JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main_menu">Main Menu


<div id="submenu">Some submenu contents here</div>
</div>

CSS

#main_menu {
    width: 500px;
    height: 30px;
    background-color: #AAA;
}
#submenu {
    width: 500px;
    height: 20px;
    display: none;
    background-color: #CCC;
    position:absolute;
    top:30px;
}

JavaScript

$('#main_menu').hover(function () {
    $('#submenu').stop(true, false).slideDown();
},
function () {
    $('#submenu').stop(true, false).slideUp();
});