What is AJAX?

Ajax, or AJAX, (Asynchronous Javascript And XML) is a web development technique used for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user requests a change. 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 behaviour of the existing page. JavaScript is the scripting language in which Ajax function calls are usually made. Data is retrieved using the XMLHttpRequest object that is available to scripting languages run in modern browsers. There is, however, no requirement that the asynchronous content is formatted in XML.

Ajax is a cross-platform technique usable on many different operating systems, computer architectures, and web browsers as it is based on open standards such as JavaScript and the DOM. There are free and open source implementations of suitable frameworks.

Usage
Some uses for Ajax interactions are the following:

• Real-time form data validation: Form data such as user IDs, serial numbers, postal codes, or even special coupon codes that require server-side validation can be validated in a form before the user submits a form. See Realtime Form Validation for details.
• Autocompletion: A specific portion of form data such as an email address, name, or city name may be autocompleted as the user types.
• Load on demand: Based on a client event, an HTML page can fetch more data in the background, allowing the browser to load pages more quickly.
• Sophisticated user interface controls and effects: Controls such as trees, menus, data tables, rich text editors, calendars, and progress bars allow for better user interaction and interaction with HTML pages, generally without requiring the user to reload the page.
• Refreshing data and server push: HTML pages may poll data from a server for up-to-date data such as scores, stock quotes, weather, or application-specific data. A client may use Ajax techniques to get a set of current data without reloading a full page. Polling is not the most efficient means of ensuring that data on a page is the most current. Emerging techniques such as Comet are being developed to provide true server-side push over HTTP by keeping a persistent connection between the client and server.
• Partial submit: An HTML page can submit form data as needed without requiring a full page refresh.
• Mashups: An HTML page can obtain data using a server-side proxy or by including an external script to mix external data with your application's or your service's data. For example, you can mix content or data from a third-party application such as Google Maps with your own application.
• Page as an application: Ajax techniques can be made to create single-page applications that look and feel much like a desktop application.