JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="top-menu-first-ul">
    <li>
        <a href="http://google.com">Google</a>
        <ul class="top-menu-second-ul">
            <li><a href="http://translate.google.com">Google Translate</a></li>
            <li><a href="http://images.google.com">Google Images</a></li>
            <li><a href="http://gmail.google.com">Gmail</a></li>
            <li><a href="http://plus.google.com">Google Plus</a></li>
        </ul>
    </li>
    <li>
        <a href="http://facebook.com">Facebook</a>
    </li>
    <li>
        <a href="http://twitter.com">Twitter</a>
    </li>
</ul>

CSS

ul.top-menu-second-ul{
    display:none;
}

JavaScript

$(document).ready( function() {
            $('.top-menu-first-ul li').mouseenter(mouseIn);
            $('.top-menu-first-ul li').mouseleave(mouseOut);
});
function mouseIn(e){
    var target = $(this);
    var timer = setTimeout(function() {
           target.find('ul').slideDown(100);
        }, 500);
    target.attr("data-timer", timer);
}

function mouseOut(e){
    var target = $(this);
    var timer = target.attr("data-timer");
    if(timer){
        clearTimeout(timer);
    }
    target.find('ul').slideUp(100)
}