JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="btn">toggle tlacitko</div>
  <div class="menu">menu</div>
</div>

CSS

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.container {
  position: relative;
  /*background-color: yellow;*/
  border:2px solid yellow;
  width: 50vw;
  height: 50vh;
}
.container .btn {
    position: absolute;
    top: 0;
    right: 0;
    z-index: 1;
    width: 100px;
    height: 100px;

      border:2px solid red;
    cursor:pointer;
  }

.container .menu {
    position: absolute;
    top: 0;
    left: 0;
    z-index: 2;
    /*width: 0;*/
    height: 100vh;
    background-color: blue;
    display:none;
  }


.container .show-menu {
  /*width: 25vw;*/
      display:block;
}

JavaScript

var btn = document.querySelector('.btn');
var menu = document.querySelector('.menu');

btn.addEventListener('click', function(event) {
  var el;
  el = event.target;
  el.innerHTML = 'kliknuto';
//if(el.closest('.btn')) {
    menu.classList.toggle('show-menu');
 // }
});