jQuery Interview Questions and Answers

jQuery selectors are used to select html elements to in order to operate on the element based on their id, class, element type, attributes, and many more. There are many types of selectors but important selectors are:

Element Selector (“element”):Selects all elements with the given tag name
$(“div”).css(“color”,”red”);// text color of all div elements will be red

All Selector (“*”): select all HTML elements at a time.
$(“*”).css(“color”, “blue”);//change all elements text color to blue

Id Selector (“#id”): Selects a single element with the given id attribute.
$(“#id”).css(“background”, “black”);

Class Selector (“.class”): Selects all elements with the given class.
$(“.class”).css(“color”, “black”);

Attribute Selector [name=”value”]: Select elements based on its attribute value.

Multiple Selector (“selector1, selector2, selectorN”): Select elements that match all selector condition.

jQuery is an open source JavaScript library that simplify Html document traversing and manipulation browser event handling, DOM animations, Ajax interaction and cross bowser JavaScript development.

jQuery is just a JavaScript library and has built in JavaScript code functionality that optimized development, reduce effort and provided cross browsers supports.

jQuery philosophy is “Write less, do more”

Open source and has large community of users and contributors

Tested and optimized for modern browsers

Lightweight, robust and expandable

Offers plug-ins and ready-made functionality

Can be used for event-driven programming

AJAX support

We can use jQuery in web page by two way

Use of content delivery network (CDN) to include a version of jQuery.
Downloaded version of jQuery in local file-system from jQuery.com.

Content delivery network is geographically distributed worldwide high-speed collection of servers, which can serve content to end-users in lowround-trip times.

Advantage:

We can start using jQuery from CDN simply by adding a <script> tag in web page without locally downloaded.

Browsers reused cached jQuery files for web sites that are located in different domains, which significantly improve performance.

CDN offers reliable high-speed access on jQuery library withminimum round-trip time.

Google, Microsoft and jQuery are popular jQuery CDN.

To load a hosted library from CDN, copy and paste the below HTML snippet for that library (shown below) in your web page

Google (https://developers.google.com/speed/libraries/#web-font-loader)
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.3/jquery.min.js"></script>

Microsoft (http://www.asp.net/ajax/cdn)
<script src="http://ajax.aspnetcdn.com/ajax/jquery/jquery-1.9.0.min.js"></script>

JQuery (https://code.jquery.com/)
<script src="http://code.jquery.com/jquery-1.9.1.min.js"></script>

To allow to loading jQuery from a local path on your own website if the CDN happens to be unavailable, add the following element immediately after the element referencing the CDN:

<script src=" /jquery/jquery-1.9.0.min.js"></script>

Ex:

<script src="http://ajax.aspnetcdn.com/ajax/jquery/jquery-1.9.0.min.js"></script>

<script src=" /jquery/jquery-1.9.0.min.js"></script>

jQuery and $ are the same thing and work same. $ is just a shortcut.

For example, the following statement will create an <a>element wrapped inside of a <p>element with a text node encapsulated inside of the <p>and <a>elements:

jQuery('<p><a>jQuery</a></p>');
which is the same as
$('<p><a>jQuery</a></p>');

We can replace $ by using no Conflict () method, see below snippet to replace $ to jqwrite

var jqwrite = $.noConflict()

Everything inside the $(document).ready() function will load as soon as the DOM (Document Object Model) is loaded and before the page contents are loaded.

The ready() event handler method is jQuery’s replacement for using the JavaScript core window.onloadevent.

jQuery Document Ready Snippet
$(document).ready(function() {  // Put javascript here});

Different ways to write jQuery Document Ready Snippet
$(document).ready(function() { ... });
$(function() { ... });
jQuery(document).ready(function() { ... });
jQuery(function() { ... });

jQuery document ready function can be used as many times in page as you like.

jQuery ready() function only waits for the DOM tree to be loaded before executing the code. Where JavaScript onload event not only waits for DOM to be created but also waits for all external resources to load fully including heavy images, audios and videos etc.
If loading images and other third party elements takes significant amount of time then user can experience significantly be impacted on execution of code defined in window.onloadevent.

As mentioned above jQuery ready() function recommended over JavaScript onload event. However, there are valid use cases that justifies use of JavaScript onload over jQuery ready().

The elements which are selected using ID are the performance responsive selector as the ID is unique throughout the rendered page.

jQuery provided $.ajax({...}); function to support Ajax functionality.

Below an example of making an AJAX call and alerting the response (or error):

$.ajax({
url:'pAjaxReqServer.php',
success:function(response) {
alert(response);
       },
error:function(xhr) {
alert('Error!  Status = '+xhr.status);
       }
});

jQuery.get() and jQuery.post() contain features that are subsets of jQuery.ajax().

jQuery.ajax()is the method that provides the most flexibility of the three.The biggest difference from an implementation point is that you can pass an objecttojQuery.ajax(), which contains the necessary parameters.WherejQuery.get()andjQuery.post() are both shorthand forjQuery.ajax().

Ex:jQuery.ajax()

jQuery.ajax({
url:'callAjax.php,
dataType:'text',
type:‘GET’,
success:function(response) {
alert(response);
  }
});

Ex:jQuery.get()

jQuery.get('callAjax.txt',function(response){ 
alert(response) 
},'text');

Ex:jQuery.post()

jQuery.post('callAjax.txt',function(response){
alert(response)
},'text');