code: function addClass(element, className) {
      if (!hasClassNameProperty(element) || hasClass(element, className)) {
          return;
      }
      element.className = (element.className + ' ' + className).replace(/^\s+|\s+$/g, '');
  }
docs: Adds a class name to the element's list of class names.
  
  @static
  @param  {DOMElement} element The DOM element to modify.
  @param  {string} className The name to append.
  @example
  
  hasClass(el, 'container');
  // => false
  
  addClass(el, 'container');
  
  hasClass(el, 'container');
  // => true