User:FatalError/Ajax (programming)

"AJAX" redirects here. For other uses, see Ajax.

Ajax (asynchronous JavaScript and XML), or AJAX, is a group of interrelated web development techniques used for creating interactive web applications. A primary characteristic is the increased responsiveness of web pages achieved by exchanging small amounts of data with the server "behind the scenes" so that entire web pages do not have to be reloaded each time there is a need to fetch data from the server. This is intended to increase the web page's interactivity, speed, functionality and usability.

Ajax is asynchronous, in that extra data is requested from the server and loaded in the background without interfering with the display and behavior of the existing page. JavaScript is the scripting language in which Ajax function calls are usually made. Data is retrieved using the XMLHttpRequest object, or, alternatively, through the use of Remote Scripting in browsers that do not support it. In any case, it is not required that the content be formatted in XML or retrieved asynchronously.

History

While the term "Ajax" was coined in 2005 by Jesse James Garrett, techniques for the asynchronous loading of content date back to 1996, when Internet Explorer introduced the IFrame element. Microsoft's Remote Scripting, introduced in 1998, acted as a more elegant replacement for these techniques, with data being pulled in by a Java applet with which the client side could communicate using JavaScript. This technique worked on both Internet Explorer 4 and Netscape Navigator 4 and onwards. Microsoft then created the XMLHttpRequest object in Internet Explorer 5 and first took advantage of these techniques using XMLHttpRequest in Outlook Web Access supplied with the Microsoft Exchange Server 2000 release.

The Web development community, first collaborating for remote scripting to enable consistent results across different browsers, created in 2002 a user-community modification to Microsoft Remote Scripting which was created to replace the Java applet with XMLHttpRequest.

Remote Scripting frameworks such as ARSCIF surfaced in 2003 not long before Microsoft introduced callbacks in ASP.NET.

In addition, the World Wide Web Consortium (W3C) has several recommendations that also allow for dynamic communication between a server and user agent, though few of them are well supported. These would include:

• The object element defined in HTML 4 for embedding arbitrary content types into documents (replaces inline frames under XHTML 1.1)
• The Document Object Model (DOM) Level 3 Load and Save Specification

Rationale

The core reason to use Ajax-style programming is to overcome the page loading requirements of HTML/HTTP-mediated web pages. Ajax creates the necessary initial conditions for the evolution of complex, intuitive, dynamic, data-centric user interfaces in web pages. Indeed, much of the Web's innovation and evolution during the Web 2.0 era has relied upon and benefited immensely from the capabilities of an Ajax platform.

Web pages, unlike native applications, are loosely coupled, meaning that the data they display are not tightly bound to data sources and must be first marshaled (set out in proper order) into an HTML page format before they can be presented to a user agent on the client machine. For this reason, web pages have to be re-loaded each time a user needs to view different datasets. By using the XMLHttpRequest object to request and return data without a re-load, a programmer bypasses this requirement and makes the loosely coupled web page behave much like a tightly coupled application, but with a more variable lag time for the data to pass through a longer "wire" to the remote web browser.

For example, in a classic desktop application, a programmer has the choice of populating a tree view control with all the data needed when the form initially loads, or with just the top-most level of data—which would load more quickly, especially when the dataset is very large. In the second case, the application would fetch additional data into the tree control depending on which item the user selects. This functionality is difficult to achieve in a web page without Ajax. To update the tree based on a user's selection would require the entire page to re-load, leading to a very jerky, non-intuitive feel for the web user who is browsing the data in the tree.

Advantages

Bandwidth usage

By generating the HTML locally within the browser, and only bringing down JavaScript calls and the actual data, Ajax web pages can appear to load relatively quickly since the payload coming down is much smaller in size, and the rest of the layout does not have to be redrawn on each update. An example of this technique is a large result set where multiple pages of data exist. With Ajax, the HTML of the page (e.g., a table structure with related <tr>...</tr> and <td>...</td> tags) can be produced locally in the browser, not brought down with the first page of the document. In addition to "load on demand" of contents, some web-based applications load stubs of event handlers and then load the functions on the fly. This technique significantly cuts down the bandwidth consumption for web applications.

Separation of data, format, style, and function

A less specific benefit of the Ajax approach is that it tends to encourage programmers to clearly separate the methods and formats used for the different aspects of information delivery via the web. Although Ajax can appear to be a jumble of languages and techniques, and programmers are free to adopt and adapt whatever works for them, they are generally propelled by the development motive itself to adopt separation among the following:

1. Raw data or content to be delivered, which is normally embedded in XML and sometimes derived from a server-side database.
2. Format or structure of the webpage, which is almost always built in HTML or XHTML and is then reflected and made available to dynamic manipulation in the DOM.
3. Style elements of the webpage: everything from fonts to picture placement are derived by reference to embedded or referenced CSS.
4. Functionality of the webpage, which is provided by a combination of:
   1. Javascript on the client browser (Also called DHTML),
   2. Standard HTTP and XMLHttp or client-to-server communication, and
   3. Server-side scripting and/or programs using any suitable language preferred by the programmer to receive the client's specific requests and respond appropriately.

Disadvantages

Browser integration

The dynamically created page does not register itself with the browser history engine, so triggering the "Back" function of the users' browser might not bring the desired result; rather than returning the user to an earlier state of the Ajax-enabled page, the back button would return the user to the last page visited before the Ajax-enabled page.

Developers have implemented various solutions to this problem. These solutions can involve using invisible IFrames to invoke changes that populate the history used by a browser's back button. Google Maps, for example, performs searches in an invisible IFrame and then pulls results back into an element on the visible web page.

Dynamic web page updates also make it difficult for a user to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the portion of a URL after the '#') to keep track of, and allow users to return to, the application in a given state. This is possible because many browsers allow JavaScript to update the fragment identifier of the URL dynamically, so that Ajax applications can maintain it as the user changes the application's state. This solution also improves back-button support. It is not, however, a complete solution.

Response-time

Network latency, the interval between user request and server response, must be considered carefully during Ajax development. Without clear feedback to the user, preloading of data and proper handling of the XMLHttpRequest object, users might experience delays in the interface of the web application, something which they might not expect or understand. Additionally, when an entire page is rendered there is a brief moment of re-adjustment for the eye as content changes. The lack of this re-adjustment with smaller portions of the screen changing makes the latency more apparent. The use of visual feedback (such as throbbers) to alert the user of background activity and/or preloading of content and data are often-suggested solutions to these latency issues.

Search engine optimization

Websites that use Ajax to load data which should be indexed by search engines must be careful to provide equivalent data for clients without JavaScript, as search engines do not generally execute the JavaScript code required for Ajax functionality. This problem is not specific to Ajax, as the same issue occurs with sites that provide dynamic data as a full-page refresh in response to, say, a form submit. (The general problem is sometimes called the deep web.)Using mod_rewrite, this issue can be resolved.

Reliance on JavaScript and the DOM

Ajax relies on JavaScript and the browser's Document Object Model (DOM), which is often implemented differently by different browsers or versions of a particular browser. Because of this, sites that use JavaScript may need to be tested in multiple browsers to check for compatibility issues. It's common to see JavaScript code written twice—one part for Internet Explorer, another part for Mozilla compatibles—although this is less true with the release of IE7 and with the now-common use of JavaScript libraries such as Prototype or jQuery, which abstract browser-specific differences from the web developer.

The level of IDE support for JavaScript used to be poor, although it is changing with more wide-spread use of tools like Drosera, Firebug, Microsoft Script Debugger and Venkman.

Finally, if the user has switched off JavaScript support, pages designed to use Ajax may not function.

Web analytics

Many web analytics solutions expect a new page to be loaded whenever new or updated content is displayed to the user, and after each step in a process such as a check-out. Since Ajax alters this process, developers must take care to instrument a page or a portion thereof so that it can be accurately tracked. Analytics systems which can track events other than simple page views, such as clicks of buttons or links, are most able to accommodate Ajax-using sites.

Accessibility

Main article: WAI-ARIA

Non-Ajax users would ideally continue to load and manipulate the whole page as a fall back, enabling the developers to preserve the experience of users in non-Ajax environments (including all relevant accessibility concerns) while giving those with capable browsers a much more responsive experience. For this reason it is advised to first develop a full application without Ajax, and implement Ajax enhancements as an addition only (see: Progressive enhancement methodology). The same counts for JavaScript in general, which can be disabled in most browsers.

Because many people consider it unreasonable to advise against the use of JavaScript in modern web applications, the W3C Web Accessibility Initiative (WAI) is working on a specification for Accessible Rich Internet Applications (ARIA). In effect, ARIA allows web pages, or portions of web pages, to declare themselves as applications rather than as documents. This allows Ajax interactions and other types of JavaScript-enabled UI controls to interact with assistive technology, like screen readers for the vision-impaired.

One early user agent implementation is Firefox 3, which maps WAI-ARIA to the accessibility API of the underlying operating system, making it possible for screen readers to treat ARIA widgets in much the same way as the widgets found in client-side software. Another early implementation is Fire Vox, an extension that turns Firefox into a self-voicing browser. Since Fire Vox operates directly on the DOM and does not use the operating system's accessibility API, it can handle WAI-ARIA features, such as live regions, which have no mapping to traditional desktop applications.

Ajax libraries and toolkits are also adopting ARIA. Notable examples include Google Web Toolkit, Backbase and Dojo Toolkit. In addition, AxsJAX, a JavaScript library which focuses on web-application accessibility, includes several abstractions built on top of ARIA.

See also

• Ajax framework
• Google Gears
• Progressive Enhancement
• Reverse Ajax
• Rich Internet application
• XMLHttpRequest

External links

• AJAX at the Open Directory Project.
• Jesse James Garrett. “Ajax: A New Approach to Web Applications”, Adaptive Path
• Ajax Tutorial with get, post, text and XML examples.
• Attacking AJAX Applications Presentation on Ajax Security issues given at the Black Hat security conference.