When listening to transition End event, the event object comes with propertyName. This is useful when detecting just what transition has completed.

elem.addEventListener( 'transitionend' function( event ) {
  console.log( event.propertyName + 'transition completed' );
});

Also interesting is how the transition end event will only trigger once if a property gets changed again, during a previous transition.

See the Pen transtionEnd by David DeSandro (@desandro) on CodePen