JSFiddle - React, Tailwind, and code Playground

HTML

<div>
      <input type="button" value="Click" onClick="buttonHandler()"/>
      
      <div id="progress">I show the activity of the button above</div>

JavaScript

function defer(f, callback) {
  var proc = function() {
    result = f();
    if (callback) {
      callback(result);
    }
  }
  setTimeout(proc, 50);
}

function buttonHandler() {
  var elm = document.getElementById("progress");
  elm.innerHTML = "thinking...";
  defer(longPrimeCalc, function (isPrime) {
    if (isPrime) {
      elm.innerHTML = "It was a prime!";
    }
    else {
      elm.innerHTML = "It was not a prime =(";
    }
  });
}
  
function longPrimeCalc() {
  function isPrime (n) {
      if (n < 2) return false;
      var q = Math.sqrt (n);
      for (var i = 2; i <= q; i++)
          if (n % i === 0)
              return false;
      return true;
  }
  return isPrime(1125899839733759);
}