1.6 Client-Side JavaScript Features

Another possible use of JavaScript is for writing programs to perform arbitrary computations. You can write simple scripts, for example, that compute Fibonacci numbers, or search for primes. In the context of the Web and web browsers, however, a more interesting application of the language might be a program that computed the sales tax on an online order, based on information supplied by the user in an HTML form. As mentioned earlier, the real power of JavaScript lies in the browser and document-based objects that the language supports. To give you an idea of JavaScript's potential, the following sections list and explain the important capabilities of client-side JavaScript and the objects it supports.

1.6.1 Control Document Appearance and Content

The JavaScript Document object, through its write( ) method, which we have already seen, allows you to write arbitrary HTML into a document as the document is being parsed by the browser. For example, you can include the current date and time in a document or display different content on different platforms.

You can also use the Document object to generate documents entirely from scratch. Properties of the Document object allow you to specify colors for the document background, the text, and the hypertext links within it. This amounts to the ability to generate dynamic and conditional HTML documents, a technique that works particularly well in multiframe documents. Indeed, in some cases dynamic generation of frame content allows a JavaScript program to replace a traditional server-side script entirely.

Internet Explorer 4 and Netscape 4 support proprietary techniques for producing Dynamic HTML effects that allow document content to be dynamically generated, moved, and altered. IE 4 also supports a complete DOM that gives JavaScript access to every single HTML element within a document. And IE 5.5 and Netscape 6 support the W3C DOM standard (or at least key portions of it), which defines a standard, portable way to access all of the elements and text within an HTML document and to position them and modify their appearance by manipulating their Cascading Style Sheets (CSS) style attributes. In these browsers, client-side JavaScript has complete power over document content, which opens an unlimited world of scripting possibilities.

1.6.2 Control the Browser

Several JavaScript objects allow control over the behavior of the browser. The Window object supports methods to pop up dialog boxes to display simple messages to the user and get simple input from the user. This object also defines a method to create and open (and close) entirely new browser windows, which can have any specified size and any combination of user controls. This allows you, for example, to open up multiple windows to give the user multiple views of your web site. New browser windows are also useful for temporary display of generated HTML, and, when created without the menu bar and other user controls, these windows can serve as dialog boxes for more complex messages or user input.

JavaScript does not define methods that allow you to create and manipulate frames directly within a browser window. However, the ability to generate HTML dynamically allows you to programmatically write the HTML tags that create any desired frame layout.

JavaScript also allows control over which web pages are displayed in the browser. The Location object allows you to download and display the contents of any URL in any window or frame of the browser. The History object allows you to move forward and back within the user's browsing history, simulating the action of the browser's Forward and Back buttons.

Yet another method of the Window object allows JavaScript to display arbitrary messages to the user in the status line of any browser window.

1.6.3 Interact with HTML Forms

Another important aspect of client-side JavaScript is its ability to interact with HTML forms. This capability is provided by the Form object and the form element objects it can contain: Button, Checkbox, Hidden, Password, Radio, Reset, Select, Submit, Text, and Textarea objects. These element objects allow you to read and write the values of the input elements in the forms in a document. For example, an online catalog might use an HTML form to allow the user to enter his order and could use JavaScript to read the input from that form in order to compute the cost of the order, the sales tax, and the shipping charge. JavaScript programs like this are, in fact, very common on the Web. We'll see a program shortly that uses an HTML form and JavaScript to allow the user to compute monthly payments on a home mortgage or other loan. JavaScript has an obvious advantage over server-based scripts for applications like these: JavaScript code is executed on the client, so the form's contents don't have to be sent to the server in order for relatively simple computations to be performed.

Another common use of client-side JavaScript with forms is for validating form data before it is submitted. If client-side JavaScript is able to perform all necessary error checking of a user's input, no round trip to the server is required to detect and inform the user of trivial input errors. Client-side JavaScript can also perform preprocessing of input data, which can reduce the amount of data that must be transmitted to the server. In some cases, client-side JavaScript can eliminate the need for scripts on the server altogether! (On the other hand, JavaScript and server-side scripting do work well together. For example, a server-side program can dynamically create JavaScript code on the fly, just as it dynamically creates HTML.)

1.6.4 Interact with the User

An important feature of JavaScript is the ability to define event handlers -- arbitrary pieces of code to be executed when a particular event occurs. Usually, these events are initiated by the user, when, for example, she moves the mouse over a hypertext link, enters a value in a form, or clicks the Submit button in a form. This event-handling capability is a crucial one, because programming with graphical interfaces, such as HTML forms, inherently requires an event-driven model. JavaScript can trigger any kind of action in response to user events. Typical examples might be to display a special message in the status line when the user positions the mouse over a hypertext link or to pop up a confirmation dialog box when the user submits an important form.

1.6.5 Read and Write Client State with Cookies

A cookie is a small amount of state data stored permanently or temporarily by the client. Cookies may be transmitted along with a web page by the server to the client, which stores them locally. When the client later requests the same or a related web page, it passes the relevant cookies back to the server, which can use their values to alter the content it sends back to the client. Cookies allow a web page or web site to remember things about the client -- for example, that the user has previously visited the site, has already registered and obtained a password, or has expressed a preference about the color and layout of web pages. Cookies help you provide the state information that is missing from the stateless HTTP protocol of the Web.

When cookies were invented, they were intended for use exclusively by server-side scripts; although stored on the client, they could be read or written only by the server. JavaScript changed this, because JavaScript programs can read and write cookie values and can dynamically generate document content based on the value of cookies.

1.6.6 Still More Features

In addition to the features I have already discussed, JavaScript has many other capabilities, including the following:

• JavaScript can change the image displayed by an <img> tag to produce image rollover and animation effects.

• JavaScript can interact with Java applets and other embedded objects that appear in the browser. JavaScript code can read and write the properties of these applets and objects and can also invoke any methods they define. This feature truly allows JavaScript to script Java.

• As mentioned at the start of this section, JavaScript can perform arbitrary computation. JavaScript has a floating-point data type, arithmetic operators that work with it, and a full complement of standard floating-point mathematical functions.

• The JavaScript Date object simplifies the process of computing and working with dates and times.

• The Document object supports a property that specifies the last-modified date for the current document. You can use it to automatically display a timestamp on any document.

• JavaScript has a window.setTimeout( ) method that allows a block of arbitrary JavaScript code to be executed some number of milliseconds in the future. This is useful for building delays or repetitive actions into a JavaScript program. In JavaScript 1.2, setTimeout( ) is augmented by another useful method called setInterval( ).

• The Navigator object (named after the Netscape web browser, of course) has variables that specify the name and version of the browser that is running, as well as variables that identify the platform on which it is running. These variables allow scripts to customize their behavior based on browser or platform, so that they can take advantage of extra capabilities supported by some versions or work around bugs that exist on some platforms.

• In client-side JavaScript 1.2, the Screen object provides information about the size and color-depth of the monitor on which the web browser is being displayed.

• As of JavaScript 1.1, the scroll( ) method of the Window object allows JavaScript programs to scroll windows in the X and Y dimensions. In JavaScript 1.2, this method is augmented by a host of others that allow browser windows to be moved and resized.

1.6.7 What JavaScript Can't Do

Client-side JavaScript has an impressive list of capabilities. Note, however, that they are confined to browser- and document-related tasks. Since client-side JavaScript is used in a limited context, it does not have features that would be required for standalone languages:

• JavaScript does not have any graphics capabilities, except for the powerful ability to dynamically generate HTML (including images, tables, frames, forms, fonts, etc.) for the browser to display.

• For security reasons, client-side JavaScript does not allow the reading or writing of files. Obviously, you wouldn't want to allow an untrusted program from any random web site to run on your computer and rearrange your files!

• JavaScript does not support networking of any kind, except that it can cause the browser to download arbitrary URLs and it can send the contents of HTML forms across the network to server-side scripts and email addresses.