JSFiddle - React, Tailwind, and code Playground

HTML

<div id="area">
  <button type="button" class="btn" id="btn1">Play With Me!</button>
  <button type="button" class="btn" id="btn2">Play With Me!</button>
</div>

JavaScript

var button = document.getElementsByTagName("button");

for (var i = 0; i < button.length; i++) {
 button[i].addEventListener("click", func);
}

function func() {
  this.style.backgroundColor = 'gray';
}