Detect click outside of element

<div class="popup-menu">
	<button class="popup-menu__button"></button>
	<ul class="popup-menu__list hide-popup">
		...
	</ul>
</div>

const popupWrapper = document.querySelector('.popup-menu');
const popupMenu = document.querySelector('.popup-menu__list');

// Run function when user clicks anywhere
document.addEventListener('click', hidePopup);

function hidePopup(el) {
	// Check if there is a menu and a button inside what was clicked on
	let targetInside = popupWrapper.contains(el.target);
	// If not, then hide the menu
	if (!targetInside) {
		popupMenu.classList.add('hide-popup');
	}	
}