How do I make dynamic classNames in an ember 2.0 component?

The @dmk'solution is the cleanest one, but if your scenario it is not working you can use classNameBindings:

export default Ember.Component.extend({
  classNameBindings: ['getClassNames'],
  getClassNames: Ember.computed('extra-classes', function(){
    return this.get('extra-classes').replace(',', ' ');
  })
})

You can add class names simply by specifying them inside the class attribute on your component:

{{new-div class="class1 class2"}}