Getting started



This 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:

   JavaScript statements...
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...
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.

// Define 1.0-compatible functions such as doClick() here
<SCRIPT LANGUAGE="JavaScript1.1">
// Redefine those functions using 1.1 features
// Also define 1.1-only functions
<FORM ...>
<INPUT TYPE="button" onClick="doClick(this)" ...>
. . .
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
[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.

if (navigator.userAgent.indexOf("3.0") != -1)
   jsVersion = "1.1"
   jsVersion = "1.0"
[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:

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

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: <HTML> <HEAD> <SCRIPT LANGUAGE="JavaScript"> <!--- Hide script from old browsers. document.write("Hello, net!") // End the hiding here. --> </SCRIPT> <P>That's all, folks. </BODY> </HTML>

This script displays the following in Navigator:

                    Hello, net!

                    That's all, folks.

Notice that there is no difference in appearance between the first line, generated with JavaScript, and the second line, generated with plain HTML.

You may sometimes see a semicolon at the end of each line of JavaScript. In general, semicolons are optional and are required only if you want to put more than one statement on a single line. This is most useful in defining event handlers, which are discussed in "Scripting event handlers".

Specifying a file of JavaScript code

The SRC attribute of the <SCRIPT> tag lets you specify a file as the JavaScript source (rather than embedding the JavaScript in the HTML). For example:

<SCRIPT SRC="common.js">
This attribute is especially useful for sharing functions among many different pages.

The closing </SCRIPT> tag is required.

JavaScript statements within a <SCRIPT> tag with a SRC attribute are ignored unless the inclusion has an error. For example, you might want to put the following statement between the <SCRIPT SRC="..."> and </SCRIPT> statements:

document.write("Included JS file not found")
The SRC attribute can specify any URL, relative or absolute. For example:

External JavaScript files cannot contain any HTML tags: they must contain only JavaScript statements and function definitions.

External JavaScript files should have the file name suffix .js, and the server must map the .js suffix to the MIME type "application/x-javascript", which the server sends back in the HTTP header. To map the suffix to the MIME type, add the following line to the mime.types file in the server's config directory, and then restart the server.

type=application/x-javascript          exts=js
If the server does not map the .js filename suffix to "application/x-javascript" MIME type, Navigator will not load the JavaScript file specified by the SRC attribute properly.

This requirement does not apply if you are using local files.

Using JavaScript expressions as HTML attribute values

Using JavaScript entities, you can specify a JavaScript expression as the value for an HTML attribute. Entity values are evaluated dynamically. This allows you to create more flexible HTML constructs, because the attributes of one HTML element can depend on information about elements placed previously on the page.

You may already be familiar with HTML character entities by which you can define characters with special numerical codes or names by preceding the name with an ampersand (&) and terminating it with a semicolon (;). For example, you can include a greater-than symbol (>) with the character entity &GT; and a less-than symbol (<) with &LT;.

JavaScript entities also start with an ampersand (&) and end with a semicolon (;). Instead of a name or number, you use a JavaScript expression enclosed in curly braces {}. You can use JavaScript entities only where an HTML attribute value would normally go. For example, suppose you define a variable barWidth. You could create a horizontal rule with the specified percentage width as follows:

<HR WIDTH="&{barWidth};%" ALIGN="LEFT">
So, for example, if barWidth were 50, this would create the following display:

As with other HTML, after layout has occurred, the display of a page can change only if you reload the page.

Unlike regular entities which can appear anywhere in the HTML text flow, JavaScript entities will be interpreted only on the right-hand side of HTML attribute name/value pairs. For example, <H4>&{myTitle};</H4> would display myTitle rather than the value of the variable myTitle.

Specifying alternate content with the NOSCRIPT tag

Use the <NOSCRIPT> tag to specify alternate content for browsers that do not support JavaScript. HTML enclosed within a <NOSCRIPT> tag is displayed by browsers that do not support JavaScript; code within the tag is ignored by Navigator. Note however, that if the user has disabled JavaScript by choosing Network Preferences from the Options menu, Navigator displays the code within the <NOSCRIPT> tag.

The following example shows a <NOSCRIPT> tag.

<B>This page uses JavaScript, so you need to get Netscape Navigator 2.0
or later!
<A HREF="">
<IMG SRC="NSNow.gif"></A>
If you are using Navigator 2.0 or later, and you see this message,
you should enable JavaScript by choosing Network Preferences from
the Options menu.

Defining and calling functions

For a complete description of defining and calling functions, see "Functions".
Functions are one of the fundamental building blocks in JavaScript. A function is a JavaScript procedure--a set of statements that performs a specific task. A function definition has these basic parts:

It's important to understand the difference between defining and calling a function. Defining the function simply names the function and specifies what to do when the function is called. Calling the function actually performs the specified actions with the indicated parameters.

Event handlers are introduced in "Scripting event handlers".
Generally, you should define the functions for a page in the HEAD portion of a document. That way, all functions are defined before any content is displayed. Otherwise, the user might perform an action while the page is still loading that triggers an event handler and calls an undefined function, leading to an error.

The following example defines a simple function in the HEAD of a document and then calls it in the BODY of the document:

<!-- Hide script from old browsers
function square(number) {
          return number * number
// End script hiding from old browsers -->
          document.write("The function returned ", square(5), ".")
<P> All done.
The function square takes one argument, called number. The function consists of one statement

return number * number
that indicates to return the argument of the function multiplied by itself. The return statement specifies the value returned by the function. In the BODY of the document, the statement

calls the function with an argument of five. The function executes its statements and returns the value twenty-five. The script displays the following results:

                    The function returned 25.

                    All done.

The square function used the line

to display output in Navigator. This line calls the write method of the Navigator document object with JavaScript's standard object notation:

where objectName is the name of the object, methodName is the name of the method, and arguments is a list of arguments to the method, separated by commas.

In addition to defining functions as described here, you can also define Function objects, as described in "Function object".

A method is a function associated with an object. You'll learn more about objects and methods in Chapter 8, "Object model." But right now, we will explore write a little more, because it is particularly useful.

Using the write method

As you saw in the previous example, the write method of document displays output in the Navigator. "Big deal," you say, "HTML already does that." But in a script you can do all kinds of things you can't do with ordinary HTML. For example, you can display text conditionally or based on variable arguments. For these reasons, write is one of the most often-used JavaScript methods.

The write method takes any number of string arguments that can be string literals or variables. You can also use the string concatenation operator (+) to create one string from several when using write.

Consider the following script, which generates dynamic HTML with Navigator JavaScript:

<!--- Hide script from old browsers
// This function displays a horizontal bar of specified width
function bar(widthPct) {
          document.write("<HR ALIGN='left' WIDTH=" + widthPct + "%>")
// This function displays a heading of specified level and some text
function output(headLevel, headText, text) {
          document.write("<H", headLevel, ">", headText, "</H",
          headLevel, "><P>", text)
// end script hiding from old browsers -->
<!--- hide script from old browsers
output(2, "JavaScript Rules!", "Using JavaScript is easy...")
// end script hiding from old browsers -->
<P> This is some standard HTML, unlike the above that is generated.
The HEAD of this document defines two functions:

The document BODY then calls the two functions to produce the following display:

The following line creates the output of the bar function:

document.write("<HR ALIGN='left' WIDTH=", widthPct, "%>")
Notice that the definition of bar uses single quotation marks inside double quotation marks. You must do this whenever you want to indicate a quoted string inside a string literal. Then the call to bar with an argument of twenty-five produces output equivalent to the following HTML:

<HR ALIGN="left" WIDTH=25%>
write has a companion method, writeln, which adds a newline sequence (a carriage return or a carriage return and linefeed, depending on the platform) at the end of its output. Because HTML generally ignores newlines, there is no difference between write and writeln except inside tags such as PRE, which respect carriage returns.

Printing output

Navigator 3.0 prints output created with JavaScript. To print output, the user chooses Print from the File menu.

To view HTML code that was generated with JavaScript write and writeln methods, the user must specify the view-source: protocol. If the user chooses Document Source or Frame Source from the View menu, the content displayed is that of the wysiwyg: URL. The following example shows a view-source: URL:

Navigator 2.0, does not print output created with JavaScript. For example, if the user chooses Print from the File menu to print the page in the previous example, you see only the line that reads "This is some standard HTML...," even though you see both lines onscreen.

Displaying output

JavaScript in Navigator generates its results from the top of the page down. Once text has been displayed, you cannot change it without reloading the page. In general, you cannot update part of a page without updating the entire page. However, you can update

Scripting event handlers

JavaScript applications in the Navigator are largely event-driven. Events are actions that occur usually as a result of something the user does. For example, clicking a button is an event, as is changing a text field or moving the mouse over a hyperlink. You can define event handlers, such as onChange and onClick, to make your script react to events.

Each event is recognized by certain objects (HTML tags), summarized in the following table:

Event Applies to Occurs when Event handler
User aborts the loading of an image (for example by clicking a link or clicking the Stop button)
windows, frames, and all form elements
User removes input focus from window, frame, or form element
buttons, radio buttons, checkboxes, submit buttons, reset buttons, links
User clicks form element or link
text fields, textareas, select lists
User changes value of element
images, windows
The loading of a document or image causes an error
windows, frames, and all form elements
User gives input focus to window, frame, or form element
document body
User loads the page in the Navigator
areas, links
User moves mouse pointer out of an area (client-side image map) or link
User moves mouse pointer over a link
onMouse- Over
User resets a form (clicks a Reset button)
text fields, textareas
User selects form element's input field
submit button
User submits a form
document body
User exits the page

If an event applies to an HTML tag, then you can define an event handler for it. The name of an event handler is the name of the event, preceded by "on." For example, the event handler for the focus event is onFocus.

To create an event handler for an HTML tag, add an event handler attribute to the tag; Put JavaScript code in quotation marks as the attribute value. The general syntax is

<TAG eventHandler="JavaScript Code">
where TAG is an HTML tag and eventHandler is the name of the event handler.

For example, suppose you have created a JavaScript function called compute. You can cause Navigator to perform this function when the user clicks a button by assigning the function call to the button's onClick event handler:

<INPUT TYPE="button" VALUE="Calculate" onClick="compute(this.form)">
You can put any JavaScript statements inside the quotation marks following onClick. These statements are executed when the user clicks the button. If you want to include more than one statement, separate statements with a semicolon (;).

Notice in the preceding example this.form refers to the current form. The keyword this refers to the current object, which is the button. The construct this.form then refers to the form containing the button. The onClick event handler is a call to the compute function, with the current form as the argument.

In general, it is good practice to define functions for your event handlers:

Using quotation marks

Use single quotation marks ( ' ) to delimit string literals so that scripts can distinguish the literal from attribute values enclosed in double quotation marks. In the following example, the function bar contains the literal "left" within a double-quoted attribute value:

function bar(widthPct) {
          document.write("<HR ALIGN='left' WIDTH=" + widthPct + "%>")
Here's another example:

<INPUT TYPE="button" VALUE="Press Me" onClick="myfunc('astring')">
Be sure to alternate double quotation marks with single quotation marks. Because event handlers in HTML must be enclosed in quotation marks, you must use single quotation marks to delimit string arguments. For example:

<FORM NAME="myform">
<INPUT TYPE="button" NAME="Button1" VALUE="Open Sesame!"
          onClick="'mydoc.html', 'newWin')">

Example: using an event handler

In the following example, you can enter an expression (for example, 2 + 2) in the first text field, and then click the button. The second text field then displays the value of the expression (for example, 4).

<!--- Hide script from old browsers
function compute(f) {
          if (confirm("Are you sure?"))
                    f.result.value = eval(f.expr.value)
                    alert("Please come back again.")
// end hiding from old browsers -->
Enter an expression:
<INPUT TYPE="text" NAME="expr" SIZE=15 >
<INPUT TYPE="button" VALUE="Calculate" onClick="compute(this.form)">
<INPUT TYPE="text" NAME="result" SIZE=15 >
The display in Navigator will look like this:

Enter an expression:

The HEAD of the document defines a single function, compute, taking one argument, f, which is a Form object. The function uses the Navigator JavaScript method confirm to display a Confirm dialog box with OK and Cancel buttons.

If the user clicks OK, then confirm returns true, and the value of the result text field is set to the value of eval(f.expr.value). The built-in JavaScript function eval evaluates its argument, which can be any string representing any JavaScript expression or statements.

If the user clicks Cancel, then confirm returns false and the alert method displays another message.

The form contains a button with an onClick event handler that calls the compute function. When the user clicks the button, JavaScript calls compute with the argument this.form that denotes the current Form object. In compute, this form is referred to as the argument f.

Calling event handlers explicitly

In JavaScript for Navigator 3.0, you can reset an event handler specified by HTML, as shown in the following example.

function fun1() {
function fun2() {
<FORM NAME="myForm">
<INPUT TYPE="button" NAME="myButton"
Note that event handlers are function references, so you must assign fun2, not fun2() (the latter calls fun2 and has whatever type and value fun2 returns).

Also, since the event handler HTML attributes are literal function bodies, you cannot use <INPUT onClick=fun1> in the HTML source to make fun1 the onClick handler for an input; you must call fun1 instead, as in the example.

Finally, because JavaScript is case-sensitive, you must spell event handler names in lowercase in JavaScript.

Validating form input

If you can't run server-based programs, skip this section.

One of the most important uses of JavaScript is to validate form input to server-based programs such as LiveWire applications or CGI programs. This is useful because

Generally, you'll want to validate input in (at least) two places:

Example validation functions

The following are some simple validation functions.

<SCRIPT LANGUAGE="javascript">
function isaPosNum(s) {
          return (parseInt(s) > 0)
function qty_check(item, min, max) {
          var returnVal = false
          if (!isaPosNum(item.value))
                    alert("Please enter a postive number" )
          else if (parseInt(item.value) < min)
                    alert("Please enter a " + + " greater than " + min)
          else if (parseInt(item.value) > max)
                    alert("Please enter a " + + " less than " + max)
                    returnVal = true
          return returnVal
function validateAndSubmit(theform) {
          if (qty_check(theform.quantity, 0, 999)) {
                    alert("Order has been Submitted")
                    return true
                    else {
                       alert("Sorry, Order Cannot Be Submitted!")
                       return false
isaPosNum is a simple function that returns true if its argument is a positive number, and false otherwise.

qty_check takes three arguments: an object corresponding to the form element being validated (item) and the minimum and maximum allowable values for the item (min and max). It checks that the value of item is a number between min and max and displays an alert if it is not.

validateAndSubmit takes a Form object as its argument; it uses qty_check to check the value of the form element and submits the form if the input value is valid. Otherwise, it displays an alert and does not submit the form.

Using the validation functions

In this example, the BODY of the document uses qty_check as an onChange event handler for a text field and validateAndSubmit as the onClick event handler for a button.

<FORM NAME="widget_order" ACTION="lwapp.html" METHOD="post">
How many widgets today?
<INPUT TYPE="text" NAME="quantity" onChange="qty_check(this, 0, 999)">
<INPUT TYPE="button" VALUE="Enter Order" onClick="validateAndSubmit(this.form)">
This form submits the values to a page in a LiveWire application called lwapp.html. It also could be used to submit the form to a CGI program.

The form looks like this:

How many widgets today?

The onChange event handler is triggered when you change the value in the text field and move focus from the field by either pressing the Tab key or clicking the mouse outside the field. Notice that both event handlers use this to represent the current object: in the text field, it is used to pass the JavaScript object corresponding to the text field to qty_check, and in the button it is used to pass the JavaScript Form object to validateAndSubmit.

To submit the form to the server-based program, this example uses a button that calls validateAndSubmit, which submits the form using the submit method, if the data are valid. You can also use a submit button (defined by <INPUT TYPE="submit">) with an onSubmit event handler that returns false if the data are not valid. For example,

<INPUT TYPE="submit"
   onSubmit="return qty_check(theform.quantity, 0, 999)"
When qty_check returns false if the data are invalid, the onSubmit handler will prohibit the form from being submitted.