TweetFollow Us on Twitter

What is JavaScript?

Volume Number: 14 (1998)
Issue Number: 5
Column Tag: Scripting

What is JavaScript?

by Dori Smith

As these examples show, it's just a simple web scripting language

Introduction

What's JavaScript? What's the difference between JavaScript and Java? Isn't JavaScript just a simplified version of Java? These are common questions that I get all the time.

Netscape originally invented a simple scripting language called LiveScript, which was to be a proprietary add-on to HTML. When Sun's new language Java became unexpectedly popular, Netscape was quick to jump on the Java bandwagon, and re-christened their scripting language JavaScript. Outside of the first four letters, there are almost no other similarities between the two.

Microsoft then added their own version of JavaScript to Internet Explorer, which they named JScript. Unfortunately, the two were not identical, so Netscape then attempted to straighten matters out by turning JavaScript over to ECMA, a Switzerland-based standards body. This gave three main versions of JavaScript-based languages: JavaScript, which works primarily with Netscape's browsers, JScript, which works with Internet Explorer, and ECMAScript, with which no browser is completely compatible. Netscape and Microsoft have both stated that future versions will match the ECMAScript standard, which should lead to convergence. However, as the most-used features are common to all, compatibility is not an issue unless you are trying to use JavaScript to control DHTML.

JavaScript is growing in popularity due to its simple learning curve relative to the amount of power it provides. Complete non-programmers are able to add a little bit of interactivity to their web pages without buying an IDE or sweating over why a program won't compile. There are numerous Web sites which contain any number of scripts available for the taking, and Netscape has fairly complete documentation on their site. And of course, there's the always useful ability to view the source of Web pages.

Syntax & Basics

The syntax of JavaScript will be simple to anyone who has ever programmed in an object oriented language. The primary object is the window, which is at the top of the hierarchy. Under that are the document, frame, location, history, and navigator objects. In turn, the document object contains anchors, applets, embeds, forms, images, and links, each of which is an array of objects. So, a reference to the first image on a page would be to self.document.image[0], although it is preferable to use names instead of numbers for clarity.

JavaScript is not a stand-alone language, but rather a scripting add-on to HTML. JavaScript is added to HTML commands by use of the <SCRIPT> tag. Anything within this tag is (or should be) ignored by browsers that do not support JavaScript. JavaScript is the most popular scripting language in use due to its cross-platform and cross-browser support, although VBScript is sometimes used on intranets when the scripter knows that everyone accessing the page is on Windows. JavaScript will attempt to execute the commands within the <SCRIPT> tag if there is no LANGUAGE attribute, or if the LANGUAGE attribute is set to JavaScript; in addition, the LANGUAGE attribute also can be used to distinguish between various versions of JavaScript.

The JavaScript in Listing 1 consists of a single line: document.write("Hello, World!"). This writes the typical "Hello, World" message, using the document.write() method. This method dynamically generates text within HTML files, in lieu of hard-coded HTML. Figure 1 shows the results.

Listing 1: Everyone's first script

hello.html

<HTML>
<HEAD>
   <TITLE>Barely a script at all</TITLE>
</HEAD>
<BODY BGCOLOR=WHITE>
<H1>
<SCRIPT LANGUAGE="JAVASCRIPT">
      document.write("Hello, World!")
</SCRIPT>
</H1>   
</BODY>
</HTML>

Figure 1. The typical "Hello, World".

Using JavaScript

There are three ways that JavaScript can be used within an HTML file. It can be put within <SCRIPT> tags within the <HEAD> tag (header scripts), within <SCRIPT> tags within the <BODY> tag (body scripts), or called directly when certain events occur.

JavaScript is put inside the <HEAD> tag primarily when you are initializing variables and creating functions that will be used throughout the entire script. Code here is executed once, when the page is loaded. Within the <BODY> tag, the best use of JavaScript is to write out text on-the-fly. JavaScript handles these events:

Event Caused by
onAbort The user aborted loading the page
onBlur The user left the object
onChange The user changed the object
onClick The user clicked on an object
onError The script encountered an error
onFocus The user made an object active
onLoad The object finished loading
onMouseOver The cursor moved over an object
onMouseOut The cursor moved off of an object
onSelect The user selected the contents of an object
onSubmit The user submitted a form
onUnload The user left the window

Any of these event handlers can be added to any HTML tag which can cause the event to occur. For example, the onLoad event handler can be added to the <BODY> tag, and the event will be called when the page has completed loading.

Image Rollovers

One of the most common uses of JavaScript is the image rollover. This is a technique which requires little programming knowledge, but can greatly enhance the experience of visiting a Web site.

HTML has its place, but it's static. A user entering a web site and seeing a window like that in Figure 2 doesn't know what they should do -- are these words links, or just graphics giving us information?

Figure 2. The Navigation Bar, upon entering the page.

Adding a rollover to this gives the look in Figure 3. Now, when a user moves a cursor over any of the options, it becomes clear that this is a navigation bar, and that each of these words is a live link.

Figure 3. When doing a rollover, the arrow gives the user feedback.

Image rollovers are handled by browsers that support JavaScript versions 1.1 and later, which includes Netscape Navigator versions 3 and above, Internet Explorer 4, and some versions of Internet Explorer 3. Listing 2 shows the code to create the image rollover effect.

Listing 2: Image rollovers give users feedback

navbar.html

<HTML>
<HEAD>
   <TITLE>Navbar</TITLE>

   <SCRIPT LANGUAGE="JAVASCRIPT">
      <!-- Hide from old browsers

      if (document.images) {
         productOn = new Image
         pricingOn = new Image
         pressOn = new Image
         
         productOff = new Image
         pricingOff = new Image
         pressOff = new Image
         
         productOn.src = "products_on.gif"
         pricingOn.src = "pricing_on.gif"
         pressOn.src = "press_on.gif"
         
         productOff.src = "products_off.gif"
         pricingOff.src = "pricing_off.gif"
         pressOff.src = "press_off.gif"
      }
      else {
         productOn.src = ""
         pricingOn.src = ""
         pressOn.src = ""
         
         productOff.src = ""
         pricingOff.src = ""
         pressOff.src = ""
         
         document.products = ""
         document.pricing = ""
         document.press = ""
      }
      
      // Stop hiding from old browsers -->
   </SCRIPT>
</HEAD>

<BODY BGCOLOR=WHITE>
   <A HREF="products.html" 
      onMouseOver="products.src=productOn.src" 
      onMouseOut="products.src=productOff.src">
   <IMG src="products_off.gif" WIDTH="226" HEIGHT="37" 
      BORDER="0" NAME="products"></A><BR>
      
   <A HREF="pricing.html" 
      onMouseOver="pricing.src=pricingOn.src" 
      onMouseOut="pricing.src=pricingOff.src">
   <IMG src="pricing_off.gif" WIDTH="189" HEIGHT="80" 
      BORDER="0" NAME="pricing"></A><BR>
      
   <A HREF="press.html" 
      onMouseOver="press.src=pressOn.src" 
      onMouseOut="press.src=pressOff.src">
   <IMG src="press_off.gif" WIDTH="154" HEIGHT="36" 
      BORDER="0" NAME="press"></A>
</BODY>
</HTML>

In the <HEAD> tag, we do our initializations. The first thing needed is to hide the JavaScript commands from browsers that are too old to understand JavaScript and too old to understand that they're supposed to ignore text within tags they can't handle. This is done by starting a HTML comment, which is not closed until just before the closing script tag. This closing comment line works because it starts with a JavaScript comment symbol, which causes JavaScript to ignore it. Next, we check to see if the browser we're using supports manipulation of image objects. If it doesn't, then care must be taken to avoid giving the user errors. This is done by checking for the existence of document.images. If it exists, we're able to do the rollover. If it doesn't, they have an old browser (JavaScript 1.0) or they have JavaScript turned off.

The image rollover technique requires two versions of each image: one "on" and one "off," based on whether the cursor is or isn't on top of the image. So, for each of the three images in the window, we need to create 2 new image objects. We then assign an image URL to each of these image objects, by setting their src property. This both enables the variables to be referenced by name later within the body, and pre-loads the graphics into the browser cache, so that they appear virtually immediately when the cursor moves over the image.

We also give each image on the page a name, by setting the NAME attribute of each within the <IMG> tag. While images can be referred to as document.image[0] (for instance), it is more flexible & manageable to instead refer to document.products. This allows us to refer to the image by name later when we want to manipulate it.

If the user has a browser that does not support image manipulation, we must initialize the same variables, so that using them later does not cause an error. Consequently, we set the src of each to the empty string. In addition, we set three other variables: document.products, document.pricing, and document.press. Older browsers don't understand that images can have names, so simply defining them here lets us use the objects in all browsers without an error.

Within the body, we add event handlers to the link tags around the images, for both onMouseOver and onMouseOut. For example, when the products image first loads, it displays the image products_off.gif. When the cursor goes over the image, the src of products is reset to the src of the productOn image that we set in the header script. This displays the version of the image with an arrow. When the cursor is moved off the image area, the src of products is reset to the src of the productOff image (set in the header script).

If the browser does not support image manipulation, the onMouseOver event instead just sets the src of an empty string to the src of another empty string, with no errors and no effect.

The secret to having this look good is proper preparation of the images, avoiding the two most common mistakes. Firstly, the two images must be of identical sizes. When one replaces the other, the size of the image area doesn't change, even though the images themselves may be of different sizes. Instead, the browser will automatically resize the graphics for you, with unpredictable (and virtually always unpleasant) results. Figure 4 shows the six images actually used in this example. The "off" versions of the images, although they do not include the red arrow, have identical dimensions to their "on" versions. Secondly, the images must not have transparent areas. In this case, if the white areas were transparent, the images would look fine upon first loading, and would also look fine when the cursor was moved over them. But, when the cursor was moved off the first time, the red arrow would continue to show through the transparent area of the "off" image.

Figure 4. There are actually 6 images; 1 each with and without the arrow.

JavaScript Navigation

Another way of improving the user interface of a web site is to improve its navigation. One technique of doing this is to create a jumping popup menu, such as the ones on Apple's DevWorld site. This example requires little actual scripting, outside of handling the onChange event from the popup menu, combined with some knowledge of how JavaScript interacts with HTML.

The HTML <SELECT> is used to create the popup menu shown in Figure 5. It has four options: "Pick an area," "Products," "Pricing," and "Press." Choosing an option triggers the onChange event, which evaluates the line of JavaScript shown in Listing 3.

Figure 5. This popup menu sends you to a new page instantly.

Listing 3: Instant navigation with a popup menu

popup.html

<HTML>
   <HEAD>
      <TITLE>Pop-up Menu</TITLE>
   </HEAD>
   <BODY BGCOLOR=WHITE>
      <P>Where do you want to go?</P>
      <FORM>
      <SELECT 
         onChange="if (this.selectedIndex > 0)
window.location.href=this.options[this.selectedIndex].value">
         <OPTION>Pick an area
         <OPTION VALUE="products.html">Products
         <OPTION VALUE="pricing.html">Pricing
         <OPTION VALUE="press.html">Press
      </SELECT>
      </FORM>
   </BODY>
</HTML>

Firstly, we check the value of selectedIndex. This is a number between 0 and 3, depending on which option was chosen. If the option chosen was the first ("Pick an area"), we don't want to do anything, so we only do anything if this.selectedIndex is greater than zero.

Otherwise, changing the value of window.location.href will force the browser to load a new page. This example uses the HTML VALUE attribute of <OPTION> to store the new page that we want to jump to. Each of these values can be referenced by JavaScript via the value property of the options array. When the user chooses an option, JavaScript finds the new page by referencing into the options array with the selectedIndex, and using its value as the location of the page.

One of the major benefits of this style of handling popup menus is that changes to the menu can be made with no knowledge of JavaScript necessary. In fact, once the onChange handler has been added, the page can then be modified by WYSIWYG editors with no loss in functionality, so long as the VALUE attributes are maintained correctly.

There's one small trick here: if you decide to change the popup so that the first option on the menu is a valid location, the user won't be able to ever get there. The event handled is onChange, and there's no way for a user to cause it to be triggered when choosing the default option.

Flow Control

Like most programming languages, JavaScript has the expected ways of controlling the flow of commands. Listing 4 and Figure 6 demonstrate functions, loops, and if statements in JavaScript.

Listing 4: Controlling flow in JavaScript

factorial.html

<HTML>
<HEAD>
   <TITLE>Factorial</TITLE>

   <SCRIPT LANGUAGE="JAVASCRIPT">
      <!-- Hide script from old browsers
      
      function showFactorial(theEntry) {
         document.myForm.result.value =
            calcFactorial(theEntry.value)
      }
   
      function calcFactorial(inVal) {
         if (isNaN(inVal))                // isNan = Is Not a Number
            return("Not a number")
         
         startVal = eval(inVal)          // eval turns a string into a number
         if (startVal < 0 || startVal != Math.floor(startVal))
            return ("Invalid number")

         endVal = 1
         for (i=1; i<=startVal; i++) {
            endVal = endVal * i
         }
         return(endVal)
      }
   
      // End hiding script from old browsers -->
   </SCRIPT>

</HEAD>
<BODY BGCOLOR=WHITE>

   <H3>The factorial of
   <FORM NAME="myForm">
      <INPUT TYPE=TEXT SIZE=3
         onChange="javascript:showFactorial(this)"> is
   <INPUT TYPE=TEXT NAME="result">
   </FORM>
   </H3>
</BODY>
</HTML>

In this example, the user enters a number in the first field. Changing this field passes control to the onChange event handler, which calls the showFactorial() function, with the entry field as its parameter. The showFactorial() function calls the calcFactorial() function, passing it the value of the entry field.

The calcFactorial() function returns either an error message (if not a number, less than zero, or not an integer) or the factorial to its caller. When control passes back to showFactorial(), the result is put into the result field in the browser window.

Figure 6. Entering a number displays its factorial.

Conclusion

Another popular use of JavaScript is form validation, which will tell the user if the form they've submitted is valid without having the overhead of connecting to a Web server. This technique gives the user the impression of speedy Web access, when they haven't accessed your server at all.

JavaScript is growing in popularity, due to many factors, including its ease of learning, the growing share of JavaScript-enabled browsers, and the large number of resources available to novices. With Dynamic HTML on the horizon, JavaScript will become more important and more prevalent in the future.

Bibliography & Suggested Reading

  • Castro, Elizabeth. HTML for the World Wide Web., Visual QuickStart Guide. 2nd edn. Peachpit Press, 1997.
  • Flanagan, David. JavaScript: The Definitive Guide. 2nd edn. O'Reilly & Associates, 1997.
  • Negrino, Tom and Dori Smith. JavaScript for the World Wide Web, Visual QuickStart Guide,. 2nd edn. Peachpit Press, 1998.

Internet References


Dori Smith, dori@chalcedony.com, is a partner at Chalcedony Consulting, and is the co-author (with Tom Negrino) of JavaScript for the World Wide Web, Visual QuickStart Guide, 2nd Edition (Peachpit Press, 1998).

 
AAPL
$93.94
Apple Inc.
-0.49
MSFT
$44.84
Microsoft Corpora
+0.15
GOOG
$589.47
Google Inc.
-5.61

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Yosemite 10.10 DP4 - Developer Prev...
Note: This is a Developer Preview. You must be a registered Apple Mac Developer to download this update. OS X Yosemite is Apple's newest operating system for Mac. An elegant design that feels... Read more
FinderPop 2.5.6 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
SpiderOak 5.1.7 - Secure cloud backup, s...
SpiderOak is a multi-platform secure online backup, storage, access, and sharing solution engineered for the consumer and small businesses. You must first sign up to use SpiderOak. Running natively... Read more
Espionage 3.6 - Simple, state of the art...
Espionage offers state-of-the-art encryption and plausible deniability for your confidential data. Sometimes, encrypting your data isn't enough to protect it. That's why Espionage 3 goes beyond data... Read more
calibre 1.45.0 - Complete e-library mana...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
iFFmpeg 4.3.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
Chromium 36.0.1985.125 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
pwSafe 3.0 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
Day One 1.9.6 - Maintain a daily journal...
Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through... Read more
Google Chrome 36.0.1985.125 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more

Latest Forum Discussions

See All

HELMUT Review
HELMUT Review By Andrew Fisher on July 21st, 2014 Our Rating: :: TRUNDLE SIMULATOR 2014Universal App - Designed for iPhone and iPad HELMUT is a fun, fleeting time-sink that offers a momentary distraction and nothing else.   | Read more »
Walkr Review
Walkr Review By Jennifer Allen on July 21st, 2014 Our Rating: :: ORIGINAL WALKINGiPhone App - Designed for the iPhone, compatible with the iPad Walking is a bit more exciting thanks to this planet building/discovering sim reliant... | Read more »
Zombie Commando Review
Zombie Commando Review By Jennifer Allen on July 21st, 2014 Our Rating: :: MINDLESS SLAUGHTERUniversal App - Designed for iPhone and iPad Briefly fun but ultimately forgettable, Zombie Commando will scratch an itch then be... | Read more »
Swords & Poker Adventures Review
Swords & Poker Adventures Review By Jennifer Allen on July 21st, 2014 Our Rating: :: SOULLESS POKER PLAYUniversal App - Designed for iPhone and iPad Swords & Poker Adventures is a mishmash of Poker and RPGing, but it lacks... | Read more »
Warhammer 40,000: The Horus Heresy: Drop...
Warhammer 40,000: The Horus Heresy: Drop Assault Coming Soon to iOS Posted by Jennifer Allen on July 21st, 2014 [ permalink ] Coming soon to iOS will be an all-new Warhammer 40,000 tactical strategy game by the name of The Horus Heresy: Drop As | Read more »
A Life Worth Dying For Review
A Life Worth Dying For Review By Jordan Minor on July 21st, 2014 Our Rating: :: A BEAUTIFUL MINDUniversal App - Designed for iPhone and iPad A Life Worth Dying For is a fascinating portrait of a serious subject.   | Read more »
Zombie Puzzle Panic Review
Zombie Puzzle Panic Review By Jordan Minor on July 21st, 2014 Our Rating: :: THE MATCHING DEADUniversal App - Designed for iPhone and iPad Zombie Puzzle Panic puts some pretty neat undead twists on Match-3 puzzling.   | Read more »
This Week at 148Apps: July 14-18, 2014
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Fallen Lords Review
Fallen Lords Review By Andrew Fisher on July 18th, 2014 Our Rating: :: FALLS SHORTiPad Only App - Designed for the iPad Fallen Lords is a decent game, but its similarity and inferiority compared to Ghost Stories makes it ultimately... | Read more »
Real Boxing’s New Combo Update is a Knoc...
Real Boxing’s New Combo Update is a Knockout Posted by Blake Grundman on July 18th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

Twelve South HiRise For MacBook – Height-Adju...
If you use your MacBook as a workhorse desktop substitute, as many of us do, a laptop stand combined with an external keyboard and pointing device are pretty much obligatory if you want to avoid... Read more
Why The Mac Was Not Included In The Apple/IBM...
TUAW’s Yoni Heisler cites Fredrick Paul of Network World whoi blogged last week that the Mac’s conspicuous absence from Apple and IBM’s landmark partnership agreement represents a huge squandered... Read more
Save $100 on 13-inch Retina MacBook Pros, plu...
Adorama has 13″ Retina MacBook Pros on sale for $100 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 13″ 2.4GHz/128GB MacBook Pro with Retina Display: $1199 - 13″ 2.... Read more
Blurr it 2.3 for iOS – Quickly Blurs Selected...
Hyderabad, India based TouchLabs has announced a new update of Blurr it 2.3, their photography app for iOS users. Blurr it allows you to blur part of the image to hide potentially sensitive or... Read more
MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for up to $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only... Read more
Amazon Announces Kindle Unlimited: Unlimited...
Amazon.com has introduced Kindle Unlimited — a new subscription service which allows customers to freely read as much as they want from over 600,000 Kindle books, and listen as much as they want to... Read more
New Linksys Wireless Range Extenders Boost Wi...
Linksys has announced its new lineup of Linksys Wi-Fi Range Extenders. Consumers often experience a weak wireless signal in some parts of their house or apartment caused by blocking elements such as... Read more
MacBook Airs available starting at $719
The Apple Store has Apple Certified Refurbished 2013 & 2012 MacBook Airs in stock today starting at $719. An Apple one-year warranty is included with each MacBook, and shipping is free: 2013... Read more
Get the best deals on iPad minis with Apple r...
The Apple Store has Certified Refurbished 2nd generation iPad minis with Retina Displays available for up to $130 off the cost of new models, starting at $339. Apple’s one-year warranty is included... Read more
Best Buy’s College Student Deals: $100 off Ma...
Take an additional $100 off all MacBooks and iMacs, $50 off iPad Airs and iPad minis, at Best Buy Online with their College Students Deals Savings, valid through July 25th. Anyone with a valid .EDU... Read more

Jobs Board

*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
Mac Expert - *Apple* Online Store Mexico -...
…MUST be fluent in English and Spanish to be considered for this position At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the Read more
*Apple* Computer Technician - Fairfield Coun...
Company DescriptionWe are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over 20 Read more
Mac Expert - *Apple* Online Store - Apple (...
**Job Summary** At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the ultimate customer experience. We believe each customer Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.