If you’re like me and come from jQuery to pure JavaScript, these helper functions might be very helpful for you:

function hasClass(el, className) {
    if (el.classList)
        return el.classList.contains(className);
    else
        return !!el.className.match(new RegExp('(\\s|^)' + className + '(\\s|$)'));
}

function addClass(el, className) {
    if (el.classList)
        el.classList.add(className);
    else if (!hasClass(el, className)) 
        el.className += " " + className;
}

function removeClass(el, className) {
    if (el.classList)
        el.classList.remove(className);
    else if (hasClass(el, className))
        el.className=el.className.replace(new RegExp('(\\s|^)' + className + '(\\s|$)'), ' ');
}