jQuery Interview Questions And Answers.

1. What is jQuery?

Ans: jQuery is fast, lightweight and feature-rich client side JavaScript Library/Framework which helps in to traverse HTML DOM, make animations, add Ajax interaction, manipulate the page content, change the style and provide cool UI effect. It is one of the most popular client side library.

2. Why do we use jQuery?

Ans: Due to following advantages.
Easy to use and learn.
Easily expandable.
Cross-browser support (IE 6.0+, FF 1.5+, Safari 2.0+, Opera 9.0+)
Easy to use for DOM manipulation.
AJAX Capabilities.
Methods for changing or applying CSS, creating animations.
Event detection and handling.

3. How JavaScript and jQuery are different?

Ans: JavaScript is a language While jQuery is a library built in the JavaScript language.

4. Is jQuery replacement of Java Script?

Ans: No. jQuery is not a replacement of JavaScript. jQuery is a different library which is written on top of JavaScript. jQuery is a lightweight JavaScript library that emphasizes interaction between JavaScript and HTML.

5. Is jQuery a W3C standard?

Ans: No. jQuery is not a W3C standard.

6. What does dollar sign ($) means in jQuery?

Ans: Dollar Sign is nothing but it's an alias for JQuery. Take a look at below jQuery code.
$(document).ready(function(){
});
Over here $ sign can be replaced with "jQuery" keyword.
jQuery(document).ready(function(){
});

7. Can we have multiple document.ready() function on the same page?

Ans: YES. We can have any number of document.ready() function on the same page.

8. Can we use our own specific character in the place of $ sign in jQuery?

Ans: Yes. It is possible using jQuery.noConflict().
var $j = jQuery.noConflict();
// Use jQuery via jQuery(...)
$j(document).ready(function(){
   $j("div").hide();
});

9. Is there any difference between body onload() and document.ready() function?

Ans: document.ready() function is different from body onload() function for 2 reasons.
We can have more than one document.ready() function in a page where we can have only one body onload function.
document.ready() function is called as soon as DOM is loaded where body.onload() function is called when everything gets loaded on the page that includes DOM, images and all associated resources of the page.

10. What is a CDN?

Ans: A content delivery network or content distribution network (CDN) is a large distributed system of servers deployed in multiple data centers across the Internet. The goal of a CDN is to serve content to end-users with high availability and high performance.
There are 3 popular jQuery CDNs.
1. Google.
2. Microsoft
3. jQuery.

11. What are selectors in jQuery and how many types of selectors are there?

Ans: To work with an element on the web page, first we need to find them. To find the html element in jQuery we use selectors. There are many types of selectors but basic selectors are:

Name: Selects all elements which match with the given element Name.
#ID: Selects a single element which matches with the given ID
.Class: Selects all elements which match with the given Class.
Universal (*): Selects all elements available in a DOM.
Multiple Elements E, F, G: Selects the combined results of all the specified selectors E, F or G.
Attribute Selector: Select elements based on its attribute value.

12. What are the fastest selectors in jQuery?

Ans: ID and element selectors are the fastest selectors in jQuery.

13. What are the slow selectors in jQuery?

Ans: class selectors are the slow compare to ID and element.

14. Which is fast document.getElementByID('txtName') or $('#txtName').?

Ans: Native JavaScipt is always fast. jQuery method to select txtName "$('#txtName')" will internally makes a call to document.getElementByID('txtName'). As jQuery is written on top of JavaScript and it internally uses JavaScript only So JavaScript is always fast.

15. Difference between $(this) and 'this' in jQuery?

Ans: this and $(this) refers to the same element. The only difference is the way they are used. 'this' is used in traditional sense, when 'this' is wrapped in $() then it becomes a jQuery object and you are able to use the power of jQuery.
$(document).ready(function(){
    $('#spnValue').mouseover(function(){
       alert($(this).text());
  });
});
In below example, this is an object but since it is not wrapped in $(), we can't use jQuery method and use the native JavaScript to get the value of span element.
$(document).ready(function(){
    $('#spnValue').mouseover(function(){
       alert(this.innerText);
  });
});

16. How do you check if an element exists or not in jQuery? 

Ans: Using jQuery length property, we can ensure whether element exists or not.
$(document).ready(function(){
    if ($('#element').length > 0){
       //Element exists
  }
});

17. How do you check if an element is empty?

Ans: There are 2 ways to check if element is empty or not. We can check using ":empty" selector.
$(document).ready(function(){
    if ($('#element').is(':empty')){
       //Element is empty
  }
});
And the second way is using the "$.trim()" method.
$(document).ready(function(){
    if($.trim($('#element').html())=='') {
       //Element is empty
  }
});

18. What is the difference between jquery.size() and jquery.length?

Ans: jQuery .size() method returns number of element in the object. But it is not preferred to use the size() method as jQuery provide .length property and which does the same thing. But the .length property is preferred because it does not have the overhead of a function call.

19. What is the difference between $('div') and $('<div/>') in jQuery?

Ans: $('<div/>') : This creates a new div element. However this is not added to DOM tree unless you don't append it to any DOM element.
$('div') : This selects all the div element present on the page.

20. What is the difference between parent() and parents() methods in jQuery?

Ans: The basic difference is the parent() function travels only one level in the DOM tree, where parents() function search through the whole DOM tree.

21. What is the difference between .empty(), .remove() and .detach() methods in jQuery?

Ans: All these methods .empty(), .remove() and .detach() are used for removing elements from DOM but they all are different.

.empty(): This method removes all the child element of the matched element where remove() method removes set of matched elements from DOM.

.remove(): Use .remove() when you want to remove the element itself, as well as everything inside it. In addition to the elements themselves, all bound events and jQuery data associated with the elements are removed.

.detach(): This method is the same as .remove(), except that .detach() keeps all jQuery data associated with the removed elements. This method is useful when removed elements are to be reinserted into the DOM at a later time.

22. Explain .bind() vs .live() vs .delegate() vs .on()

Ans: All these 4 jQuery methods are used for attaching events to selectors or elements. But they all are different from each other.

.bind(): This is the easiest and quick method to bind events. But the issue with bind() is that it doesn't work for elements added dynamically that matches the same selector. bind() only attach events to the current elements not future element. Above that it also has performance issues when dealing with a large selection.

.live(): This method overcomes the disadvantage of bind(). It works for dynamically added elements or future elements. Because of its poor performance on large pages, this method is deprecated as of jQuery 1.7 and you should stop using it. Chaining is not properly supported using this method.

.delegate(): The .delegate() method behaves in a similar fashion to the .live() method, but instead of attaching the selector/event information to the document, you can choose where it is anchored and it also supports chaining.

.on(): Since live was deprecated with 1.7, so new method was introduced named ".on()". This method provides all the goodness of previous 3 methods and it brings uniformity for attaching event handlers.

23. What is event.PreventDefault?

Ans: The event.preventDefault() method stops the default action of an element from happening. For example, Prevents a link from following the URL.

24. What is the difference between event.PreventDefault and event.stopPropagation?

Ans: event.preventDefault(): Stops the default action of an element from happening.
event.stopPropagation(): Prevents the event from bubbling up the DOM tree, preventing any parent handlers from being notified of the event. For example, if there is a link with a click method attached inside of a DIV or FORM that also has a click method attached, it will prevent the DIV or FORM click method from firing.

25. Can you include multiple version of jQuery? If yes, then how they are executed?

Ans: Yes. Multiple versions of jQuery can be included in same page.

26. In what situation you would use multiple version of jQuery and how would you include them?

Ans: Well, it is quite possible that the jQuery plugins which are used are dependent on older version but for your own jQuery code, you would like to use newer version. So because of this dependency, multiple version of jQuery may required sometimes on single page.
Below code shows how to include multiple version of jQuery.
<script type='text/javascript' src='js/jquery_1.9.1.min.js'></script>
<script type='text/javascript'>
 var $jq = jQuery.noConflict();
</script>
<script type='text/javascript' src='js/jquery_1.7.2.min.js'></script>
By this way, for your own jQuery code use "$jq", instead of "$" as "$jq" refers to jQuery 1.9.1, where "$" refers to 1.7.2.

27. What is chaining in jQuery?

Ans: Chaining is one of the most powerful feature of jQuery. In jQuery, Chaining means to connect multiple functions, events on selectors. It makes your code short and easy to manage and it gives better performance. The chain starts from left to right. So left most will be called first and so on.
The above jQuery code sample can be re-written using chaining. See below.
​$(document).ready(function(){
    $('#dvContent').addClass('dummy').css('color', 'red').fadeIn('slow');  
});​

28. How to write browser specific code using jQuery?

Ans: Using jQuery.browser property, we can write browser specific code. This property contains flags for the useragent, read from navigator.userAgent. This property was removed in jQuery 1.9.

29. Can we use jQuery to make ajax request?

Ans: Yes. jQuery can be used for making ajax request.

30. What are various methods to make ajax request in jQuery?

Ans: Using below jQuery methods, you can make ajax calls.
load() : Load a piece of html into a container DOM
$.getJSON(): Load JSON with GET method.
$.getScript(): Load a JavaScript file.
$.get(): Use to make a GET call and play extensively with the response.
$.post(): Use to make a POST call and don't want to load the response to some container DOM.
$.ajax(): Use this to do something on XHR failures, or to specify ajax options (e.g. cache: true) on the fly.

31. Is there any advantage of using $.ajax() for ajax call against $.get() or $.post()?

Ans: By using jQuery post()/ jQuery get(), you always trust the response from the server and you believe it is going to be successful all the time. Well, it is certainly not a good idea to trust the response. As there can be n number of reason which may lead to failure of response.

Where jQuery.ajax() is jQuery's low-level AJAX implementation. $.get and $.post are higher-level abstractions that are often easier to understand and use, but don't offer as much functionality (such as error callbacks).

32. How can we debug jQuery?

There are two ways to debug jQuery:
Debugger keyword
Add the debugger to the line from where we have to start debugging and then run Visual Studio in Debug mode with F5 function key.
Insert a break point after attaching the process

33. What are deferred and promise object in jQuery?

Ans: Deferred and promise are part of jQuery since version 1.5 and they help in handling asynchronous functions like Ajax.