JSFiddle - React, Tailwind, and code Playground

HTML

<div class="one">Hover</div>
<div class="two">
    <ul class="list">
        <li>list item</li>
        <li>list item</li>
        <li>list item</li>
        <li>list item</li>
    </ul>
</div>

CSS

.one {
    width:70px;
    height: 70px;
    background: #ccc;
    margin-bottom: 10px;
}
.two {
    width:0;
    height: 70px;
    background: #ccc;
}
ul {
    display: none;
}

JavaScript

$(".one").hover(function () {
    $(".two").animate({
        width: "400px"
    });
    $(".list").fadeIn();
});