jquery-6: next, prev

<button class="left">아래</button>
<button class="right">위</button>


<section>
      <div class="a active"></div>
      <div class="a"></div>
      <div class="a"></div>
      <div class="a"></div>
      <div class="a"></div>
</section>

div.a {
  background-color: green;
  width: 300px;
  height: 50px;
  border: 10px solid blue;
}

div.a.active {
  border-color: black;
}

$("button.left").click(function() {
  let $now = $('div.a.active');
  let $next = $now.next();
  
  if($next.length == 0) {
    return false
  };
  
  $next.addClass('active');
  $now.removeClass('active');
})

$("button.right").click(function() {
  let $now = $('div.a.active');
  let $next = $now.prev();
  
    if($next.length == 0) {
    return false
  };
  
  $next.addClass('active')
  $now.removeClass('active');
})