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introduction to JavaScript programming

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introduction to JavaScript programming syntax


  • Navigator Is More Than a Web Browser
  • JavaScript's Place in Navigator
  • Java and JavaScript: Compare and Contrast
  • The Current State of JavaScript
  • What Is JavaScript?
  • JavaScript Is Object-Based
  • Working with Objects in JavaScript
  • Strengths of JavaScript
  • Weaknesses of JavaScript

    Navigator 3.0 is the most powerful version of Netscape's Web browser. Besides bringing together a collection of useful Internet-access tools, such as a mail client, a news reader, and improved support for the developing HTML 3 standard, Navigator 3.0 adds several features that enhance the ability of Web authors to develop complete, platform-independent applications deployed and executed in the Netscape browser. Going beyond the Web browser, Navigator Gold adds editing and development tools to the package.

    These capabilities include an applications programmer's interface (API) for plug-ins.

    Plug-ins are program modules that dynamically extend the capability of Navigator to handle new types of data and information, along with JavaScript and Java, which allow the addition of flexible programmability to Web pages.

    JavaScript is also incorporated into Netscape's servers, such as LiveWire, which allows it to be used to develop server-end programs instead of the traditional CGI-BIN interface used in today's Web servers. In addition, Microsoft Internet Explorer 3.0, the latest Web browser for Windows 95 and Windows NT from Microsoft, incorporates numerous features found in Navigator 3 including JavaScript.

    In this chapter we take a detailed look at the main features and aspects of JavaScript, as well as review the major strengths and weaknesses of the JavaScript language and its suitability to particular tasks.

    We then dive deeper into objects and how they work and take a look at properties and methods-the building blocks of objects. We also look at the built-in objects in JavaScript and what they offer the programmer.

    In this chapter we take a broad look at Navigator and consider how JavaScript fits into the puzzle. You'll learn about the following topics:

    • Frames: The ability to divide a window into multiple, independent sections
    • Plug-ins: Third-party add-ons for Navigator that extend the browser's ability to handle new data and information
    • Java: An object-oriented programming language for distributed applications
    • JavaScript: A simple, object-based programming language incorporated into Navigator 3.0 (and the subject of this book)
    • The similarities and differences between Java and JavaScript
    • JavaScript as a scripting language
    • Objects, properties, and methods
    • The Navigator Object Hierarchy and other built-in objects
    • Strengths and weaknesses of JavaScript

    Navigator Is More Than a Web Browser

    Although Netscape Navigator started out its life as a basic Web browser, as it has grown increasingly popular, it has become much more.

    Unlike earlier browsers and today's basic Web applications, Navigator now provides authors with numerous tools to step beyond the traditional constraints of HTML. Instead of simply combining text, pictures, sound, and video, authors now have finer control over document layout, fonts, and color; they are able to extend the functionality of the browser using plug-ins and Java; and they can produce interactive applications using JavaScript.

    A quick look at the Netscape Web site shows that today's Navigator can do so much more than previous versions-even without special programming by Web developers. With freely available plug-ins from leading software companies, Web authors can include native CorelDRAW! graphics or Microsoft Word files in their documents, as well as view VRML (Virtual Reality Modeling Language) worlds, and view documents formatted in Adobe's device-independent Acrobat format.

    On top of all this, Navigator provides several tools that Web page developers and authors can take advantage of to enhance their documents and add dynamic interaction with the information they provide on the Internet.


    Frames are the most visually noticeable extension of HTML in Navigator 2.0 and 3.0. Using frames, authors are able to partition the screen into multiple rectangular sections. Each section is independent of all the others, and a different URL is loaded into each frame. In addition, links in one frame can be used to update another frame, without disturbing any others.

    Frames are being used today to produce fixed, bannerlike mastheads for Web Pages, for menus that always stay on the screen and don't need to be reloaded or redrawn, and for forms-based searches where the form is always available.

    In addition to the obvious visual appeal of frames technology, this extension to HTML may have an added benefit in that it reduces the amount of data that may need to be requested from the server and sent to clients. On the increasingly congested World Wide Web, this may prove to make Web browsing slightly more efficient and reduce some of the load on over-burdened Web servers.

    Figure 1.1 shows an example of how frames can divide a screen and provide both a fixed masthead and a permanent menu next to several frames of dynamic information. Figure 1.2 shows how a menu selection updates a section of the screen.


    By providing an open application programmer's interface for plug-ins, Netscape has made it possible for third-party software vendors to provide the ability to view a wide range of data and documents in the Navigator browser window. Netscape refers to objects displayed by plug-ins as Live Objects.

    Netscape integrates plug-ins-and Java applets-into the browser environment using LiveConnect. LiveConnect provides mechanisms for plug-ins, Java applets, and JavaScript scripts to communicate with each other and to share information with each other.

    To date, a significant number of vendors including Corel, Paper Software, and Adobe have produced plug-ins for their own formats. (Figure 1.3 shows one such company.) Many of these will likely be extended to support JavaScript, enabling JavaScript programs to interact with plug-ins directly and further enhancing the power of JavaScript in future versions of Netscape Navigator. At the present time there are no tools for doing this.

    The number of available plug-ins is constantly changing. Netscape provides an up-to-date list of available plug-ins at this site:

    Interactivity with Java and JavaScript

    Using JavaScript and Java, Web developers have come up with further enhancements to Web pages beyond the ability to load multiple documents in separate frames and view new file formats.

    Some of the common Java applets available today include the following:

    The term applet has come into common use since Sun Microsystems introduced Java to the Web community in 1995. Applets are small applications that are included in Web pages and downloaded on demand to be executed by the client browser. Although the term technically only refers to the type of Java programs described here, with the introduction of JavaScript, the term is now being used by some people on the Web to describe the type of integrated scripts developed with JavaScript.

    • Scrolling text applets: Java is being used to add marquees to Web pages.
    • Live data feeds: Current examples include a live stock ticker, such as the financial portfolio demonstration by Sun Microsystems' Jim Graham, shown in Figure 1.4.

    • Search engines: Java applets can be used to build database queries, which are then sent to remote databases.
    • Adding protocols to Web browsers: One of the potential possibilities offered by Java is that, as protocols are developed on the Internet, Java can be used to add functionality to Web browsers on demand.
    • New file formats: Java allows Web browsers to display new file formats by downloading applets to display the files as needed.

    The widespread popularity of Java means that the range of uses will increase in the future, but today Java is being used primarily for cosmetic enhancements to Web pages.

    By comparison, JavaScript is used to produce scripts designed to react to user and environment events-as well as in the future being the glue to hook Java applets more seamlessly into Web pages. The following are some examples:

    • An interactive color picker for Web developers to test different background and text colors in their documents.
    • Calculators: Examples on the Web include a unit conversion calculator and loan interest calculators.
    • Dynamic output based on the current environment and the user's previous surfing history.
    • Forms verification: JavaScript can be used to ensure that form data is entered properly before sending it to the server, rather than relying on the server to verify form content after it is submitted. Figure 1.5 illustrates the form verification.

    • Building URLs: JavaScript is used to build custom URLs based on user choices in forms.
    • JavaScript can be used to replace many CGI scripts for client-side processing, easing bandwidth demands, and decreasing server load for busy Web servers.
    • Newsgroup-like discussion groups can incorporate sophisticated features such as threading.

    JavaScript's Place in Navigator

    JavaScript is one of the least-used components of Netscape's suite of interactivity tools (although that is quickly changing), but it could be the most significant. Not only does it bring interactive programming within the reach of the average Web author with no formal programming experience, it can also be used to move much of the processing away from over-burdened servers to increasingly powerful client workstations.

    JavaScript Programs Are Built into Web Pages

    As opposed to the independent application files used to deliver Java applets to Web browsers, the actual source code for JavaScript scripts can be included directly in Web pages. This is distinct from Java applets, which exist independently of the HTML Web pages.

    JavaScript Gives Programmers Access to Browser Properties

    JavaScript is integrated tightly into HTML and the Navigator browser. Developers have available to them a wide range of tools and information to interact with the currently loaded HTML document, as well as the current browser session.

    JavaScript exposes properties related to the document windows, the history list, the loaded documents, frames, forms, and links to the programmer. In addition, JavaScript can be used to trap user events, such as changing form values or pointing at links, so that appropriate programs can be developed for each event.

    Java and JavaScript: Compare and Contrast

    Given the tremendous popularity of Java since its introduction in 1995, it is important to take a look at the differences between Java and JavaScript. Although they are related-JavaScript borrows most of Java's syntax, for instance-they are fundamentally different and serve different purposes. They are complementary rather than competing with each other.

    Using Java-A Complex, Complete Object-Oriented Programming Language

    Java is much more than a language for developing Web-based applications. It is designed to compete in a market of full-fledged, general-purpose programming languages such as C, C++, Pascal, and FORTRAN. Unlike its predecessors, Java's claims to fame include the fact that it is platform-independent and that it can be used for both applications development and the development of in-line applets, or small applications, for Web pages.

    Like C++ and Smalltalk, Java is object-oriented and relies heavily on the syntax and style of C++. With this comes the steep learning curve of a high-end object-oriented programming language.

    A Compiled Language

    Unlike most other general-purpose programming languages, Java is not compiled in the traditional sense. Instead of compiling to native machine code, the Java compiler converts source code into Java byte codes (known as architecture-neutral byte-codes)-a platform-independent representation of the Java program code-which are then run on a machine-dependent runtime interpreter. In this way, developers only need to develop and maintain one set of source code and compile it once, and the code can then be run using the runtime interpreters for any machine.

    Like all compiled languages, though, this adds the complexity of a compilation cycle to development and especially to debugging. However, to a certain degree like other compiled languages, an efficient runtime engine means that Java should offer better performance than general-purpose, interpreted scripting languages.

    Fully Extensible

    A fundamental feature of true object-oriented languages is that they are extensible. That is, programmers can create their own classes-or groupings of objects and data structures-to extend the basic classes that are part of the programming languages.

    A class is a term used in object-oriented programming to refer to a set of related objects that share common characteristics. Classes, and the ability to create new classes, are what make object-oriented programming a powerful and flexible programming model.

    Java is no exception to this rule. Java programmers routinely create their own extensions to the base set of tools or classes.

    Steep Learning Curve

    As mentioned before, object-oriented programming languages tend to have steep learning curves, especially for nonprogrammers. Java is not exempt from this difficulty.

    The general consensus among beginning programmers is that learning Java is a formidable task, especially considering the complexity of the available on-line documentation on the Internet.

    Enables Client-Server Interaction

    The base set of classes that comes with the Java distribution make it ideally suited to client-server interactions. The ability to work with URLs and talk to HTTP servers already exists. The support for applets adds the ability to interact with user events in the client Web browser.

    In addition, HotJava, the demonstration browser from Sun Microsystems, demonstrates how Java can become the means by which browsers dynamically learn to handle new protocols as that ability is needed.

    Sun developed HotJava when Java was still in alpha development to demonstrate the potential of Java for distributed applications on the World Wide Web and to show how browsers could dynamically learn to handle new protocols and file types. HotJava is available for Solaris and 32-bit Windows (Windows NT and Windows 95) from the Java home page at

    Developing Stand-Alone Applications and Applets

    Java is famous because it can be used to develop applets that are delivered on the World Wide Web and executed in client Web browsers. However, Java can also be used to develop complete, platform-independent GUI applications using the Java runtime interpreter.

    Offers Sophisticated Security

    Because of the extremely open and public nature of the World Wide Web, security is a major issue for Java and Java applets. After all, allowing application code from unknown remote machines to be downloaded and executed on your computer system is potentially dangerous. Not only is there potential for applets to contain viruses, but they could simply be malicious applications intent on destroying your data and rendering your computer inoperable.

    To address this, Sun implemented tight security features from the earliest stages of Java development. These features include verification of bytecodes (to ensure they don't violate access restrictions and more), as well as configurable network security that ranges from disabling network access to limiting access by an applet only to the host where the code originated, all the way to completely free network access.

    There have been reports of security holes in the Java environment, but both Sun and Netscape have been responsive to these reports and concerns. With each release of Java, security should be improved.

    Distinct from HTML

    Even though applets are a feature of the World Wide Web and are included as in-line applets in HTML files, they are distinct and separate from HTML and HTML files. New HTML tags force a Web browser to initiate a new connection to the server and tell the browser where to display the applet's output on the Web page, but beyond that, applets are separate and distinct from HTML.

    Using JavaScript: A Simple, Object-Based Scripting Language

    In contrast to Java, JavaScript joins the ranks of simple, easy-to-use scripting languages. JavaScript promises an easier learning curve than Java along with powerful tools to add interactivity to Web pages with little effort.

    Derived from Java

    JavaScript owes a lot to Java. Its syntax and basic structure are similar to Java, even if the range of functions and the style of programming can differ greatly. JavaScript started life as Netscape's own scripting language with the name LiveScript, but in late 1995, Sun endorsed the language, and it became JavaScript.

    JavaScript keeps more than just the basic syntax and structure of Java-it also borrows most of Java's flow constructs and implements some of the same security precautions, such as preventing applets from writing to the local disk.

    An Interpreted Language

    Unlike Java, JavaScript is an interpreted language. Whereas in Java, source code is compiled prior to runtime, in an interpreted language, source code files are executed directly at runtime in JavaScript.

    Interpreted languages offer several advantages-as well as several drawbacks. Interpreted languages such as JavaScript are generally simpler than compiled languages and are easy to learn. It is often easier to develop, change, and trouble-shoot programs because the need to recompile with each change is removed.

    On the negative side, the need to interpret commands as the program is run can produce a performance hit with some interpreted languages. In the case of JavaScript, this doesn't seem to be a problem.

    JavaScript scripts are compiled to byte codes (similar to Java) as the script is downloaded and evaluated, and for most scripts, performance feels quite snappy.

    Not Fully Extensible

    Unlike Java, JavaScript is not fully extensible. The JavaScript model is one of a limited set of base objects, properties, methods, and data types, which provide enough capabilities to create client-side or server-side applications.

    While users can create their own objects and write functions, this is not the same as the classes and inheritance offered in Java and other object-oriented programming languages.

    Since JavaScript is an object-based scripting language, this book will be devoted to learning about the objects, properties, methods, and data types available in JavaScript.

    Limited Client-Server Interaction

    JavaScript in its current form is not designed for complete client-server interaction. Beyond analyzing, building, and invoking URLs, JavaScript can't talk directly to servers or talk different protocols. Essentially, JavaScript is well suited to handling client-end activity.

    Still, there are indications that future versions of JavaScript will support protocols such as HTTP and ftp, but several security issues surrounding client-server interaction through JavaScript need to be addressed by Netscape before it implements these features.

    In addition, JavaScript is part of the LiveWire Web server offered by Netscape. LiveWire is aimed at groups and organizations developing interactive Web applications. In this role, JavaScript provides an alternative to traditional CGI scripting for server-end programming. In this sense, JavaScript can be used for client-server interaction.

    Integrated into HTML

    Where Java is only loosely tied to HTML, JavaScript is tightly integrated into HTML files. Typically, entire scripts are in the same files as the HTML that defines a page, and these scripts are downloaded at the same time as the HTML files.

    The Current State of JavaScript

    In undertaking the task of learning JavaScript, it is important to keep in mind the current status of the language and where its development appears to be headed.

    Under Development

    Both Navigator 3.0 and JavaScript are under intense development and are constantly evolving. So, JavaScript is, by definition, a language under development (as are most programming languages). In practical terms, that means that the complete language specification is not implemented yet.

    In a very real way, JavaScript is a moving target. From Navigator 2.0 to Navigator 3.0, the feature set has been expanded greatly and changed noticeably. In addition, some existing methods and properties have been altered. There is no reason to believe that this development and changing won't extend into future versions of Navigator.

    On top of this, Microsoft's implementation of JavaScript differs subtly from Netscape's, and there is no guarantee that these differences won't persist or change over time.

    Supported by Sun

    In late November 1995, LiveScript became JavaScript in a joint announcement by Netscape and Sun that Sun would be supporting JavaScript as the Java-based open scripting standard for the Internet.

    Given this support, there is little doubt that JavaScript will continue to resemble Java and that it will become the choice tool for gluing Java applets into Web pages, at least in the Netscape environment.

    Essentially, this endorsement from Sun breathed life into a scripting language that very few people were paying attention to, but which is now the hot topic on the World Wide Web.

    Endorsed by Many Companies

    Sun Microsystems isn't the only company to support JavaScript. At the same time as Netscape and Sun made their announcement, more than 28 companies including America Online, Apple Computers, Oracle, Silicon Graphics, Architext, and SCO announced that they would also be endorsing JavaScript as the open scripting standard for the Internet, and many indicated they were considering licensing the technology to include in their own products.

    Potential Use in Different Products

    In addition to including JavaScript in its Navigator Web browser, Netscape is including JavaScript in its server-end application development environment called LiveWire.

    This makes it possible that the momentum for CGI programming using Perl and Bourne or C shell may shift toward JavaScript, which would provide a consistent development environment at both the client and server ends.

    In addition, the endorsements from numerous companies mean that JavaScript will start to appear in products from companies other than Netscape and in products other than Web browsers and servers. As mentioned earlier, JavaScript is already supported in Microsoft's Internet Explorer.

    In addition, Netscape has proposed JavaScript as a scripting standard for the World Wide Web.

    JavaScript Today: Scripting for the Netscape Web Browser

    In this book, we are focusing on using JavaScript in the Navigator Web browser because this is where most JavaScript development is taking place and because this is the easiest environment in which to learn and practice JavaScript. JavaScript is also available in Microsoft's new Internet Explorer version 3. Internet Explorer also includes another scripting language VBScript based on the company's Visual Basic programming language. VBScript is not covered in this book.

    What Is JavaScript?

    By now, you probably have some idea of what JavaScript is and what it isn't, but let's look more closely at the features you'll be learning and using throughout the course of this book.

    JavaScript Is a Scripting Language

    Scripting languages have been in use long before the Web came around. In the UNIX environment, scripts have been used to perform repetitive system administration tasks and to automate many tasks for less computer-literate users. In addition, scripting languages are the basis of much of the CGI-BIN programming that is currently used to add a limited form of interactivity to Web pages.

    Examples of scripting languages include Perl, well known in CGI programming, Awk and SED (designed for intensive text processing), and even HyperTalk which, like JavaScript, is an object-oriented scripting language.

    Of course, this still doesn't tell you what the main advantages of scripting languages are. Like all scripting languages, JavaScript is interpreted, which provides an easy development process; it contains a limited and easy-to-learn command set and syntax; and it is designed for performing a well-defined set of tasks.

    Designed for Simple, Small Programs

    Because JavaScript is a scripting language, it is well suited to implementing simple, small programs. For instance, JavaScript would ideally be suited to developing a unit conversion calculator between miles and kilometers or pounds and kilograms. These tasks can be easily written and performed at acceptable speeds with JavaScript and would be easily integrated into a Web page. A more robust language such as Java would be far less suitable for the quick development and easy maintenance of these types of applications.

    By contrast, JavaScript would not be well suited to implementing a distributed CAD document display and manipulation environment. While eventually JavaScript will be a tool for integrating this type of Java applet or plug-in into a Web page, to attempt to develop the actual applet in JavaScript would be at best, difficult and inefficient and, more likely, would be impossible.

    Of course, this doesn't mean that sophisticated applications can't be-and aren't being-developed with JavaScript. Nonetheless, scripting languages are generally used for smaller tasks rather than for full, compiled programs.

    Performs Repetitive Tasks

    Just as JavaScript is suited to producing small programs, it is especially well designed for repetitive, event-invoked tasks. For example, JavaScript is ideal for calculating the content of one field in a form based on changes to the data in another field. Each time the data changes, the JavaScript program to handle the event is invoked, and the new data for the other field is calculated and displayed.

    Designed for Programming User Events

    Because of the way in which JavaScript is integrated into the browser and can interact directly with HTML pages, JavaScript makes it possible to program responses to user events such as mouse clicks and data entry in forms.

    For instance, a JavaScript script could be used to implement a simple help system. Whenever the user points at a button or a link on the page, a helpful and informative message can be displayed in the status bar at the bottom of the browser window.

    This adds interactivity to Web pages, makes forms dynamic, and can decrease the bandwidth requirements and server load incurred by using forms and CGI programming.

    Easy Debugging and Testing

    Like other scripting languages, JavaScript eases development and trouble-shooting because it is not compiled. It is easy to test program code, look at the results, make changes, and test it again without the overhead and delay of compiling.

    The Java Glue

    When Netscape announced JavaScript, it referred to the language as the tool to "glue Java applets into Web pages." With Navigator 3.0 this has become possible. JavaScript can trigger events in Java applets, and Java applets can call JavaScript methods and functions.

    JavaScript Is Object-Based

    Object-oriented is a term that has been overused by the media and marketing arm of the computer and software industries. Nonetheless, the fact that JavaScript has a limited object-oriented model is an important distinction.

    JavaScript's Object Model

    In order to understand what it means for JavaScript to be object-based, we need to look at objects and how they work.

    Fundamentally, objects are a way of organizing information, along with the methods for manipulating and using that information.

    Objects provide a way to define specific pieces of data related to the item in question; these pieces are known as properties. In addition, these are supplemented by tasks that can be performed on or with that information, known as methods. Together properties and methods make up objects.

    Because of the general nature of objects, specific instances can be created for each case where they are needed. For instance, a car object could then have several instances for Toyotas, Fords, and Volkswagens.

    A Comparison with Procedural Languages

    Defining objects in this way differs greatly from the way in which information is handled in traditional procedural programming languages such as FORTRAN and C.

    In these languages, information and procedures (similar to methods) are kept separate and distinct and are not linked in the way that objects are. Also, the concept of creating instances isn't as well developed in procedural languages.

    Working with Objects in JavaScript

    JavaScript includes built-in objects to work with elements of the currently loaded HTML document, as well as performing other useful tasks, such as mathematical calculations. It also offers the programmer the chance to create her own objects.

    Built-In Objects

    JavaScript offers a set of built-in objects that provide information about the currently loaded Web page and its contents, as well as the current session of Navigator. In addition, these objects provide methods for working with their properties.

    The Navigator Object Hierarchy

    Most of the built-in objects in JavaScript are part of the Navigator Object Hierarchy. The Navigator Object Hierarchy is built from a single base object called the window object, as illustrated in the following outline.


    Table 1.1 highlights the major JavaScript objects.

    Table 1.1. Overview of the Navigator Object Hierarchy.

    navigator The navigator object provides properties that expose information about the current browser to JavaScript scripts.
    window The window object provides methods and properties for dealing with the actual Navigator window, including objects for each frame.
    location The location object provides properties and methods for working with the currently open URL.
    history The history object provides information about the history list and enables limited interaction with the list.
    document The document object is one of the most heavily used objects in the hierarchy. It contains objects, properties, and methods for working with document elements including forms, links, anchors, and with applets.

    Other Built-In Objects

    In addition to the objects in the Navigator Object Hierarchy, JavaScript provides several objects that are not related to the current windows or loaded documents. Table 1.2 outlines the major features of these objects.

    Table 1.2. Other built-in objects.

    string The string object enables programs to work with and manipulate strings of text, including extracting substrings and converting text to upper- or lowercase characters.
    Math The Math object provides methods to perform trigonometric functions, such as sine and tangent, as well as general mathematical functions, such as square roots.
    Date With the Date object, programs can work with the current date or create instances for specific dates. The object includes methods for calculating the difference between two dates and working with times.

    Extending JavaScript: Creating Your Own Objects

    In addition to a wide range of built-in objects, as a JavaScript programmer, you can create your own objects to use in your scripts.


    For example, if you need to build an object in JavaScript to represent the different types of airplanes sold by an aircraft manufacturer, you would have several pieces of information related to the airplane that you would want included in the object, including the following:

    • Model
    • Price
    • Normal seating capacity
    • Normal cargo capacity
    • Maximum speed
    • Fuel capacity

    Properties are like variables in traditional languages, such as C and Pascal. Variables are named containers which are used to hold pieces of data, such as numbers or text. Variables are discussed in more detail in "Working with Data and Information."

    So, in JavaScript, if you call your object airplane, these properties might be referred to as:


    As you will learn later, in JavaScript, the properties of objects are referred to by the structure


    Of course, having this information isn't worth much without ways to use the information. For instance, in this example, you want to be able to print out a nicely formatted description of the aircraft or be able to calculate the maximum distance the plane can travel based on the fuel capacity.

    In object-oriented terminology, these tasks are known as methods. Like properties, your methods might be referred to as:


    Objects within Objects

    Objects can also include other objects in much the same way as properties and methods. For instance, the airplane manufacturer may want to include an object inside his object definition to handle information about the number of planes in use worldwide, who is using them, and their safety records. This information, along with methods for working with the information, could be combined into an object called airplane.record. This object could then include properties and methods such as:



    What you have created is a general description of an object that defines the information you want to work with and the ways you want to work with it. These general descriptions of objects are known as classes. In object-oriented programming, then, you can create specific instances of the class as needed.

    In object-oriented programming, creating specific copies of classes is known as creating instances. JavaScript itself is classless, but provides mechanisms to create instances of objects (thus providing the same basic functionality). We cover the details of creating objects and instances in "Functions and Objects-The Building Blocks of Programs."

    For instance, in this example, the airplane manufacturer might want to create an instance of the airplane object for its newest aircraft, the SuperPlane. If this instance were created, then a program could assign specific values to the properties of the new instance by referring to superplane.price, superplane.model, and so on. Likewise, a description of the new plane could be printed out using superplane.description().

    Strengths of JavaScript

    JavaScript offers several strengths to the programmer including a short development cycle, ease-of-learning, and small size scripts. These strengths mean that JavaScript can be easily and quickly used to extend HTML pages already on the Web.

    Quick Development

    Because JavaScript does not require time-consuming compilation, scripts can be developed in a relatively short period of time. This is enhanced by the fact that most of the interface features, such as dialog boxes, forms, and other GUI elements, are handled by the browser and HTML code. JavaScript programmers don't have to worry about creating or handling these elements of their applications.

    Easy to Learn

    While JavaScript may share many similarities with Java, it doesn't include the complex syntax and rules of Java. By learning just a few commands and simple rules of syntax, along with understanding the way objects are used in JavaScript, it is possible to begin creating fairly sophisticated programs.

    Platform Independence

    Because the World Wide Web, by its very nature, is platform-independent, JavaScript programs created for Netscape Navigator are not tied to any specific hardware platform or operating system. The same program code can be used on any platform for which Navigator 2.0 is available.

    Small Overhead

    JavaScript programs tend to be fairly compact and are quite small, compared to the binary applets produced by Java. This minimizes storage requirements on the server and download times for the user. In addition, because JavaScript programs usually are included in the same file as the HTML code for a page, they require fewer separate network accesses.

    Weaknesses of JavaScript

    As would be expected, JavaScript also has its own unique weaknesses. These include a limited set of built-in methods, the inability to protect source code from prying eyes, and the fact that JavaScript still doesn't have a mature development and debugging environment.

    Limited Range of Built-In Methods

    Early versions of the Navigator 2.0 beta included a version of JavaScript that was rather limited. In the final release of Navigator 2, the number of built-in methods had significantly increased, but still didn't include a complete set of methods to work with documents and the client windows. With the release of Navigator 3, things have taken a further step forward with the addition of numerous methods, properties, and event handlers.

    No Code Hiding

    Because the source code of JavaScript script presently must be included as part of the HTML source code for a document, there is no way to protect code from being copied and reused by people who view your Web pages.

    This raises concerns in the software industry about protection of intellectual property. The consensus is that JavaScript scripts are basically freeware at this point in time.

    Lack of Debugging and Development Tools

    Most well-developed programming environments include a suite of tools that make development easier and simplify and speed up the debugging process.

    Currently, there are some HTML editors and programming editors that provide JavaScript support. In addition, there are some on-line tools for debugging and testing JavaScript scripts. However, there are really no integrated development environments such as those available for Java, C, or C++. LiveWire from Netscape provides some development features but is not a complete development environment for client-side JavaScript.


    JavaScript is one of several key components in Netscape Navigator designed to help Web developers produce interactive applications on the Internet. Other features include support for plug-ins, frames, and of course, Java.

    Where Java is compiled, object-oriented, complex, and distinct from HTML, JavaScript is interpreted, object-based, simple to use and learn, and tightly integrated with HTML.

    JavaScript exposes properties of the current browser session to the programmer, including elements of the currently loaded HTML pages, such as forms, frames, and links.

    In this chapter we also took a look at the implications of the fact that JavaScript is a scripting language, took a close look at what objects are and how they are used in JavaScript, and took a broad look at the built-in objects in JavaScript.

    You learned that, while JavaScript offers many benefits including a quick development cycle and platform independence, it still has drawbacks.

    In the next chapter, you will begin taking a look at the specifics of developing JavaScript scripts and will work on your first script.


    QI've used only visual programming tools for making Web pages. Can I learn JavaScript? How will knowing JavaScript make my home page better?
    AWhile it's true that programming in JavaScript is more similar to programming in C than using a visual programming tool, most JavaScript programming is simple enough to make it easy for the complete beginner.
    In addition, the development of interfaces to JavaScript programs almost entirely involves the use of HTML and HTML forms, which can be developed in a visual environment using several HTML development tools, including Navigator Gold from Netscape.
    If you learn JavaScript, you will be able to add interactivity to Web pages. For instance, if a user enters data in a form, a result can be calculated and displayed for the user. Similarly, if the user moves the mouse over a link or button, help information can be displayed in the status bar of the Navigator window.
    QI want to add interactivity to my Web pages. Should I learn Java or JavaScript?
    AJava and JavaScript are not competitors. They are complementary programming languages which both extend the functionality of Web browsers-in this case Netscape Navigator.
    You will find that Java is suited to a set of tasks different from JavaScript. For instance, a viewing tool for CAD documents would need to be developed using Java applets while an interactive HTML form could only be developed using JavaScript.

introduction to JavaScript programming code practice

introduction to JavaScript programming tutorial

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The anchor Object [C|2|3|I]

The applet Object [C|3]

The area Object [C|3]

The Array Object [C|3|I]

The button Object [C|2|3|I]

The checkbox Object [c|2|3|I]

The combo Object [C|I]

The Date Object [C|2|3|I]

The document Object [C|2|3|I]

The FileUpload Object [C|3]

The form Object [C|2|3|I]

The frame Object [C|2|3|I]

The Function Object [C|3]

The hidden Object [C|2|3|I]

The history Object [C|2|3|I]

The Image Object [C|3]

The link Object [C|2|3|I]

The location Object [C|2|3|I]

The Math Object [C|2|3|I]

The mimeType Object [C|3]

The navigator Object [C|2|3|I]

The Option Object [C|3]

The password Object [C|2|3|I]

The plugin Object

The radio Object [C|2|3|I]

The reset Object [C|2|3|I]

The select Object [C|2|3]

The String Object [C|2|3|I]

The submit Object [C|2|3|I]

The text Object [C|2|3|I]

The textarea Object [C|2|3|I]

The window Object [C|2|3|I]

Independent Functions, Operators, Variables, and Literals

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