function getClass(key = '', classes, classPrefix) {
  if (!isUndefined(classes) && !isUndefined(classes[key])) {
    if (classes[key] === false) {
      return '';
    }
    return classes[key];
  } else if (classPrefix) {
    return `${classPrefix}-${key}`;
  } else {
    return key;
  }
}
Get class string based on previously determined classes
@param  {String} [key=''] - default value for the classes object
@param  {Object} classes
@param  {String} classPrefix