Jquery .addClass

.addClass()
Returns: jQuery
.addClass(className)
className One or more class names to be added to the class attribute of each matched element.
.addClass(function(index, currentClass))
function(index, currentClass) A function returning one or more space-separated class names to be added. Receives the index position of the element in the set and the old class value as arguments.
Adds the specified class(es) to each of the set of matched elements.
It's important to note that this method does not replace a class. It simply adds the class, appending it to any which may already be assigned to the elements.

More than one class may be added at a time, separated by a space, to the set of matched elements, like so:

$("p").addClass("myClass yourClass");
This method is often used with .removeClass() to switch elements' classes from one to another, like so:

$("p").removeClass("myClass noClass").addClass("yourClass");
Here, the myClass and noClass classes are removed from all paragraphs, while yourClass is added.

As of jQuery 1.4, the .addClass() method's argument can receive a function.

$("ul li:last").addClass(function() {
return "item-" + $(this).index();
});
Given an unordered list with five

elements, this example adds the class "item-4" to the last

.

Example Demo < 1 >
Adds the class "selected" to the matched elements.

Hello

and

Goodbye