Ajax

Getting Started
An introduction to Ajax.

Asynchronous JavaScript + XML, while not a technology in itself, is a term coined in 2005 by Jesse James Garrett, that describes a "new" approach to using a number of existing technologies together, including: HTML or XHTML, Cascading Style Sheets, JavaScript, The Document Object Model, XML, XSLT, and most importantly the XMLHttpRequest object.
When these technologies are combined in the Ajax model, web applications are able to make quick, incremental updates to the user interface without reloading the entire browser page. This makes the application faster and more responsive to user actions.

Although X in Ajax stands for XML, JSON is used more than XML nowadays because of its many advantages such as being lighter and a part of JavaScript. Both JSON and XML are used for packaging information in Ajax model.

Documentation

Getting Started
This article guides you through the Ajax basics and gives you two simple hands-on examples to get you started.
Using the XMLHttpRequest API
The XMLHttpRequest API is the core of Ajax. This article will explain you how to use some Ajax techniques, like:
Server-sent events
Traditionally, a web page has to send a request to the server to receive new data; that is, the page requests data from the server. With server-sent events, it's possible for a server to send new data to a web page at any time, by pushing messages to the web page. These incoming messages can be treated as Events + data inside the web page. See also:  Using server-sent events.
Pure-Ajax navigation example
This article provides a working (minimalist) example of a pure-Ajax web site composed only of three pages.
Sending and Receiving Binary Data
The responseType property of the XMLHttpRequest object can be set to change the expected response type from the server. Possible values are the empty string (default), "arraybuffer", "blob", "document", "json", and "text". The response property will contain the entity body according to responseType, as an ArrayBuffer, Blob, Document, JSON, or string. This article will show some Ajax I/O techniques.
XML
The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language for creating special-purpose markup languages. It is a simplified subset of SGML, capable of describing many different kinds of data. Its primary purpose is to facilitate the sharing of data across different systems, particularly systems connected via the Internet.
JXON
JXON stands for lossless Javascript XML Object Notation, it is a generic name by which is defined the representation of Javascript object trees (JSON) using XML.
Parsing and serializing XML
How to parse an XML document from a string, a file or via javascript and how to serialize XML documents to strings, Javascript Object trees (JXON) or files.
XPath
XPath stands for XML Path Language, it uses a non-XML syntax that provides a flexible way of addressing (pointing to) different parts of an XML document. As well as this, it can also be used to test addressed nodes within a document to determine whether they match a pattern or not.
The FileReader API
The FileReader API lets web applications asynchronously read the contents of files (or raw data buffers) stored on the user's computer, using File or Blob objects to specify the file or data to read. File objects may be obtained from a FileList object returned as a result of a user selecting files using the <input> element, from a drag and drop operation's DataTransfer object, or from the mozGetAsFile() API on an HTMLCanvasElement.
HTML in XMLHttpRequest
The W3C XMLHttpRequest specification adds HTML parsing support to XMLHttpRequest, which originally supported only XML parsing. This feature allows Web apps to obtain an HTML resource as a parsed DOM using XMLHttpRequest.
Other resources
Other Ajax resources you may find useful.

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See also

Alternate Ajax Techniques
Most articles on Ajax have focused on using XMLHttp as the means to achieving such communication, but Ajax techniques are not limited to just XMLHttp. There are several other methods.
Ajax: A New Approach to Web Applications
Jesse James Garrett, of adaptive path, wrote this article in February 2005, introducing Ajax and its related concepts.
A Simpler Ajax Path
"As it turns out, it's pretty easy to take advantage of the XMLHttpRequest object to make a web app act more like a desktop app while still using traditional tools like web forms for collecting user input."
Fixing the Back Button and Enabling Bookmarking for Ajax Apps
Mike Stenhouse has penned this article, detailing some methods you can use to fix back button and bookmarking issues when developing Ajax applications.
Ajax Mistakes
Alex Bosworth has written this article outlining some of the mistakes Ajax application developers can make.
Tutorial with examples.
 
XMLHttpRequest specification
W3C Working draft