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Chapter 1What Is JavaScript?JavaScript is a lightweight object-based scripting language created by Netscape Communications Corporation for developing Internet applications. JavaScript is lightweight in that there isn't a great deal to learn and you can be productive with it very quickly, in contrast to much more complex languages such as Java. As a scripting language, JavaScript is meant to tell an application what to do. Unlike languages used to create applications, it cannot do anything without the application. You can develop server applications or client applications with JavaScript. In this book, the term "server" refers to the computer where your Web pages reside. The term "client" refers to the browser application that loads and displays your Web pages. This book focuses on teaching you to create client applications with JavaScript-specifically, documents (Web pages) on the World Wide Web. You can embed JavaScript statements in Web pages, which are written in HTML (Hypertext Markup Language). JavaScript is an extension to HTML that lets you create more sophisticated Web pages than you ever could with HTML alone. To appreciate this, it helps to know a little history. The history of JavaScriptStrictly speaking, HTML is a Standard Generalized Markup Language (SGML), Document Type Definition (DTD). An SGML document has three parts. The first part defines the character set to be used and tells which characters in that set distinguish text from markup tags. Markup tags specify how the viewer application, or browser, should present the text to the user. The second part of an SGML document specifies the document type and states which markup tags are legal. The third part of an SGML document, called the document instance, contains the actual text and markup tags. Because there is no requirement that the three parts of an SGML document reside in the same physical file, we can concentrate on the document instance. The Web pages you create are document instances. Most HTML browsers assume a common definition about the character set used, and about which characters distinguish text from markup tags. They also generally agree about a core set of legal markup tags. They then diverge on which additional new markup tags to permit. HTML 1.0 and 2.0HTML 1.0 refers to the original set of markup tags. HTML 1.0 is so limited that a browser that restricted HTML documents to HTML 1.0 would be a museum piece. HTML 2.0 includes a more generous set of markup tags than HTML 1.0; in particular, it allows markup tags that define user input fields. As of this writing, HTML 2.0 defines the de facto common core of markup tags. You can create a relatively sophisticated Web page with HTML 2.0 markup tags. HTML 3.0 and NHTMLHTML 3.0, still in the process of standardization, adds additional markup tags to those defined in HTML 2.0, such as tags to define tables, figures, and mathematical equations. HTML 3.0 expands some tags to include more functionality, such as centering text or images in the browser, and adding background colors and images. NHTML, a nickname for Netscape's extension of HTML 2.0, is another set of markup tags that goes beyond those defined in HTML 2.0. Netscape, like other developers of cutting edge Web browsers, is trying to influence the development of the HTML 3.0 standard, and has developed extensions of its own. At the same time, Netscape is making an effort to conform to the evolving HTML 3.0 specification. Furthermore, Netscape continues to support markup tags that the draft HTML 3.0 specification has declared obsolete. Netscape's browser, Netscape Navigator, is not precisely HTML 3.0-compliant. The best way to find out whether Netscape Navigator supports a particular markup tag is to get the latest version and try a document containing the tag. Why NHTML isn't proper SGMLFormally, a Web page is the third part of an SGML DTD, and as such, should conform to the SGML DTD specification. A few features of NHTML do not conform to the rules of the SGML DTD specification. If browsers actually treated a Web page as the third part of an SGML DTD, this would be a problem. However, browsers typically accept a certain hard-coded level of HTML-typically HTML 2.0 with some HTML 3.0 extensions and some NHTML extensions-and ignore markup tags that they do not recognize. Where this nonconformity does present a problem is in writing tools that validate Web pages. These tools typically use an SGML parser, and they require a page to be part of a properly conforming SGML DTD for the level of HTML they check. Should you validate your Web pages? I think so, although as of this writing, it's not a viable option for JavaScript pages. So far, no one has come up with a validation tool that recognizes and validates the syntax of JavaScript. But count on it; someone will. And when they do, try it on your pages. Validation tools have saved me countless hours of headaches trying to figure out why my HTML code wasn't working correctly. Just for the record, here are the nonconforming parts of NHTML:
LiveScriptNetscape began working on a scripting language called LiveScript, which quickly evolved into what is now JavaScript. Although JavaScript and Java are not the same thing, Netscape intends JavaScript to tie into Java; hence the name change. Netscape and Sun Microsystems (the developers of Java) are working closely on the development of the two languages. There are few other major differences between LiveScript and JavaScript, the biggest being that LiveScript was case-insensitive and JavaScript is case-sensitive. What JavaScript isn'tJavaScript can provide a high degree of user interaction like some other systems, including CGI and Java. CGIThe Common Gateway Interface (CGI) provides a mechanism for a program on the server to interact with the client's browser. You can use any language to write CGI programs, and CGI programs may be interpreted (PERL scripts, for instance) or compiled (C or C++). One popular use of CGI is in hit counters-programs that modify the page to show how many times that page has been visited. Another popular use of CGI is in form handling, where a program on the server reads the data from the user input fields and does some work based on that data. JavaScript, which does its work in the client's browser, cannot entirely replace CGI. For instance, a hit counter has to update a file on the server so it can remember how many times the page has been visited by all visitors. That's a little difficult for JavaScript, but a JavaScript Web page can keep track of how many times a given visitor has visited the page. So can CGI, but only if given an endless supply of disk space on the server. JavaScript can do a lot of the same things CGI can do, and it can often do them much more efficiently. For example, JavaScript can do form validation more efficiently than CGI. When a non-JavaScript page has user input fields, it sends all the field values to a CGI server application. The CGI application then has to figure out whether the data in each field makes sense before doing something with the data. A JavaScript page, however, can validate the data entered before it is sent to the server. If the data is invalid, JavaScript can block transmission to the server. Because all of this work is performed on the client side, JavaScript does not waste bandwidth transmitting bad data and then receiving an error page from the server. JavaScript can also replace some of the animation and page-reloading functionality of CGI. To perform animation and page-reloading, CGI provides mechanisms called "server push" and "client pull." With server push, the Web page server maintains a connection between the client (the browser) and server. Server push restricts the number of simultaneous connections the Web page server can maintain-a popular page using server push will frequently reward potential visits with a "sorry, not now, try later" message. Client pull, on the other hand, involves the client frequently re-establishing its connection to the server, artificially adding to the traffic at the server. You can use JavaScript to create dynamic documents that would have required either server push or client pull in CGI, but that involve no additional traffic or long, drawn-out connections between the client and the server. JavaScript is independent of the server platform, the hardware and software that your server uses. CGI has to be written in a language that your server platform supports. No single language is supported by all server platforms. If your Web pages rely on CGI, what happens if the company that provides your server changes platforms? What happens if they go bankrupt and you have to take your pages and their CGI applications to another server? Finally, not all Web space providers allow Web pages to use CGI. CGI requires that the program be executed on the server, but some Web space providers are nervous about the possible side effects of badly written or maliciously written CGI programs being executed on their machines. Some providers only allow the use of a limited set of applications. Many providers do not support server push CGI. JavaScript running on the client browser is perfectly safe to the server, and affords you, the creator of the JavaScript document, much greater flexibility in how your document interacts with the reader. JavaMany people confuse JavaScript with Java, which is a programming language developed by Sun Microsystems, Inc. Each has its own Usenet newsgroup, yet people frequently post questions about Java to the JavaScript newsgroup, and vice versa. Java is a programming language and JavaScript is a scripting language. Java programs are compiled on the server. You can write stand-alone programs in Java. Scripts written in JavaScript are interpreted by the browser. You cannot write stand-alone programs in JavaScript-you need a browser to interpret JavaScript. Java is object-oriented. It employs classes and inheritance. It provides encapsulation of data. JavaScript is object-based. There are no classes. There is no inheritance. Data within objects is readily accessible. Java is compiled into "applets" that are accessed from HTML pages. JavaScript is embedded in HTML. Java requires that data types be strongly typed (if a function expects one of its arguments to be a number, the function will not accept a character string). JavaScript is loosely typed. JavaScript has numbers, character strings, and Booleans (logical yes/no, true/false, on/off data) and freely interchanges them. Java can be used to create very powerful applications. JavaScript scripts cannot really do all the neat things that Java applets can. On the other hand, it is much more difficult to write programs in Java than it is to write scripts in JavaScript. Platforms and browsersJavaScript, as described in this book, is supported by Netscape Navigator 2.01 and later releases. It is supported on several architectures, as you can see in Table 1.1.
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