AJAX

From DocForge

AJAX is an acronym for Asynchronous JavaScript And XML. Using JavaScript a web page makes requests to a web server using the XMLHttpRequest object, often responding with the results to update page content. AJAX use became widespread after all of the required components were built into the most common web browsers and a few popular web sites used it to add user-friendly features.

Common uses include auto-complete text boxes and dynamic updates of RSS feeds.

[edit] Benefits

[edit] User Interface

A typical reason for using Ajax is an improvement to the user experience. Pages using Ajax can behave more like a standalone application than a typical web page. Pages can be updated dynamically, allowing a faster response to user interaction. Ajax can make web sites more interactive.

[edit] Bandwidth Usage

By generating HTML and altering the DOM locally within the browser, and only downloading JavaScript and the actual data, Ajax web pages can appear to load relatively quickly since the payload coming down is much smaller in size than a complete page. 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 TD and TR tags, can be produced locally in the browser and not downloaded with the first page of the document.

In addition to "load on demand" of contents, some web applications load stubs of event handlers and then load functions on the fly. This technique significantly cuts down the bandwidth consumption for web applications that have complex logic and functionality.

[edit] 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 between: (1) the raw data or content to be delivered - which is normally embedded in XML and sometimes derived from a server-side database; (2) the format or structure of the webpage - which is almost always built in HTML (or better, XHTML) and is then reflected and made available to dynamic manipulation in the DOM; (3) the style elements of the webpage - everything from fonts to picture placement - are derived by reference to embedded or referenced CSS; and (4) the functionality of the web page is provided by a combination of (A) Javascript on the client browser (also called DHTML), (B) Standard HTTP and XMLHttp for client-to-server communication, and (C) server-side scripting and/or programs using any suitable language preferred by the programmer to receive the client's specific requests and respond appropriately.

[edit] Drawbacks

[edit] 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.

Developers have implemented various solutions to this problem. These solutions can involve using invisible iframe elements 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. The World Wide Web Consortium (W3C) did not include an iframe element in its XHTML 1.1 Recommendation; the Consortium recommends the object element instead.

Another issue is that dynamic web page updates 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, however, not a complete solution.

[edit] Response-Time

Network latency - or the interval between user request and server response - needs to be considered carefully during Ajax development. Without clear feedback to the user, smart preloading of data and proper handling of the XMLHttpRequest object, users might experience delay 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 when the 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.

[edit] Search Engine Optimization

Websites that use Ajax to load data which should be indexed by search engines must be careful to provide equivalent data at a public, linked URL and in a format that the search engine can read, 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, for example, a form submit.

[edit] JavaScript Reliability

Ajax relies on JavaScript, which may be 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 not uncommon to see a JavaScript code written twice, one part for Internet Explorer, another part for Firefox. JavaScript libraries such as prototype and mootools help overcome this by providing common APIs which internally handle browser differences.

The level of IDE support for JavaScript has also been very poor in the past, though this is improving.

An issue also arises if the user has switched off JavaScript support in the web browser, thus disabling the functionality of the page.

[edit] Libraries and Frameworks

These libraries and frameworks aid developers in implementing AJAX.

• Prototype is a small JavaScript library which simplifies the process of adding AJAX to web pages.
• Pyjamas is a python AJAX framework
• Mootools is another small JavaScript library which implements almost all of the features of Prototype. This svelte response to Prototype has many new features of its own and a very active developer community.
• DojoToolkit is a powerful cross-browser javascript toolkit which handles XmlHttpRequests (the foundation of AJAX) and also provides a full widget stack.