<div class="burger"> 
  <ul class="burger-list">
    <li class="burger-list__item">1</li>
    <li class="burger-list__item">2</li>
    <li class="burger-list__item">3</li>
  </ul>
</div>

.burger:before, .burger_active:before {
  content: "☰";
  position: absolute;
  left: 5px;
  top: 5px;
}

.burger_active:before {
  content: "x";
}

$(".burger").click(function(){
  $(this).toggleClass("burger_active");
});