<div id="app">
  <button type="button" id="toggle" aria-expanded="false">
    Toggle Panel
  </button>
  <p class="hello">hello</p>
</div>

#app {
  margin: 50px;
}

p {
  padding: 10px 12px;
  border: 1px solid #ccc;
  width: 100px;
}

$(function() {
  $('#toggle').on('click', function() {
    $('.hello').toggle();
    $(this).attr('aria-expanded', ($(this).attr('aria-expanded') == "false" ? true : false));
  });
});