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Getting started

his chapter introduces JavaScript, discusses some of the fundamental concepts of JavaScript in Navigator and provides basic examples. It shows JavaScript code in action, so you can begin writing your own scripts immediately, using the example code as a starting point.


What is JavaScript?

JavaScript is Netscape's cross-platform, object-based scripting language for client and server applications. There are two types of JavaScript:

JavaScript in Navigator

Netscape Navigator 2.0 (and later versions) can interpret JavaScript statements embedded in an HTML page. When Navigator requests such a page, the server sends the full content of the document, including HTML and JavaScript statements, over the network to the client. The Navigator then displays the HTML and executes the JavaScript, producing the results that the user sees. This process is illustrated in the following figure.

Client-side JavaScript statements embedded in an HTML page can respond to user events such as mouse-clicks, form input, and page navigation. For example, you can write a JavaScript function to verify that users enter valid information into a form requesting a telephone number or zip code. Without any network transmission, the HTML page with embedded JavaScript can check the entered data and alert the user with a dialog box if the input is invalid.

JavaScript in LiveWire

LiveWire is an application development environment that uses JavaScript for creating server-based applications similar to CGI (Common Gateway Interface) programs. In contrast to Navigator JavaScript, LiveWire JavaScript applications are compiled into bytecode executable files. These application executables are run in concert with a Netscape server (version 2.0 and later) that contains the LiveWire server extension.

The LiveWire server extension generates HTML dynamically; this HTML (which may also include client-side JavaScript statements) is then sent by the server over the network to the Navigator client, which displays the results. This process is illustrated in the following figure.

For more information on LiveWire, see the LiveWire Developer's Guide.

In contrast to standard CGI programs, LiveWire JavaScript is integrated directly into HTML pages, facilitating rapid development and easy maintenance. LiveWire JavaScript contains an object framework that you can use to maintain data that persist across client requests, multiple clients, and multiple applications. LiveWire JavaScript also provides objects and methods for database access that serve as an interface to Structured Query Language (SQL) database servers.

JavaScript, the language

As described in the previous sections, client and server JavaScript differ in numerous ways, but they have the following elements in common:

So, if you have LiveWire, you will often be able to write functions that work on either the client or the server.

Different versions of JavaScript work with specific versions of Navigator. For example, JavaScript 1.1 is for Navigator 3.0. For information, see "Specifying the JavaScript version".

JavaScript and Java

JavaScript and Java are similar in some ways but fundamentally different in others. The JavaScript language resembles Java but does not have Java's static typing and strong type checking. JavaScript supports most Java expression syntax and basic control-flow constructs. In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a simple, instance-based object model that still provides significant capabilities. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.

Java is an object-oriented programming language designed for fast execution and type safety. Type safety means, for instance, that you can't cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java's object-oriented model means that programs consist exclusively of classes and their methods. Java's class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript authoring.

In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages like HyperTalk and dBASE. These scripting languages offer programming tools to a much wider audience because of their easier syntax, specialized built-in functionality, and minimal requirements for object creation.

JavaScript Java
Interpreted (not compiled) by client.
Compiled bytecodes downloaded from server, executed on client.
Object-based. Uses built-in, extensible objects, but no classes or inheritance.
Object-oriented. Applets consist of object classes with inheritance.
Code integrated with, and embedded in, HTML.
Applets distinct from HTML (accessed from HTML pages).
Variable data types not declared (loose typing).
Variable data types must be declared (strong typing).
Dynamic binding. Object references checked at runtime.
Static binding. Object references must exist at compile-time.
Cannot automatically write to hard disk.
Cannot automatically write to hard disk.


Embedding JavaScript in HTML

You can embed JavaScript in an HTML document in the following ways:

Unlike HTML, JavaScript is case sensitive.

Using the SCRIPT tag

The <SCRIPT> tag is an extension to HTML that can enclose any number of JavaScript statements as shown here:

<SCRIPT>
   JavaScript statements...
</SCRIPT>

A document can have multiple SCRIPT tags, and each can enclose any number of JavaScript statements.

Specifying the JavaScript version

The optional LANGUAGE attribute specifies the scripting language and JavaScript version:

<SCRIPT LANGUAGE="JavaScriptVersion">
   JavaScript statements...
</SCRIPT>

JavaScriptVersion specifies one of the following to indicate which version of JavaScript your code is written for:

Statements within a <SCRIPT> tag are ignored if the user's browser does not have the level of JavaScript support specified in the LANGUAGE attribute; for example:

If the LANGUAGE attribute is omitted, Navigator 2.0 assumes LANGUAGE="JavaScript". Navigator 3.0 assumes LANGUAGE="JavaScript1.1"

You can use the LANGUAGE attribute to write scripts that contain Navigator 3.0 features, and these scripts will not cause errors if run under Navigator 2.0. The following examples show some techniques for using the LANGUAGE attribute.

Example 1. This example shows how to define functions twice, once for JavaScript 1.0, and once using JavaScript 1.1 features.

<SCRIPT LANGUAGE="JavaScript">
// Define 1.0-compatible functions such as doClick() here
</SCRIPT>
<SCRIPT LANGUAGE="JavaScript1.1">
// Redefine those functions using 1.1 features
// Also define 1.1-only functions
</SCRIPT>

<FORM ...>
<INPUT TYPE="button" onClick="doClick(this)" ...>
. . .
</FORM>

Example 2. This example shows how to use two separate versions of a JavaScript document, one for JavaScript 1.0 and one for JavaScript 1.1. The default document that loads is for JavaScript 1.0. If the user is running Navigator 3.0, the replace method replaces the page.

<SCRIPT LANGUAGE="JavaScript1.1">
// Replace this page in session history with the 1.1 version
location.replace("js1.1/mypage.html")
</SCRIPT>
[1.0-compatible page continues here...]

Example 3. This example shows how to test the navigator.userAgent property to determine whether the user is running Navigator 3.0. The code then conditionally executes 1.1 features.

<SCRIPT LANGUAGE="JavaScript">
if (navigator.userAgent.indexOf("3.0") != -1)
   jsVersion = "1.1"
else
   jsVersion = "1.0"
</SCRIPT>
[hereafter, test jsVersion == "1.1" before use of any 1.1 extensions]

Example 4. In many cases, you can test the differences between JavaScript 1.0 and 1.1 by comparing new properties (such as navigator.javaEnabled or window.focus) to null. In JavaScript 1.0 and 1.1, an undefined property compares equal to null, so the 1.1 function references will be non-null in Navigator 3.0.

if (navigator.javaEnabled != null && navigator.javaEnabled()) {
   // must be 3.0 and Java is enabled, use LiveConnect here...
} else {
   // 2.0, no Java connection
}

Hiding scripts within comment tags

Only Netscape Navigator versions 2.0 and later recognize JavaScript. To ensure that other browsers ignore JavaScript code, place the entire script within HTML comment tags, and precede the ending comment tag with a double-slash (//) that indicates a JavaScript single-line comment:

<SCRIPT>
<!-- Begin to hide script contents from old browsers.
JavaScript statements...
// End the hiding here. -->
</SCRIPT>

Since browsers typically ignore unknown tags, non-JavaScript-capable browsers will ignore the beginning and ending SCRIPT tags. All the script statements in between are enclosed in an HTML comment, so they are ignored too. Navigator properly interprets the SCRIPT tags and ignores the line in the script beginning with the double-slash (//).

Although you are not required to use this technique, it is considered good etiquette so that your pages don't generate unformatted script statements for those not using Navigator 2.0 or later.

Note

For simplicity, some of the examples in this book do not hide scripts, because the examples are written specifically for Navigator 2.0.

Example: a first script

Here is a simple script:

<SCRIPT LANGUAGE="JavaScript"> <!--- Hide script from old browsers. document.write("Hello, net!") // End the hiding here. --> </SCRIPT>

That's all, folks.


file: /Techref/language/java/SCRIPT/getstart.htm, 16KB, , updated: 2009/2/2 14:27, local time: 2022/9/24 19:33,
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