How to get class name through JavaScript

Here i am explaining how to get class name through JavaScript using id and u can also change class name.
How do get the class name through JavaScript given no id with this span.

Like: This is a simple span, here i am getting class name using id of span.

    var className=document.getElementById("xyz").className;
    if(className=="xyz")
    {
       alert(className);
    }
    else
    {
        document.write("This is not a valid class name");
        alert(className);
    }