鼠标单击时折叠箭头

var arr = document.querySelector('.arrow');
arr.addEventListener('click', function(event) {
  event.target.classList.toggle('down');
});

.arrow {
  margin: 1em;
}

.arrow::before {
  position: absolute;
  content: '';
  width: 0;
  height: 0;
  border: .5em solid transparent;
  border-left-color: gray;
  transform-origin: 0 50%;
  transition: transform .25s;
}

.arrow.down::before {
  transform: rotate(90deg);
  transition: transform .25s;
}

<div class="arrow"></div>