that depends, but one way is like this
$('.Navigation-listItem').click(function(e) {
if( $(this).children('.Navigation-list--dropdown').hasClass('is-hidden') ){
$(".Navigation-list--dropdown").addClass('is-hidden');
$(this).children('.Navigation-list--dropdown').removeClass('is-hidden');
} else {
$(".Navigation-list--dropdown").addClass('is-hidden');
}
}).children('a.Navigation-link--dropdownTrigger').click(function(e){e.preventDefault();}); // add this line