JSFiddle - React, Tailwind, and code Playground

HTML

<body>
<div id="detection">
adblock not detected
</div>
<br><br><br><br>
<div class="bottom-ads">
SOMETHING INSIDE AD ELEMENT (getting blocked)
</div>
SOMETHING OUTSIDE AD ELEMENT (visible)
</body>

JavaScript

setInterval(function(){
	var adblock = isHidden(document.getElementsByClassName('bottom-ads'));

	if (adblock)
  	document.getElementById('detection').innerHTML = "ADBLOCK DETECTED";
}, 1000);

function isHidden(el) {
  var style = window.getComputedStyle(el[0], null);
  return (style.display === 'none')
}