Instead of adding a class via javascript for implementing some kind of animation, we can 'simulate' this extending a 'Silent class' with Sass.

gistfile1.scss

.popup-container:hover > .popup{
  @extend %fadeIn;
}
.popup{
  visibility: hidden;
  /*Some other styles*/
}

/*Silent Class*/
%fadeIn{
  visibility: visible;
  animation-name: fadeIn;
  animation-duration: .2s;
  animation-timing-function: ease-in-out ;
}

/*Some Keyframe animation*/
@keyframes fadeIn{
  0%{
    opacity: 0.0;
    transform: scale(0);
  }
  60%{
    transform: scale(1.1);
  }
  80%{
    transform: scale(0.9);	
    opacity: 1;
  }
  100%{
    transform: scale(1);
    opacity: 1;
  }
}