Primeiros Passos

Uma introdução ao AJAX

JavaScript Assíncrono e XML, enquanto não uma tecnologia em si, é um termo criado em 2005 por Jesse James Garret, que descreve uma "nova" abordagem para utilizar uma série de tecnologias existentes em conjunto, incluindo HTML ou XHTML, Cascading Style Sheets, JavaScript, Document Object Model, XML, XSLT e o objeto XMLHttpRequest.
Quando estas tecnologias são combinadas no modelo AJAX, as aplicações da Web são capazes de efetuar atualizações incrementais e rápidas na interface do utilizador sem recarregar toda a página. Isto torna a aplicação mais rápida e mais responsiva para as ações do do utilizador.

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.

Primeiros Passos
This article guides you through the Ajax basics and gives you two simple hands-on examples to get you started.
Utilizar a API XMLHttpRequest
The XMLHttpRequest API is the core of Ajax. This article will explain how to use some Ajax techniques, like:
API Fetch
The Fetch API provides an interface for fetching resources. It will seem familiar to anyone who has used XMLHTTPRequest, but this API provides a more powerful and flexible feature set.
Eventos de envio do servidor
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 website composed only of three pages.
Enviar e Receber Dados Binário
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.
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 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.
Analisar e serializar 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 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.
A API FileReader
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 no 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.
Outros recursos
Outros recursos Ajax que poderão ser úteis.

Consulte também

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."
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