JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="menu">
    <li>dfmvdfvdfv</li>
</ul>
<ul class="submenu"></ul>

CSS

.submenu,
.menu {
    height: 20px;
    background: red;
    margin-top: 10px;
}

.submenu {display: none;}
.submenu.fast {display: block;}

JavaScript

$('.menu li').hover(function(){
		$('.submenu').addClass('fast')
		}, function(){
        $('.submenu').removeClass('fast')
});