How to Improve jQuery Selector Performance ?

What is a jQuery ?

• jQuery is a lightweight, "Write Less, Do More", JavaScript library.
• The purpose of jQuery is to make it much easier to use JavaScript on your website.
• jQuery takes a lot of common tasks that requires many lines of JavaScript code to accomplish, and wraps it into methods that you can call with a single line of code.
• jQuery also simplifies a lot of the complicated things from JavaScript, like AJAX calls and DOM manipulation.

                              What is a DOM ?
                                      - Document Object Model
                                      - is the Backbone of every webpage
                                      - web browser parses HTML and Javascript and maintains
                                        a Data Model of what should be displayed to the user 

The jQuery Library Contains the Following Features:

• HTML/DOM manipulation
• CSS manipulation
• HTML event methods
• Effects and animations
• AJAX
• Utilities
• jQuery has plugins for almost any task out there.

What is a jQuery Selector?

• A Selector identifies an HTML element tag that is used to manipulate with jQuery code. 
• Selectors allow page elements to be selected

Selector Syntax 

$(Selector Expression)          OR      jQuery(Selector Expression)

What are the Types of jQuery Selectors?

1. Class Selectors
       $(".intro")  - All elements with class="intro"

2. ID Selectors
       $("#lastname")  - The element with id=lastname

3. Element Selectors
       $("p")   - All p elements

How to Caching jQuery Selectors ?

What is Happening When We use Selector ?

• jQuery Traverses through all Elements of DOM every time

How Can Overcome this?

• You should Always Cache your Selections to some variable

Its Like this :

      var myList = $('.myList');

      myList.text("cached");

When to Use ?

• Using the Same Selection More than Once in your DOM Tree. 
• e:g: Inside a jQuery Function Likes below

BAD Way

$(".myList").click( function() {

   $(".myList").hide(); 

});

BEST Way

var myList  = $(".myList");

myList.click( function() {

       myList.hide();  // No need to re-select .myList since we cached it

});

Performance of  jQuery Selector when Caching

More Tips for Optimizing jQuery Selector Performance :

1. Qualify your Selector strings when searching by class
       - give Selector's Context as 2nd Parameter

       - when the branches of your DOM tree become very deep 

                       What is a Context ?
                            - jQuery has an optional second argument called context
                            - Its Limit the search within a specific node
                            - Great when you have a very Large DOM tree and need to find
                             e.g: All the <p> tags within a specific part of the DOM tree
                            - You can check to see what is the context of Selector by using context
                               Property.
                              e.g:    $('p').context; //out put is => document

BAD Way

$(".your-class")

BEST Way

// get the node for the context

var context = $('#myContainer')[0];

$(".your-class", context )

                 

2. Don't Qualify your selector when searching by Id.

      - searching by Id uses the browsers native getElementById method (which is very fast)

BAD Way

$("div#your-id")

BEST Way

$("#your-id")

Performance of jQuery ID vs Class  Selectors

Conclusion

• Always Try to use Id of a Selector when its possible
• Always Do Caching for jQuery Selectors before it use
• Always Use Context with class Selectors

I hope this helps to You.Comments and feedback greatly appreciated.