Styling application in JavaScript

Let's make a button, by clicking on which the element will be either shown or hidden. Let the element be hidden by default (this is implemented using display: none), and it will be shown by adding the class active. This class will be either added or removed using classList.toggle:

html<button id="button">click me</button>
<div id="elem"></div>

css#elem {
  display: none;
  width: 200px;
  height: 200px;
  border: 1px solid green;
}
#elem.active {
  display: block;
}

javascriptlet button = document.querySelector('#button');
let elem = document.querySelector('#elem');

button.addEventListener('click', function() {
  elem.classList.toggle('active');
});

Результат: