Ребят ! Почему не работает этот код?

var elem = $(".elem"), 
  elemWidth = elem.width(), 
  elemHeight = elem.height(); 
var props = { 
  "left": "calc(50% - " + elemWidth / 2 + "px)", 
  "top": "calc(50% - " + elemHeight / 2 + "px)" 
}; 
//console.log(props); 
elem.css(props);

.elem { 
  width: 200px; 
  height: 100px; 
  border: 1px solid black; 
  background: lightgreen; 
  position: absolute; 
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<div class="elem"></div>