code: function addClass (el, className) {
    if (!className) return
  
    if (Array.isArray(className)) {
      each(className, function (className) {
        addClass(el, className)
      })
  
      return
    }
  
    if (el.classList) {
      var classNames = className.split(' ').filter(Boolean)
      each(classNames, function (className) {
        el.classList.add(className)
      })
    } else {
      el.className += ' ' + className
    }
  }
docs: addClass : addClass(el, className)
  Adds a class name to an element. Compare with `$.fn.addClass`.
  
  var addClass = require('dom101/add-class');
  
  addClass(el, 'active');