如何从Vanilla Javascript打开Boostrap 3模式框

<!-- 按钮触发模态框 -->
<button type="button" class="btn btn-primary" id="openModalBtn">
  打开模态框
</button>

<!-- 模态框结构 -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">模态框标题</h4>
      </div>
      <div class="modal-body">
        模态框内容
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">关闭</button>
        <button type="button" class="btn btn-primary">保存</button>
      </div>
    </div>
  </div>
</div>

document.getElementById('openModalBtn').addEventListener('click', function() {
  $('#myModal').modal('show');
});