Add a class to a DIV with javascript

Question

Example HTML:

<div id="foo" class="class_one"></div> 

How can I add the class class_two without replacing class_one?

end result:

<div id="foo" class="class_one class_two"></div> 

Answer 1

See deekshith's answer below if you only need to support new browsers

Standard javascript:

document.getElementById('foo').className += ' class_two' 

or JQuery:

$('#foo').addClass('class_two');