The term Ajax is used to describe a set of technologies that allow browsers to provide users with a more natural browsing experience. Before Ajax, Web sites forced their users into the submit/wait/redisplay paradigm, where the user 's actions were always synchronized with the server 's "think time". Ajax provides the ability to communicate with the server asynchronously, thereby freeing the user experience from the request/response cycle. With Ajax, when a user clicks a button, you can use JavaScript and DHTML to immediately update the UI, and spawn an asynchronous request to the server to perform an update or query a database. When the request returns, you can then use JavaScript and CSS to update your UI accordingly without refreshing the entire page. Most importantly, users don't even know your code is communicating with the server: the Web site feels like it's instantly responding.

While the infrastructure needed by Ajax has been available for a while, it is only recently that the true power of asynchronous requests has been leveraged. The ability to have an extremely responsive Web site is exciting as it finally allows developers and designers to create "desktop-like" usability with the standard HTML/CSS/JavaScript stack.

Defining Ajax

Jesse James Garrett at Adaptive Path defined Ajax as follows:

Ajax isn't a technology. It's really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:

  • Standards-based presentation using XHTML and CSS
  • Dynamic display and interaction using the Document Object Model
  • Asynchronous server communication using XMLHttpRequest
  • JavaScript binding everything together

This is all fine and dandy, but why the name Ajax? Well, the term Ajax was coined by Jesse James Garrett, and as he puts it, it is "short-hand for Asynchronous JavaScript + XML."

How Does Ajax Work?

The kernel of Ajax is the XmlHttpRequest JavaScript object. This JavaScript object was originally introduced in Internet Explorer 5, and it is the enabling technology that allows asynchronous requests. In short, XmlHttpRequest lets you use JavaScript to make a request to the server and process the response without blocking the user.

By performing screen updates on the client, you have a great amount of flexibility when it comes to creating your Web site. Here are some ideas for what you can accomplish with Ajax:

  • Dynamically update the totals on your shopping cart without forcing the user to click Update and wait for the server to resend the entire page.
  • Increase site performance by reducing the amount of data downloaded from the server. For example, on Amazon's shopping cart page, when I update the quantity of an item in my basket, the entire page is reloaded, which forces 32K of data to be downloaded. If you use Ajax to calculate the new total, the server can respond with just the new total value, thereby reducing the required bandwidth 100 fold.
  • Eliminate page refreshes every time there is user input. For example, if the user clicks Next on a paginated list, Ajax allows you to just refresh the list with the server data, instead of redrawing the entire page.
  • Edit table data directly in place, without requiring the user to navigate to a new page to edit the data. With Ajax, when the user clicks Edit, you can redraw the static table into a table with editable contents. Once the user clicks Done, you can spawn an Ajax request to update the server, and redraw the table to have static, display-only data.

The possibilities are endless! Hopefully you are excited to get started developing your own Ajax-based site. Before we start, however, let's review an existing Web site that follows the old paradigm of submit/wait/redisplay and discuss how Ajax can improve the user's experience.

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