如何在列表打开或关闭时添加减号图标？

<ul class="expandable">
  <li>列表项1</li>
  <li>列表项2</li>
  <li>列表项3</li>
</ul>

.expandable > li:before {
  content: "+";
  background-image: url(加号图标路径);
  background-repeat: no-repeat;
  background-position: center;
  padding-right: 10px;
}

.expanded > li:before {
  content: "-";
  background-image: url(减号图标路径);
}

var expandableList = document.querySelector('.expandable');
expandableList.addEventListener('click', function(event) {
  if (event.target.tagName === 'LI') {
    event.target.parentNode.classList.toggle('expanded');
  }
});