavaScript. (JavaScript 3.0, which should
incorporate the functions that are currently lacking, is due out in June, give or take a
month; no firm date had been set at press time.) But that doesn’t mean that no one is
using it; bleeding-edge Web hackers have been experimenting with the crippled
JavaScript for months, and many are impressed with what they see.
What is JavaScript?
Java and JavaScript are distinct languages, with different purposes and features.
JavaScript was designed to provide an easy way for Web authors to create interactive
Web pages. Java is a programming language which is used to create stand-alone
applications, called applets. Unlike Java, which is meant for experienced
programmers with an understanding of C++, JavaScript is a simpler “scripting”
language (like dBASE and AppleScript) aimed at those with less programming
experience.
Both are powerful languages, but they each offer distinct functionalities.
According to Andy Augustine, vice president of technology for Frequency Graphics, a
Web development and hosting service provider, “JavaScript is aimed at the people who
don’t have C++ experience, people who don’t have much experience with CGI, but want
to create some sort of interactive Web page.”
“Contrary to popular misconceptions, JavaScript was not meant to be a
scaled-down version of Java, nor was it intended to be a replacement for CGI
(server-side) scripts. Instead, JavaScript functions as an outstanding way to enhance
both,” he said.
JavaScript can be used to manage user input as well as to show text, play sounds,
display images, or communicate with a plug-in in response to “events” such as a
mouse-click or exiting or entering a Web page.
JavaScript’s syntax is loosely based on the Java language, but those who have
used both languages say the similarity is very loose indeed. In fact, the closest
resemblance the two languages have is their names. JavaScript was originally called
Mocha while being developed at Netscape. It was later dubbed LiveScript, and when
Netscape Communications partnered with Sun to develop Java, LiveScript was renamed
JavaScript.
Java vs. JavaScript
JavaScript programs are interpreted and run entirely on the client side. This means
fewer hits and less processing time on the server than with Java (where applets are
compiled on the server before being executed on the client) or CGI (which requires the
server to do the work and rack up hits).
There are other differences between the two languages. In Java, applets are files
distinct from HTML pages; with JavaScript, the code is integrated into the HTML using
special tags. (This can cause problems with some Web browsers that do not properly
handle JavaScript code.) Like Java, JavaScript is a cross-platform language that can
work with any compatible browser.
Here’s an example HTML file with an embedded JavaScript. Between the
tags, the Web browser displays a form with a space to enter a number. When the user
enters a number in the field, the browser starts the script, located between the
Please enter a number from 1 to 10:
onChange="if (!checkNum(this.value, 1, 10))
{this.focus();this.select();} else {thanks()}">
All this only takes one hit on the server - to download the HTML code and the
embedded script. Doing this with a CGI script would require multiple hits on the
server, once for the HTML file, and again to verify the input every time the user types
into the form. JavaScript allows the client to perform more significant calculations
without weighing down the server.
Other differences are more technical. JavaScript is object-based: its code uses
built-in, extensible objects, but unlike Java, there are no classes or object
inheritance. In JavaScript, object references are checked at run-time; in Java, object
references must exist upon compilation. In JavaScript, variable data types are not
declared, unlike in Java, where they must be declared before use.
Which language is better? Until a JavaScript interpreter is complete, there is
no fair comparison. Although it was originally intended as a tool for fledgling
programmers, even experienced programmers who know Java will find JavaScript
useful. The languages can work together to complement each other.
Currently, the only Web browser with any support for JavaScript is Netscape
2.0, and it lacks much of the functionality promised for the final release. More than
30 other companies, including Microsoft, have agreed to include support for
JavaScript in their Web browsers. With the current version of JavaScript, you can
send information to the browser (such as a dialog box) and get the status of objects
(such as mouse movements), but some of the most exciting functions, such as the
abilities to read input streams from documents and to interact with Java applets, are
still unimplemented.
When JavaScript includes all of the tools the specs say will be included, including
something called the “applet object” (which will allow scripts to interact with Java
applets), the two languages will be a mighty team. “As soon as the applet object is
released, you will be able to control Java applets with JavaScript, and that will make
them work perfectly together,” Augustine says. This means that you will be able to
control a Java applet using a Web-based form, with a JavaScript doing the
behind-the-scenes translation between the Web page and the applet. The user will be
able to use a radio button to select how a Java applet will run - for instance, changing
the color and speed of a rotating logo. JavaScript will pass information from the form
to the Java applet. JavaScript’s applet object will also allow scripts to send
information to and get information from other scripts and CGI programs.
Another tool that will be available in the final release of JavaScript is the
“history object”, which will allow the script to see what Web pages the user has
recently visited (although to protect the user’s privacy, the script will not be able to
share that information with the server). This would allow a page to offer an
intelligent “back” button, letting the user move quickly back to any previously visited
Web page.
As with Java, security in JavaScript is likely to be an ongoing battle. Several
security problems were quickly discovered in JavaScript 2, including a bug that let
JavaScript send a blank electronic mail message to someone without the sender’s
knowledge. Another bug allowed JavaScript to get a listing of files on the user’s hard
drive (although the script could not read the contents of any files).
Now or Later?
Should you bother learning JavaScript now, or is it currently just a toy for
experimenters? Should you wait until the final version of the JavaScript interpreter
ships before trying to learn the ins and outs of the language? Augustine thinks that
now is the time to get a jump start on JavaScripting. “Learn JavaScript right now if
you want to capture the market while it is viable; you’ll be ahead of the field. If you
wait until it is fully functional before you learn it, I think you’ll be behind the game,”
he says. Online tutorials can get you started with the basics of the language, and
Internet mailing lists and newsgroups devoted to the topic are already home to thriving
communities of developers and experimenters.
Will the serious Web page developer need to understand both Java and
JavaScript? Because JavaScript can interact directly with the user anywhere on a
Web page (rather than being limited to a single window, as with Java), most Java
developers will probably want to learn both. But JavaScript should also be viable as a
stand-alone tool for less experienced programmers who want to add interactivity to
their Web pages.
Example Scripts
There are already dozens of example JavaScripts on the Web. Many, such as the
1040EZ tax form and body mass calculator, simply compute values input from an
HTML form. Others are more inspired, such as the currency exchange calculator,
which lets you select two countries from a list and then computes the exchange rate. I
am particularly fond of the Zen koan generator, even though it occasionally causes
Netscape 2.0 to crash.
http://www.homepages.com/fun/1040EZ.html
http://www.iohk.com/UserPages/acheng/javascript.html
http://www.superprism.net/doc/EXPRESS/util/currency.html
http://www.wrldpwr.com/javascriptzen.html
The color picker shows how JavaScript can work with frames and color: pass the
mouse over the name of a color in one frame and the background of another frame will
become that color. Other examples are downright inspirational, such as the JavaScript
VRML clock, which uses JavaScript to display a three-dimensional clock if you have a
VRML browser.
http://www.lsi.usp.br/doc/color/picker.html
http://www.phoenix.net/~jalonso/applets/vrml
Many more example applications are available at http://www.gamelan.com/.
Remember how interesting (and sometimes annoyingly funky) Web pages became
when authors started using and abusing sundry new tools like backgrounds and frames?
Expect to see another round of enlightening and obscure Web sites as developers and
hackers begin to understand and use JavaScript.
For More Information
A growing number of information sources about JavaScript are online. Andy
Augustine’s JavaScript FAQ is at:
http://www.freqgrafx.com/411/jsfaq.html
There is also a JavaScript mailing list, a high-volume, sometimes technical list
for discussion about using the language. To subscribe, send a message to
majordomo@obscure.org with a message body of “subscribe javascript”. A digest
version of the list is also available. To subscribe, use a message body of “subscribe
javascript-digest”. Further discussion takes place on the newsgroup
comp.lang.javascript.
Netscape’s official JavaScript documentation is required reading if you plan to
learn your way around JavaScript. The files are also downloadable for reading offline.
http://home.netscape.com/eng/mozilla/2.0/handbook/javascript/index.html
http://home.netscape.com/eng/mozilla/2.0/handbook/javascript/jsdoc.zip
Augustine maintains a list of online tutorials, example applications and other
information at JavaScript 411. Finally, the ubiquitous Yahoo naturally has an index of
JavaScript-related goodies.
http://www.freqgrafx.com/411/
http://www.yahoo.com/Computers_and_Internet/Languages/JavaScript/