This function will append or remove a class from an element cleanly by ensuring whitespace is how it should be.

classChanger.js

/**
 * Attribution: Alexander Gryson (agryson.net)
 * Appends or removes a class to/from an element
 * @param  {boolean} bool        True if appending class, false if removing
 * @param  {string} targetID    ID of target element
 * @param  {string} classString Class to be added / removed (case sensitive)
 */

function classChange(bool, targetID, classString){
    var target = document.getElementById(targetID);
    var targetPresent;
    var newClass;
    if (target.className.indexOf(classString) > 0){
        newClass = ' ' + classString;
        targetPresent = true;
    } else if(target.className.indexOf(classString) === -1 && target.className.length > 0){
        newClass = ' ' + classString;
        targetPresent = false;
    }else if(target.className.indexOf(classString) === -1){
        newClass = classString;
        targetPresent = false;
    }else{
        newClass = classString;
        targetPresent = true;
    }

    if(targetPresent && !bool){
        target.className = target.className.replace(newClass, '');
    } else if (!targetPresent && bool){
        target.className += newClass;
    }
}

Alternatively, as I've recently discovered, all of this is also natively supported by pure JavaScript