Ajax (programming)

Ajax (also AJAX ; short for "Asynchronous JavaScript and XML") is a set of web development techniques using many web technologies on the client-side to create asynchronous web applications. With Ajax, web applications can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behaviour of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows web pages and, by extension, web applications, to change content dynamically without the need to reload the entire page. In practice, modern implementations commonly utilize JSON instead of XML.
Ajax is not a single technology, but rather a group of technologies. HTML and CSS can be used in combination to mark up and style information. The webpage can then be modified by JavaScript to dynamically display—and allow the user to interact with—the new information. The built-in XMLHttpRequest object, or since 2017 the new function within JavaScript, is commonly used to execute Ajax on webpages, allowing websites to load content onto the screen without refreshing the page. Ajax is not a new technology, or a different language, just existing technologies used in new ways.
History.
In the early-to-mid 1990s, most Web sites were based on complete HTML pages. Each user action required that a complete new page be loaded from the server. This process was inefficient, as reflected by the user experience: all page content disappeared, then the new page appeared. Each time the browser reloaded a page because of a partial change, all of the content had to be re-sent, even though only some of the information had changed. This placed additional load on the server and made bandwidth a limiting factor on performance.
In 1996, the iframe tag was introduced by Internet Explorer; like the object element, it can load or fetch content asynchronously. In 1998, the Microsoft Outlook Web Access team developed the concept behind the XMLHttpRequest scripting object. It appeared as XMLHTTP in the second version of the MSXML library, which shipped with Internet Explorer 5.0 in March 1999.
The functionality of the Windows XMLHTTP ActiveX control in IE 5 was later implemented by Mozilla, Safari, Opera and other browsers as the XMLHttpRequest JavaScript object. Microsoft adopted the native XMLHttpRequest model as of Internet Explorer 7. The ActiveX version is still supported in Internet Explorer, but not in Microsoft Edge. The utility of these background HTTP requests and asynchronous Web technologies remained fairly obscure until it started appearing in large scale online applications such as Outlook Web Access (2000) and Oddpost (2002).
Google made a wide deployment of standards-compliant, cross browser Ajax with Gmail (2004) and Google Maps (2005). In October 2004 Kayak.com's public beta release was among the first large-scale e-commerce uses of what their developers at that time called "the xml http thing". This increased interest in AJAX among web program developers.
The term "AJAX" was publicly used on 18 February 2005 by Jesse James Garrett in an article titled "Ajax: A New Approach to Web Applications", based on techniques used on Google pages.
On 5 April 2006, the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official Web standard.
The latest draft of the XMLHttpRequest object was published on 6 October 2016, and the XMLHttpRequest specification is now a living standard.
Technologies.
The term "Ajax" has come to represent a broad group of Web technologies that can be used to implement a Web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax, Jesse James Garrett explained that the following technologies are incorporated:
Since then, however, there have been a number of developments in the technologies used in an Ajax application, and in the definition of the term Ajax itself. XML is no longer required for data interchange and, therefore, XSLT is no longer required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange, although other formats such as preformatted HTML or plain text can also be used. A variety of popular JavaScript libraries, including JQuery, include abstractions to assist in executing Ajax requests.
Examples.
JavaScript example.
An example of a simple Ajax request using the GET method, written in JavaScript.
get-ajax-data.js:
// This is the client-side script.
// Initialize the HTTP request.
var xhr = new XMLHttpRequest();
xhr.open('GET', 'send-ajax-data.php');
// Track the state changes of the request.
xhr.onreadystatechange = function () {
 var DONE = 4; // readyState 4 means the request is done.
 var OK = 200; // status 200 is a successful return.
 if (xhr.readyState === DONE) {
 if (xhr.status === OK) {
 console.log(xhr.responseText); // 'This is the output.'
 } else {
 console.log('Error: ' + xhr.status); // An error occurred during the request.
// Send the request to send-ajax-data.php
xhr.send(null);
send-ajax-data.php:
<?php
// This is the server-side script.
// Set the content type.
header('Content-Type: text/plain');
// Send the data back.
echo "This is the output.";
Many developers disliked the syntax used in the XMLHttpRequest object, so some of them applied workarounds that are no longer needed since Fetch.
Fetch example.
Fetch is a new native JavaScript API. According to Google Developers Documentation, "Fetch makes it easier to make web requests and handle responses than with the older XMLHttpRequest."
fetch('send-ajax-data.php')
 .then(data => console.log(data))
 .catch (error => console.log('Error:' + error));
ES7 async/await example:
async function doAjax() {
 try {
 const res = await fetch('send-ajax-data.php');
 const data = await res.text();
 console.log(data);
 } catch (error) {
 console.log('Error:' + error);
doAjax();
As seen above, fetch relies on JavaScript promises.
The codice_1 specification differs from codice_2 in the following significant ways: