How do I add a class to a given element?

Question

I have an element that already has a class:

Answer 1

If you don't want to use jQuery and want to support older browsers:

function addClass(elem, clazz) {
    if (!elemHasClass(elem, clazz)) {
        elem.className += " " + clazz;
    }
}

function elemHasClass(elem, clazz) {
    return new RegExp("( |^)" + clazz + "( |$)").test(elem.className);
}