function addClass(element,value) {
   if (!element.className) {
      element.className = value;
} else {
   newClassName = element.className;
   newClassName+= " ";
   newClassName+= value;
   element.className = newClassName;
   }
}

Description: A function that add a class to an element if the element doesn't have anyone, if it already has one, add a new class name.
Arguments: The elemento to which you want to add the class and the name of the class you want to add.

From book Dom Scripting by Jeremy Keith.

addClass function

javascript, style

JavaScript