JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu" id="menu">
    Text
<button onclick="zobrazSkryVseobecne('hidden');" onfocusout="zobrazSkryVseobecne('hidden')">Menu</button>
        </div>
        <div class="skryt" id="hidden"><a href="#">Hidden text</a></div>
        <div class="normalny">
        Normal text
        </div>

CSS

.menu + div {height: 200px; background-color: rgb(0, 0, 0); width: 100%; position: absolute}
.menu + div a {display: block; width: 100%; color: rgb(255, 255, 255)}
.menu button {display: block; float: right; height: 100%; width: 50%}
.skryt {display: none;}

JavaScript

function zobrazSkryVseobecne(idecko) {
    var miesto = document.getElementById(idecko);
    miesto.className = (miesto.className === "skryt" ? "" : "skryt");
}