JQuery

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For more info:-http://jquery.com/

A basic introduction to jQuery and the concepts that you need to know to use it.

As almost everything we do when using jQuery reads or manipulates the document object model (DOM), we need to make sure that we start adding events etc. as soon as the DOM is ready.

To do this, we register a ready event for the document.

$(document).ready(function() {
   // do stuff when DOM is ready
 });

$(document).ready(function() {
   $("a").click(function() {
     alert("Hello world!");
   });
 });
This should show the alert as soon as you click on the link. You are ready now to copy and paste this script into your custom.js file. Then, open starterkit.html in the browser and click any link. You should see a pop-up window with "Hello world!" message regardless of what link was clicked.
Let's have a look at what we are doing: $("a") is a jQuery selector, in this case, it selects all aelements. $ itself is an alias for the jQuery "class", therefore $() constructs a new jQuery object. The click() function we call next is a method of the jQuery object. It binds a click event to all selected elements (in this case, a single anchor element) and executes the provided function when the event occurs.
This is similar to the following code:
<a href="" onclick="alert('Hello world')">Link</a>
The difference is quite obvious: We don't need to write an onclick for every single element. We have a clean separation of structure (HTML) and behavior (JS), just as we separate structure and presentation by using CSS.