select.js

function select(selector) {
  // Checks select(01) and select('')
	if(typeof selector !== 'string' || selector.length < 2) {
  	return false;
  }

	var el = document.querySelector(selector) || null;
  var allEl = document.querySelectorAll(selector) || [];

  if (el === null && allEl.length === 0) {
  	return false;
  }
  
  if(allEl.length === 1) {
  	return el;
  } else {
  	return allEl;
  }
}

const btn = select('.ad');

if (btn) {
  btn.addEventListener('click', (e) => {
    console.log(e);
  })
}