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).

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Tidy Up 5.3.7 - Find duplicate files and...
Tidy Up is a full-featured duplicate finder and disk-tidiness utility. Features: Supports Lightroom: it is now possible to search and collect duplicates directly in the Lightroom library. Multiple... Read more
Pinegrow 5.97 - Mockup and design web pa...
Pinegrow (was Pinegrow Web Designer) is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation,... Read more
BlueStacks 4.210.0 - Run Android applica...
BlueStacks App Player lets you run your Android apps fast and fullscreen on your Mac. Feature comparison chart How to install Bluestacks on your Mac Go to MacUpdate and click the green "Download"... Read more
WhatsApp 2.2027.10 - Desktop client for...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
Art Text 4.0.1 - $29.99
Art Text is graphic design software specifically tuned for lettering, typography, text mockups and various artistic text effects. Supplied with a great variety of ready to use styles and materials,... Read more
Adobe Dreamweaver CC 2020 20.2 - Build w...
Dreamweaver CC 2020 is available as part of Adobe Creative Cloud for as little as $20.99/month (or $9.99/month if you're a previous Dreamweaver customer). Adobe Dreamweaver CC 2020 allows you to... Read more
Adobe Acrobat DC 20.009.20074 - Powerful...
Acrobat DC is available only as a part of Adobe Creative Cloud, and can only be installed and/or updated through Adobe's Creative Cloud app. Adobe Acrobat DC with Adobe Document Cloud services is... Read more
beaTunes 5.2.10 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
DiskCatalogMaker 8.1.5 - Catalog your di...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
Meteorologist 3.4.1 - Popular weather ap...
Meteorologist is a simple interface to weather provided by weather.com. It provides the ability to show the weather in the main menu bar, displaying more detail in a pop-up menu, whose contents are... Read more

Latest Forum Discussions

See All

Steam Link Spotlight - Disco Elysium
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was Signs of the Sojourner Read about how it plays using Steam Link over here. | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
Pokemon Go's July Community Day wil...
Pokemon Go developers have announced the details concerning the upcoming Gastly Community Day. This particular event was selected by the players of the game after the Gas Pokemon came in second place after a poll that decided which Pokemon would... | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Detective Di is a point-and-click murder...
Detective Di is a point-and-click murder mystery set in Tang Dynasty-era China. You'll take on the role of China's best-known investigator, Di Renjie, as he solves a series of grisly murders that will ultimately lead him on a collision course with... | Read more »
Dissidia Final Fantasy Opera Omnia is se...
Dissidia Final Fantasy Opera Omnia, one of Square Enix's many popular mobile RPGs, has announced a plethora of in-game events that are set to take place over the summer. This will include several rewards, Free Multi Draws and more. [Read more] | Read more »
Sphaze is a neat-looking puzzler where y...
Sphaze is a neat-looking puzzler where you'll work to guide robots through increasingly elaborate mazes. It's set in a visually distinct world that's equal parts fantasy and sci-fi, and it's finally launched today for iOS and Android devices. [... | Read more »
Apple Arcade is in trouble
Yesterday, Bloomberg reported that Apple is disappointed in the performance of Apple Arcade and will be shifting their approach to the service by focusing on games that can retain subscribers and canceling other upcoming releases that don't fit... | Read more »
Pixel Petz, an inventive platform for de...
Pixel Petz has built up a sizeable player base thanks to its layered, easy-to-understand creative tools and friendly social experience. It revolves around designing, trading, and playing with a unique collection of pixel art pets, and it's out now... | Read more »
The King of Fighters Allstar's late...
The King of Fighters ALLSTAR, Netmarble's popular action RPG, has once again been updated with a plethora of new content. This includes battle cards, events and 21 new fighters, which increases the already sizeable roster even more. [Read more] | Read more »

Price Scanner via MacPrices.net

$219 Apple AirPods Pro are back at Verizon, s...
Verizon has Apple AirPods Pro on sale again for a limited time for $219.99 on their online store. Their price is $30 off Apple’s MSRP, and it’s the lowest price we’ve seen for AirPods Pro. Available... Read more
Apple’s $779 13″ MacBook Air deal returns to...
Apple has clearance, Certified Refurbished, 2019 13″ MacBook Airs available again starting at $779. Each MacBook features a new outer case, comes with a standard Apple one-year warranty, and is... Read more
$200 13″ MacBook Pro discounts are back at Am...
Amazon has 2020 13″ 2.0GHz MacBook Pros on sale again today for $150-$200 off Apple’s MSRP. Shipping is free. Be sure to purchase the MacBook Pro from Amazon, rather than a third-party seller, and... Read more
Deal Alert! Apple AirPods with Wireless Charg...
Sams Club has Apple AirPods with Wireless Charging Case on sale on their online store for only $149.98 from July 6, 2020 to July 9, 2020. Their price is $50 off Apple’s MSRP, and it’s the lowest... Read more
Xfinity Mobile promo: Apple iPhone XS models...
Take $300 off the purchase of any Apple iPhone XS model at Xfinity Mobile while supplies last. Service plan required: – 64GB iPhone XS: $599.99 save $300 – 256GB iPhone XS: $749.99 save $300 – 512GB... Read more
New July 2020 promo at US Cellular: Switch an...
US Cellular has introduced a new July 2020 deal offering free 64GB Apple iPhone 11 smartphones to customers opening a new line of service. No trade-in required, and discounts are applied via monthly... Read more
Apple offers up to $400 Education discount on...
Apple has launched their Back to School promotion for 2020. They will include one free pair Apple AirPods (with charging case) with the purchase of a MacBook Air, MacBook Pro, iMac, or iMac Pro (Mac... Read more
July 4th Sale: Woot offers wide range of Macs...
Amazon-owned Woot is blowing out a wide range of Apple Macs and iPads for July 4th staring at $279 and ranging up to just over $1000. Models vary from older iPads and 11″ MacBook Airs to some newer... Read more
Apple Pro Display XDR with Nano-Texture Glass...
Abt Electronics has Apple’s new 32″ Pro Display XDR model with the nano-texture glass in stock and on sale today for up to $144 off MSRP. Shipping is free: – Pro Display XDR (nano-texture glass): $... Read more
New 2020 Mac mini on sale for up to $100 off...
Amazon has Apple’s new 2020 Mac minis on sale today for $40-$100 off MSRP with prices starting at $759. Shipping is free: – 2020 4-Core Mac mini: $759 $40 off MSRP – 2020 6-Core Mac mini: $998.99 $... Read more

Jobs Board

Physical Therapist Assistant - *Apple* Hill...
Physical Therapist Assistant - Apple Hill Rehab - Full Time Tracking Code 62519 Job Description General Summary: Under the direct supervision of a licensed Physical Read more
Operating Room Assistant, *Apple* Hill Surg...
Operating Room Assistant, Apple Hill Surgical Center - Full Time, Day Shift, Monday - Saturday availability required Tracking Code 62363 Job Description Operating Read more
Perioperative RN - ( *Apple* Hill Surgical C...
Perioperative RN - ( Apple Hill Surgical Center) Tracking Code 60593 Job Description Monday - Friday - Full Time Days Possible Saturdays General Summary: Under the Read more
Product Manager, *Apple* Commercial Sales -...
Product Manager, Apple Commercial Sales Austin, TX, US Requisition Number:77652 As an Apple Product Manager for the Commercial Sales team at Insight, you Read more
*Apple* Mac Product Engineer - Barclays (Uni...
Apple Mac EngineerWhippany, NJ Support the development and delivery of solutions, products, and capabilities into the Barclays environment working across technical Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.