js-amsterdam

script.js

$('.circle-slider').find('.slider-arrow.next').on('click', function () {
  $this = $(this).closest('.circle-slider');
  var index = parseInt($this.attr('data-state')) + 1;

  if (index <= $this.find('.item').length) {
    $this.attr('data-state', index);
  } else {
    $this.attr('data-state', 1);
  }
});

$('.circle-slider').find('.slider-arrow.prev').on('click', function () {
  $this = $(this).closest('.circle-slider');
  var index = parseInt($this.attr('data-state')) - 1;

  if (index < 1) {
    $this.attr('data-state', $this.find('.item').length);
  } else {
    $this.attr('data-state', index);
  }
});

// код который правим
var
  pageY = 0,
  porog = 20;

$('.circle-slider').on('mousedown', '.item', function (e) {

  pageY = e.pageY;

  var that = e.delegateTarget;
  var $this = $(that);

  $(that).on('mousemove', function (e) {

    if (e.pageY > pageY + porog) {
      pageY = e.pageY;
      var index = parseInt($this.attr('data-state')) + 1;
      if (index <= $this.find('.item').length) {
        $this.attr('data-state', index);
      } else {
        $this.attr('data-state', 1);
      }
      $(that).off('mousemove');
    } else if (e.pageY < pageY - porog) {
      pageY = e.pageY;
      var index = parseInt($this.attr('data-state')) - 1;

      if (index < 1) {
        $this.attr('data-state', $this.find('.item').length);
      } else {
        $this.attr('data-state', index);
      }
      $(that).off('mousemove');
    }
  });
});

$('.circle-slider').on('mouseup mouseleave', function (e) {
  $(this).off('mousemove');
});