仅当打开时切换sidenav菜单

<div class="sidenav">
  <!-- 导航菜单内容 -->
</div>
<button class="open-btn" onclick="toggleSidenav()">打开</button>

.sidenav {
  width: 200px;
  height: 100%;
  background-color: #f1f1f1;
  position: fixed;
  left: -200px;
  top: 0;
  transition: left 0.3s;
}

.open-btn {
  position: fixed;
  left: 0;
  top: 0;
  z-index: 999;
}

function toggleSidenav() {
  var sidenav = document.querySelector('.sidenav');
  if (sidenav.style.left === '-200px') {
    sidenav.style.left = '0';
  } else {
    sidenav.style.left = '-200px';
  }
}