JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<div class="btn">Click</div>
<div class='dropdown'>
    <ul>
        <li>asdf asdf asdf asdf asdf</li>
        <li>asdf asdf asdf asdf asdf asd</li>
        <li>asdf asdf asdf asdf asdf asdf</li>
    </ul>
</div>

CSS

.btn {
    background-color:#ccc;
    width:60px;
}
.dropdown {
    display:none;
    background-color:#ccc;
    display:block;
    width:300px;
}

JavaScript

$(".btn").click(function () {
    $(".dropdown").show();
});

$(document).click(function (e) {
    if (!$(e.target).hasClass("btn") 
        && $(e.target).parents(".dropdown").length === 0) 
    {
        $(".dropdown").hide();
    }
});