JSFiddle - React, Tailwind, and code Playground

HTML

<div id='panel1'>
text to hidden
</div>

<button onclick="testFunction()">Hide</button>

JavaScript

function simulateComplexOperation(sleepDuration) {
  var now = new Date().getTime();
  while (new Date().getTime() < now + sleepDuration) { /* do nothing */ }}		
   
function testFunction() {   
document.getElementById('panel1').style.display = 'none';
console.log('before');
simulateComplexOperation(2000);
console.log('after');
}