Bootstrap 4 dropdown hover

CSS

.dropdown:hover>.dropdown-menu {
    display: block;
}

or

JavaScript

function toggleDropdown (e) {
  const _d = $(e.target).closest('.dropdown'),
    _m = $('.dropdown-menu', _d);
  setTimeout(function(){
    const shouldOpen = e.type !== 'click' && _d.is(':hover');
    _m.toggleClass('show', shouldOpen);
    _d.toggleClass('show', shouldOpen);
    $('[data-toggle="dropdown"]', _d).attr('aria-expanded', shouldOpen);
}, e.type === 'mouseleave' ? 150 : 0);
}

$('body')
  .on('mouseenter mouseleave','.dropdown',toggleDropdown)
  .on('click', '.dropdown-menu a', toggleDropdown);