Code inside DOMContentLoaded event not working

Question

I have used

Answer 1

https://learnwithparam.com/blog/vanilla-js-equivalent-of-jquery-ready/

function ready(callbackFunc) {
  if (document.readyState !== 'loading') {
    // Document is already ready, call the callback directly
    callbackFunc();
  } else if (document.addEventListener) {
    // All modern browsers to register DOMContentLoaded
    document.addEventListener('DOMContentLoaded', callbackFunc);
  } else {
    // Old IE browsers
    document.attachEvent('onreadystatechange', function() {
      if (document.readyState === 'complete') {
        callbackFunc();
      }
    });
  }
}

ready(function() {
  // your code here
});