Возвращение значения, которое было передано в метод

  var totalItems = $('.item').length;
            var currentIndex = $('div.item.active').index() + 1;

            var down_index;
            $('.num').html(''+currentIndex+'/'+totalItems+'');

                $(".next").click(function(){
                currentIndex_active = $('div.item.active').index() + 2;
                if (totalItems >= currentIndex_active)
                {
                    down_index= $('div.item.active').index() + 2;
                    $('.num').html(''+currentIndex_active+'/'+totalItems+'');
                }
            });

                $(".prev").click(function(){
                    down_index=down_index-1;
                if (down_index >= 1 )
                {
                    $('.num').html(''+down_index+'/'+totalItems+'');
                }
            });

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>


 <div class="num"></div>

<div id="carouselExampleIndicators" class="carousel slide" data-ride="carousel" data-wrap="false" data-interval="false">
  <ol class="carousel-indicators">
    <li data-target="#carouselExampleIndicators" data-slide-to="0" class="active"></li>
    <li data-target="#carouselExampleIndicators" data-slide-to="1"></li>
    <li data-target="#carouselExampleIndicators" data-slide-to="2"></li>
  </ol>
  <div class="carousel-inner">
    <div class="carousel-item item active">
      <img class="d-block w-100" src="https://via.placeholder.com/500x300" alt="First slide">
    </div>
    <div class="carousel-item item">
      <img class="d-block w-100" src="https://via.placeholder.com/500x300" alt="Second slide">
    </div>
    <div class="carousel-item item">
      <img class="d-block w-100" src="https://via.placeholder.com/500x300" alt="Third slide">
    </div>
  </div>
  <a class="carousel-control-prev prev" href="#carouselExampleIndicators" role="button" data-slide="prev">
    <span class="carousel-control-prev-icon" aria-hidden="true"></span>
    <span class="sr-only">Previous</span>
  </a>
  <a class="carousel-control-next next" href="#carouselExampleIndicators" role="button" data-slide="next">
    <span class="carousel-control-next-icon" aria-hidden="true"></span>
    <span class="sr-only">Next</span>
  </a>
</div>