Client-side dynamic behavior

Static Web pages never change. Every time they are retrieved from the server, the user sees exactly the same page as the author posted. Dynamic behavior can be introduced by programs that execute on the client computer, the server or both. Currently, there are four general options for client side dynamic behavior.
  1. Javascript. With Javascript, a Web page that is downloaded contains small event handlers that can change the displayed page. The most common uses for Javascript are changing the image or text that is displayed when the user rolls the mouse over something on the screen and checking the validity of information entered into a form.
  2. AJAX. AJAX, Asynchronous Javascript and XML combines Javascript with downloading content without displaying it while the user looks at a page. Javascript event handlers can then display or use this data without delay. The data is marked up with XML tags to facilitate retrieval. Complex AJAX applications are almost always built using Javascript libraries like Jquery.
  3. Java applets. A Java applet is a program that is automatically downloaded and executed when the user links to an HTML page that contains it.
  4. Plug-ins. You can install a plug-in like the Macromedia Flash player which enables your Web browser to show animations and movies. Once you install the plug-in, your browser will use it to play Flash movies from any Web site. Youtube uses Flash to play movies.
  5. Custom clients. For complex tasks rich user interfaces and specialized server software and databases, the user may be asked to independently install a client program. That program is used for the interaction, not the Web browser. Examples include Apple's iTunes client, Google Earth and Microsoft's Virtual Earth 3D.
  6. Desktop vs browser and data synchronization. We are about to see several new toolkits and browser extensions that allow one to develop applications that are downloaded and run on the desktop, like Java applets, but which do not run in the browser. They will have the advantage of being able to access files and other resources on the desktop and provide faster execution and more user interface polish than javascript in the browser. I have not yet looked at applications built with any of these tools, so cannot tell you much except to keep an eye on them. This note will be revised!

    We are also seeing new tools like Google Gears which will automatically synchronize server and desktop data so we can use our network-based applications off line. For example, using Gears, one can now disconnect from the Internet and continue using the Google RSS reader. When you re-connect to the Internet, the items you had read while offline would automatically be marked as read online and new feed items would be automatically downloaded.

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