plain javascript, document ready

<script type="text/javascript">
document.onreadystatechange = () => {
  if (document.readyState === 'complete') {
	// Script to hide/show menu
	var button = document.querySelector('#menu-button');
	console.log('b11:', button);
	var menu = document.querySelector('#menu');
	button.addEventListener('click', function (event) {
		  if (menu.style.display == "") {
			  menu.style.display = "none";
			  button.innerHTML = "Show Menu";
		  } else {
			  menu.style.display = "";
			  button.innerHTML = "Hide Menu";
		  }
		}
	  );

  }
};
</script>
<button id="menu-button">Hide Menu</button>
<nav id="menu">
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
  </ul>
</nav>