如何加载一个全部关闭的手风琴，然后打开onclick？

<div class="accordion">
  <div class="panel">
    <div class="panel-header" onclick="toggleAccordion(0)">标题1</div>
    <div class="panel-content">
      内容1
    </div>
  </div>
  <div class="panel">
    <div class="panel-header" onclick="toggleAccordion(1)">标题2</div>
    <div class="panel-content">
      内容2
    </div>
  </div>
  <div class="panel">
    <div class="panel-header" onclick="toggleAccordion(2)">标题3</div>
    <div class="panel-content">
      内容3
    </div>
  </div>
</div>

.panel {
  border: 1px solid #ccc;
  margin-bottom: 10px;
}
.panel-header {
  background-color: #f0f0f0;
  padding: 10px;
  cursor: pointer;
}
.panel-content {
  display: none;
  padding: 10px;
}

function toggleAccordion(index) {
  var panels = document.getElementsByClassName('panel');
  var content = panels[index].getElementsByClassName('panel-content')[0];

  if (content.style.display === 'block') {
    content.style.display = 'none';
  } else {
    content.style.display = 'block';
  }
}