JQuery

jQuery is a JavaScript library designed to simplify HTML DOM tree traversal and manipulation, as well as event handling, CSS animation, and Ajax. It is free, open-source software using the permissive MIT License. As of May 2019, jQuery is used by 73% of the 10 million most popular websites. Web analysis indicates that it is the most widely deployed JavaScript library by a large margin, having at least 3 to 4 times more usage than any other JavaScript library.
jQuery's syntax is designed to make it easier to navigate a document, select DOM elements, create animations, handle events, and develop Ajax applications. jQuery also provides capabilities for developers to create plug-ins on top of the JavaScript library. This enables developers to create abstractions for low-level interaction and animation, advanced effects and high-level, themeable widgets. The modular approach to the jQuery library allows the creation of powerful dynamic web pages and Web applications.
The set of jQuery core features—DOM element selections, traversal and manipulation—enabled by its "selector engine" (named "Sizzle" from v1.3), created a new "programming style", fusing algorithms and DOM data structures. This style influenced the architecture of other JavaScript frameworks like YUI v3 and Dojo, later stimulating the creation of the standard "Selectors API". Later, this style has been enhanced with a deeper algorithm-data fusion in an heir of jQuery, the D3.js framework.
Microsoft and Nokia bundle jQuery on their platforms. Microsoft includes it with Visual Studio for use within Microsoft's ASP.NET AJAX and ASP.NET MVC frameworks while Nokia has integrated it into the Web Run-Time widget development platform.
Overview.
jQuery, at its core, is a Document Object Model (DOM) manipulation library. The DOM is a tree-structure representation of all the elements of a Web page. jQuery simplifies the syntax for finding, selecting, and manipulating these DOM elements. For example, jQuery can be used for finding an element in the document with a certain property (e.g. all elements with an h1 tag), changing one or more of its attributes (e.g. color, visibility), or making it respond to an event (e.g. a mouse click).
jQuery also provides a paradigm for event handling that goes beyond basic DOM element selection and manipulation. The event assignment and the event callback function definition are done in a single step in a single location in the code. jQuery also aims to incorporate other highly used JavaScript functionality (e.g. fade ins and fade outs when hiding elements, animations by manipulating CSS properties).
The principles of developing with jQuery are:
History.
jQuery was originally created in January 2006 at BarCamp NYC by John Resig, influenced by Dean Edwards' earlier cssQuery library. It is currently maintained by a team of developers led by Timmy Willison (with the jQuery selector engine, Sizzle, being led by Richard Gibson).
jQuery was originally licensed under the CC BY-SA 2.5, and relicensed to the MIT license in 2006. At the end of 2006, it was dual-licensed under GPL and MIT licenses. As this led to some confusion, in 2012 the GPL was dropped and is now only licensed under the MIT license.
Features.
jQuery includes the following features:
Browser support.
jQuery 3.0 and newer supports "current−1 versions" (meaning the current stable version of the browser and the version that preceded it) of Firefox (and ESR), Chrome, Safari, and Edge as well as Internet Explorer 9 and newer. On mobile it supports iOS 7 and newer, and Android 4.0 and newer.
Distribution.
The jQuery library is typically distributed as a single JavaScript file that defines all its interfaces, including DOM, Events, and Ajax functions. It can be included within a Web page by linking to a local copy, or by linking to one of the many copies available from public servers. jQuery has a content delivery network (CDN) hosted by MaxCDN. Google in Google Hosted Libraries service and Microsoft host the library as well.
Example of linking a copy of the library locally (from the same server that hosts the Web page):
<script src="jquery-3.5.1.min.js"></script>
Example of linking a copy of the library from jQuery's public CDN:
<script
 src="https://code.jquery.com/jquery-3.5.1.min.js"
 integrity="sha256-9/aliU8dGd2tb6OSsuzixeV4y/faTqgFtohetphbbj0="
 crossorigin="anonymous"></script>
Interface.
Functions.
jQuery provides two kinds of functions, static utility functions and jQuery object methods. Each has its own usage style.
Both are accessed through jQuery's main identifier: codice_3. This identifier has an alias named codice_4. All functions can be accessed through either of these two names.
jQuery methods.
The codice_3 function is a factory for creating a jQuery object that represents one or more DOM nodes. jQuery objects have methods to manipulate these nodes. These methods (sometimes called "commands)", are "chainable" as each method also returns a jQuery object.
Access to and manipulation of multiple DOM nodes in jQuery typically begins with calling the codice_6 function with a CSS selector string. This returns a jQuery object referencing all the matching elements in the HTML page. codice_7, for example, returns a jQuery object with all the codice_8 elements of class codice_9. This node set can be manipulated by calling methods on the returned jQuery object.
Static utilities.
These are utility functions and do not directly act upon a jQuery object. They are accessed as static methods on the jQuery or $ identifier. For example, codice_10 is a static method.
No-conflict mode.
jQuery provides a codice_11 function, which relinquishes control of the codice_4 name. This is useful if jQuery is used on a Web page also linking another library that demands the codice_4 symbol as its identifier. In no-conflict mode, developers can use codice_3 as a replacement for codice_4 without losing functionality.
Typical start-point.
Typically, jQuery is used by putting initialization code and event handling functions in codice_16. This is triggered by jQuery when the browser has finished constructing the DOM for the current Web page.
$(function () {
 // This anonymous function is called when the page has completed loading.
 // Here, one can place code to create jQuery objects, handle events, etc.
or
$(fn); // The function named fn, defined elsewhere, is called when the page has loaded.
Historically, codice_17 has been the de facto idiom for running code after the DOM is ready. However, since jQuery 3.0, developers are encouraged to use the much shorter codice_18 signature instead.
Chaining.
jQuery object methods typically also return a jQuery object, which enables the use of "method chains":
$('div.test')
 .on('click', handleTestClick)
 .addClass('foo');
This line finds all codice_19 elements with class attribute codice_20 , then registers an event handler on each element for the "click" event, then adds the class attribute codice_21 to each element.
Certain jQuery object methods retrieve specific values (instead of modifying state). An example of this is the codice_22 method, which returns the current value of a text input element. In these cases, a statement such as codice_23 cannot be used for chaining as the return value does not reference a jQuery object.
Creating new DOM elements.
Besides accessing existing DOM nodes through jQuery, it is also possible to create new DOM nodes, if the string passed as the argument to $() factory looks like HTML. For example, the below code finds an HTML codice_24 element, and creates a new codice_25 element with value "VAG" and label "Volkswagen", which is then appended to the select menu:
$('select#car-brands')
 .append($('<option>')
 .attr({ value: 'VAG' })
 .text('Volkswagen')
Ajax.
It is possible to make Ajax requests (with cross-browser support) with codice_26 to load and manipulate remote data.
$.ajax({
 type: 'POST',
 url: '/process/submit.php',
 data: {
 name : 'John',
 location : 'Boston',
}).then(function(msg) {
 alert('Data Saved: ' + msg);
}).catch(function(xmlHttpRequest, statusText, errorThrown) {
 alert(
 'Your form submission failed.\n\n'
 + 'XML Http Request: ' + JSON.stringify(xmlHttpRequest)
 + ',\nStatus Text: ' + statusText
 + ',\nError Thrown: ' + errorThrown);
This example posts the data codice_27 and codice_28 to codice_29 on the server. When this request finishes the success function is called to alert the user. If the request fails it will alert the user to the failure, the status of the request, and the specific error.
The above example uses the codice_30 and codice_31 methods to register callbacks that run when the response has completed. These promise callbacks must be used due to the asynchronous nature of Ajax requests.
jQuery plug-ins.
jQuery's architecture allows developers to create plug-in code to extend its function. There are thousands of jQuery plug-ins available on the Web that cover a range of functions, such as Ajax helpers, Web services, datagrids, dynamic lists, XML and XSLT tools, drag and drop, events, cookie handling, and modal windows.
An important source of jQuery plug-ins is the plugins sub-domain of the jQuery Project website. The plugins in this subdomain, however, were accidentally deleted in December 2011 in an attempt to rid the site of spam. The new site is a GitHub-hosted repository, which required developers to resubmit their plugins and to conform to new submission requirements. jQuery provides a "Learning Center" that can help users understand JavaScript and get started developing jQuery plugins.
In order to create these plug-ins, developers can either choose to write their own code from scratch or build on top of an existing structure such as the jQuery Boilerplate.
Testing framework.
QUnit is a test automation framework used to test the jQuery project. The jQuery team developed it as an in-house unit testing library. The jQuery team uses it to test its code and plugins, but it can test any generic JavaScript code, including server-side JavaScript code.
As of 2011, the jQuery Testing Team uses QUnit with TestSwarm to test each jQuery codebase release.